SWITCHING METHODS WITH A PLURALITY OF TIME INTERVALS

Abstract: A time frame switching method and system of data units that utilize a common time reference, which is divided into a plurality of periodic time frames. The system is designed to operate with time frames, which have a plurality of predefined time durations. The plurality of data units that are contained in each of the time frames are switched through a switch fabric and then forwarded in a pipelined manner through the network. The switch fabric is implemented in various manners, such as, electronic cross connect and optical cross connect, wherein the optical cross connect can be a combination of at least one of the following: an optical cross-bar, an optical banyan network, a Lithium-Niobate optical switch, an Indium Phosphate optical switch, a 2-D MEMS optical switch, a 3-D MEMS optical switch, a semiconductor optical amplifier (SOA) based optical switch, an holographic optical switch, Bubble optical switch, plurality of tunable lasers, and plurality of optical stars. The system operates with high-speed wavelength division multiplexing (WDM) links, i.e., with multiple lambdas, having a plurality of transmission capacities. The outcome of this method is called fractional lambda switching with a plurality of time frame durations.

Description: SWITCHING METHODS WITH A PLURALITY OF TIME INTERVALS RELATED APPLICATIONS: This is a application is based upon the continuation-in-part application, filed with the U.S.P.T.O. on September 21, 2001, for "SWITCHING METHODS WITH COMMON TIME REFERENCE AND PLURALITY OF TIME FRAME DURATIONS," under 37 C.F.R. §1.53, of pending prior application Serial No. 09/120,700, filed on 07/22/98, for "DSfTERCONNECTING A SYNCHRONOUS SWITCHING NETWORK THAT UTILIZES A COMMON TIME REFERENCE WITH AN ASYNCHRONOUS SWITCHING NETWORK," and further claims priority of pending provisional application Serial No. 60/235,765, filed on 09/27/2000, for "SWITCHING, GROOMING, AND DEGROOMING METHODS AND LINK TRANSMISSION CONTROL WITH COMMON TIME REFERENCE," and of pending provisional application Serial No. 60/261,133, filed on 01/10/2001, for "SWITCHING METHODS WITH COMMON TIME REFERENCE AND PLURALITY OF TIME FRAME DURATIONS." FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT: Not Applicable. BACKGROUND OF THE INVENTION: This invention relates generally to a method and apparatus for switching and grooming data units in a communications network in a timely manner while providing low switching complexity and performance guarantees, over a plurality of communications links with a plurality of transmission rates. Circuit-switching networks are still the main carrier for real-time traffic, are designed for telephony service, and cannot be easily enhanced to support multiple services and carry multimedia traffic in their native packet formats. Circuit-switching is based on extremely accurate clock frequencies needed for byte-by-byte switching. This enables circuit-switching networks to transport data streams at constant rates with a small delay jitter. Finally, the clock accuracy for SONET requires a heightened amount of accuracy as line transmission speed increases. Due to the statistical multiplexing of packet streams, packet switching networks handle bursty data more efficiently than do circuit-switching networks. However, current packet switches and routers operate asynchronously, and only provide "best effort" service in which end-to-end delay and jitter can be neither guaranteed nor bounded. Furthermore, statistical variations of traffic intensity in packet switching networks often lead to congestion that results in excessive delays and loss of packets, thereby significantly reducing the fidelity of real-time streams at their respective points of reception. Finally, since current packet switches and routers electronically process the header of each packet to be routed and switched, a high processing power is required and the scalability of the packet switching network is limited. In circuit switches routing is time-based. A time period, called frame, is divided into very small slices of time, called slots that contain only one byte of information. The absolute position of each time slice within each time period determines where its respective byte is routed. Time driven switching/routing, in accordance with some aspects of the present invention, supports more sophisticated and flexible timing than circuit switching. Consequently, time driven switching provides better support of video-based multimedia applications. In accordance with the present invention, the size of time frames used for time driven switching is larger than the size of time slots used in circuit switching; consequently, time driven switching is far less complicated than circuit switching. In accordance with the present invention, time driven routing is based on control information included in at least one of the headers and trailers of selected ones of the time frames, which current circuit switching cannot provide for. In accordance with the present invention, time driven switching/routing uses a Common Time Reference (CTR). The CTR, though used in time-driven switching, is not used in circuit switching. The above discrepancy between time driven switching and circuit switching has far reaching implications during comparison of the two. For example, use of a CTR in a network deterministically ensures that time frames as defined by different nodes do not slip, thus enabling the deterministic pipeline forwarding of time frames. The above assurances of a CTR exist in contrast to the properties of a circuit-switching network, where (1) there are time slot slips, and (2) deterministic pipeline forwarding is not possible. In U.S. Pat No. 5,418,779 Yemini et al. disclose a switched network architecture that uses time. Time is used in order to determine when a plurality of switches can transmit over a predefined routing tree to one destination. This kind of tree is known as "sink" tree since the destination switch functions as a "sink" for the transmission from all switches. The time interval in which the plurality of switches transmits to a selected "sink" destination switch is called time band. In different time bands the plurality of switches are transmitting to a different single "sink" destination switch. Network switches change their configuration between time bands in order to build the proper "sink" tree during each time band. The present invention does use neither "sink" trees nor time bands for transmission over "sink" trees. Yemini's invention may not be realizable in communications networks with end- to-end propagation delays that are not much smaller than the time band durations. In general, in Yemini's invention the end-to-end propagation delays introduce a non-trivial scheduling problem that may or may not have a solution. Furthermore, Yemini's invention does not discuss or specify how to take into consideration the link propagation delays and the end-to-end propagation delays. Consequently, general topology switched network cannot be built the way it is taught by Yemini's et al. invention. Yemini's invention has another problem, which is congestion, that is the direct result of using "sink" trees. Data units received from different upstream switches contend for a single outgoing link towards the root of the "sink" tree. The present invention does not have any congestion. This is a direct consequence of using in the current invention completely different system operation principles and methods. For example, in Yemini's et al. patent there is no pipeline forwarding: data units do not proceed in a lock-step fashion through the communications network, as it is the case in the present invention. The lack of pipeline forwarding leads to the above mentioned scheduling and congestion problems. Such problems are due to the fact that incoming time bands of Yemini's invention are not aligned in different input ports of the network's switches. Furthermore, it was not specified what are the temporal relationship of the same and different time bands on different "sink" tree switches when the link propagation delay and the end-to-end propagation delay are not zero. In contrast, time frames in the present invention are aligned with a Common Time Reference (CTR) on every switch. Optical data communications using a single wavelength consist of the transmission of a single data stream into a series of pulses of light that are carried over an optical fiber. These pulses of light are of a single wavelength. Use of a single wavelength vastly under-utilizes the capacity of an optical fiber, which is capable of carrying a large number of signals each at a unique wavelength. Due to the nature of propagation of light signals, an optical fiber can carry multiple wavelengths simultaneously. The process of carrying multiple discrete signals via separate wavelengths of light on the same optical fiber is known in the art as wavelength division multiplexing (WDM). Many optical components, including, but not limited to, WDM multiplexers, WDM de-multiplexers, star couplers, tunable lasers, filters, wavelength waveguide grating routers (WGRs) are deployed in optical networks featuring WDM, and consequently used in the embodiments presented in this disclosure. [T. E. Stern and K. Bala, "Multiwavelength Optical Networks: a Layered Approach," Prentice Hall PTR, Upper Saddle River, NJ, USA, ISBN 020130967X. R. Ramaswami and K. N. Sivarajan, "Optical Networks: a Practical Perspective," Morgan Kaufmann Publishers, San Francisco, CA, USA, ISBN 1-55860-445-6. H. J. R. Dutton, "Understanding Optical Communications," Prentice Hall PTR, Upper Saddle River, NJ, USA, ISBN 0-13-020141-3]. The present invention contains the novel combination of: (1) time-based switching and routing and (2) WDM technology. WDM includes the capabilities for (1) dynamic tunable wavelength transmission, (2) dynamic and static wavelength switching, and (3) tunable wavelength reception. The increasing demand for communications capacity has led to general deployment of Wavelength Division Multiplexing (WDM), which requires extremely high capacity switches. Lambda or static wavelength switches address this need for extremely high switching capacity by switching a whole wavelength from an input optical fiber link to an output optical fiber link without requiring any processing of the transmitted data units. In accordance with the present invention, WDM with whole lambda switching will be deployed in the network's optical core. However, switching of whole lambdas (e.g., lambdas of OC-192) is inefficient and costly because of three inherent problems it causes: N square problem: the number of lambdas needed to accommodate all the possible connections among all access points is on the order of the square of the number of such access points. This will limit the size of the optical core. Bandwidth mismatch problem: there is a substantial bandwidth mismatch when extremely high capacity backbone networks feed low capacity access links. As data leaves the core and is moved by packet switches towards the edge, buffers at access links frequently become congested, causing increased delays and dropped packets. Traffic unbalancing problem: the traffic load across the network is not evenly distributed, i.e., it is not balanced. Thus, trying to satisfy the traffic load requirements using whole lambda switching remains both inflexible and inefficient. Time driven switching adds the capability of switching fractions of lambdas, or Fractional Lambda Pipes (FLPs), thereby solving the above three problems. This approach, also called Fractional Lambda Switching (FLSw), will permit the optical core to be extended far closer to the network edges while reaching the lower speed network access devices with a bandwidth that matches their operation capability. FLSw dynamically switches lambda fractions while carrying data units (e.g., IP data packets, and SONET STS1 frames), in a heterogeneous (mix of very high speed and very low speed links) meshed network, while providing deterministic performance guarantees. The size of fractional lambda pipes can be dynamically allocated to satisfy the specific needs of the access networks to which a fractional lambda pipe is connected. Small capacity FLPs can be used at the periphery to access low speed subnetworks, such as, cable modems, xDSL, VoIP gateways and wireless. Fractional Lambda Switching (FLSw) combines the advantages of circuit switching and packet switching. FLSw is used for constructing a Fractional Lambda Pipe (FLP). A FLP is equivalent to a leased line in circuit switching. A FLP is realized by two simple elements: A Common Time Reference (CTR) throughout the network that is globally aligned with Coordinated Universal Time (UTC); and Pipeline Forwarding (PF) of time frames (logical containers of data packets) across FLPs. The CTR is a reference clock used to realize pipeline forwarding of time frames, both within switches and across FLPs. The CTR is received via either the Global Positioning System (GPS), which is globally available at a low cost with an accuracy of 10-20 nanoseconds, or any other UTC (Coordinated Universal Time) distribution system such as the GLONASS (Russia) and, in the future, Galileo (Europe). The common time reference or more specifically, the UTC second, is partitioned into time frames. The duration of a time frame is a link parameter - fast links might use shorter time frames, while slow links might use longer time frames. Contiguous time frames are grouped into time cycles, and contiguous time cycles are grouped together into contiguous super cycles. The duration of a super cycle is one UTC second, as shown in Fig. 2, and both the duration of time frames and the number of time frames in a cycle can be chosen for convenience. For example, a 1 Gb/s link might use time cycles of 100 time frames, each time frame having a duration of 125 μs; while a 10 Gb/s link might use time cycles of 1000 time frames, each time frame having a duration of 12.5 microseconds. For both links, each time frame will carry the same 15,625-byte payload, and there will be 80 time cycles in each super cycle or one UTC second, as shown in Fig. 2. The common time reference can be realized by using UTC (Coordinated Universal Time), which is globally available via, for example GPS (Global Positioning System). By international agreement, UTC is the same all over the world. UTC is the scientific name for what is commonly called GMT (Greenwich Mean Time), the time at the 0 (root) line of longitude at Greenwich, England. In 1967, an international agreement established the length of a second as the duration of 9, 192,631 ,770 oscillations of the cesium atom. The adoption of the atomic second led to the coordination of clocks around the world and the establishment of UTC in 1972. The Time and Frequency Division of the National Institute of Standards and Technologies (NIST) (see http://www.boulder.nist.gov/timefreq) is responsible for coordinating UTC with the International Bureau of Weights and Measures (BIPM) in Paris. Each FLP's switching schedule is simple, and repeats every time cycle and/or super cycle. Thus, FLPs, together with the predictability provided by the CTR and pipeline forwarding, eliminate the complexity of data packet header processing. Each FLP transports data packets of one protocol, such as IP, MPLS, ATM, FR, or FC. However, different FLPs may carry data units of different protocols, i.e., data units of different protocols can travel across a selected link. Fractional lambda switches have significantly lower complexity than both packet switches and circuit switches with comparable switching capability for the following reasons: Minimum switch fabric complexity is implemented using a Banyan network, which has the complexity of a-N-lg„N switching elements, where N is the total number of optical channels and 'a' is the size (number of inputs outputs) of each switching element. Optimal speed-up of the switch fabric that operates at the same speed as the optical channels (e.g., 10 Gb/s with OC-192 links). Optimal memory access bandwidth is equal to the optical channel bandwidth - the switch architecture enables that, with only 3 input queues, a queue is never used for reading and writing at the same time, i.e., memory access with a speedup of 1. (Very) small input memory for each optical channel (e.g., a 10 Gb/s channel requires 3 input queues with a total of 48 Kbytes of memory) and no buffering at the output port are needed. (Very) simple control of the switch fabric suffices, since its configuration changes at a relatively low frequency (e.g., 80,000 times per second) and it is known in advance. This operation complexity is comparable to that of a TI multiplexer. Though highly efficient, a Banyan Network is subject to what is known as switch blocking: it may be impossible to connect an idle input with an idle output because a switching element is not available on the path between input and output. An interesting attribute of fractional lambda switching is the nearly complete elimination of blocking through Banyan-based switches. Advances in optical transport have led to the realization of high-speed optical channels (e.g., OC-192 at 10 Gb/s); however, a single source transmitting to a single destination will not utilize the capacity of such channels. In accordance with the present invention, the following two basic requirements have been formed to enable the full utilization of such high capacity channels: (i) Grooming and degrooming: the need to aggregate (i.e., grooming) traffic from multiple sources into one optical channel and to separate (i.e., degrooming) an optical channel traffic to different destinations; (ii) Dynamic optical switching: the need to route portions from one optical channel (i.e., a lambda or a wavelength) on different optical paths to different destinations. Dynamic all-optical switching is appealing for a number of reasons, stemming from the transparency of the transported data stream to the switching system itself; such reasons being: (i) intrinsically protocol independent (multi-protocol) transport; (ii) high scalability, since the transmission rate of each optical channel is transparent to the optical switching system; and (iii) no processing performed on switched data units, thus eliminating processing bottlenecks. Dynamic all-optical switching is possible when the optical switch reconfiguration time is significantly smaller than the time between two successive switch configuration changes. The latest advances in optical switching have resulted in decreasing reconfiguration times of optical switch fabrics. However, taking full advantage of such advances for dynamic optical switching is not obvious, for several reasons: (i) Processing of in band control information, e.g., packet headers, is not possible; (ii) Dynamic optical storage is not available to assist in coping with output port contention and switch control and reconfiguration time; (iii) Optical switch reconfiguration time should be significantly smaller than the time between two successive reconfigurations. Due to the above limitations it is not possible to realize an asynchronous packet switching system, and therefore, using time is necessary. However, time-based techniques deployed in circuit switching (e.g., SONET, based on byte switching, i.e., byte de-multiplexing and byte multiplexing) are not applicable to all-optical switches. The most comprehensive solution to the above-mentioned problems is use of a common time reference (CTR) for pipeline forwarding (PF) in order to facilitate dynamic all-optical switching. CTR provides the synchronization needed to orchestrate the control of network switches while eliminating the need for optical storage and processing. Dvnamic all-optical switching of time frames Time is divided into time frames and any time frame of a sequence of incoming time frames over one optical channel can be optically switched to any outgoing optical channel. Such time frame switching is the basis of fractional lambda switching (FLSw). FLSw is used for constructing Fractional lambda pipes (FLPs), i.e., fractions of a wavelength. Each FLP transports data units of one protocol — such as, IP, MPLS, ATM, FR, FC, and SONET frames (e.g., STS1 frame) -, thereby realizing the desired protocol independent property of all-optical switching. An all-optical switch realizes PF in two operational phases. Data units belonging to a whole time frame received from each of the optical channels during the first phase, Phase 1, are switched through the switch in the second phase, Phase 2. In a possible embodiment, if Phase 1 begins in time frame t, Phase 2 takes place in time frame t+1. In another embodiment, if Phase 1 ends in time frame t, Phase 2 takes place in time frame t+1. The 2-phase operation ensures that data units received from the various optical channels are aligned with the CTR before being switched. Phase 2 can be performed during either the time frame immediately following Phase 1, during time frame t+1 — immediate forwarding operation, or at a later time frame — non-immediate forwarding operation. Alignment involves aligning the beginning and end of each time frame from each optical channel with the beginning and end of the CTR time frames. Alignment can be performed either independently on each one of the channels carried by an optical link, or collectively on all of the channels carried by an optical link. Alignment is needed because the propagation delay on optical links between switches is not an integer multiple of time frames. An optical alignment subsystem is part of the all-optical fractional lambda switch, operates on all the wavelengths carried by each optical fiber, and contributes to the realization of Phase 1 of the PF. The optical alignment subsystem is based on a programmable optical delay line guaranteeing that the overall delay experienced through the optical fiber and the delay line is an integer number of time frames. Data units leave the switch at the transmitting end of a selected optical fiber aligned with the CTR. When they arrive at the output of the optical alignment subsystem at the receiving end of the selected optical fiber, they are still aligned with the CTR. The alignment system is comprised of a controller that detects time frame delimiters and adjusts the delay by using a programmable optical delay line (note: the alignment changes only when the propagation delay on the optical link changes). The availability of a common time reference (CTR) on a global scale for all network nodes enables the implementation of dynamic optical switches with a simple architecture. This architecture is used to realize fractional lambda switching (FLSw). FLSw is based on a common time reference (CTR) for pipeline forwarding (PF) of time frames. In FLSw, the synchronization provided by the CTR is leveraged to orchestrate the operation of subsystems within a switch and across the whole network. SUMMARY OF THE INVENTION: Some aspects of the present invention utilize an alignment feature within an input port for aligning incoming data units to a time frame boundary prior to the entry to a switch fabric. In a possible embodiment the alignment feature is designed using electrical components, such as random access memory (RAM) and digital circuitry. In another embodiment the alignment feature is designed using optical components, such as optical delay lines. According to one aspect of the invention disclosed herein, the switching method can be applied to the switching of data units between optical channels wherein the time frame duration on one optical channel is different from the time frame duration on the other optical channels. Moreover, the switching method can be applied to the switching of data units between optical channels wherein capacity of one optical channel is different from the capacity of the other optical channels. Advances in optical transport have lead to the realization of ever-larger optical channels (40 Gb/s trials are being conducted) whose capacity is utilized by many traffic sources. The increasing amount of optical channel capacity has resulted in the need for the following: (i) grooming - the aggregation of traffic from a large number of sources into one optical channel; (ii) degrooming - the separation of traffic from one optical channel to many different destinations. For example, when considering a 10 Gb/s high-speed optical channel and a 1 Mb/s "high-speed" residential access, such as the one provided through xDSL services, the traffic from 10,000 such accesses will be groomed in order to fill up a single optical channel. This creates the demand for a large amount of grooming and degrooming equipment with high interface density and low cost per interface. Grooming of data units from a large number of low speed sources to fill up a high capacity optical channel is a major challenge for network designers. Grooming and degrooming of optical channels cannot be performed in the all-optical domain (i.e., it should be done electronically with either a packet-based or a time-based approach for multiplexing and de-multiplexing). This patent discloses that the availability of a common time reference (CTR) enables the implementation of simple and scalable time-based grooming and degrooming equipment that will interface to various optical core networks. In accordance with the present invention, the synchronization requirements are independent of the physical link transmission speed while in circuit switching the synchronization becomes more and more difficult as the link speed increases. In accordance with some aspects of the present invention routing is performed based on timing information; in accordance with some aspects of the present invention routing is to be additionally based on information contained in at least one of an header and a trailer of time frames. A control mode is provided by the present invention where at least one time frame or a fraction of a time frame — called control time frame and control sub-time frame, respectively — comprises signal information to establish, maintain, and disestablish (or destroy) a reserved traffic channel. The system decodes and is responsive to the control information in the time frame or time frame fraction. The switches of the present invention respond, when able, by at least one of establishing a reserved data channel, establishing a reserved transfer bandwidth, and reserving capacity for the traffic associated with the control information. Analogously, terminating control signals to each switch in a plurality of connected switches causes the switches of the present invention to respond by at least one of destroying, reallocating, and reclaiming the data transfer capacity or bandwidth that had been made available to the traffic channel. A method is provided for coupling control information to each time frame. Such control information is used by coupled transmitting systems and receiving systems to identify at least one of: the boundaries of the time frames, the ordinal number of the time frame within the ordinal number of the time cycle within the super cycle. Deployment of this method enables the requirement on the accuracy of the common time reference to be relaxed which thereby increases the robustness and reliability of the switching systems operating responsive to the common time reference. A system design and a method is provided for switching time frames using control information, such as a label, coupled with each time frame. In one aspect of the disclosed invention the label is contained within a header associated with each time . frame. A method and system is provided for mapping a time frame onto the proper switching time and forwarding time, wherein the method for mapping is responsive to the control information, such as a label, coupled with the time frame, wherein switching and forwarding is responsive to the common time reference. Coupling control information with each time frame enables an increase of reliability of the switching system and a reduction of accuracy requirement on the common time reference signal. In addition, one aspect of this invention comprises methods for providing protection switching by pre-allocating protection channels. In a possible embodiment, the protection channel is idle while the corresponding primary channel is carrying traffic. In an alternative embodiment, the protection channel carries the same traffic as the primary channel, while the latter is working properly. In another possible embodiment, the same protection channel is shared by a plurality of primary channels. In an alternative embodiment, a protection channel carries low priority traffic while the primary channel operates normally. One aspect of the invention described in the present disclosure is a method and system for switching time frames using different time references in different switches. One aspect of the invention is a method for a switching system to derive the common time reference from neighboring switches in the event that the common time reference is not available through an external signal, such as the GPS. A method is also provided for enabling a plurality of switching systems to operate with the time reference generated by one of the switching systems called a reference node. The invention encompasses a method for electing the reference node in a distributed fashion. The method for switching with different time references provides the switching systems with increased robustness and independence with regards to the availability of an external common time reference signal. In accordance with some aspects of the present invention, a novel time frame switch fabric controller is provided which stores a predefined sequence of switch fabric configurations that are responsive to a high level controller coordinating multiple switching systems. The high level controller applies the stored predefined sequence of switch fabric configurations on a cyclical basis, having at least one of both simple periodicity and complex periodicity. The application of the stored predefined switch fabric configurations permits the switches of the present invention to relay data over predefined, scheduled, and/or reserved data channels sans the computational overhead of computing those schedules ad infinitum within each switch. This frees the switch computation unit to operate relatively autonomously so that it can handle new traffic reservation requests without changing the predefined switch fabric configurations at large, due to the fact that the switch computation unit provides for finding routes for such new requests by deteπr ning how to utilize unused switch bandwidth. The computational requirements of determining a small incremental change to a switch fabric are far less than the computational requirements when having to re-compute the entire switch fabric configuration. Moreover, the bookkeeping operations associated with the incremental changes are significantly less time-consuming to track than tracking the entire state of the switch fabric as it changes over time. A switching method is disclosed for designing multi-terabit SONET/SDH (Synchronous Optical METwork/Synchronous Digital Hierarchy) switches. A method is provided for encapsulating SONET/SDH frames within time frames that are groomed, switched, and degroomed responsive to the common time reference. Transforming SONET/SDH frame switching into time frame switching provides for the scalability needed to design switches with aggregated switching capacity of multiple terabits/second. Electronic components are one of the limiting factors in designing switching systems operating at very high switching rates. Optical components are independent of the bit rate of information carried by the optical signal upon which they operate. Thus, optical switch fabrics, optical filters, and waveguide grating routers enable the design of very high capacity switching systems. Components of the above listed types are changing from being static, i.e., their configuration can be changed on a long time scale, to dynamic wherein their configuration can be changed on a very short time scale. When designing dynamic optical switching, an unresolved issue is the control of the switching configuration. Due to the lack of flexible and simple optical storage capability, optical packet switching - which provides a way of controlling the switching configuration responsive to the control information contained in the packet header - is impractical. Switching of time frames responsive to the common time reference provides a solution to the control of dynamic reconfigurable optical components. Quickly tunable lasers are being implemented and are going to be commercially available in the near future. Switch designs based on tunable lasers and a method to control them responsive to the common time reference are disclosed in the present invention. The present invention also discloses switch designs based on wavelength conversion and a method to control the wavelength conversion responsive to the common time reference. Optical components such as wavelength converters, tunable lasers, tunable receivers, tunable filters, passive star couplers, and passive waveguide grating routers are utilized in the disclosed designs. Some of the disclosed embodiments are based exclusively on optical components (i.e., the disclosed systems are all-optical dynamic switching systems). The designs and methods disclosed in the present invention provide a unique path to prompt deployment and utilization of such dynamic optical components. These and other aspects and attributes of the present invention will be discussed with reference to the following drawings and accompanying specification. BRIEF DESCRIPTION OF THE DRAWINGS: FIG. 1 shows two switching systems interconnected with links of different speeds, operating according to the present invention; FIG. 2A contains a timing diagram of a common time reference (CTR) that is aligned with the coordinated universal time (UTC) standard, as utilized by the present invention, wherein the CTR is divided into a plurality of contiguous periodic super cycles each comprised of 100 contiguous time cycles which are then each comprised of 800 contiguous time frames; FIG. 2B contains a timing diagram of a common time reference (CTR) that is aligned with the coordinated universal time (UTC) standard, as utilized by the present invention, wherein the CTR is divided into a plurality of contiguous periodic super cycles each comprised of 100 contiguous time cycles which are then each comprised of 100 contiguous time frames; FIG. 3A shows an optical switching system with the following basic components: a plurality of WDM DMUXes (de-multiplexers), a plurality of either electronic or optical alignment subsystem, an either electronic or optical switch fabric, a plurality of WDM MUXes (multiplexers). FIG. 3B shows a two phase forwarding method illustrated across an either electronic or optical switching system, where phase 1 is for Receiving & Alignment and phase 2 is for Transmission & Switching (across the optical fabric); FIG. 4 shows a switching system operating according to the disclosed invention wherein data units arriving during one time frame over a plurality of low capacity input channels are switched and forwarded over one high capacity output channel during a plurality of time frames, wherein the data units received during a plurality of time frames over one high capacity input channel are switched and forwarded over a plurality of low capacity output channels during one time frame; FIG. 5 shows a switching system operating according to the disclosed invention wherein data units arriving during one time frame, over a plurality of low capacity input channels, are switched and forwarded over one high capacity output channel, during a plurality of time frames and over a plurality of low capacity output channels, during one time frame, wherein data units received, during a plurality of time frames over one high capacity input channel, are switched and forwarded, over a plurality of low capacity output channels, during one time frame and over one high capacity output channel, during a plurality of time frames; FIG. 6 shows a switching system operating according to the disclosed invention wherein data units arriving during one time frame, over a plurality of low capacity input channels, are switched and forwarded, over a plurality of low capacity output channels, during one time frame, wherein the data units received, during one time frame over one high capacity input channel, are switched and forwarded, over one high capacity output channel, during one corresponding time frame; FIG. 7 shows the high level architecture of a time driven switch capable of multiplexing and de-multiplexing data units received and transmitted over channels with different capacity, wherein the architecture includes one alignment subsystem for each input channel, one switch fabric with a transfer rate equal to the transmission rate of the highest capacity channel, as many inputs as the number of input channels of the switch, and one rate matching buffer for each low capacity output channel; FIG. 8 is a block diagram of an alignment subsystem comprising a plurality of queues in which incoming- data-units are stored responsive to the input channel unique time reference (UTR) and the time structure (with sub-time frame subdivision) in use within the switch, wherein data units are retrieved from the queues responsive to both the common time reference (CTR) and the time structure deployed within the switch (with sub-time frame subdivision); FIG. 9 depicts the block diagram of a rate matching buffer (RMB) comprising at least one of a plurality of queues in which incoming data units are stored responsive to both the common time reference (CTR) and the time structure (time frame subdivision) in use on the channel coupled with the RMB, wherein data units are retrieved from the queues responsive to both the common time reference (CTR) and the time structure (time frame subdivision) in use on the channel coupled to the RMB; FIG. 10 is the architecture of a possible implementation of a Fractional Lambda Interface. The fractional lambda interface is installed at the ingress of a network that deploys time driven switching from a networks that does not deploy time driven switching. The fractional lambda interface is responsible for mapping incoming data units on fractional lambda pipes; FIG. 11 shows a grooming scenario in which multiple low capacity channels (OC-3, 155 Mb/s) are aggregated in a single high capacity channel (OC-768, 40 Gb/s); FIG. 12 shows a pictorial representation of a grooming operation according to the disclosed invention wherein data units arriving during one time frame over a plurality of low capacity input channels are forwarded over one high capacity output channel during a plurality of shorter time frames; FIG. 13 shows a functional architecture of the grooming system comprising a grooming module and a grooming controller responsive to the Common Time Reference (CTR 002) and the Unique Time References (UTRs) of each input channel; FIG. 14 shows a block diagram of the grooming module comprising a plurality of alignment subsystems, each responsive to the Common Time Reference (CTR 002) and the Unique Time Reference-(UTR) of the input channel coupled to the alignment subsystem; FIG. 15 shows a degrooming scenario in which multiple low capacity channels (OC-3, 155 Mb/s) are derived from a single high capacity channel (OC-768, 40 Gb/s); FIG. 16 shows a pictorial representation of a degrooming operation according to the disclosed invention wherein data units arriving over a single high capacity input channel are forwarded over a plurality of low capacity input channels; FIG. 17 shows the functional architecture of a degrooming system comprising a degrooming module a degrooming controller responsive to both the Common Time Reference (CTR 002) and the Unique Time Reference (UTR) of a high capacity input channel; FIG. 18 shows a block diagram of the degrooming module comprising at least one of a plurality of alignment subsystems, each responsive to both the Common Time Reference (CTR 002) and the Unique Time Reference (UTR) of an input channel, and at least one of a plurality of routing modules; wherein a routing module has one input channel and a plurality of output channels over which it routes data units received from an input channel; FIG. 19 is a schematic diagram of the routing module, comprising a routing table, a plurality of queues, and a, routing controller that determines to which output port an incoming data unit should be switched; FIG.20 shows a scenario in which a time driven switching network, also called fractional lambda switching network, provides transport for SONET/SDH channels thus offering SONET connectivity among SONET cross-connects, SONET Add/Drop Multiplexers, and SONET equipment, such as PBXs, wherein fractional lambda interfaces are deployed at the interface between the SONET devices and the time driven switching network; FIG. 21 shows four possible scenarios of time framing and multiplexing of SONET STS-48 frames received from a SONET network and forwarded on a time driven switching network; FIG.21A exemplifies the time framing of whole STS-48 frames - one STS-48 frame for each time frame; FIG. 2 IB exemplifies the time framing of STS- 12 frames de-multiplexed from the original STS-48 frames - one or more STS-12 frames for each time frame; FIG. 21C shows the multiplexing of entire STS-48 frames time framed in different time frames; FIG. 21D shows the multiplexing of STS-12 frames de-multiplexed from the original STS-48 frame; FIG. 22 shows four possible scenarios of time framing and multiplexing of SONET STS-48 frames received from a SONET network and forwarded over a time driven switching network; FIG. 22A exemplifies the time framing of a fraction of an STS-48 frames - one STS-48 frame is time framed over a plurality of consecutive time frames; FIG. 22B exemplifies the time framing of a fraction of an STS-48 frames - one STS-48 frame is time framed over a plurality of consecutive time frames, wherein the same time frame possibly contains fractions from a plurality of STS-48 frames; FIG.22C shows the multiplexing of entire STS-48 frames time framed in different time frames, wherein each STS-48 frame is time framed over a plurality of consecutive time frames; FIG. 22D shows the multiplexing of fractions of STS-48 frames time framed in a plurality of non-consecutive time frames, wherein fractions of different STS-48 frames can be time framed in consecutive time frames; FIG. 23 A shows a diagram of the functions to be performed in order to provide SONET/SDH services over a time driven switched network, i.e., in order to carry STS- N frames over a time driven switched network and reconstruct the original OC-N channel at the egress of the time driven switching network; FIG. 23B is the high level architecture of a fractional lambda interface used to provide SONET services; FIG.24 shows the architecture of the ingress module of a fractional lambda interface for mapping SONET channels onto fractional lambda pipes; FIG. 25 shows the architecture of the egress module of a fractional lambda interface for mapping fractional lambda pipes onto SONET channels; FIG. 26 shows four possible scenarios of time de-framing of SONET STS-48 frames received from a time driven switching network and forwarded on a SONET network; FIG. 27 shows four possible scenarios of time de-framing of SONET STS-48 frames received from a time driven switching network and forwarded on a SONET network; FIG. 28A depicts the structure of a SONET (Synchronous Optical NETwork) frame used for transmission at about 50 Mb/s (STS-1 channel; FIG. 28B depicts the structure of a SONET (Synchronous Optical NETwork) frame used for transmission integer multiple N of STS-1 - which is STS-N; FIG. 29 is a pictorial representation of the alignment principle wherein unaligned time frames on all the inputs are aligned to the common time reference prior to being switched; FIG. 39A is a block diagram of a time driven tunable laser comprising a tunable laser-scheduling controller for changing the laser wavelength responsive to the common time reference; FIG. 30B is a timing diagram showing the wavelength generated by the time driven tunable laser during subsequent time frames; FIG. 30C shows an output port selected on the downstream switch as a consequence of the usage of a selected wavelength during a selected time frame; FIG. 31A is an architecture of a switching system responsive to the common time reference (CTR) based on tunable lasers and comprising a switch controller, a plurality of WDM de-multiplexers, a plurality of optical alignment subsystems — one for each input line — , a plurality of WDM multiplexers, and a plurality of optical interconnections; FIG. 3 IB is a timing diagram of a switching operation that is responsive to the common time reference with two pipeline forwarding phases: (i) receiving & alignment and (ii) switching & transmitting; FIG. 32 shows a communications system responsive to the common time reference wherein data units are associated to a specific time frame, wherein such data units are transmitted over a specific wavelength across a Wavelength Division Multiplexing (WDM) network (or lambda switching network) whose network nodes (called wavelength routers or lambda routers) possibly route different wavelengths towards different destinations; FIG. 33 is an architecture of a switching system responsive to the common time reference (CTR) based on wavelength conversion (WLC) subsystems and comprising a switch controller, a plurality of optical alignment subsystems — one for each input line — , a plurality of star couplers, a plurality of wavelength division multiplexers (WDMs), and an optical interconnection subsystem between WLC subsystems and WDMs; FIG. 34A is the block diagram of a wavelength conversion subsystem comprising a wavelength conversion (WLC) scheduling controller for changing the converted wavelength and possibly the emitted wavelength responsive to the common time reference and to a wavelength mapping table; FIG. 34B is a timing diagram showing the wavelength received by the wavelength conversion subsystem during subsequent time frames; FIG. 35A is a block diagram of a possible embodiment of tunable wavelength conversion subsystem comprising a tunable receiver (TR) and a tunable laser (TL) both responsive to a color control signal; FIG. 35B is a timing diagram showing the wavelength received by the tunable receiver (TR) and the wavelength generated by the tunable laser (TL) during subsequent time frames; FIG. 35C is a block diagram of a possible embodiment of tunable wavelength conversion subsystem comprising a tunable receiver (TR) responsive to a color control signal, an alignment subsystem responsive to the common time reference (CTR), and a fixed laser; and FIG. 36 shows a possible embodiment of optical programmable delay system based on a programmable optical switching matrix and a set of fiber connections between switch outputs and switch inputs. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT: While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will be described herein in detail, specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated. The present invention relates to a system and method for switching and forwarding data units over a network with optical WDM (wavelength division multiplexing) links. The switches of the network maintain a common time reference (CTR), which is obtained either from an external source (such as GPS — Global Positioning System, or GLONASS, or Galileo) or is generated and distributed internally. The commoii time reference is used to define time intervals, which include super cycles, time cycles, time frames, sub-time frames, and other kinds of time intervals. The time intervals are arranged in both simple periodicity and complex periodicity (i.e., like seconds and minutes of a clock). A data unit that arrives to an input port of a switch or a grooming or degrooming system is switched to an output port based on either arrival time information and/or specific routing information in the data unit' s-header (e.g., IPv4 destination address in the Internet, VCI/VPI labels in ATM, MPLS-multi-protocol label switching-labels). Each switch along a route from a source to a destination forwards data units in periodic time intervals that are predefined using the common time reference. A system is provided for managing data transfer of data units from a source to a destination. The transfer of the data units is provided during a predefined time interval, comprised of a plurality of predefined time frames. The system is further comprised of a plurality of switches. A common time reference signal is coupled to each of the switches, and a time assignment controller assigns selected predefined time frames for transfer into and out from each of the respective switches responsive to the common time reference signal. Each communications channel may use a different time frame duration generated from the common time reference signal. Data units received during at least one of a plurality of time frames over at least one of a plurality of input channels can be transmitted during a single time frame over a single output channel. Data units received during a single time frame from an input link are transmitted during at least one of a plurality of time frames over at least one of a plurality of output links. For each switch, there is a first predefined (optical) channel and first predefined time frame within which a respective data unit is transferred into the respective switch, and a second predefined (optical) channel and a second predefined time frame within which the respective data unit is forwarded out of the respective switch, wherein the first and second predefined time frames may have different durations. The time assignment provides consistently fixed time intervals between the input to and output from the fractional lambda pipe. In a preferred embodiment, there is a predefined subset of the predefined time frames during which the data units are transferred in the switch, and for each of the respective switches, there is a predefined subset of the predefined time frames during which the data units are transferred out of the switch. For each of the data units, there is an associated time of arrival to a respective input port. The time of arrival is associated with a particular predefined time frame. For each of the mappings by the routing controller, there is an associated mapping by a scheduling controller, which maps each of the data units between the time of arrival and forwarding time out. The forwarding time out is associated with a specified predefined time frame. There is a fixed time difference between the time frames for the associated time of arrival and forwarding time out for each of the data units. A predefined interval is comprised of a fixed number of contiguous time frames comprising a time cycle. Data units that are forwarded over a given fractional lambda pipe are forwarded from an output port within a predefined subset of time frames in each time cycle. The time frames associated with a particular switches within the fractional lambda pipe are associated with the same switch for all the time cycles, and are also associated with one of input into or output from the particular respective switch. In one embodiment of the present invention, there is a constant fixed time between the input into and output from a respective one of the switches for each of the time frames within each of the time cycles. A fixed number of contiguous time cycles comprise a super cycle, which is periodic. Data units that are forwarded over a given fractional lambda pipe are forwarded from an output port within a predefined subset of time frames in each super cycle. Furthermore, the number of data units that can be forwarded in each of the predefined subsets of time frames within a super cycle for a given fractional lambda pipe is also predefined. In the preferred embodiment, the common time reference signal is devised from the GPS (Global Positioning System), and is in accordance with the UTC (Coordinated Universal Time) standard. The UTC time signal does not have to be received directly from GPS. Such a signal can be received through various means, as long as the delay or time uncertainty associated with that UTC time signal does not exceed half a time frame. In an alternative embodiment the UTC signal is received from at-least-one of the Russian Global Navigation Satellite System (GLONASS) and the European Galileo system. In one embodiment, the super cycle duration is equal to one second as measured using the UTC (Coordinated Universal Time) standard. In an alternate embodiment the super cycle duration spans multiple UTC seconds. In another alternate embodiment the super cycle duration is a fraction of a UTC second. In a preferred embodiment, the super cycle duration is a small integer number of UTC seconds. Data units can be Internet Protocol (IP) data packets, multi-protocol label switching (MPLS) data packets, Point-to-Point Protocol (PPP) frames, High-level Data Link Control (HDLC) frames, Frame Relay frames, fiber channel data units, asynchronous transfer mode (ATM) cells, or SONET/SDH frames. In accordance with one aspect of the present invention, a system is provided for transferring data units across a data network while maintaining for reserved data traffic, constant bounded jitter (or delay uncertainty), and no congestion-induced loss of data units. Such properties are essential for many multimedia applications, such as telephony and video teleconferencing. In accordance with one aspect of an illustrated implementation of the present invention, as shown in FIG. 1, two or more switching systems 52 can be connected through one or more links comprising a plurality of channels having at least one of a plurality of transmission capacities. Each switching system is responsive to a common time reference (CTR) signal 002 which can be implemented, among other ways, by using the timing signal provided by the global positioning system (GPS) or other positioning systems such as GLONASS and Galileo. The switching systems move data units received at input ports to selected output ports according to the time driven switching operating principles. Time Frames and Time Cycles FIG. 2 is an illustration of a common time reference (CTR) that is aligned to -UTC. Consecutive time-frames are grouped into time cycles. FIG.2A and FIG. 2B provide examples of the common time reference (CTR) organized according to time frames of two different durations. As shown in the example illustrated in FIG.2 A, there are 800 time frames in each time cycle, where each time frame lasts 12.5 microseconds. For illustration purposes, the time frames within a time cycle are numbered 1 through 800. According to the example shown in FIG. 2B, there are 100 time frames in each time cycle, where each time frame lasts 125 microseconds. For illustration purposes, the time frames within a time cycle are numbered 1 through 100. Time frames having a different duration can be used for transmission over channels with a different capacity. FIG.2A provides an example in which 15.325 microseconds time frames are coupled to OC-192 (2.4 Gb/s) channels, while FIG. 2B exemplifies the coupling of 125 microseconds time frames with OC-3 (155 Mb/s) channels. In FIG. 2 the ratio c between the transmission speed of a high capacity channel and the transmission speed of a low capacity channel is defined. In the example in FIG. 2, c is 64. As shown in FIG. 2, consecutive time cycles are grouped together into super cycles, and there are 100 time cycles in each super cycle. For illustration purposes, time cycles within a super cycle are numbered 0 through 99. Super cycles 0 and m are shown in FIG. 2. Time cycles of different durations can be coupled with channels that deploy time frames of different durations. Equivalently, super cycles comprised of a different number of time cycles can be coupled with different channels that deploy time frames having different durations. FIG. 2 is illustrative of the relationship between time frames, time cycles, and super cycles; in alternate embodiments, the number of time frames within a time cycle may be different than 100 or 800, and the number of time cycles within a super cycle may be different than 100. FIG. 2 illustrates how the common time reference signal can be aligned with the -UTC (Coordinated Universal Time) standard. In this illustrated example, the duration of every super cycle is exactly one second as measured by the UTC standard. Moreover, as shown in FIG. 2, the beginning of each super cycle coincides with the beginning of a UTC second. Consequently, when leap seconds are inserted or deleted for UTC corrections (due to changes in the earth's rotation period), the cycle and super cycle periodic scheduling will not be affected. The time frames, time cycles, and super cycles are each associated in the same manner with all respective switches within a fractional lambda pipe at all times. In the embodiment illustrated in FIG. 2, the super cycle duration is equal to one second as measured using the UTC (Coordinated Universal Time) standard. In an alternate embodiment the super cycle duration spans multiple UTC seconds. In another alternate embodiment the super cycle duration is a fraction of a UTC second. In another embodiment, the super cycle duration is a small integer number of UTC seconds. A time frame may be further divided into time slots in the preferred embodiment, not illustrated in FIG. 2. Fractional Lambda Interface A time driven switch 52, also called fractional lambda switch, in FIG. 1 is capable of switching the data units received through a selected input optical channel to at least one of a plurality of output optical channels (also called wavelength or lambdas) coupled to at least one of a plurality of optical fibers (also called optical links). As shown in FIG. 10, data units that time driven switches are required to switch in the same way (from the same input channel to the same output channel) are associated with a Fractional Lambda Pipe (FLP) 2910 in FIG. 10. Data units belonging to a plurality of protocols can be carried on the same optical link (i.e., switched by one switching system). Data units belonging to different protocols are associated with different FLPs. FIG. 10 shows 3 Fractional Lambda Pipes (FLPs) defined across the time driven switches A and B. FLP 1 carries Gigabit Ethernet (GE) frames that have to be delivered to destination Z, FLP 2 carries IP packets to destination Y, and FLP 3 carries Fiber ChanneL(FC) data units to, destination X. A deterministic quality of service is guaranteed by reserving resources to each FLP. In the context of time driven switching, also called fractional lambda switching, resources are reserved to an FLP by requiring and being granted exclusive access to an output channel during at least one of a plurality of time frames or time frame fractions. A Fractional Lambda Interface (2900 in FIG. 10) at the boundary of a subnetwork that is not deploying time driven switching is responsible for mapping incoming data units on fractional lambda pipes (FLPs). The mapping is based on either control information carried in the data units' header-in case of packet switching-or the data units' arrival time-in case of circuit switching. FIG. 10 shows the block diagram of the preferred embodiment of fractional lambda interface 2900. A Packet Scheduling Controller 2930 processes data units (for example asynchronous packets or SONET frames) arriving from at least one of a plurality of input channels 2931. Based on information contained in the packet header — such as an MPLS label, or the destination address in an IP packet, or the VCI/VPI in an ATM cell, or other header fields - or the time position of a SONET STS-N frame in a multiplexed STS-M frame, where M>N, the Packet Scheduling Controller 2930 identifies the fractional lambda pipe 2910 to which the data unit belongs. The relevant header information is used for example, as a lookup key, to retrieve fractional lambda pipe information from a pre-computed table. Once processed by the Packet Scheduling Controller 2930 in FIG. 10, data units are stored in a per fractional lambda pipe (FLP) queuing system 2940. The per fractional lambda pipe (FLP) queuing system 2940 comprises a multiplicity of queues 2945. Each queue is associated with one fractional lambda pipe. The Forwarding Controller 2920 retrieves the data units contained in a specific queue 2945 during each of the time frames reserved to the associated fractional lambda pipe 2910. Another implementation of a fractional lambda interface features a per time frame queuing system that contains one queue for each time frame in the time cycle. For each data unit, the Packet Scheduling Controller 2930 in FIG. 10 uses the control information associated with the data unit as a key to a fractional lambda pipe schedules table to retrieve the pointers to the queue in which the data unit should be stored. Multiple ways exist according to which the Packet Scheduling Controller 2930 can choose the specific queue in which to store a data unit. One possible implementation consists in choosing the first queue that will be served (Le., the one associated with the next time frame to come which is reserved for the fractional lambda pipe 2910 to which the data unit belongs). At each time frame, the Forwarding Controller 2920 in FIG. 10 retrieves and forwards on the line 2932 towards a time driven switch 52 data tmits stored in the queue associated with the fractional lambda pipe 2910 to which the given time frame has been reserved. The current time frame is identified in accordance with the Common Time Reference 002. At the beginning of a new time frame the Forwarding Controller 2920 in FIG. 10 may change the queue 2945 from which data units are retrieved. The new queue 2945 is identified by consulting the FLP schedules database 2925, which contains among other information, the fractional lambda pipe to which each time frame had been reserved. Each of the per-fractional lambda pipe queues 2945 can be logically organized in a plurality of sub-queues. When retrieving data units from each of the queues 2945, the Forwarding Controller 2920 can apply a variety of scheduling algorithms, such as FIFO, simple priority, round robin, weighted fair queuing, to the plurality of sub- queues. The order in which data units are retrieved from the various sub-queues (i.e., the relative priority of the sub-queues) depends on the adopted queue management policy. With reference to the preferred embodiment comprising a per FLP queuing system 2940, all the data units that happen to be remaining in a queue 2945 by the end of a time frame will be served during the upcoming time frames reserved to the fractional lambda pipe 2910 associated with the given queue 2945. The fractional lambda interface 2900 can have multiple lower capacity input lines 2931 that are aggregated on the same higher speed output line 2932. In other words, data units are received from multiple input lines 2931, sorted in the queues 2945 of the same per-FLP queuing system 2940, from which the Forwarding Controller 2920 retrieves data units for transmission on the output channel 2932. The Forwarding Controller 2920 can be comprised of a plurality of Forwarding Controllers, each one associated with at least one of the output channels 2932. There can be a plurality of sets of queues 2940, each set comprising at least one queue 2945, wherein each set 2940 is associated with one of the Forwarding Controllers 2920. The Forwarding Controller 2920 in FIG. 10 can retrieve data units from more than one queue 2945 and forward them on more than one output channel 2932. In this case the FLP Schedules database 2925 provides for each selected time frame, the FLP 2910, to which the selected time frame has been reserved on each of the output channels 2932. Thus, each time frame can be reserved to zero (not reserved) FLPs, to one FLP, to as many FLPs 2910 as the number of output channels 2932, or a number of FLPs larger than the number of output channels 2932, in the case where more than one FLP share the same time frame on the same channel 2932. The Fractional Lambda Interface 2900 in FIG. 10 can comprise a plurality of Forwarding Controller Modules 2920 each associated with at least one of a plurality of asynchronous data units streams (packet streams) or synchronous streams (circuit switched channels). Switching with Pipeline Forwarding with a Plurality of Time Frame Durations and a Plurality of Channel Capacities One of the aspects of the disclosed invention is related to the capability of forwarding, over the same output channel during the same time frame, data units received during at least one of a plurality of time frames over at least one of a multiplicity of incoming channels. This operation is known as multiplexing and takes place at least at one of two different levels. Multiplexing can be done at the channel capacity level by aggregating data units from a plurality of low capacity channels on a single high capacity channel. In a possible scenario of channel capacity level multiplexing, the sum of the capacity of the incoming channels equals the capacity of the output channel. Multiplexing can be done at the time frame level by transmitting, during the same time frame, data units received during the same time frame on a plurality of input channels. In another scenario, time frame level multiplexing is realized by transmitting, during the same time frame, data units received during a plurality of time frames on a single input channel. In another scenario, time frame level multiplexing is realized by transmitting, during the same time frame, data units received during a plurality of time frames over a plurality of input channels. In a possible scenario of time frame level multiplexing, the sum of the time frame durations employed at the incoming channels equals the duration of the time frame employed at the output channel. Multiplexing at the channel capacity level or at both the channel capacity and time frame level is called grooming. Grooming refers in general to the capability of aggregating data units received on a plurality of low capacity channels over a single higher capacity channel. Data units received by a time driven switch 52 in FIG. 1 over one of its input channels during a first time frame are forwarded by the time driven switch over a predefined one of its output channels during a predefined second time frame, wherein the second time frame is either the one immediately following the first one, such that this operation is called immediate forwarding, or the second time frame is another one of the time frames following the first time frame, such that this operation is called non- immediate forwarding. If the first time frame in which the data units are received is predefined, the second time frame is consequently predefined. This in turn makes the time frame, in which the data units will be received by a downstream time driven switch 52, predefined. For example, in FIG. 10 once the time frame during which the Fractional Lambda Interface transmits on its output channel 2932 data units belonging to a FLP is known, given the type of forwarding (immediate or non-immediate) performed by the time driven switches A and B on the path of the FLP, the time frame during which the data units will be forwarded on each of the communications channels 2933 and 2934 will be uniquely determined. In other words the time frames, in which data units belonging to a given FLP are transmitted over each of the traversed links, are strictly coupled. Once a time frame used on any one of the communications links traversed by the FLP is determined, each corresponding one of the time frames used on all the other links traversed by the FLP is univocally determined, given the type of forwarding (immediate or non-immediate) performed by the time driven switches traversed by the FLP. In order to properly reserve switching capacity in the time driven switches 52 (A and B in the example shown in FIG. 10) and transmission capacity over the communications channels (2932, 2933, and 2934 in the example shown in FIG. 10) traversed by an FLP, a scheduling problem has to be solved for finding the number of time frames required to provide the desired capacity to the FLP, wherein the chosen time frames are not already used for switching and transmitting data units belonging to another FLP on any of the traversed switches 52 and communications channels, wherein each chosen time frame on each one of the links traversed by the FLP is properly coupled to the corresponding time frame chosen on the other communications links traversed by the FLP, given the type of forwarding (immediate or non-immediate) performed by the time driven switches traversed by the FLP. The Two Phases Operation In a (all-optical) time driven switch, pipeline forwarding (PF) is realized in two operational phases, as shown in FIG. 3B. Data units belonging to a whole time frame received from each of the optical channels 470 during the first phase, Phase 1, are switched through the switch fabric 490 in the second phase, Phase 2. In a possible embodiment, if Phase 1 begins in time frame t, Phase 2 takes place in time frame t+1. In another embodiment, if Phase 1 ends in time frame t, Phase 2 takes place in time frame t+1. The 2 phase operation ensures that data units received from the various optical channels are aligned with the CTR before being switched. Phase 2 can be performed during either the time frame immediately following Phase 1, during time frame t+1 — immediate forwarding operation, or at a later time frame — non-immediate forwarding operation. Alignment serves to align the beginning and end of each time frame on each optical channel with the beginning and end of the CTR time frames. The alignment is needed since the propagation delay on optical links between switches 52 is not an integer number of time frames. The alignment subsystem 420 shown as part of the fractional lambda switch in FIG. 3A operates on one of the wavelengths carried by each optical fiber 450, and is part of Phase 1 of PF. The alignment subsystem is based on a plurality of queues guaranteeing that the overall delay experienced through the optical fiber 450 and the alignment subsystem 420 is an integer number of time frames. As a result, when data units - that have left the switch 52 at the transmitting end of a fiber 455 aligned with the CTR - exit the alignment subsystem 420 at the receiving end are again aligned with respect to CTR. Various embodiments of the alignment subsystem 420 will be presented in the reminder of this disclosure. The alignment principle is exemplified in FIG. 29. Time frames received on the input links 6530 are not aligned with the CTR. Each time frame contains a payload 6540; an idle time acts as a safety margin separating the payloads 6540 of adjacent time frames. The payloads 6540u of the time frames on the input links 6530 are not aligned with the CTR. Time frame payloads received from different input links 6530 are not necessarily aligned among themselves (see for example 6540u-l and 6540u-N in FIG. 29). An alignment subsystem 6520 coupled with each input 6530 delays incoming, unaligned time frame payloads 6540u such that time frame payloads 6540a are aligned upon exiting the alignment subsystem 6520. Time frame payloads 6540a on all the inputs 6525 of the switch fabric 50 are aligned to the CTR. Time frame payloads 6540 switched to the outputs 6535 are all aligned to the CTR. The ratio between the switch reconfiguration time (i.e., the idle time between time frames) and the time frame duration represents the switching overhead. In order to keep the switching overhead acceptable, the time frame duration must be significantly larger than the switch reconfiguration time. On the other hand, the time cycle size (i.e., the number of time frames in each time cycle) determines the minimum capacity that can be allocated to an FLP. The channel capacity, time frame duration, and time cycle size determine the maximum access time to the FLP, (i.e., the maximum delay a data unit experiences before entering an FLP while waiting for the first time frame allocated to its FLP). Time Frame Switching with Different Time Frame Durations FIG.4, FIG. 5, and FIG. 6 provide a logical view of the operation of a time driven switch 52 equipped with channels with different capacities. The switching systems depicted in FIG. 4, FIG. 5, and FIG. 6 have channels of two different capacities: four low capacity input channels 920-1 through 920-4 and four low capacity output channels 920-11 through 920-14, all eight of capacity: Low_capacit ; and one high capacity input channel 920-5 and one high capacity output channel 920-15, both of capacity: High_capacity. For example the four low capacity channels could be OC-48 SONET channels (Low_capacity=2.4 Gb/s) and high capacity channels could be OC- 192 SONET channels (High_capacity=9.6 Gb/s). Channels 1 920-1 though 4920-4 and channels 11 920-11 through 14920-14 are low capacity channels; channels 5920-5 and 15 920-15 are high capacity channels. Time frames of duration TF2 are deployed on low capacity channels; time frames of duration TFl are employed on high capacity channels. Sample durations are TF1=12.5 microseconds and TF2=25 microseconds, yielding Time driven switches could be equipped with channels having more than two different capacities and employing more than two different time frame durations. The principles underlying the switching system operation and the multiplexing and de-multiplexing of data units are not conceptually different from the ones exemplified in FIG. 4, FIG. 5, and FIG. 6. FIG. 4, FIG. 5, and FIG. 6 are examples of how data units received on a mixture of low capacity and high capacity channels can be forwarded by a time driven switch over a mixture high capacity and low capacity channels, wherein data units from separate input channels could be aggregated on the same output channel and data units on the same input channel can be separated for transmission over a plurality of output channels. In the example in FIG. 4, data units received during a predefined time frame TF2-1 of duration TF2 on the low capacity input channels 920-1 to 920-4 are transmitted on the high capacity output channel 920-15. The data units received on channel 1 920-1, comprising X bits, are transmitted on the output channel 920-15 during a first predefined sub-time frame h - having duration subTF - of a first predefined time frame TF1-3 of duration TFl. The X bits of data units received on channel 2920-2 are forwarded over the output channel 920-15 during the second sub- time frame g of the first predefined time frame TF1-3. Data units received during the predefined time frame TF2-1 on input channels 3 920-3 and 4920-4 are transmitted on the output channel 920-15 during specific predefined sub- time frames f and e, respectively, within a second predefined time frame TF1-4. The mapping between a specific time frame on a specific input channel and a specific sub-time frame on an output channel is repeated cyclically, for example over each time cycle, or each super cycle, or a multiple thereof. The specific time frame and sub-time frame therein upon which an incoming time frame and input channel are mapped can be fixed or changed in each cyclical mapping. Each time frame in a time cycle or super cycle can have a different mapping. The same mapping can be used for more than one time frame in the same cycle or super cycle. Data units can be identified as belonging to different time frames and sub-time frames by means of delimiters introduced during the transmission operation. Explicit delimiters can be realized by one of a plurality of different methods. There can be a different delimiter control word to signal the beginning of a new time frame (i.e., a time frame delimiter - TFD), sub-time frame, time cycle (i.e., a time cycle delimiter - TCD) and super cycle (i.e., a super cycle delimiter - SCD). The explicit delimiter signaling can be realized by the SONET/SDH path overhead field that was design to carry control, signaling and management information. An implicit delimiter can be realized by measuring the UTR time with respect to the CTR. An alternative way of implementing an implicit delimiter is by counting the number of bytes from an explicit delimiter. Alternatively, time frame delineation can be based on time frame delimiter in the optical signal carried on the communications link coupled to input i. A possible embodiment of time frame delimiter consists of dedicating one of the wavelengths of the communications link for transmission of the delimiter. Data units can also be identified as belonging to different time frames and sub- time frames by means of their time of arrival or time of transmission, or by counting the amount of data received since the end of the previous time frame or sub-time frame. The sub-time delimiter can comprise a time frame identifier implemented, for example, according to one of the methods outlined above. The time frame identifier identifies the sub-time frame within its time frame or the time frame within its time cycle. The time frame identifier can carry control information regarding the data units belonging to the corresponding time frame or sub-time frame. For example, when the bits transmitted during a time frame have been received either over a plurality of input channels or during a plurality of time frames, the time frame identifier can identify at least one of the input channels over which the bits had been received and the time frame during which the bits had been received. In the example shown in FIG.4, the first selected sub-time frame h of the first time frame TF1-3 during which the X bits received from Channel 1 920-1 are transmitted, has an identifier uniquely identifying at least one of the sub-time frames h of the first selected time frame TF1-3 during which the X bits are transmitted, the channel 920-1 on which the X bits had been received, and the time frame TF2-1 during which the X bits had been received from the input channel 1 920-1. In a possible embodiment, time frame identifiers are constructed hierarchically to carry information about the time frames during which the corresponding data units were received in each time driven switch that has aggregated (or groomed) such data units. Time frame and input channel information is added by each aggregating (or grooming) time driven switch, and removed by each de-aggregating (or degrooming) time driven switch. In the example in FIG. 4, the data units received during two predefined time frames, TF1-1 and TF1-2 of duration TFl on the high capacity input channel.5 920-5, are transmitted on the low capacity output channels 920-11 through 920-14. The data units, comprising 2-X bits, received on channel 5 920-5 during time frame TF1-1 are transmitted on the output channels 920-12 and 920-13 during a predefined time frame TF2-2 of duration TF2. The X bits of data units received on channel 5920-5 during a first predefined sub-time frame d - having duration subTF-of the first predefined time frame TFl-1 are forwarded over the output channel 920-12. The X bits of data units received during the second sub-time frame c of the first predefined time frame TFl-1 are transmitted over output link 920-13 during the same time frame TF2-2. The data units received during the predefined sub-time frames b and a within the time frame TF1-2 on input channel 5920-5 are transmitted on the output channels 920-11 and 920- 14 respectively, during the predefined time frame TF2-2. In the example depicted in FIG. 5, the data units received during a predefined time frame TF2-1 of duration TF2 on the low capacity input channels 920-1 and 920-4 are transmitted on the high capacity output channel 920-15; the data units received on the low capacity input channels 920-2 and 920-3 during the predefined time frame TF2- 1 are transmitted over the low capacity channels 920-12 and 920-13, respectively, during a fifth predefined time frame TF2-2 of duration TF2. The data units, comprising X bits, received on channel 1 920-1 are transmitted on the output channel 920-15 during a first predefined sub-time frame h of duration subTF within a first predefined time frame TF1-3 of duration TFl. The X bits of data units received on channel 4920-4 are forwarded over the output channel 920-15 during a first sub-time frame e of a second predefined time frame TF1-4. In the example in FIG. 5, the data units received during two predefined time frames TFl-1 and TF1-2 of duration TFl on the high capacity input channel 5 920-5 are transmitted partly over the high capacity output channel 15920-15 and partly on the low capacity output channels 920-11 and 920-14. The data units received on channel 5 920-5 during a third predefined time frame TFl-1, comprising 2-X bits, are transmitted partly on the low capacity output channel 920-14 during a fifth predefined time frame TF2-2 of duration TF2, and partly on the high capacity output channel 920-15 during a predefined sub-time frame of duration subTF within the first predefined time frame TF1-3. The X bits of data units received on channel 5920-5 during a first predefined sub-time frame d of duration subTF within the first predefined time frame TFl-1 are forwarded over the output channel 920-14. The X bits of data units received during a second sub-time frame c of the third predefined time frame TFl-1 are transmitted over output link 920-15 during a predefined sub-time frame g of duration subTF within the first predefined time frame TFl-3. The data units received during the predefined sub-time frames b and a within a fourth predefined time frame TF1-2 on input channel 5 920-5 are transmitted on the output channel 920-11 during the predefined time frame TF2-2 and on the output channel 920-15 during the predefined sub-time frame f within the second predefined time frame TFl-4 respectively. In the example depicted in FIG. 6, the data units received during a predefined time frame TF2-1 of duration TF2 on the low capacity input channels 920-1 through 920-4 are transmitted over the low capacity channels 920-11 through 920-14 during a predefined time frame TF2-2 of duration TF2. The data units received on channel 1 920-1, comprising X bits, are transmitted on the output channel 920-14; the X bits of data units received on channel 2920-2 are forwarded over the output channel 920-12; the data units received on channel 3 920-3, comprising X bits, are transmitted on the output channel 920-13; the X bits of data units received on channel 4920-4 are forwarded over the output channel 920-11. In the example in FIG. 6, the data units received during two predefined time frames TFl-1 and TF1-2 of duration TFl on the high capacity input channel 5 920-5 are transmitted over the high capacity output channel 15920-15 during two predefined time frames TFl-3 and TFl-4, respectively. The data units received on channel 5 920-5 during time frame TFl-1 and time frame TF1-2 are organized into different sub-time frames. The X bits of data units received on channel 5920-5 during a first predefined sub-time frame d of duration subTF within the first predefined time frame TFl-1 are forwarded over the output channel 920-15 during a first predefined sub-time frame h of duration subTF. The X bits of data units received during a second sub-time frame c of the first predefined time frame TFl-1 are transmitted over output link 920-15 during a predefined sub-time frame g of duration subTF within a second predefined time frame TFl-3. The data units received during predefined sub-time frames b and a within a third predefined time frame TF1-2 on input channel 5920-5 are transmitted on the output channel 920-15 during the predefined sub-time frames f and e, respectively, within a fourth predefined time frame TFl-4. Time driven Switch with Rate Matching Buffer (RMB) FIG. 7 shows the high level architecture of one possible embodiment of a time driven switch with a plurality of input and output channels having different transmission capacity. The switching system of the example presented in FIG. 7 has the same input output channel configuration as the switches presented in the examples contained in FIG. 4, FIG. 5 and FIG. 6, including the duration of time frames deployed on each channel. The total number of input (and output) channels is N. In the sample switch configuration presented in FIG. 5A, N=256; in the sample switch configuration presented in FIG. 7, N=5. The architecture depicted in FIG. 7 is characterized by being based on a N-by-N switch fabric with an input/output transfer capacity equal to the transmission capacity High_capacity of the high capacity channels. In other words, the input lines 940-H and the output lines 1120-H of the switch fabric have transmission capacity High_capacity. Low capacity input channels 920-1 through 920-4 are connected to a first type of alignment subsystem 1500, while the high capacity input channel 920-5 is connected to a second type of alignment subsystem 1400. Other configurations are also feasible. For example, a single type of alignment subsystem 1500 could be used for both low capacity and high capacity input channels 920. The high capacity output channel 920-15 is connected directly to one of the output lines 1120-H of the switch fabric 50. Each of the low capacity output channels 920-11 through 920-14 is fed by a Rate Matching Buffer (RMB) 1600 which receives over one output line 1120-H data transferred through the switch fabric 50. After the alignment subsystem has aligned the data units received on each input channel 920-1 through 920-5 with the common time reference, they can be transferred through the switch fabric. During each time frame all the data units received during one previous time frame are moved from a buffer in the alignment subsystem to the output lines 1120-H of the switch fabric 50. With reference to the switch architecture presented in FIG. 7, data units that are to be routed over the high capacity channel 920-15 are immediately transmitted over the output channel 920-15. This switching and forwarding method is called 2 phase switching and forwarding and is illustrated in FIG. 3B. With reference to the switch architecture presented in FIG. 7, data units that are to be routed over the low capacity channels 920-11 through 920-14 are buffered in a rate matching buffer (RMB) 1600 and forwarded in a following time frame, which can be the next time frame in a possible embodiment. This switching and forwarding method is called 3 phase switching and forwarding and it is illustrated in FIG. 3B. There are a plurality of methods for transferring data units from input 940-H to output 1120-H through the switch fabric 50. In a first method, all the data units received on one of the input channels 920-1 through 920-5 during a first predefined time frame are transferred to the same output line 1120-H during a second predefined time frame, wherein the second predefined time frame follows the first predefined time frame. In a second method, the data units received during a first predefined time frame over one of the input channels 920-1 through 920-5 can be transferred to different output lines 1120-H, wherein data units received during the same sub-time frame of the first predefined time frame are transferred to the same output line 1120-H. Data units to be transmitted on one of the low speed output channels 920-11 through 920-14 are stored in a Rate Matching Buffer (RMB) 1600 which they enter through a line 1120-H having capacity High_capacit , and are transmitted through the low capacity channel 920-11 through 920-14 coupled to the RMB at a lower rate Low_capacit . FIG.9 shows the block diagram of a Rate Matching Buffer 1600 receiving data units from one of the output lines 1120 of the switch fabric 50, and transmitting data units on one of the output channels 920 of the time driven switch 52. The data flows through input 1120 and through output 920 have different rates. In the switch configuration presented in FIG. 7, the output lines 1120 from the switch fabric 50 have capacity High_capacity, while the output channels 920-11 through 920-14 have capacity Low_capacity, where High_capacity > Low_capacity. Thus, the RMBs 1600 deployed in this configuration receive data units in the queues at a rate higher than the rate at which data units are transmitted out of the queues. The RMB 1600 depicted in FIG. 9 comprises two buffer queues 1650 which are deployed according to the following principles: During each time frame of duration TFi_j as deployed on channel j of link i associated to the RMB, - each queue is associated to either the input 1120 or the output 920 of the RMB; - the input line 1120 is associated to a respective one of the TF queues 1650; - the output line 920 is associated to a respective one of the TF queues 1650. In FIG. 9, a l-to-2 de-multiplexer (DMUX) 1620 is responsible for coupling the input line 1120 with a respective one of the TF queues 1650 during each time frame TFi _j according to the principles listed above, responsive to the Select-RMB-in signal 1610 from the fabric controller 55, wherein the Select-RMB-in signal 1610 is aligned to the CTR 002. In FIG. 9, a 2-to-l multiplexer (MUX) 1640 is responsible for coupling the output line 920 with a respective one of the TF queues 1650 during each time frame TFij according to the principles listed above, responsive to the Select-RMB-out signal 1630 from the fabric controller 55, wherein the Select-RMB-in signal 1630 is aligned to the CTR 002. Consequently during each time frame TFij, data units are received in the selected one of the TF queues 1650 at the rate of the input line 1120 and are transmitted out of another TF queue 1650 at the rate of the output line 920. Another possible implementation of an RMB — not shown in FIG. 9 — comprises a single buffer queue in which data units are concurrently stored while received from the input line 1120 at the rate of the line (for example High_capacity, in the switch configuration depicted in FIG.7) and are retrieved and then transmitted on the output line 920 at the line rate (for example Low_capacity, in the configuration depicted in FIG.7), where the rate at which data units are stored is different from the rate at which data units are retrieved. This alternative implementation with single buffer does not require data units to be stored in the RMB for a whole time frame, thus allowing for the implementation of 2 phase switching and forwarding (see FIG. 3B) on low capacity output channels. On the other hand, this alternative implementation requires at least one of a double access memory and a memory with access speedup (i.e., access speed higher than the rate of each of the input 1120 and output 920 data lines). Unique Time Reference (UTR) and Alignment to CTR In FIG. 8, a plurality of buffer queues 1550 are part of each alignment subsystem 1500, wherein each of the respective buffer queues is associated, for each of the subTFs, with a unique combination of one of the incoming optical channels and one of the outgoing optical channels. A mapping controller within the switch controller, in FIG. 3 logically maps, for each of the (UTR-i) subTFs, selected incoming optical channels 920 to selected buffer queues 1550, in FIG. 8, and logically maps, for each of the CTR TFs, selected ones of the plurality of buffer queues 1550 to selected outgoing channels 940. In FIG. 8, the Select-in signal 1410 generated by the switch controller 410 (see FIG. 3) determines which of the buffers 1550 will receive data units from the channel 920 at every sub-time frame subTF as it is defined by the UTR-i. The selection process by the alignment subsystem 1500 is responsive to the Select-in signal 1410 received from the fabric controller 55. The Select-in signal 1410 is fed into a l-to-3-Pi_j DMUX (de-multiplexer) 1420 that selects one of 3-Pij buffer queues 1550. Pi_j is the number of sub-time frames of duration subTF contained in one TF of duration TFi_j employed on the incoming channel 920. Prior to output, the buffer queues in the alignment subsystem 1500 for each time frame and sub-time frame can be filled to an arbitrary level with data units arranged in an arbitrary order. The alignment subsystem 1500 in FIG. 8 is comprised of a plurality of TF queues 1550, wherein each of the sub-time frame queues comprises means to determine that the respective sub-time frame queue 1550 is empty, wherein each of the sub-time frame queues further comprises a means to determine that the respective sub-time frame queue is not empty. The empty (and not empty) signal 950 is provided to the switch controller 410, in FIG. 3. In FIG. 8, the mapping controller within the fabric controller 55 further provides for the coupling of selected sub-time frame queues 1550 to their respective ones of the outgoing channels 940, for transfer of the respective stored data units during the respective CTR sub-time frames. This operation is performed responsive to the Select- out signal 1430, as shown in FIG. 8. During each of the subTFs of the CTR, only one of the buffer queues 1550 is associated with the outgoing line 940. For each of the subTFs of the UTR-i, only one of the buffer queues is associated with the incoming channel 920. The same buffer queue 1550 is never associated at the same time with both the incoming channel 920 and the outgoing line 940. In FIG. 8, the alignment subsystem 1500 can have more than 3-Pij TF queues 1550 - this can be used for the non-immediate forwarding method: in this method a data unit is delayed in the input port until there is an available sub-time frame for it so that it may be switched to the selected one of the outgoing optical channels 920. In this method, the delay is increased (i.e., more time frames may be needed to get from input to output). The non-immediate forwarding method adds flexibility to the scheduling process of fractional lambda pipes. Grooming and Degrooming Communications networks are engineered to support different amounts of traffic in different areas of the network. Usually network users (individuals or organizations) access the network through dedicated low capacity channels. Traffic generated by multiple users is aggregated over higher capacity channels that span metropolitan and regional areas. Traffic generated in metropolitan and regional areas is aggregated over higher capacity channels that span national and continental areas. Dually, traffic flowing among and across continents through very high capacity channels needs to be sorted and directed towards the proper destination over the lower speed national, regional, and eventually metropolitan channels. Methods for (i) switching data units among channels with different capacity, (ii) aggregating data flows from a plurality of low capacity channels to a single high capacity channel (grooming), and (iii) de-aggregating data flows from a single high capacity channel to a plurality of low capacity channels (degrooming) are instrumental in properly and flexibly engineering communications networks. The Switching Methods with Common Time Reference and Plurality of Time Frame Durations described above in the present disclosure fulfill (i). The Time-based Grooming Methods with Common Time Reference described in the following fulfill (ii), while Time-based Degrooming Methods with Common Time Reference that will be described below in the present disclosure fulfill (iii). Grooming is a process of multiplexing where the number of inputs is (much) larger than the number of outputs, possibly one, while degrooming is a process of demultiplexing where the number of inputs is (much) smaller, possibly one, than the number of outputs. Grooming takes place at least at one of two different levels: the channel capacity level - also called inter-time frame grooming - and the time frame level - also called intra-time frame grooming. When operating at the channel capacity level data units from a plurality of low capacity channels are aggregated on a single high capacity channel. In a possible scenario of channel capacity level multiplexing, the sum of the capacity of the incoming channels equals the capacity of the output channel. Data units received during a single time frame on a plurality of input channels are transmitted on the high capacity output channel during separate, possibly consecutive time frames. When operating at the time frame level, data units received during multiple time frames on at least one of a plurahty of input channels, are transmitted on a high capacity channel during a single time frame. Grooming can be done at both the channel capacity and time frame level in one of the following ways: by transmitting during the same time frame data units that were received during the same time frame from a plurality of input channels; by transmitting during the same time frame data units that were received during a plurality of time frames from a single input channel; by transmitting during the same time frame data units that were received during a plurality of time frames from a plurality of input channels. In a possible scenario of combined channel capacity and time frame level grooming, the time frame duration employed on the incoming channels equals the duration of the time frame employed on the output channel, wherein the size of the time frame employed on the output channel equals the sum of the time frame sizes employed on the incoming channels, wherein the time frame size is the amount of information that can be sent during a time frame over a given communications channel. Time-based Grooming and Degrooming Methods with CTR The common time reference (CTR) is partitioned into successive time frames (TFs), as shown in FIG. 2 A and FIG. 2B. Contiguous time frames are grouped into time cycles and contiguous time cycles are grouped into contiguous super cycles, wherein one super cycle is equal to and temporally aligned with one UTC second, as shown in FIG. 2A. Each time frame may be sub-divided into a plurality of sub-time frames (subTF). Pipeline forwarding (PF) of time frames through a sequence of grooming, switching, and degrooming systems is realized by pre-scheduling the forwarding time of data units contained in each time frame and/or sub-time frame. Thus, no processing of in-band information, e.g.: packet header, is necessary once data units belonging to each time frame enter a network with CTR. Data units on the low capacity input channels of the grooming and degrooming devices depicted in FIG. 11 and FIG. 15, respectively, are organized in time frames that are not aligned to the CTR, due to the propagation delay on the various channels. Thus, data units received on each channel are first aligned with the CTR and then switched and multiplexed (or de-multiplexed and switched) onto the output channels. The result of the alignment operation is that the beginning and end of each time frame after alignment occurs at the same time as the beginning and end of the CTR time frames. The time frame duration and its size (the number of bits that can be transmitted during one time frame) over the output channels may be different from the duration and size used over the input channels. In the most general case, time frame duration might not be the same on all the input channels. However, without loss of generality, in order to keep the following description simple it is assumed that the same time frame duration is used over all input channels and all output channels. Grooming Deployment A possible realization of the grooming subnetwork 4000 is provided in FIG. 11, while FIG. 15 shows a possible realization of the degrooming subnetwork 5000. FIG. 11 shows a typical grooming scenario based on channels of the SONET (Synchronous Optical NETwork) hierarchy. Data units transmitted through a plurality of OC-3 (155 Mb/s) channels are aggregated and transmitted over a single OC-768 (40 Mb/s) channel. The aggregation, or grooming architecture shown in FIG. 11 is based on grooming systems 4200-a through 4200-d. At the periphery of the network, a plurality of grooming systems 4200-a aggregate data flows from 4 OC-3 channels 4020 over a single OC-12 channel 4010 (622 Mb/s). Li a real world scenario, the network shown in FIG. 11 could be the network of a service provider offering access to a plurality of customers. The local area networks of the customers are connected to a Time driven Switch or a Time driven Priority switch 4060, which forwards their traffic on a respective one of the OC-3 links 4020. The traffic generated by each of a plurality of groups of four customers is aggregated in the provider's point of presence (POP) closest to the customers on a single OC- 12 channel 4010, and carried towards a metropolitan aggregation point where a grooming system 4200-b further aggregates traffic carried by OC-12 channels 4010 and generated by a plurality of users dispersed in the metropolitan area. Notice that the span of the links carrying the OC-12 channels 4010 can be between a few meters (when the grooming systems 4200-a and 4200-b are co-located within the same central office) and a few kilometers. In FIG. 11, each one of the plurality of grooming systems 4200-b, aggregates traffic received over four OC-12 channels 4010 for transmission over a single OC-48 (2.5 Gb/s) channel 4030. In a real world example, a service provider could use this architecture to gather traffic from customers located in various metropolitan areas geographically close and connected through OC-12 channels 4010 and transfer the aggregated data flows to a regional aggregation point over a plurality of OC-48 channels 4030. In the regional aggregation points, in FIG. 11, each one of a plurality of grooming systems 4200-c aggregates the data units received over four OC-48 channels 4030 for transmission over a single OC-192 (10 Mb/s) channel 4040. The OC-192 channel 4040 is used to carry the traffic to a continental concentration point where a grooming system 4200-d aggregates the traffic from four OC-192 channels 4040 for transmission over an intercontinental backbone of OC-768 (40 Gb/s) channels 4050 and Time driven Switches or Time driven Priority switches 4060. The grooming architecture depicted in FIG. 11 allows all the traffic generated by all the customers of the service provider to be aggregated on a single OC-768 channel 4050 that is fed in a switching system 4060 operating according to either Time driven Switching or Time driven Priority. Grooming Methods: Grooming can be performed according to at least one of two methods. (1) Intra-time frame grooming. The time frame duration on the output channel is the same as on the input channel; however, the time frame size on the output channel is larger than on the input channel. For example, if the output channel capacity is N times larger than the input channel capacity, time frames on the output channel are N times larger than are those on the input channel. Multiplexing is achieved by combining N time frames worth of data units received from all the input channels during the same time frame on the output channel. Data units received from different input channels are multiplexed in the same time frame within separate sub-time frames. (2) Inter-time frame grooming. The time frame size on the output channel is the same as on the input channels; however, due to the higher capacity of the output channel, the time frame duration is proportionally shorter. For example, if the output channel capacity is N times larger than the input channel capacity, time frames on the output channel are N times shorter than are those on the input channels. Multiplexing is achieved by interleaving time frames of data units received from the various input channels Channel 1 through Channel 4 on the output channel 4110, as shown in FIG. 12. In the example depicted in FIG. 12 the output channel 4110 capacity is four times the capacity of the input channels (e.g., the output channel is an OC-192 channel, while each input channels is an OC-48 channel). Grooming that combines the above two methods, shown in FIG. 12 A, is such that grooming is obtained by interleaving time frames containing data units from different input channels (inter-time frame grooming), each time frame being composed of two sub-time frames (intra-time frame grooming). FIG. 16 shows how degrooming is performed. Data units are received on a high capacity input channel Channel5 during each time frame. When intra-time frame degrooming is performed, after data units received from the input channel Channel5 during a first time frame have been aligned, they are transmitted during a second time frame of duration TF2 on multiple output channels Channel 1 through Channel4, wherein data units received during the same sub-time frame of the first time frame are transmitted over the same channel, wherein data units received during different sub- time frames of the first time frame are transmitted over different output channels. When inter-time frame degrooming is performed, as shown in FIG. 16B, data units received during multiple consecutive time frames 5130 through 5137 of duration TF3 from a high capacity input channel Channel5 are transmitted during one selected time frame of duration TF2 over multiple lower capacity output channels Channel 1 through Channel4. Finally, as shown in FIG. 16A, a combination of the two degrooming methods, inter-frame degrooming and intra-frame degrooming, can be deployed as well. The time frame duration to be deployed on each communications channel should be chosen wisely according to the channel capacity. In fact, a small time frame size is not desirable because the grooming and degrooming system operation is simpler and more efficient if any data unit transmitted on the channels fits within a single time frame - and even simpler and more efficient if a few data units are contained in each time frame. On the other hand, time frames that are too large are undesirable as well since in order to align received data units with the CTR, all the data units received in a time frame need to be buffered. Consequently, the larger the time frames, the larger the amount of memory needed to perform CTR alignment. In summary, short time frames result in high speed control within each grooming and degrooming system, while long time frames result in longer alignment times and consequently longer delay introduced by each grooming and degrooming system. Grooming Realization FIG. 12 shows a schematic description of the operation of a grooming system 4200. The grooming system 4200 in FIG. 12 has four low capacity input channels and one high capacity output channel. Multiplexing over the output channel of data units received from the four input channels can be done at least at two levels: (i) the channel capacity level - also called inter-time frame grooming - and (ii) the channel capacity and time frame level. FIG. 12A shows an example of multiplexing (or grooming) at both the channel capacity level (inter-time frame grooming) and time frame level (intra- time frame grooming); FIG. 12B shows an example of multiplexing (or grooming) at the channel capacity level, or inter-time frame grooming. In the configuration shown in FIG. 12, time frames of duration TF2 are deployed on the four input channels Channel 1 through Channel4 that have capacity Low_capacity. Consequently time frames have size X bits. A different time frame duration and possibly time frame size is used on the output channel 4110 that has capacity High_capacity. In a sample configuration, Low_capacity could be 2.5 Gb/s (OC-48) and High_capacity could be 10 Gb/s (OC-192), with TF2=25 microseconds and TF1=12.5 microseconds. In this scenario the size of the time frames TFl is double than the size of the time frames TF2. FIG. 12A depicts such a configuration, which could be the configuration of a grooming system 4200-c in FIG. 11. In the sample configuration depicted in FIG. 12 A, the size (i.e., the amount of bits that can be transmitted) of the time frames TFl deployed on the output channel is double (2-X bits) the size of the time frames TF2 deployed on the four input channels (X bits). In FIG. 12, the grooming system 4200 receives X bits from each of the four input channels during a first selected time frame of duration TF2. As shown in FIG. 12, during a second selected time frame 4120 of duration TFl, the grooming system 4200 transmits the X bits received, during the first selected time frame, from Channel 1 and the X bits received, during the first selected time frame, from Channel2 over the output channel 4110 , wherein the second selected time frame follows the first selected time frame. In a possible embodiment, in FIG. 12A, the X bits received from Channel 1 can be transmitted during a first selected sub-time frame of the second time frame 4120 of duration subTF=TFl 2, and the X bits received from Channel2 can be transmitted during a first selected sub-time frame of the second time frame 4120 of duration subTF=TFl/2. The two sub-time frames of time frame 4120 can be delineated by a sub-time frame delimiter that can be either an implicit one or an explicit one. Explicit delimiters can be realized by one of a plurality of different methods. There can be a different delimiter control word to signal the beginning of a new time frame (i.e., a time frame delimiter - TFD), sub-time frame, time cycle (i.e., a time cycle delimiter - TCD) and super cycle (i.e., a super cycle delimiter - SCD). The explicit delimiter signaling can be realized by the SONET/SDH path overhead field that was design to carry control, signaling and management information. An implicit delimiter can be realized by measuring the UTR time with respect to the CTR. An alternative way of implementing an implicit delimiter is by counting the number of bytes from an explicit delimiter. Alternatively, delimiters can be realized as an optical signal carried on the communications link coupled to input i. A possible embodiment of time frame delimiter consists of dedicating one of the wavelengths of the communications link for transmission of the delimiter. The sub-time frame delimiter can comprise a time frame identifier that identifies the sub-time frame within its time frame or the time frame within its time cycle. The time frame identifier can carry control information regarding the data units belonging to the corresponding time frame or sub-time frame. For example, when the bits transmitted during a time frame had been received either by a previous grooming system over a plurality of input channels or during a plurality of time frames, the time frame identifier can identify at least one of the input channels over which the bits were received and the time frame during which the bits were received. In the example shown in FIG. 12A, the first selected sub-time frame of the second time frame 4120 during which the X bits received from Channell are transmitted has an identifier uniquely identifying at least one of the sub-time frames of the second time frame 4120 during which the X bits are transmitted, the channel Channell on which the X bits had been received, and the first selected time frame during which the X bits had been received from the input channel Channell. In a possible embodiment, time frame identifiers are constructed hierarchically to carry information about the time frames during which the corresponding data units where received in each grooming system they had traversed. In an alternative embodiment, X bits received from Channell can be transmitted during a plurality of sub-time frames of the second time frame 4120 multiplexed with the X bits received from Channel2 that are transmitted during a plurality of distinct sub-time frames of the second time frame 4120. The sub-time frames of the second time frame 4120 can be delineated by a sub-time frame delimiter that can be either an implicit one or an explicit one implemented according to the methods described above in this disclosure. In a third alternative embodiment, the X bits received from Channell and the X bits received from ChanneI2 are multiplexed and transmitted during the second time frame 4120. During a third selected time frame 4125 of duration TFl, the grooming system 4200 transmits the X bits received, during the first selected time frame, from Channel3 and the X bits received, during the first selected time frame, from Channel4 over the output channel 4110 , wherein the third selected time frame 4125 follows the first selected time frame. The X bits received from Channel3 and the X bits received from Channel4 during the first selected time frame can be transmitted on the output channel during the third selected time frame 4125 during either two separate sub-time frames, during a plurality of distinct multiplexed sub-time frames, or multiplexed during the time frame 4125, similarl to what was described above for the transmission of the X bits received from Channell and Channel2. In a possible configuration, the third selected time frame 4125 follows the second selected time frame 4120, as shown in the example depicted in FIG. 12A. In an alternative configuration, the second selected time frame 4120 follows the third selected time frame 4125. In a possible configuration, the second time frame 4120 is adjacent to the third time frame 4125, as shown in the example depicted in FIG. 12 A. In an alternative configuration, the second time frame 4120 is not adjacent to the third time frame 4125. In the sample configuration depicted in FIG: 12B, the size (i.e., the number of bits that can be received) of the time frames TF3 deployed on the output channel is the same as the size (i.e., the number of bits that can be transmitted) of the time frames TF2 deployed on the four input channels (X bits). The grooming system 4200 receives X bits from each of the four input channels during a first selected time frame of duration TF2. During a second selected time frame 4130 of duration TF3, as depicted in FIG. 12B, the grooming system 4200 transmits the X bits received from Channell over the output channel 4110, wherein the second selected time frame 4130 follows the first selected time frame. During a third selected time frame 4133 of duration TF3, the grooming system 4200 transmits the X bits received from Channel2 over the output channel 4110, as depicted in FIG. 12B, wherein the third selected time frame 4133 follows the first selected time frame. During a fourth selected time frame 4137 of duration TF3, the grooming system 4200 transmits the X bits received from ChanneI3 over the output channel 4110, wherein the fourth selected time frame 4137 follows the first selected time frame. During a fifth selected time frame 4135 of duration TF3, the grooming system 4200 transmits the X bits received from Channel4 over the output channel 4110, wherein the fifth selected time frame 4135 follows the first selected time frame. In a possible configuration, as depicted in FIG. 12B, the fifth selected time frame 4135 follows the fourth selected time frame 4137, that in turn follows the third selected time frame 4133, that in turn follows the second selected time frame 4130, as shown in the example depicted in FIG. 12A. In alternative configurations, the four time frames 4130 through 4137 follow each other in any other possible order. In a possible configuration, the four time frames 4130 through 4137 are adjacent, as shown in the example depicted in FIG. 12A. In an alternative configuration, the four time frames 4130 through 4137 are not all adjacent to at least one of the others. In a possible configuration, each of the four time frames 4130 through 4137 is not adjacent to any one of the others. FIG. 13 depicts the architecture of a possible embodiment of a Grooming system 4200 comprising a Grooming Controller 4220 and a Grooming Module 4300 between a plurality of low capacity input channels 4230 and a higher capacity output channel 4240. In the sample configuration shown in FIG. 13, the grooming system 4200 has c input channels at capacity Low_capacity and one output channel at capacity High_capacity, where Higft_capadty=c , Low_capadty. In FIG. 13, the grooming module 4300 aggregates data units received on the input channels 4230 for transmission over the output channel 4240 responsive to the control signals 4250, 4260, and 4270 from the grooming controller 4220. The grooming controller 4220 generates the control signals for the grooming module 4300 responsive to the common time reference (CTR) 002 received through an external control line 4280, and responsive to the unique time reference (UTR) coupled with each input channel 4230. The grooming controller 4220 devises such UTR by using the time frame and sub-time frame delimiters included in the data stream according one of the methods described above in the present disclosure. In FIG. 13, The control signals sent from the grooming controller 4220 to the grooming module 4300 comprise a Select_in signal 4250 for each one of the plurality of input channels 4230, a Select_out signal 4260 for each one of the plurality of input channels 4230, and a Select signal 4270. Each Select_in signal 4250 is aligned with the UTR coupled to the corresponding input channel 4230 and has a period equal to the time frame duration TF2 deployed on the corresponding input channel 4230. Each Select_out signal 4260 is aligned with the CTR and has a period equal to the time frame duration TF2 deployed on the corresponding input channel 4230. The Select signal 4270 is aligned with the CTR and has a period equal to the sub-time frame duration subTF deployed on the output channel 4240. The grooming systems 4200-a through 4200-d deployed in the grooming scenario presented in FIG. 11 can be implemented using the architecture shown in FIG. 13, where c=4. For example, the Grooming systems 4200-c can be implemented with the architecture shown in FIG. 13, where Low_capadt =2.5 Gb/s (OC-48 4030) and High_capadty=10 Gb/s (OC-1924040). A possible implementation of the grooming module 4300 is shown in FIG. 14. An array of alignment subsystems 1400 receives data units from the low capacity input channels 4230. After having been aligned to the CTR 002 within the alignment subsystem, the data units received over different input channels in a previous time frame are sent to the high capacity output channel 4240. Data units to be transmitted over the high capacity output channel 4240 during each predefined sub-time frame are retrieved from their respective alignment subsystem 1400 through the data lines 4320. The specific one of the alignment subsystems 1400 from which data units are to be retrieved to be transmitted over the high capacity output channel 4240 is selected by a c- by-1 selector 2110 in each sub-time frame responsive to the Select signal 4270 from the grooming controller 4220. FIG. 14 shows the sample implementation of a grooming module 4300 with c low capacity input channels 4230. The same architecture with c=4 can be used in the implementation of the grooming module 4300 comprised within the grooming systems 4200 deployed in the sample grooming scenario presented in FIG. 11. For example, the grooming module 4300 within one of the grooming systems 4200-b operates between four input channels 4230 having capacity Low_capadty=622 Mb/s (OC-12 4010) and an output channel 4240 having capacity High_capadt =2.5 Gb/s (OC-48 4030). An alternative implementation of the grooming module 4300 comprises a buffer queue between the c-by-1 selector 2110 and the output channel 4240. Such buffer queue — not shown in the block diagram depicted in FIG. 14 — decouples the transmission of data units on lines 4320 and through the c-by-1 selector 2110 from the transmission on the output channel 4240. Degrooming Deployment: FIG. 15 shows a typical degrooming scenario based on channels of the SONET (Synchronous Optical NETwork) hierarchy. Data units transmitted through a single OC- 768 (40 Mb/s) channel are de-aggregated and transmitted over a plurality of OC-3 (155 Mb/s) channels. The de-aggregation, or degrooming, architecture shown in FIG. 15 is based on degrooming systems 5200-a through 5200-d. The traffic carried over an OC-768 (40 Gb/s) channel 4050 is divided by a degrooming system 5200-a over four separate OC-192 (10 Gb/s) channels 4040. In a real world scenario, the network shown in FIG. 15 could be the network of a service provider offering access to a plurality of customers. The traffic in the intercontinental backbone of the network is handled by Time driven Switches or Time driven Priority switches 4060 that forward backbone traffic directed to a given continental area over an OC-768 channel 4050. Traffic from the intercontinental backbone is divided over four OC-192 (10 Gb/s) channels 4040 to be carried to four distinct continental de- aggregation points. In FIG. 15, each one of the degrooming systems 5200-b in the continental de- aggregation points divides the traffic received over its respective OC-192 channel 4040 over four distinct OC-48 (2.5 Gb/s) channels 4030 which are connected to respective regional de-aggregation points. In FIG. 15, each one of the degrooming systems 5200-c divides the traffic received over a respective one OC-48 channel 4030 over four distinct OC-12 (622 Mb/s) channels 4010. In a real world example, a service provider could use this architecture to divide the traffic received at a regional aggregation point through an OC- 48 channel 4030 and addressed to customers located in various metropolitan areas geographically close and connected through OC-12 channels 4010 to the regional aggregation point In FIG. 15, each of the degrooming systems 5200-d in the Point of Presence (POP) of the provider within metropolitan areas receives traffic through an OC-12 channel 4010 and divides it over four OC-3 (155 Mb/s) channels 4020, each one connected to a respective customer. The local area networks of the customers are connected to a Time driven Switch or a Time driven Priority switch 4060 which receives their traffic through a respective one of the OC-3 links 4020. The degrooming architecture depicted in FIG. 15 allows all the traffic addressed to all the customers of the service provider to be routed by a switching system 4060 operating according to either Time driven Switching or Time driven Priority on a single OC-768 channel 4050. The traffic is further divided over lower capacity channels until it reaches the intended customer over its respective OC-3 channel 4020. egroQming R j eaiføation; FIG. 16 shows a schematic description of the operation of a degrooming system 5200. The degrooming system 5200 in FIG. 16 has one high capacity input channel and four low capacity output channels. De-multiplexing of data units received from the input channel over the four output channels during transmission can be done at least at two levels: (i) the channel capacity level - also called inter-time frame degrooming - and (ii) the channel capacity and time frame level. FIG. 16A shows an example of demultiplexing (or degrooming) at both the channel capacity and time frame level; FIG. 16B shows an example of de-multiplexing (or degrooming) at the channel capacity level - also called inter-time frame degrooming. In FIG. 16, time frames of duration TF2 are deployed on the four output channels Channell through Channel4 that have capacity Low.capacity. Consequently time frames have size X bits. A different time frame duration and possibly time frame size is used on the input ChanneI5 that has capacity High_capadty. In a sample configuration, Low_capadty could be 2.5 Gb/s (OC-48) and High_capadty could be 10 Gb/s (OC-192), with TF2=25 microseconds and TF1=12.5 microseconds. In this scenario the size of the time frames TFl is double the size of the time frames TF2. FIG. 16A depicts such a configuration which could be the configuration of a degrooming system 5200-b in FIG. 15. In the sample configuration depicted in FIG. 16 A, the size of the time frames TFl deployed on the input channel Channel5 is double (2-X bits) the size of the time frames TF2 deployed on the four output channels (X bits). The degrooming system 5200 receives 2-X bits from the input channel Channel5 during a first selected time frame 5120 of duration TFl and another 2-X bits from the input channel Channel5 during a second selected time frame 5125 of duration TFl. According to the configuration presented in FIG. 16, during a third selected time frame of duration TF2, the degrooming system 5200 transmits over the output channels Channell through Channel4 the 4-X bits received from the from the input channel Channel5 during the first selected time frame 5120 and the second selected time frame 5125, wherein the third selected time frame follows the first 5120 and the second 5125 selected time frames. In a possible embodiment, the X bits received from Channel5 during a first selected sub-time frame d of the first time frame 5120 of duration subTF=TFl/2 are transmitted on a predefined output channel Channel3, as shown in the example in FIG. 17. The X bits received from Channel5 during a second selected sub-time frame c of the first time frame 5120 of duration subTF=TFl/2 are transmitted on a predefined output channel Channel2. In FIG. 16, the two sub-time frames c and d of the first selected time frame 5120 can be delineated by a sub-time frame delimiter that can be either an implicit one or an explicit one implemented according to the methods described later in this disclosure. The sub-time delimiter may comprise a time frame identifier implemented according to the methods described later in this disclosure. The time frame identifier identifies the sub-time frame within its time frame or the time frame within its time cycle. The time frame identifier can carry control information regarding the data units belonging to the corresponding time frame or sub-time frame. For example, when the bits received during a time frame had been previously aggregated by a grooming system after having been received either over a plurality of input channels or during a plurahty of time frames, the time frame identifier can identify at least one of the input channels over which the bits were received and the time frame during which the bits were received. In a possible embodiment, a degrooming system 5200 selects the output channel and the time frame during which data units are to be transmitted over the selected output channel responsive to the control information contained in the identifier of time frame or sub-time frame during which the data units were received. In the example shown in FIG. 16A, the first selected sub-time frame d of the first time frame 5120 during which X bits are received from the input channel Channel5 has an identifier uniquely associated to the X bits. Responsive to such an identifier, the degrooming system 5200 forwards the respective X bits over the output channel Channel3 during the third selected time frame. In other words, the output channel Channel3 and the third selected time frame are chosen responsive to the identifier of sub-time frame d of time frame 5120. Alternatively, delimiters can be realized as an optical signal carried on the communications link coupled to input i. A possible embodiment of time frame delimiter consists of dedicating one of the wavelengths of the communications link for transmission of the delimiter. In a third alternative embodiment, the X bits transmitted on Channel3 and the X bits transmitted on ChanneI2 are multiplexed and received from Channel5 during the first time frame 5120. The 2-X bits received from Channels during a the second selected time frame 5125 of duration TFl are transmitted by the degrooming system 5200 on two predefined output channels Channel4 and Channell during the third selected time frame of duration TF2, as shown in the example in FIG. 17. The 2-X bits transmitted over ChanneI4 and Channell can be received from ChanneI5 either during two separate sub-time frames, during a plurality of distinct multiplexed sub-time frames, or multiplexed during the time frame 5125, similarly to what was described above for the reception of the 2-X bits received from Channels. In a possible configuration, the second selected time frame 5125 follows the first selected time frame 5120, as shown in the example depicted in FIG. 16 A. In an alternative configuration, the first selected time frame 5120 follows the second selected time frame 5125. In a possible configuration, the first time frame 5120 is adjacent to the second time frame 5125, as shown in the example depicted in FIG. 16A. In an alternative configuration, the first time frame 5120 is not adjacent to the second time frame 5125. In the sample configuration depicted in FIG. 16B, the size of the time frames TFl deployed on the input channel Channel5 is the same as the size of the time frames TF2 deployed on the four output channels (X bits). The degrooming system 5200 receives 4-X bits from the input channel Channel5 during four-selected time frames 5130 through 5137 of duration TF3. During a fifth selected time frame of duration TF2, the degrooming system 5200 transmits the X bits received, during the first selected time frame 5130, from the input channel Channel5 over the output channel Channell, wherein the fifth selected time frame of duration TF2 follows the first selected time frame 5130. During a fifth selected time frame of duration TF2, the degrooming system 5200 transmits the X bits received, during the second selected time frame 5133, from the input channel Channel5 over the output channel Channel4 , wherein the fifth selected time frame of duration TF2 follows the second selected time frame 5133. During a fifth selected time frame of duration TF2, the degrooming system 5200 transmits the X bits received, during the third selected time frame 5137, from the input channel Channel5 over the output channel Channel2 , wherein the fifth selected time frame of duration TF2 follows the third selected time frame 5137. During a fifth selected time frame of duration TF2, the degrooming system 5200 transmits the X bits received, during the fourth selected time frame 5135, from the input channel Channel5 over the output channel Channell , wherein the fifth selected time frame of duration TF2 follows the fourth selected time frame 5135. In a possible configuration, the fourth selected time frame 5135 follows the third selected time frame 5137, that in turn follows the second selected time frame 5133, which in turn follows the first selected time frame 5130, as shown in the example depicted in FIG. 16A. In alternative configurations, the four time frames 5130 through 5137 follow each other in any other possible order. In a possible configuration, the four time frames 5130 through 5137 are adjacent, as shown in the example depicted in FIG. 16A. In an alternative configuration, the four time frames 5130 through 5137 are not all adjacent to at least one of the others. In a possible configuration, each of the four time frames 5130 through 5137 is not adjacent to any one of the others. FIG. 17 depicts the architecture of a possible embodiment of a degrooming system 5200 comprising a Degrooming Controller 5220 and a Degrooming Module 5300 between a high capacity input channel 5240 and a plurality of lower capacity output channels 5230. In the sample configuration shown in FIG. 17, the degrooming system 5200 has c output channels at capacity Low_capadty and one input channel at capacity High_capadty, where High_capadty=c , Low_capadty. The degrooming module 5300 de-aggregates data units received from the input channel 5240 for transmission over the output channels 5230 responsive to the control signals 5250, 5260, and 5270 from the degrooming controller 5220. The degrooming controller 5220 generates the control signals for the degrooming module 5300 responsive to the common time reference (CTR) 002 received through an external control line 5280 and responsive to the unique time reference (UTR) coupled with the input channel 5240. The degrooming controller 5220 devises the UTR by means of at least one of the time frame delimiters and sub-time frame delimiters embedded in the flow of data according to at least one of the methods described later in the present disclosure. The control signals sent from the degrooming controller 5220 to the degrooming module 5300 comprise a Select__in signal 5250 for each one of the plurality of output channels 5230, a Select_out signal 5260 for each one of the plurality of output channels 5230, and a Select signal 5270. Each Select_in signal 5250 is aligned to the UTR coupled with the input channel 5240 and has a period equal to the time frame duration TF2 deployed on the corresponding output channel 5230. Each Select_out signal 5260 is aligned to the CTR and has a period equal to the time frame duration TF2 deployed on the corresponding output channel 5230. The Select signal 5270 is aligned to the UTR devised from the input channel 5240 and has a period equal to the sub-time frame duration subTF deployed on the input channel 5240. The degrooming systems 5200-a through 5200-d deployed in the degrooming scenario presented in FIG. 15 can be implemented using the architecture shown in FIG. 17, where c=4. For example, the Degrooming systems 5200-b can be implemented with the architecture shown in FIG. 17, where Low_capadty=2.5 Gb/s (OC-48 4030) and High_capadt =10 Gb/s (OC-1924040). Another alternative implementation of the degrooming module 5300 is shown in FIG. 18. The embodiment of degrooming module 5300 shown in FIG. 18 comprises a plurality of alignment subsystems 1400, at least one of a plurality of routing modules 5600, and a 1-by-c selector 2210. The embodiment of degrooming module 5300 depicted in FIG. 18 combines synchronous (time-based) degrooming with asynchronous (routing-based) degrooming. In FIG. 18, each Alignment Subsystem 1400 and each routing module 5600 handle data units received during a specific sub-time frame of a first time frame from the high capacity channel 5240. Data units are received by each Alignment Subsystem 1400 and each routing module 5600 through a respective one of the data lines 5320 from the 1-by-c selector 2210. In the embodiment shown in FIG. 18, after having been aligned to the CTR 002 within the alignment subsystem 1400, the data units received over the high capacity input line 5320 during a respective sub-time frame of a first time frame are transmitted over the corresponding low capacity output channel 5230 during a second time frame, wherein the second time frame follows the first time frame. The specific one of the alignment subsystems 1400 in which data units are to be transferred through a respective one of the data lines 5320 is selected by the 1-by-c selector 2210 in each sub- time frame responsive to the Select signal 5270 from the degrooming controller 5220. In the embodiment shown in FIG. 18, the data units received by each of the routing modules 5600 over the high capacity input line 5320 during a respective sub- time frame of a first selected time frame are transmitted over at least one of a plurality of output channels 5510 at a non previously selected time following the first selected time frame. The specific one of the routing modules 5600 in which data units are to be transferred through a respective one of the data lines 5320 is selected by the 1-by-c selector 2210 in each sub-time frame responsive to the Select signal 5270 from the degrooming controller 5220. The routing module 5600 is equipped with at least one output channel 5510. In one possible implementation, each of the n output channels 5510 has capacity Low_capadty/w. In a possible alternative implementation, each of the output channels 5510 has capacity Low_capadty. In an alternative implementation, each of the output channels 5510 has capacity High_capadty. In an alternative embodiment, output channels 5510 have different capacities, including but not limited to, Low_capadty, High_capadty, and a fraction or multiple thereof. FIG. 19 shows the architecture of a possible embodiment of the routing module 5600 comprising one input queue 5620, a 1-by-n selector 5640, a plurality of output queues 5630, and a routing controller 5610. Data units received from the high capacity data line 5320 are stored in the input queue 5620 while waiting for the routing controller 5610 to process them. Based on the control information associated to the data unit at the head of the input queue 5620, the routing controller 5610 determines the output channel 5510 on which said data unit is to be transmitted. In a possible embodiment, the routing controller 5610 retrieves the control information associated with the data units in the input queue 5620 through a data line 5670 and looks up the retrieved control information in a routing table 5615. The selected routing table entry identifies the output channel 5510 on which the corresponding data unit is to be transmitted. The routing controller 5610 selects the corresponding output 5680 of the selector 5640 and the processed data unit is stored in the corresponding output queue 5630 until the corresponding output channel 5510 is available for transmission. In a possible implementation data units are packets and the control information is contained in the packet's he * ader. In a possible embodiment, packets are IP (Internet Protocol) packets and the control information processed by the routing controller 5610, as in FIG. 19, is the destination address contained in the respective field of the IP header. In another embodiment, data units are ATM (Asynchronous Transfer Mode) cells and the control information is at least one of the VPI (Virtual Path Identifier) and VCI (Virtual Connection Identifier) in the cell header. SONET Switching with Common Time Reference Time driven switching can be used to implement very high capacity switching systems for SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy) channels. FIG. 20 shows a scenario where at least one of a time driven switching network i.e., a network composed of a plurality of time driven switches interconnected in an arbitrary topology by communications links and a time driven priority network [C-S Li, Y. Ofek and M. Yung, "Time driven Priority Flow Control for Real-time Heterogeneous Internetworking," IEEE INFOCOM'96, Apr. 1996.] is deployed to provide SONET/SDH services. A SONET channel, also called path, can be created across native SONET equipment (such as SONET cross connects 8040 and SONET add/drop multiplexers 8010) and the time driven switches used to realize a time driven switching subnetwork 8020. SONET frames, whose structure is depicted in FIG. 28, can be exploited to embed control information in the flow of data units. FIG. 28A is the structure of a STS- 1 (Synchronous Transport Signal) frame used for transmission on channels at speed 51.84 Mb/s, and FIG. 28B shows the structure of an STS-N frame used for transmission on channels at speed N-51.84 Mb/s. The STS-1 frame is organized in rows and columns; the frame is transmitted on a serial communications channel, e.g., 920 in FIG. 1, by rows. The STS-1 frame is composed of 9 lines of 90 bytes. The first 3 bytes of every row carry control information called Transport Overhead TOH. The control information in the Transport Overhead TOH is used by the network elements at the end points of sections (repeaters) and lines (add-drop multiplexers). The remaining 87 bytes on each row are transported between the end points of a communication. Data traveling end to end is not put in the STS-1 frame simply starting from column 3 of row 1. Instead, data is inserted in what is called a SONET Payload Environment SPE that can start at any position within the rightmost 87 columns of the STS-1 frame. The SPE is said to float within the payload of the STS-1 frame. Two of the bytes in the TOH, called bytes HI and H2, are a pointer to the position of the SPE within the STS-1 frame. The SPE, comprising 9 rows of 87 bytes, is further divided in a payload part PAYLOAD and an overhead part, called Path Overhead POH that consists of the first byte of each row — first column. User data traveling on a communications channel defined over a SONET network are carried within the PAYLOAD part. The STS-N frame, shown in FIG. 28B, is obtained by byte interleaving N STS-1 frames. As a result, the first 3-N columns of an STS-N frame contain the transport overhead TOH, while the remaining N-87 columns contain N SPEs, each one possibly floating independently. An STS-N frame (with N > 1) is transmitted in 125 microseconds, resulting in a transmission speed of N-51.84 Mb/s. As shown in FIG. 20, native SONET devices access the services of the time driven switching subnetwork 8020 through a fractional lambda interface 8300. In the ingress direction, the fractional lambda interface 8300 provides time framing and synchronization with the CTR (common time reference) of STS-N frames prior to transmission within the time driven switching or time driven priority subnetwork 8020. In the egress direction the fractional lambda interface 8300 provides time de-framing and synchronization to the SONET network time reference of STS-N frames prior to transmission within a SONET subnetwork. Native SONET devices, for example, SONET cross-connects 8040, and SONET Add/Drop Multiplexers 8010, that are part of SONET rings 8070 in FIG. 20, and SONET CPE (customer premise equipment; for example PBXs 8050, in FIG. 20) are connected to fractional lambda interfaces 8300 through SONET channels (for example, OC-48 8035, OC-1928030) in the scenario shown in FIG. 20, and fractional channels (e.g., 5 bundled, or concatenated, TI channels 8060) in the sample scenario shown in FIG. 20. Since the fractional lambda interfaces 8300 deployed in this scenario are used to provide SONET services, they are also called SONET fractional lambda interfaces (or SONET FLI) in the reminder of this document In a sample scenario, with reference to FIG. 20, a SONET channel created through a SONET cross-connect 8040 is routed over a link connected to a respective SONET fractional lambda interface 8300, switched through the TDS subnetwork 8020 towards an egress point and through the corresponding fractional lambda interface 8300 over a link connected to a SONET Add/Drop multiplexer 8010 that inserts the channel over a SONET metropolitan ring 8070. The time driven switching network 8020 operates as a SONET Network Element (NE) providing section level termination. In an alternative embodiment, the TDS network 8020 is transparent to the SONET equipment. In another alternative embodiment, the TDS network 8020 acts as a SONET user providing path level termination. A SONET channel entering the TDS subnetwork 8020 through a fractional lambda interface 8300 can be switched to a single egress point, or its component SONET sub-channels can be switched to different egress points. For example, a whole OC-192 8030 channel entering the TDS subnetwork can be switched to an egress point to which native SONET equipment is connected through an OC- 1928030 or higher capacity SONET channel. Alternatively, an OC-1928030 channel entering the TDS subnetwork can be de-multiplexed in its four OC-48 components channel and each component channel can be switched separately to a plurality of egress points to which native SONET equipment is connected via an OC-48 8035, OC-1928030, or higher capacity SONET channel. If the original OC-48 channel exits the networks through an egress point connected to an OC-1928030 or higher capacity channel, the egress fractional lambda interface 8300 is responsible for multiplexing the OC-48 channel 8035 with others within the outgoing SONET channel. Multiple SONET channels entering the TDS subnetwork 8020 through a selected fractional lambda interface 8300 multiplexed over the same access link - for example using WDM (Wavelength Division Multiplexing) - can be switched to a single egress point or to different egress points. FIG. 21, FIG. 22, FIG. 26, and FIG. 27 show a schematic description of the time framing and multiplexing operations as performed by alternative embodiments of a fractional lambda interface 8300 providing SONET services. FIG. 21 A and FIG. 2 IB show a fractional lambda interface (FLI) 8300 with one OC-48 (2.5 Gb/s) input channel 8110 and one output channel 8120 that has a capacity about three times the capacity of the respective OC-48 input channel 8110. FIG. 22A and FIG. 22B show a fractional lambda interface (FLI) 8300 with one OC-48 (2.5 Gb/s) input channel 8210 and one output channel 8220 that has about the same capacity as the respective OC-48 input channel 8210. FIG. 21C, FIG. 21D, FIG. 22C, and FIG. 22D show a fractional lambda interface (FLI) 8300 with three OC-48 (2.5 Gb/s) input channels 8130 and 8230, and one output channel 8140 and 8240, respectively, that has a capacity about three times the capacity of each of the respective OC-48 input channels 8130 and 8230. FIG. 26A and FIG. 26B show a fractional lambda interface (FLI) 8300 with one OC-48 (2.5 Gb/s) output channel 8610 and one input channel 8620 that has a capacity about three times that of the respective OC-48 output channel 8610. FIG. 27A and FIG. 27B show a fractional lambda interface (FLI) 8300 with one OC-48 (2.5 Gb/s) output channel 8710 and one input channel 8720 that has about the same capacity as the respective OC-48 output channel 8710. FIG. 26C, FIG. 26D, FIG. 27C, and FIG. 27D show a fractional lambda interface (FLI) 8300 with three OC-48 (2.5 Gb/s) output channels 8630 and 8730 and one input channel 8640 and 8740, respectively, that has a capacity about three times that of the respective OC-48 output channels 8630 and 8730. FIG. 21A shows a scenario in which each STS-48 frame 8115 received on the OC-48 input channel 8110 is transmitted over the output channel 8120 during a single time frame 8125. Given the capacity of the output channel 8120 (three times the capacity of the input channel 8110) and the duration of time frames 8125 on the output channel 8120, one STS-48 frame is transmitted over the output channel 8120 on average every three time frames. In a possible embodiment, the whole STS-48 frame 8115 received on the input channel 8110 is transmitted on the output channel 8120. In an alternative embodiment, only the Synchronous Payload Environment (see FIG. 28) is transmitted over the output channel 8120. The mapping between the STS-48 frames and the time frames during which they are transmitted on the output channel 8120 in FIG. 21 repeats every time cycle or super cycle. The time frames 8125 on the output channel 8120 are delineated according to at least one of the methods presented earlier in this disclosure. In a possible embodiment, time frame delineation on the output channel 8120 is provided at the network layer (as defined within the ISO OSI layered model) — for example, carrying control information for time frame delineation within the Internet Protocol (IP) payload or header. Each STS-48 frame transmitted on the output channel 8120 is encapsulated in an IP packet. In an alternative embodiment, time frame delineation is provided at the data link layer — for example, carrying control information for time frame delineation within the Point-to-Point Protocol (PPP) payload or header. Each STS-48 frame transmitted on the output channel 8120 is encapsulated in a PPP packet. In another alternative embodiment, time frame delineation is provided at the physical layer — for example, carrying control information for time frame delineation within the SONET frame. In a possible implementation, the SPE of the STS-48 frames 8115 received on the input channel 8110 are aligned to the common time reference (CTR) when transmitted on the output channel 8120. On the output channel 8120 and within the TDS network, the bytes HI and H2 in the transport overhead TOH, which point to the position of the SPE within the STS frame, can be used as a time frame delimiter D-frame. In this implementation, the beginning of a time frame, sub-time frame, control time frame, or time cycle — depending on the delimiter being implemented — must coincide with the beginning of the SPE. L other words, the SPE must be aligned with the common time reference (CTR) employed on the output channel 8120 and within the TDS network. If time frame delineation is provided at the physical layer using a digital wrapper - for example by carrying control information for time frame delineation within the digital wrapper - the STS-48 frames 8115 or their SPEs are encapsulated in the payload of the digital wrapper. FIG. 26A shows how the STS-48 frames 8615, time framed and transmitted according to the embodiment presented in FIG. 21 A, are time de-framed and forwarded on a SONET channel 8610. In the scenario depicted in FIG. 26A each STS-48 frame received from the input channel 8620 during a single time frame 8125 is transmitted over the output channel 8610 according to the SONET standard. Given the capacity of the input channel 8820 (three times the capacity of the output channel 8610) and the duration of time frames 8125 on the input channel 8620, one STS-48 frame is received from the input channel 8620 on average every three time frames. In a possible embodiment of the system presented in FIG. 26, whole STS-48 frames are received from the input channel 8620 and transmitted on the output channel 8610. In an alternative embodiment, only the Synchronous Payload Environment (see FIG. 28) are received from the input channel 8620 and a corresponding complete STS- 48 frame 8615 must be constructed prior to transmission over the output channel 8610. The pattern of time frames during which STS-48 frames are received on the input channel 8620 in FIG. 26 repeats every time cycle or super cycle. The time frames 8125 on the input channel 8620 are delineated according to at least one of the methods presented earlier in this disclosure. FIG. 21B shows a scenario in which each STS-48 frame i 8115 received on the OC-48 channel 8110 is de-multiplexed in its four STS-12 tributaries Fi-1, Fi-2, Fi-3, and Fi-4. Each STS-12 tributary frame is transmitted over the output channel 8120 during a single time frame 8127. Given the capacity of the output channel 8120 (three times the capacity of the input channel 8110) and the duration of time frames 8127 on the output channel 8120, the four tributary STS-12 frames composing one STS-48 frame are transmitted over the output channel 8120 during selected ones of six consecutive time frames 8127. In the example shown in FIG. 2 IB, the first STS-48 frame Frame 1 received on the input channel 8110 is de-multiplexed in its four STS-12 tributaries Fl-1, Fl-2, Fl- 3, and Fl-4. Frames Fl-1 and Fl-3 are transmitted over the output channel 8120 during a selected first time frame 8127. Frame Fl-2 is transmitted during a second selected time frame 8127, while Fl-4 is transmitted during a third selected time frame, wherein the third selected time frame follows the second one, and the second selected time frame follows the first one, wherein the three selected time frame are not consecutive. In an alternative embodiment, at least two of the three selected time frames are consecutive. In a possible embodiment, the mapping between the STS-12 frames and the time frame during which they are transmitted on the output channel 8120 repeats every time cycle or super cycle. As shown in FIG. 21B, more than one STS-12 frame can be transmitted during the same time frame 8127. In a possible implementation SONET frames are transmitted in an arbitrary order during the relative time frame. In another possible implementation, each SONET frame is transmitted in a separate pre-defined sub-time frame. The time frames 8127 and sub-time frames (containing different STS-12 frames) on the output channel 8120 are delineated according to at least one of the methods presented earlier in this disclosure. In a possible embodiment, whole STS-12 frames are transmitted on the output channel 8120. In an alternative embodiment, only the Synchronous Payload Environment (see FIG. 28) of each STS-12 frame is transmitted over the output channel 8120. In a possible embodiment, selected STS-12 frames are transmitted during the same time frame 8127. For example, STS-12 frames belonging to concatenated STS-12 channels can be sent during the same time frame 8127, possibly without having been previously byte de-multiplexed. FIG. 26B shows how STS-48 frames 8615 de-multiplexed, time framed, and transmitted according to the embodiment presented in FIG. 2 IB are later time de- framed, multiplexed, and forwarded on a SONET channel 8610 by a FLI 8300. In the scenario depicted in FIG. 26B, each of the four STS-12 tributaries of an STS-48 frame 8615 is received from the input channel 8620 during a single time frame 8127. The four STS-12 tributaries of each STS-48 frame 8615 are multiplexed according to the SONET standard to obtain the corresponding STS-48 frame. Given the capacity of the input channel 8620 (three times the capacity of the output channel 8610) and the duration of time frames 8127 on the input channel 8620, the four tributary STS-12 frames composing one STS-48 frame 8615 are received from the input channel 8620 during selected ones of six consecutive time frames 8127. In the example shown in FIG. 26B, a first STS-48 frame Frame 1 was previously de-multiplexed in its four STS-12 tributaries: Fl-1, Fl-2, Fl-3, and Fl-4. Frames Fl-1 and Fl-3 are received from the input channel 8620 during a selected first time frame 8127. Frame Fl-2 is received during a second selected time frame 8127, while Fl-4 is received during a third selected time frame, wherein the third selected time frame follows the second one, and the second selected time frame follows the first one, wherein the three selected time frames are not consecutive. In an alternative embodiment, at least two of the three selected time frames are consecutive. Once the four tributaries Fl-1, Fl-2, Fl-3, and Fl-4 being part of one STS-48 frame have been received, they are multiplexed according to the SONET standard to compose the original STS-48 frame that is subsequently transmitted on the SONET OC- 48 output channel 8610. In a possible embodiment, the mapping between the tributary STS-12 frames belonging to a given STS-48 frame and the time frame during which they are received on the input channel 8620 repeats every time cycle or super cycle. More than one STS-12 frame can be received during the same time frame 8127. In a possible implementation SONET frames are received in an arbitrary order during the relative time frame. In another possible implementation, each SONET frame is received in a separate pre-defined sub-time frame. The time frames 8127 and sub-time frames (containing different STS-12 frames) on the input channel 8620 are delineated according to at least one of the methods presented earlier in this disclosure. In a possible embodiment, whole STS-12 frames are received from the input channel 8620. In an alternative embodiment, only the Synchronous Payload Environment (see FIG. 28) of each STS-12 frame is received over the input channel 8620, in which case the fractional lambda interface 8300 is responsible for reconstructing the whole STS-12 frame before multiplexing the tributaries in the original STS-48 frame. In a possible embodiment, selected STS-12 frames are received during the same time frame 8127. For example, STS-12 frames belonging to concatenated STS- 12 channels can be received during the same time frame 8127, possibly without having been previously byte de-multiplexed. FIG. 21C shows a scenario in which each STS-48 frame received on the OC-48 input channels 8130-a through 8130-c is transmitted over the output channel 8140 during a single time frame 8145. Given the capacity of the output channel 8140 (three times the capacity of each input channel 8130-a through 8130-c) and the duration of time frames 8145 on the output channel 8140, one STS-48 frame from a selected one of the input channels 8130-a through 8130-c is transmitted over the output channel 8140 on average every three time frames. STS-48 frames from different input channels 8130-a through 8130-c are multiplexed at the time frame level, i.e., the STS-48 SONET frames are transmitted during different consecutive time frames. In the presented embodiment, the mapping between each of the STS-48 frames from the plurality of input channels and the respective time frame during which it is transmitted on the output channel 8140 repeats every time cycle or super cycle. In the sample scenario shown in FIG. 21C, an STS-48 frame Framela received from a selected input channel 8130-a is transmitted on the output channel 8140 during a first selected time frame 8145. An STS-48 frame Framelb received from another selected input channel 8130-b is transmitted during a second selected time frame 8145, wherein the second selected time frame immediately follows the first selected time frame. An STS-48 frame Framelc received from another selected input channel 8130-c is transmitted during a third selected time frame 8145, wherein the third selected time frame immediately follows the second selected time frame. FIG. 26C shows a scenario in which each STS-48 frame to be forwarded on the OC-48 output channels 8630-a through 8630-c is received from the input channel 8640 during a single time frame 8145. This scenario takes place when STS-48 frames were previously time framed and transmitted over a TDS network according to the method depicted in FIG. 21C. Given the capacity of the input channel 8640 in FIG. 26C (three times the capacity of each output channel 8630-a through 8630-c) and the duration of time frames 8145 on the input channel 8640, one STS-48 frame is received from the input channel 8640 on average every three time frames. STS-48 frames destined for the different output channels 8630-a through 8630-c are multiplexed over the input channel 8640 at the time frame level, i.e., the SONET STS-48 frames are received during different consecutive time frames. In a possible implementation, the mapping between the time frame during which each of the STS-48 frames is received from the input channel 8640 and the output channel 8630-a through 8630-c on which the STS-48 frame is to be transmitted repeats every time cycle or super cycle. In the sample scenario shown in FIG. 26C, an STS-48 frame Framela to be transmitted on a selected output channel 8630-a is received from the input channel 8640 during a first selected time frame 8145. An STS-48 frame Frame lb to be transmitted on another selected output channel 8630-b is received during a second selected time frame 8145, wherein the second selected time frame immediately follows the first selected time frame. An STS-48 frame Framelc to be transmitted on another selected output channel 8630-c is received during a third selected time frame 8145, wherein the third selected time frame immediately follows the second selected time frame. In an alternative embodiment, at least one of the first selected time frame, the second selected time frame, and the third selected time frame is does not immediately follow the other. FIG. 2 ID shows a scenario in which each STS-48 frame received on the OC-48 input channels 8130-a through 8130-c is de-multiplexed in its four STS-12 tributaries. Each STS-12 tributary frame from the plurality of input channels 8130-a through 8130- c is transmitted over the output channel 8140 during a single time frame 8147. Given the capacity of the output channel 8140 (three times the capacity of each input channel 8130-a through 8130-c) and the duration of time frames 8147 on the output channel 8140, the four tributary STS-12 frames composing one STS-48 frame are transmitted over the output channel 8140 during selected ones of six consecutive time frames 8147. In the example shown in FIG. 2 ID, the first STS-48 frame Frame la received on a first input channel 8130-a is de-multiplexed in its four STS-12 tributaries Fla-1, Fla-2, Fla-3, and Fla-4; the first STS-48 frame Frame lb received on a second input channel 8130-b is de-multiplexed in its four STS-12 tributaries Flb-1, Flb-2, Flb-3, and Flb-4; the first STS-48 frame Frame lc received on a third input channel 8130-c is de-multiplexed in its four STS-12 tributaries Flc-1, Flc-2, Flc-3, and Flc-4. Frames Fla-1 and Fla-3 are transmitted over the output channel 8140 during a first selected time frame 8147. Frame Fla-2 is transmitted during a second selected time frame 8147, while Fla-4 is transmitted during a third selected time frame, wherein the third selected time frame follows the second one, and the second selected time frame follows the first one, wherein the three selected time frames are not consecutive. L another possible configuration, at least two of the three selected time frames are consecutive. STS-12 frames Flb-3 and Flc-3 are transmitted in a fourth selected time frame 8147 that follows the first selected time frame, precedes the second selected time frame, and is adjacent to both of the first selected time frame and the second selected time frame. STS-12 frame Flb-2 is transmitted on the output channel 8140 during the second selected time frame, together with frame Fla-2. In a possible embodiment, the mapping between the STS-12 frames and the time frame during which they are transmitted on the output channel 8140 repeats every time cycle or super cycle. As shown in FIG. 2 ID, more than one STS-12 frame can be transmitted during the same time frame 8147. In a possible implementation, multiple SONET frames are transmitted in an arbitrary order during the respective time frame. In another possible implementation, each SONET frame is transmitted in a separate pre-defined sub-time frame. The time frames 8147 and sub-time frames (containing different STS-12 frames) on the output channel 8140 are delineated deploying delimiters implemented according to at least one of the methods presented earlier in this disclosure. In a possible embodiment, whole STS-12 frames are transmitted on the output channel 8140. In an alternative embodiment, only the Synchronous Payload Environment (see FFIG. 28) of each STS-12 frame is transmitted over the output channel 8140. In a possible embodiment, selected STS-12 frames are transmitted during the same time frame 8147. For example, STS-12 frames belonging to concatenated STS-12 channels can be sent during the same time frame 8147, possibly without having been previously byte de-multiplexed. FIG. 26D shows a scenario in which each STS-48 frame to be transmitted on the OC-48 output channels 8630-a through 8630-c was previously de-multiplexed in its four STS-12 tributaries according to the method depicted in FIG. 2 ID. Each STS-12 tributary frame is received from the input channel 8640 in FIG. 26D during a single time frame 8147. Given the capacity of the input channel 8640 (three times the capacity of each output channel 8630-a through 8630-c) and the duration of time frames 8147 on the input channel 8640, the four tributary STS-12 frames composing one STS-48 frame are received from the input channel 8640 during selected ones of six consecutive time frames 8147. In the example shown in FIG. 26D, the first STS-48 frame Frame la transmitted on a first output channel 8630-a had been previously de-multiplexed in its four STS-12 tributaries Fla-1, Fla-2, Fla-3, and Fla-4; the first STS-48 frame Frame lb to be transmitted on a second output channel 8630-b had been previously de- multiplexed in its four STS-12 tributaries Flb-1, Flb-2, Flb-3, and Flb-4; the first STS-48 frame Frame lc to be transmitted on a third output channel 8630-c had been previously de-multiplexed in its four STS-12 tributaries Flc-1, Flc-2, Flc-3, and Flc- 4. Frames Fla-1 and Fla-3 are received from the input channel 8640 during a selected first time frame 8147. Frame Fla-2 is received during a second selected time frame 8127, while Fla-4 is received during a third selected time frame, wherein the third selected time frame follows the second one, and the second selected time frame follows the first one, wherein the three selected time frame are not consecutive. In an alternative configuration, the three selected time frame are consecutive. STS-12 frames Flb-3 and Flc-3 are received in a fourth selected time frame 8147 that follows the first selected time frame, precedes the second selected time frame, and is adjacent to both of the first selected time frame and the second selected time frame. STS-12 frame Flb-2 is received from the input channel 8640 during the second selected time frame, together with frame Fla-2. In the preferred embodiment, the mapping between the tributary STS-12 frames and the time frame during which they are received on the input channel 8640 repeats every time cycle or super cycle. More than one STS-12 frame can be received during the same time frame 8147. In a possible implementation, multiple SONET frames are received in an arbitrary order during their respective time frame. In another possible implementation, each SONET frame is received in a separate pre-defined sub-time frame of the respective time frame. The time frames 8147 and sub-time frames (containing different STS-12 frames) on the input channel 8640 are delineated by means of delimiters implemented according to at least one of the methods presented earlier in this disclosure. In a possible embodiment, whole STS-12 frames are received from the input channel 8640. In an alternative embodiment, only the Synchronous Payload Environment (see FIG. 28) of each STS-12 frame is received over the input channel 8640, in which case the fractional lambda interface 8300 is responsible for reconstructing the whole STS-12 frame before multiplexing the tributaries in the original STS-48 frame. In a possible embodiment, selected STS-12 frames are received during the same time frame 8147. For example, STS-12 frames belonging to concatenated STS-12 channels can be received during the same time frame 8147, possibly without having been previously byte de-multiplexed. FIG. 22A shows a scenario in which each STS-48 frame 8115 received on the OC-48 input channel 8210 is transmitted over the output channel 8220 during a plurality of time frames 8225. Given the capacity of the output channel 8220 (the same as the input channel 8210) and the duration of time frames 8225 on the output channel 8220, one STS-48 frame is transmitted over the output channel 8220 during four time frames 8225. Each STS-48 frame 8115 is divided into four parts, which will be called "fractions" in the following, where each part is transmitted during a selected time frame 8225. In the embodiment presented in FIG. 22A, one single STS-48 fraction is transmitted during each time frame 8225. The time frames 8225 on the output channel 8220 are delineated by means of delimiters implemented according to at least one of the methods presented earlier in this disclosure. In the preferred embodiment, the mapping between the fractions of STS-48 frames and the time frames 8225 during which they are transmitted on the output channel 8220 repeats every time cycle or super cycle. In the example shown in FIG. 22A, the first STS-48 frame Frame 1 received on the input channel 8210 is divided into four fractions. The first fraction is transmitted over the output channel 8220 during a selected first time frame 8225, the second fraction is transmitted during a second selected time frame 8225, the third fraction is transmitted during a third selected time frame 8225, and the fourth fraction is transmitted during a fourth selected time frame 8225; wherein the fourth selected time frame immediately follows the third one, the third selected time frame immediately follows the second one, and the second selected time frame immediately follows the first one, wherein the four selected time frame are consecutive. In an alternative configuration, the first selected time frame, the second selected time frame, the third selected time frame, and the fourth selected time frame follow each other in a different order than the one above. In a possible embodiment, fractions of whole STS-48 frames 8115 received on the input channel 8210 are transmitted on the output channel 8220. In an alternative embodiment, only fractions of the Synchronous Payload Environment (see FIG. 28) are transmitted over the output channel 8220. FIG. 27A shows how STS-48 frames 8615 time framed and transmitted according to the embodiment presented in FIG. 22A are time de-framed and forwarded on a SONET channel 8710. FIG. 27A shows a scenario in which each STS-48 frame 8615 to be transmitted on the OC-48 output channel 8710 is received from the input channel 8720 during a plurality of time frames 8225. Given the capacity of the input channel 8720 (the same as the output channel 8710) and the duration of time frames 8225 on the input channel 8720, one STS-48 frame is received from the input channel 8720 during four time frames. Each STS-48 frame 8615 was previously divided into four parts, which we call "fractions", where each part is received during a selected time frame 8225. In the embodiment presented in FIG. 27 A, one single STS-48 fraction is received during each time frame 8225. The time frames 8225 on the input channel 8720 are delineated by means of delimiters implemented according to at least one of the methods presented earlier in this disclosure. In the preferred embodiment, the mapping between the fractions of STS-48 frames and the time frames during which they are transmitted on the output channel 8720 repeats every time cycle or super cycle. In the example shown in FIG. 27 A, the first STS-48 frame Frame 1 to be transmitted on the output channel 8710 was previously divided into four fractions. The first fraction is received over the output channel 8720 during a first selected time frame 8225, the second fraction is received during a second selected time frame 8225, the third fraction is received during a third selected time frame 8225, and the fourth fraction is received during a fourth selected time frame 8225, wherein the fourth selected time frame immediately follows the third one, the third selected time frame immediately follows the second one, and the second selected time frame immediately follows the first one; wherein the four selected time frames are consecutive. In a possible embodiment, fractions of whole STS-48 frames 8615 to be transmitted on the output channel 8710 are received from the input channel 8720. In an alternative embodiment, only fractions of the Synchronous Payload Environment (see FIG. 28) are received from the input channel 8720. FIG. 22B shows a scenario in which each STS-48 frame 8115 received from the OC-48 channel 8210 is transmitted over the output channel 8220 during a plurality of time frames 8227, wherein the number of time frames during which a single STS-48 frame is transmitted can vary. Given the capacity of the output channel 8220 (the same as the input channel 8210) and the duration of time frames 8227 on the output channel 8220, one STS-48 frame is transmitted over the output channel 8220 during either three or four time frames. Each STS-48 frame 8115 is divided into a plurality of parts and each part is transmitted on the output channel 8220 during a selected time frame 8227. In the embodiment presented in FIG. 22B, fractions from one or two different STS-48 frames are transmitted during each time frame 8227. The time frames 8227 on the output channel 8220 are delineated by means of delimiters implemented according to at least one of the methods presented earlier in this disclosure. Li the preferred embodiment, the mapping between each selected fraction of STS-48 frames and the respective time frame during which the selected fraction is transmitted on the output channel 8220 repeats every time cycle or super cycle. In a possible embodiment, fractions of the entire STS-48 frames 8115 received on the input channel 8210 are transmitted on the output channel 8220. In an alternative embodiment, fractions of only the Synchronous Payload Environment (see FIG. 28) are transmitted over the output channel 8220. In the configuration shown in FIG. 22B, fractions of the same STS-48 frame can be of different size. The transmission of a fraction of an STS-48 frame can require either a whole time frame 8227, or part of it. When transmission of an STS-48 frame fraction is completed before the end of a selected time frame 8227, a fraction from another STS-48 frame can be transmitted during the reminder of the same selected time frame 8227. Each SONET frame fraction is transmitted in a separate pre-defined sub- time frame. The time frames 8227 and sub-time frames (containing fractions from different STS-48 frames) on the output channel 8220 are delineated by means of delimiters implemented according to at least one of the methods presented earlier in this disclosure. In the example shown in FIG. 22B, the first STS-48 frame Frame 1 received on the input channel 8210 is divided into three fractions. The first fraction is transmitted over the output channel 8220 during a selected first time frame 8227, the second fraction is transmitted during a second selected time frame 8227, while the third fraction is transmitted during a third selected time frame, wherein the third selected time frame immediately follows the second one, the second selected time frame immediately follows the first one, and the three selected time frame are consecutive. In an alternative embodiment, at least two of the three selected time frames are not consecutive. Since, as shown in FIG. 22B, the third selected time frame is not completely filled up, i.e., the transmission of the third fraction of the STS-48 frame Framel does not require a whole time frame 8227, a fraction from the next received STS-48 frame Frame2 is transmitted during the third selected time frame 8227. The other two fractions of frame Frame2 are transmitted during the two time frames 8227 following the third selected time frame 8227. FIG. 27B shows how STS-48 frames 8615 time framed and transmitted according to the embodiment presented in FIG. 22B are time de-framed and forwarded on a SONET channel 8710. As shown in FIG. 27B, each STS-48 frame 8615 to be transmitted on the OC-48 channel 8710 is received from the input channel 8720 during a plurality of time frames 8227, wherein the number of time frames during which a single STS-48 frame is received is not necessarily fixed. Given the capacity of the input channel 8720 (the same as the output channel 8710) and the duration of time frames 8227 on the input channel 8720, one STS-48 frame 8615 is received from the input channel 8720 during either three or four time frames. Each STS-48 frame 8615 had been divided into a plurality of parts, each of which will be called a "fraction", and each part is received during a selected time frame 8227. In the embodiment presented in FIG. 27B, fractions from one or two different STS-48 frames 8615 are received during each time frame 8227. The time frames 8227 on the input channel 8720 are delineated by means of delimiters implemented according to at least one of the methods presented earlier in this disclosure. In the preferred embodiment, the mapping between the fractions of STS-48 frames 8615 and the time frames during which they are received from the input channel 8720 repeats every time cycle or super cycle. In a possible embodiment, fractions of the entire STS-48 frames 8615 to be transmitted on the output channel 8710 are received from the input channel 8720. In an alternative embodiment, fractions of the Synchronous Payload Environment (see FIG. 28) are received from the input channel 8720. Fractions of the same STS-48 frame 8615 can be of different size. An STS-48 frame fraction can be received during either a whole time frame 8227 or part of it. When reception of an STS-48 frame fraction is completed before the end of a selected time frame 8227, a fraction from another STS-48 frame can be received during the reminder of the same selected time frame 8227. In a possible embodiment, each SONET frame fraction is transmitted in a separate pre-defined sub-time frame. The time frames 8227 and sub-time frames (containing fractions from different STS-48 frames) on the input channel 8720 are delineated by means of delimiters implemented according to at least one of the methods described earlier in this disclosure. In the example shown in FIG. 27B, the first STS-48 frame Frame 1 to be transmitted on the output channel 8710 is divided into three fractions. The first fraction is received from the input channel 8720 during a first selected time frame 8227, the second fraction is received during a second selected time frame 8227, while the third fraction is received during a third selected time frame, wherein the third selected time frame immediately follows the second one, the second selected time frame immediately follows the first one, and the three selected time frame are consecutive. In an alternative embodiment, at least two of the three selected time frames are not consecutive. Since the third selected time frame is not completely filled up, i.e., the reception of the third fraction of the STS-48 frame Framel does not require a whole time frame 8227, a fraction from the STS-48 frame Frame2 to be transmitted next is received during the third selected time frame 8227. The other two fractions of frame Frame2 are received during the two time frames 8227 following the third selected time frame 8227. FIG. 22C shows a scenario in which each STS-48 frame received on the OC-48 input channels 8230-a through 8230-c is transmitted over the output channel 8240 during a plurality of time frames 8245. Given the capacity of the output channel 8240 (three times the capacity of each input channel 8230-a through 8230-c) and the duration of time frames 8245 on the output channel 8240, one STS-48 frame from a selected one of the input channels 8230-a through 8230-c is transmitted over the output channel 8240 during three selected time frames 8245 every nine time frames. During each time frame 8245 bits from a single STS-48 frame are transmitted. In the configuration shown in FIG. 22C, STS-48 frames from different input channels 8230-a through 8230-c are multiplexed at the time frame level, i.e., transmitted during different consecutive time frames. In the preferred embodiment, the mapping between the STS-48 frames from the plurality of input channels and the time frame during which they are transmitted on the output channel 8240 repeats every time cycle or super cycle. In the sample scenario shown in FIG. 22C, an STS-48 frame Framela received from a first selected input channel 8230-a is divided into three fractions. The three fractions are transmitted on the output channel 8240 during a first set of three selected time frames 8245, with one fraction in each time frame 8245. The fraction from a single STS-48 frame is transmitted during one time frame 8245. An STS-48 frame Framelb received from a second selected input channel 8230-b is divided into three fractions. The three fractions of frame Framelb are transmitted on the output channel 8240 during a second set of three selected time frames 8245, with one fraction in each time frame 8245, wherein the second set of selected time frames immediately follows the first set. An STS-48 frame Framelc received from a third selected input channel 8230-c is divided in three fractions. The three fractions of frame Framelc are transmitted on the output channel 8240 during a third set of three selected time frames 8245, one fraction in each time frame 8245, wherein the third set of selected time frames immediately follows the second set. In a possible embodiment, fractions of entire STS-48 frames are transmitted on the output channel 8240. In an alternative embodiment, fractions of only the Synchronous Payload Environment (see FIG. 28) of each STS-48 frame are transmitted over the output channel 8240. FIG. 27C shows a scenario in which each STS-48 frame to be transmitted on the OC-48 output channels 8730-a through 8730-c is received from the input channel 8740 during a plurality of time frames 8245. This scenario takes place when STS-48 frames had previously been time framed and transmitted over at least one of a TDS network and a TDP network according to the method depicted in FIG. 22C. Given the capacity of the input channel 8740 (three times the capacity of each output channel 8730-a through 8730-c) and the duration of time frames 8245 on the input channel 8740, one STS-48 frame to be transmitted on a selected one of the output channels 8730-a through 8730-c is received from the input channel 8740 during three selected time frames 8245 every nine time frames. During each time frame 8245 bits from a single STS-48 frame are received. STS-48 frames destined for different output channels 8730-a through 8730-c are multiplexed on the input channel 8740 at the time frame level, i.e., they are received during different consecutive time frames. Li the preferred embodiment, the mapping between the STS-48 frames destined for each output channel 8730-a through 8730-c and the time frame during which they are received from the input channel 8740 repeats every time cycle or super cycle. In the sample scenario shown in FIG. 27C, an STS-48 frame Framela to be transmitted on a first selected output channel 8730-a was previously divided into three fractions. The three fractions are received from the input channel 8740 during a first set of three selected time frames 8245, one fraction in each time frame 8245. During each time frame 8245 a fraction from a single STS-48 frame is received. An STS-48 frame Framelb to be transmitted on a second selected output channel 8730-b is divided into three fractions. The three fractions of frame Framelb are received from the input channel 8740 during a second set of three selected time frames 8245, with one fraction in each time frame 8245, wherein the second set of selected time frames immediately follows the first set. An STS-48 frame Framelc to be transmitted on a third selected output channel 8730-c is divided into three fractions. The three fractions of frame Framelc are received from the input channel 8740 during a third set of three selected time frames 8245, with one fraction in each time frame 8245, wherein the third set of selected time frames immediately follows the second set. In a possible embodiment, fractions of entire STS-48 frames are received from the input channel 8740. In an alternative embodiment, fractions of only the Synchronous Payload Environment (see FIG. 28) of each STS-48 frame are received from the input channel 8740. FIG. 22D shows a scenario in which each STS-48 frame received on the OC-48 input channels 8230-a through 8230-c is divided into three fractions. Each STS-48 frame fraction from any of the plurality of input channels 8230-a through 8230-c is transmitted over the output channel 8240 during a single time frame 8247. Given the capacity of the output channel 8240 (three times the capacity of each input channel 8230-a through 8230-c) and the duration of time frames 8247 on the output channel 8240, the three fractions of one STS-48 frame are transmitted over the output channel 8240 during three selected ones of nine consecutive time frames 8247. Each fraction is transmitted during a single time frame 8247 and a single fraction of a single STS-48 frame is transmitted during each time frame. Fractions of STS-48 frames received on different input channels 8230-a through 8230-c are multiplexed at the time frame level on the output channel 8240. In other words, fractions of different STS-48 frames can be transmitted during adjacent time frames 8247. In the example shown in FIG. 22D, the first STS-48 frame Frame la received on a first input channel 8230-a is divided into three fractions Fla-1, Fla-2, and Fla-3; the first STS-48 frame Frame lb received on a second input channel 8230-b is divided into three fractions Flb-1, Flb-2, and Flb-3; the first STS-48 frame Frame lc received on a third input channel 8230-c is divided into three fractions Flc-1, Flc-2, and Flc-3. Fraction Fla-1 is transmitted during a first selected time frame 8247, fraction Flb-1 is transmitted during a second selected time frame 8247, fraction Flc-1 is transmitted during a third selected time frame 8247, wherein the third selected time frame follows the second one, and the second selected time frame follows the first one, wherein the three selected time frame are consecutive. In an alternative embodiment the three selected time frames follow each other in a different order .than the one above, hi an alternative configuration, at least 2 of the three selected time frame are not consecutive. Fractions Fla-1, Fla-2, and Fla-3 are transmitted during three selected time frames 8247, wherein the selected time frames are not consecutive and fractions of STS-48 frames received on the second input channel 8230-b and third input channel 8230-c are transmitted during the time frames between the three selected time frames. In the preferred embodiment, the mapping between fractions of STS-48 frames and the time frame during which they are transmitted on the output channel 8240 repeats every time cycle or super cycle. In another possible embodiment, fractions of more than one STS-48 frame can be transmitted during the same time frame 8247. Li a possible implementation, different fractions are transmitted in an arbitrary order during their respective time frame. In another possible implementation, each STS-48 frame fraction is transmitted in a separate pre-defined sub-time frame. The time frames 8247 and sub-time frames (containing fractions of different STS-48 frames) on the output channel 8240 are delineated by means of delimiters implemented according to at least one of the methods presented earlier in this disclosure. In a possible embodiment, STS-48 frames are divided into fractions of different sizes. In a possible embodiment, fractions of entire STS-48 frames are transmitted on the output channel 8240. In an alternative embodiment, fractions of only the Synchronous Payload Environment (see FIG. 28) of each STS-48 frame are transmitted over the output channel 8240. FIG. 27D shows a scenario in which each STS-48 frame to be transmitted on the OC-48 output channels 8730-a through 8730-c has been previously divided into three fractions. This scenario takes place when STS-48 frames have been time framed and transmitted over at least one of a TDS network and a TDP network according to the method depicted in FIG. 22D. As shown in FIG. 27D, each STS-48 frame fraction destined to any of the plurality of output channels 8730-a through 8730-c is received from the input channel 8740 during a single time frame 8247. Given the capacity of the input channel 8740 (three times the capacity of each output channel 8730-a through 8730-c) and the duration of time frames 8247 on the input channel 8740, the three fractions of one STS-48 frame are received from the output channel 8740 during three selected ones of nine consecutive time frames 8247. Each fraction is received during a single time frame 8247, and a single fraction of a single STS-48 frame is received during each time frame. Fractions of STS-48 frames destined for different output channels 8730-a through 8730-c are multiplexed at the time frame level on the input channel 8740. In other words, fractions of different STS-48 frames can be received during adjacent time frames 8247. In the example shown in FIG. 27D, the first STS-48 frame Frame la to be transmitted on a first selected output channel 8730-a was previously divided into three fractions Fla-1, Fla-2, and Fla-3; the first STS-48 frame Frame lb to be transmitted on a second selected output channel 8730-b was previously divided into three fractions Flb-1, Flb-2, and Flb-3; the first STS-48 frame Frame lc to be transmitted on a third selected output channel 8730-c was previously divided into three fractions Flc-1, Flc- 2, and Flc-3. Fraction Fla-1 is received during a first selected time frame 8247, fraction Flb- 1 is received during a second selected time frame 8227, fraction Flc-1 is received during a third selected time frame 8227, wherein the third selected time frame follows the second one, and the second selected time frame follows the first one, wherein the three selected time frame are consecutive. In an alternative configuration, the three selected time frames follow each other in a different order than the one above. In an alternative embodiment, at least two of the three selected time frame are not consecutive. In the scenario depicted in FIG. 22D, fractions Fla-1, Fla-2, and Fla-3 are received during three selected time frames 8247, wherein the selected time frames are not consecutive and fractions of STS-48 frames to be transmitted on output channels 8730-b and 8730-c are received during the time frames between the three selected ones. In the preferred embodiment, the mapping between fractions of STS-48 frames and the time frame during which they are received from the input channel 8740 repeats every time cycle or super cycle. In another possible embodiment, fractions of more than one STS-48 frame can be received during the same time frame 8247. In a possible implementation, different fractions are received in an arbitrary order during the relative time frame. In another possible implementation, each STS-48 frame fraction is received in a separate predefined sub-time frame. The time frames 8247 and sub-time frames (containing fractions of different STS-48 frames) on the input channel 8740 are delineated by means of delimiters implemented according to at least one of the methods presented earlier in this disclosure. Li a possible embodiment, STS-48 frames are divided into fractions of different sizes. In a possible embodiment, fractions of entire STS-48 frames are received from the input channel 8740. In an alternative embodiment, fractions of the Synchronous Payload Environment (see FIG. 28) of each STS-48 frame are received from the input channel 8740. SONET Fractional Lambda Interface FIG. 23A shows a high level functional description of the transmission and switching of SONET frames over time driven switching networks or time driven priority networks. An STS-N SONET frame undergoes a number of manipulations 8330 prior to entering a time driven switching network or a time driven priority network and another set of manipulations 8380 prior to exiting the network. Each set of manipulations is performed by a fractional lambda interface 8300 that is installed at the boundary between a time driven switching network - or a time driven priority network - and a SONET network. Since the set of manipulations 8330 to be performed on an STS-N frame prior to entering a time driven switching network is different from the set of manipulations 8380 to be performed on an STS-N frame prior to exiting the time driven switching network, the fractional lambda interface for SONET (or SONET FLI) is functionally comprised of two modules, as shown in FIG. 23B. An ingress module 8400 performs the manipulations required prior to entering the time driven switching network, while the egress module 8500 performs the manipulations required prior to exiting the time driven switching network. As shown in FIG. 23B, the SONET fractional lambda interface 8300 is connected to at least one SONET device from which it receives STS-N frames through at least one input channel 8430 and to which it transmits STS-N frames through at least one output channel 8550. The SONET FLI is also connected to at least one time driven switch 52 to which it sends data units through at least one output channel 8440 and from which it receives data units through at least one input channel 8540. STS-N frames are received from the SONET network through input channel 8430 by the ingress module 8400 of the FLI 8300. The ingress module 8400 performs ingress manipulations 8330 on the received STS-N frames and transmits them to a connected time driven switch 52 through the output channel 8440. The egress module 8500 of the FLI 8300 receives data units from a time driven switch 52 through the input channel 8540 and performs egress manipulations 8380 required to devise corresponding STS-N frames that are transmitted to the connected SONET network through the output channel 8550. . As shown in FIG. 23A, an STS-N frame undergoing ingress manipulations 8330 is first de-multiplexed 8310 in its STS-M tributaries, where M≤N. In an alternative embodiment, de-multiplexing of incoming STS-N frames is not performed, and following processing is performed on STS-M frames where M=N. In an alternative embodiment, the SONET de-multiplexing function 8330 is replaced by another demultiplexing - or segmentation - function, which divides each incoming STS-N frame into a plurality of parts, or fractions, that are transmitted separately through the time driven switching network. The resulting fractions can have fixed size or variable size. The STS-M frames to be transmitted on the TDS network are time framed 8320. In other words, each STS-M frame is transmitted on a channel 8440 to a time driven switch 52, as shown in FIG. 23B, during a selected time frame or sub-time frame, wherein the selected time frame or sub-time frame is delineated by means of delimiters implemented according to at least one of the time frame delineation methods described before in this disclosure. In the preferred embodiment, the mapping between an STS-M frame and the time frame or sub-time frame during which it is transmitted towards a time driven switch 52 on channel 8440 is repeated cyclically each time cycle or super cycle. In a possible embodiment, control information can be associated to each STS-M frame or fraction thereof, being time framed 8320 and transmitted on a channel 8440 to a time driven switch 52, as shown in FIG. 23B, during a selected time frame or sub-time frame. Said control information is used to at least route the STS-M frame through the time driven switching or time driven priority network 8250 in FIG. 23A. The time framed STS-M frames (i.e., the STS-M frames transmitted during selected time frames or sub-time frames) are switched and routed towards their destination 8350 according to at least one of the time driven switching principles of operation and the time driven priority principles of operation. STS-M frames (or fractions thereof) exiting the time driven switching network (i.e., received by the egress module 8500 of a SONET fractional lambda interface 8500) are time de-framed 8360. In other words, the original STS-M frame (or fraction thereof) is obtained after having possibly stripped out at least one of the delimiter of the time frame during which it was transmitted, the delimiter of the sub-time frame during which it was transmitted, and the associated control information and after having possibly reassembled a plurality of fractions. A plurality of STS-M frames are multiplexed 8370 according to the SONET standard and the original STS-N frame is transmitted to the piece of SONET equipment to which the STS-N frame is to be routed. The piece of SONET equipment is connected to the egress module 8500 of the fractional lambda interface 8300 through channel 8550 shown in FIG. 23B. In another embodiment, STS-M frames received through the time driven switching network do not need to be multiplexed in order to build higher order STS-N frames (i.e., M=N in the example shown in FIG. 23A) and the SONET MUX function 8370 is not performed. In an alternative embodiment, a plurality of fractions are composed to build the original STS-N frame that is then transmitted to the piece of SONET equipment to which the STS-N frame is to be routed. FIG. 24 depicts the functional block diagram of the ingress module 8400 of a SONET FLI 8300, comprising a SONET DMUX and FLP mapping module 8410, a per FLP queuing system 2940, and a Forwarding and Transmit Delineation Controller with position logic and position counter 8420. The ingress module 8400 of a SONET FLI 8300 performs the functions 8330 represented in FIG. 23A. The SONET STS-N frames received through at least one of a plurality of input channels 8430 depicted in FIG. 24 are processed by a module 8410 that performs SONET de-multiplexing (DMUX) and maps each de-multiplexed tributary STS-M frame onto a selected fractional lambda pipe (FLP) 2910, responsive to a Cycle- mapping Controller 8417. In a possible embodiment, the mapping performed by the Cycle-mapping Controller 8417 is based on a table 8415 providing the mapping between each SONET OC-M tributary channel and a corresponding FLP. Li an alternative embodiment, the STS-N frames are divided into fractions and a table provides the mapping between each STS-N frame fraction and the corresponding FLP. Based on the mapping information, the SONET DMUX and FLP mapping module 8410 stores the STS-M frames (or STS-N fractions) in the proper FLP queue 2945. The Forwarding and Transmit Delineation Controller 8420 retrieves data units (STS-M frames or STS-N fractions) from the Per FLP queuing system 2940 and transmits them over at least one of a plurality of output channels 8440 towards an time driven switch 52. Upon transmission on the output channel 8440, the Forwarding and Transmit Delineation controller 8420 delineates time frames by means of delimiters implemented according to at least one of the methods previously described in this disclosure. The selected one of the FLP queues 2945 from which data units are retrieved during each time frame by the Forwarding and Transmit Delineation controller 8420 is determined based on the content of an FLP schedules database 8425 that contains information regarding reservation of transmission resources to FLPs 2910 during each time frame. Data units (STS-M tributary frames or STS-N fractions) transmitted over an FLP traverse the time driven switching network (i.e., are switched and routed through a plurality of time driven switches 52 and communications channels 8445) and reach the egress point of the TDS network configured for the corresponding FLP 2910. In an alternative embodiment, data units (STS-M tributary frames or STS-N fractions) transmitted over a pipe - also called synchronous virtual pipe (SVP) - traverse the time driven priority network (i.e., are switched and routed through a plurality of time driven priority switches 52 and communications channels 8445 in FIG. 24) and reach the egress point of the TDP network configured for the corresponding FLP 2910. At the TDS network egress point an FLP 2910 is terminated on an FLI 8300. The data units exiting the FLP 2910 are processed by the egress module 8500 of the FLI 8300. FIG. 25 depicts the functional block diagram of a possible implementation of the egress module 8500 of an FLI 8300, comprising a Receive Delineation and De-Framing Controller 8510, a Per SONET channel queuing system 8560, and a SONET multiplexer 8520. The egress module 8500 of a SONET FLI 8300 performs the functions 8380 represented in FIG. 23A. Data units that have traveled through the network of time driven switches 52 and communications channels 8445 over an FLP 2910 are fed into the egress module 8500 of the FLI 8300 through at least one of a plurality of input channels 8540. Data units received through an input channel 8540 are first processed by the Receive Delineation and De-framing controller 8510. The Receive Delineation and De- framing controller 8510 utilizes delimiters embedded in the data stream according to at least one of the methods presented above in this disclosure, to identify time frame boundaries and uniquely associate data units (STS-M frames of STS-N fractions) with the time frame to which they belong. Time frame delineation is based on delimiters implemented according to at least one of the methods described earlier in this disclosure. The Receive Delineation and de-framing controller 8510 is responsible for stripping the delineation information from the data stream (i.e., to perform time de- framing) and to map each data unit received onto the corresponding SONET channel. The mapping is based on the content of the FLP to SONET channel table 8515. In fact, the time frame or sub-time frame to which a data unit belongs univocally associates the data unit with the FLP on which it traveled through the TDS network. The FLP to SONET channel table 8515 associates the time frame or sub-time frame (and, ultimately, the corresponding FLP 2910) to the SONET channel to which the received data unit belongs. Before transmitting the mapped data unit on the proper output line 8530, the Receive Delineation and de-framing controller 8510 stores the data unit in the per SONET channel queue 8565 corresponding to the SONET channel to which it belongs. As shown in FIG. 25, a SONET multiplexer 8520 retrieves tributary STS-M frames from the per SONET Channel queuing system 8560 and multiplexes them in STS-N frames (where N≥M) to be transmitted to a piece of SONET equipment connected to at least one of a pluraUty of output channels 8550. In an alternative embodiment, fractions of SONET frames are received over the input channels 8540 of the egress module 8500 of the FLI 8300. The Receive Delineation and De-framing controller 8510 devises time frame delineation information and stores each SONET frame fraction in the channel queue 8565 corresponding to the FLP 2910 over which the SONET frame fraction was received. SONET frame fractions are retrieved from the per SONET channel queuing system 8560 and are composed to obtain the original STS-N frame to be transmitted to a piece of SONET equipment connected to at least one of a plurality of output channels 8550 shown in FIG. 25. The SONET network can perform at least one of SONET switching, SONET multiplexing, and SONET de-multiplexing. In a possible embodiment, SONET switching is implemented by using at least one of time driven switching (TDS) technology and time driven priority technology, as described above in this disclosure. Time driven Tunable Laser-Based Switching with Common Time Reference Advances in components for optical networking feature dynamic optical switching fabrics — e.g., based among others on electro-mechanical micro mirrors, holographic techniques, bubbles — and tunable lasers. The time required for changing the input/output configuration of dynamic optical switching fabrics is currently larger than the time required for changing the wavelength generated by a tunable laser. As a consequence, optical switch architectures based on tunable lasers rather than optical switching fabrics are appealing. The present disclosure describes a number of optical switch architectures based on tunable lasers. FIG. 31 A shows three possible architectures of a fractional lambda switch implemented using an array of tunable lasers 10200 and comprising a switch controller 13030. In FIG. 31 A there is 16 input and output ports, each terminating a Wavelength Division Multiplexing (WDM) optical fiber 10010 carrying 16 wavelengths. Li each of the architectures, an optical de-multiplexer (WDM DMUX) 10040 separates the 16 wavelengths. The WDM DMUX 10040 is coupled with an alignment subsystem 10100 or 10900, as show in FIG. 31 A. In an alternative embodiment sub-carrier multiplexing (SCM) is used to provide for multiple channels on each fiber. SCM multiplexers and SCM de-multiplexers - instead of WDM multiplexers (MUXes) 10050 WDM de-multiplexers (DMUXes) 10040 - combine and separates the various optical channels on the fibers. As shown in FIG. 31 A during each time frame, aligned data units retrieved from the alignment subsystem are transmitted by a tunable laser 10200 to an output port where a WDM multiplexer (WDM MUX) 10050 combines the wavelength generated by the tunable laser with other 15 wavelengths onto the corresponding output fiber. As shown by the block diagram of a time driven tunable laser 10200 depicted in FIG. 30A, the control of the tunable laser is based on a wavelength-mapping table 10210 that is downloaded into each tunable laser scheduling controller 10220 by the switch controller 13030. The wavelength-mapping table 10210 indicates to the tunable laser scheduling controller 10220 the wavelength to be used during each time frame of the time cycle or super cycle. As shown in FIG. 30B the tunable laser 10230 can change wavelength every time frame. The wavelength mapping follows a predefined pattern that repeats itself every time cycle or every super cycle. The switch controller 13030 centrally computes the wavelength mapping table 10210 for all the tunable lasers 10200 guaranteeing that the same wavelength is not used during the same time frame by more than one tunable laser 10200 that is connected to the same output port WDM MUX 10050, as shown in FIG. 31 A . The wavelength- mapping table 10210, shown in FIG. 30, of a tunable laser 10200 is changed at the FLP control level, i.e., each time an FLP is set up or torn down. The optical alignment subsystem 10900 in FIG. 31 A is based on a programmable delay system 10930, guaranteeing that the overall delay experienced through the optical fiber and the programmable delay system 10930 is an integer number of time frames. As a result, when data units that have left the switch at the transmitting end of the fiber aligned with the CTR arrive at the WDM DMUX 10040 (see for example FIG. 31 A) at the receiving end are still aUgned with respect to then CTR. In the switch depicted in FIG. 31 A the tunable lasers 10200 are statically connected to the same input WDM DMUX 10040 and the same output WDM MUX 10050. Each WDM DMUX 10040 is coupled with tunable lasers 10200 connected to each one of WDM MUXes 10050. Due to this interconnection configuration, the wavelength on which data units are carried on the input fiber determines their routing within the switch. Thus, the wavelength deployed by a tunable laser 10200 during a time frame determines the route the data units belonging to the time frame will take in the downstream switch on the path to their destination. This WDM wavelength switching method (by using tunable lasers) is equivalent to label switching in ATM and MPLS. The label chosen in a switch determines the routing of a cell or MPLS packet in the next switch. Equivalently, the wavelength chosen in a switch determines the routing of a time frame worth of data in the following switch. Therefore, the wavelength entry in the tunable laser's wavelength mapping table 10210 (shown in FIG. 30) is equivalent to the Next Hop Label Forwarding Entry (NHLFE) of an MPLS switch. The switch architecture presented in FIG. 31 A is inexpensive and scalable because it is based on simple components and static interconnections among them. The control complexity is very low, since lasers are tuned no more than once every time frame. However, the resulting switch is not flexible, since only one wavelength from an input 10010 (WDM DMUX 10040) can be switched to a given output 10020 (WDM MUX 10050). A more flexible architecture can be obtained in two ways: 1. When the number of wavelengths per port is larger than the number of ports, and 2. When the switch architecture depicted in FIG. 31 A is generalized, which adds configurable connections between the tunable lasers 10200 and the outputs 10020 by means of an optical cross connect 10510. The number of wavelengths on an input 10010 switched to a given output 10020 during the same time frame can be changed dynamically — even though on a possibly long time scale — to accommodate uneven traffic patterns. Detailed description of switch architectures FIG. 30A shows the block diagram of a possible implementation of a time driven tunable laser 10200 comprising a tunable laser (TL) scheduling controller 10220 responsive to the CTR 002 and a control signal 13033 from the switch controller, and a tunable laser transmitter 10230 responsive to a color control signal 10240. The tunable laser 10200 receives data units from its input line 10165 and a tunable laser transmitter 10230 transmits them on a selected wavelength over the output line 10030. The selected wavelength used by the tunable laser transmitter 10230 is determined responsive to the color control signal 10240 generated by the TL scheduling controller 10220. In a possible embodiment the color control signal 10220 selects a different wavelength for transmission of data units by the tunable laser transmitter 10230 during each time frame. FIG. 30B is a sample timing diagram describing the operation of a tunable laser transmitter 10230 in this embodiment. The timing diagram shows a sequence of CTR time frames TF, and, for each time frame, the wavelength used for transmission by the tunable laser transmitter 10230, wherein each wavelength is identified by the color (green, yellow, red, and blue) of the corresponding light beam. FIG. 30C shows the implication of the choice of a selected wavelength on the routing of data units transmitted over the wavelength. Different wavelengths are routed to different outputs 10020 of the downstream switching system 10000 due to the interconnections between the output lines 10160 of the WDM DMUXes 10040 and the input lines 10030 of the WDM MUXes 10050 of each output 10020. For example, in a possible configuration data units transmitted by the tunable laser 10200 on the green wavelength during a first selected time frame are going to be routed to and — during a second selected time frame, wherein the second time frame follows the first one — forwarded through output 1 of the next switching system 10000 traversed by the data units, i.e., the switching system 10000 connected to the output 10020 with to the output line 10030 of the tunable laser 10200 is coupled. Instead, data units transmitted by the tunable laser 10200 on the yellow wavelength during a third selected time frame are going to be routed to and — during a fourth selected time frame, wherein the fourth time frame follows the third one — forwarded through output 3 of the next switching system 10000 traversed by the data units, i.e., the switching system 10000 connected to the output 10020 to which the output line 10030 of the tunable laser 10200 is coupled. In an alternative embodiment the color control signal 10220 selects a different wavelength for transmission of data units by the tunable laser transmitter 10230 during each sub-time frame. The TL scheduling controller 10220 generates the color control signal 10240 responsive to both the CTR 002 and the content of a wavelength mapping table 10210. The wavelength mapping table 10210 contains the mapping between each CTR time frame or sub-time frame and the wavelength to be used by the tunable laser transmitter 10230 for transmitting during the selected CTR time frame or sub-time frame. The above mentioned mapping is calculated and written in the wavelength mapping table 10210 of each one of the TL scheduling controllers 10220 by the switch controller 13030 through control signal 13033. The switch controller 13030 calculates time frame or sub-time frame schedules on all the wavelengths of all the inputs 10010 and outputs 10020. Li a possible embodiment, the above mentioned mapping repeats each time cycle or each super cycle. An alternative embodiment features a centralized TL scheduling controller that generates the color control signal 10240 for all the tunable lasers 10200 in the respective switching system 10000. In such an embodiment the centralized TL scheduling controller can be implemented within the switch controller 13030. FIG. 31 A is the architecture of an alternative embodiment of a time driven switch based on tunable lasers 10200. The switching system 10000 presented in FIG. 31 A has a plurality of inputs 10010 and outputs 10020, each one consisting of an optical link with a plurality of wavelengths. The switching system 10000 in FIG. 31 A comprises a switch controller 13030 a plurality of optical alignment subsystems 10900, WDM (wavelength division multiplexing) de-multiplexers (DMUX) 10040, tunable lasers 10200, and WDM multiplexers (MUXes) 10050, and connection lines 10030 between each one of the tunable lasers 10200 and a respective one of the WDM multiplexers 10050. The WDM DMUXes 10040, optical alignment subsystems 10900, tunable lasers 10200, and WDM MUXes 10050, are controlled by the switch controller 13030, responsive to the CTR 002, through four bi-directional control lines 13031, 13737, 13033, and 13034, respectively. Each of the four control lines provides configuration information from the switch controller 13030 to the WDM DMUXes 10040, optical alignment subsystems 10900, tunable lasers 10200, and WDM MUXes 10050; and via the four bi-directional control lines 13031, 13737, 13033, and 13034, the switch controller 13030 receives various status and control information from the WDM DMUXes 10040, alignment subsystems 10100, tunable lasers 10200, and WDM MUXes 10050. A respective one of the plurality of the optical alignment subsystems 10900 is associated to each respective one of the inputs 10010. The Optical Alignment Subsystem 10900 aligns to the common time reference (CTR) data units received over the plurality of wavelengths of its respective input 10010. Each WDM DMUX 10040 divides each of the wavelengths received from the corresponding optical input line 10320 and directs it to a corresponding tunable laser 10200. In the configuration shown in FIG. 31A the switching system 10000 comprises 16 inputs 10010 and outputs 10020, each one comprising 16 wavelengths. Consequently, each WDM DMUX 10040 has 16 output lines 10310 and each WDM MUX 10050 has 16 input lines 10030. For example, WDM DMUX i has 16 output lines (i,l) through (i,16) and WDM MUX; has 16 input lines (j,l) through (j,16). The switch in FIG. 31 A performs PF that is reaUzed in two operational phases, as shown in FIG. 3 IB. Data units belonging to a whole time frame received from each of the optical channels during Phase 1 are switched through the switch in Phase 2. Li a possible embodiment, if Phase 1 begins in time frame t, Phase 2 takes place in time frame t+1. In another embodiment, if Phase 1 ends in time frame t, Phase 2 takes place in time frame t+1. The 2 phase operation ensures that data units received from the various optical channels are ahgned with the CTR before being switched. Phase 2 can be performed during either the time frame immediately following Phase 1, during time frame t+1 — immediate forwarding operation, or at a later time frame — non- immediate forwarding operation. During each time frame, the tunable laser 10200 in FIG. 31 A receives from its respective line 10310 data units to be switched during the current time frame and transmits them over a pre-selected wavelength on the connection line 10030 to its respective one of the WDM MUXes 10050. Each WDM MUX 10050 multiplexes the wavelengths received on its respective input lines 10030 from the tunable lasers 10200 and transmits them on its respective output 10020. For example, WDM MUX 10050 multiplexes on output; 10020 the wavelengths received on the connection lines 10030 ( ,1) through (/,16). As shown in FIG. 30A, each tunable laser 10200 can change the wavelength on which it transmits for each time frame according to the information stored in a (sub)- time frame table 10210 downloaded in the tunable laser controller 10220 by the switch controller 13030 through control line 13033. By properly building the (sub)-time frame tables 10210 for all the tunable lasers 10200 of the switch in FIG. 31 A, the switch controller 13030 ensures that no more than one among the plurality of tunable lasers 10200 connected to the same WDM MUX 10050 transmits over the same wavelength during the same time frame. The topology of the interconnections 10030 between each tunable laser 10200 and a respective one of the WDM MUXes 10050, determines the route of the data units received on each wavelength from each input 10010. For example, with reference to FIG. 31 A, the data units received on a first selected wavelength of input 1 10010 which is de-multiplexed by the WDM DMUX 10040 on its output line 1 (1,1) are going to be transmitted by the respective first tunable laser 10200 on output 1 10020. This is a consequence of the fact that the respective first tunable laser 10200 is connected via a first one of the connection lines 10030 to input line (1,1) of the WDM MUX 1 10050 that is coupled to output 1 10020. Instead, with reference to FIG. 31 A, data units received on a second selected wavelength of input 1 10010 which is de-multiplexed by the WDM DMUX 10040 on its output line; (1/) are going to be transmitted by the respective second tunable laser 10200 on output; 10020. This is a consequence of the fact that the respective second tunable laser 10200 is connected via a second one of the connection lines 10030 to input line ø,l) of the WDM MUX; 10050 that is coupled to output; 10020. In other words, the first wavelength over which data units are carried on a selected input link 10010 determines the selected output 10020 on which those data units will be forwarded. The second wavelength on which the data units are transmitted by the tunable laser 10200 coupled to the selected input 10010 and first wavelength from which they are received determines the routing in the switching system 10000 coupled to the selected output 10020. In a possible embodiment, the OXC configuration changes reoccur periodically with a period which is an integer multiple of the time cycle or the super cycle. In a possible implementation the number of wavelengths of each input 10010 is smaller than the number of inputs 10010. In another possible implementation the number of wavelengths of each input 10010 is larger than the number of inputs 10010. Transmission of data units responsive to a common time reference 002 can deploy multiple wavelengths-also called optical channels or lambdas-across a Wavelength Division Multiplexing (WDM) link and throughout a lambda routed network. FIG. 32 shows the architecture of a communications system responsive to the common time reference wherein data units are associated to a specific time frame or sub-time frame, wherein such data units are transmitted over a specific wavelength across a WDM network 7310 whose network nodes (called wavelength routers or lambda routers) possibly route different wavelengths towards different destinations. In other words, the lambda routing network 7310 couples an optical channel on an ingress link 1230-1 with an optical channel (possibly the same optical channel if the lambda routing network 7310 does not have wavelength conversion capability) on a selected egress link 1230-E. The transmission system in FIG. 32 couples data units from an output port 7320 to an input port 7330 through a communications network 7310 deploying lambda routing. Within the WDM communications network 7310 multiple optical channels are multiplexed over links among nodes; nodes, also called lambda routers, route different channels over different paths. The output port 7320 and the input port 7330 are connected to the communications network 7310 through WDM links 1230-1 and 1230- E, respectively, comprising a plurality of optical channels. The system receives a common time reference 002 and comprises a transmission delineation controller 7325-source of delimiter signals-responsive to the CTR 002; a serial transmitter 6012, responsive to the delimiter signals 6030 through 6034 and the CTR 002 for sending the control signals and the data units over a line 7345 to a tunable laser 7340. The tunable laser 7340 transmits the bit stream received on line 7345 on the output link 1230-1 on a selected optical channel responsive to the Select-WL signal 7328. The transmission system in FIG. 32 is connected to a lambda switching network 7310. In fact the communications network 7310 routes different wavelengths entering the network from the same ingress point (e.g., the communications link 1230-1) to different egress points (among which, for example, the communications link 1230-E) of the communications network 7310. For example, data units transmitted during a first respective time frame over the green wavelength are routed by the communications network 7310 to the egress point X, while data units transmitted during a second respective time frame over the yellow wavelength are routed by the communications network 7310 to egress point Y. The system depicted in FIG. 32 further comprises a tunable optical receiver 7350 that receives data through the input link 1230-E over a selected optical channel , responsive to the Select-WL 7358. The received data stream is passed through line 7355 to a serial receiver 6022, responsive to the CTR 002. The serial receiver 6022 is coupled to a receive delineation controller 7335, responsive to the CTR 002, through delimiter signals 6040 through 6044. The input port 7330 in FIG. 32 further comprises an Alignment Subsystem 6600 for storing the data units received from the input link 1230- E while sorting them out according the time frame or sub-time frame during which they were sent out of the output port 7320. The output port 7320 and the input port 7330 in FIG. 32 are connected to a lambda switching network 7310. The communications network 7310 can route different wavelengths entering the network from various ingress points (among which, for example, the communications link 1230-1) to one egress points (e.g., the communications link 1230-E) of the communications network 7310. For example, data units received during a first respective time frame over the green wavelength had been routed by the communications network 7310 from ingress point X, while data units received during a second respective time frame over the yellow wavelength had been routed by the communications network 7310 from ingress point Y. The ahgnment subsystem 6600 in FIG. 32 receives data units over the data line 6020 from the serial receiver 6022. The data units that exit from the alignment subsystem 6600 are transferred to the switching fabric over its input lines 940. The control data units, namely the data units transmitted over the communications channel 920 during a control time frame, are transferred to the switch controller 13030 through line 980. In FIG. 32, the Transmit Delineation Controller 7325, responsive to the CTR 002, generates control signals 6030 through 6034 to indicate to the serial transmitter (TX) 6012 to insert control information in the data flow. The serial transmitter 6012 receives data units over line 6010 and transmits them on the communications channel 920. Responsive to the control signals 6030 through 6034 from the Transmit Delineation Controller 7325, the serial transmitter 6012 combines the data units to be transmitted on the communications channel 920 with control information such as time frame delimiters, time cycle delimiters, and time stamps according to at least one of the plurality of methods to encode such information in the data stream. Upon receiving the data stream, the serial receiver 6022 on the receiving side of the communications channel 1230-E separates data units from control signals. The serial receiver 6022 outputs the received data units on the data Une 6020 and notifies the receive delineation controller 7335 of the received control signals over the lines 6040 through 6044. The Transmit Delineation Controller 7325, responsive to the CTR 002, generates control signals 6030 through 6034 to indicate the serial transmitter (TX) 6012 to insert control information in the data flow. The Transmit Delineation Controller 7325 generates the control signals 6030 through 6034 according to predefined operation principles that aim at providing a receiving input port to identify the boundaries of TFs and time cycles. In addition, the Transmit Delineation Controller 7325 generates the Select-WL signal 7328 to indicate the Tunable Laser 7340 on which optical channel the data units belonging to the current time frame or sub-time frame should be transmitted through the link 1230-1 at the ingress of the DWM network 7310. Whenever, according to the common time reference 002, a new time frame or sub-time frame is beginning, the Transmit Delineation Controller 7325 uses the Select-WL signal 7328 to select the optical channel on which the data units belonging to the current time frame or sub-time frame are going to be transmitted. The optical channel on which the data units are being transmitted determines the egress link 1230-E from which such data units are going to exit the lambda routing network 7310 and ultimately the input port 7330 on which they are going to be received. The Receive Delineation Controller 7335 receives the control signals 6040 through 6044 and handles them according to operation principles that aim at identifying the boundaries of TFs and time cycles. In addition, the Receive Delineation Controller 7335 generates the Select-WL signal 7358 to indicate the Tunable Optical Receiver 7350 on which optical channel the data units belonging to the current time frame or sub-time frame should be received through the link 1230-E at the egress of the DWM network 7310. Whenever, according to the control signals 6040 through 6044, a new UTR (Unique Time Reference) time frame or sub-time frame is beginning, the Transmit Delineation Controller 7325 uses the Select-WL signal 7358 to select the optical channel on which the data units belonging to the current time frame or sub-time frame are going to be received. The data stream received over the selected optical channel are passed to the serial receiver through the line 7355. In an alternative embodiment, the input port 7330 comprises a fixed receiver (instead of a tunable one 7350) or the tunable optical receiver 7350 is kept tuned on the same wavelength. In this embodiment the lambda switching network 7310 provides a wavelength merging service, i.e., a selected wavelength of an egress WDM link 1230-E carries data units transmitted on the selected wavelength from different ingress points (among which, possibly, the WDM link 1230-1) in different moments. In a possible embodiment, each of the time frames of the UTR of link 1230-E carry data units transmitted from a different ingress point, i.e., the wavelength received during each of the time frames of the link 1230-E UTR had been generated by a different respective output port 7320. Time driven Tunable Wavelength Conversion-Based Switching Advances in components for optical networking have led to the realization of dynamic optical switching fabrics — e.g., based among others on electro-mechanical micro mirrors, holographic techniques, bubbles — , tunable lasers, tunable receivers, wavelength converters, and tunable wavelength converters. The time required for changing the input/output configuration of dynamic optical switching fabrics is currently larger than the time required for changing the wavelength received by a tunable receiver, the wavelength transmitted by a tunable laser, and the wavelength emitted by a tunable wavelength converter. As a consequence, optical switch architectures based on tunable receivers, tunable lasers, and tunable wavelength converters, rather than optical switching fabrics, are appealing. The present disclosure describes a number of optical switch architectures based on wavelength conversion achieved through the deployment of at least one of tunable receivers, tunable lasers, and tunable wavelength converters. Architectures based on tunable wavelength converters can be appealing especially because they can provide all-optical solutions for scenarios and appHcations in which similar architectures deploying electronic components fail to provide the needed scalability. FIG.33 is the architecture of a possible embodiment of a time driven switch 13000 based on a wavelength conversion (WLC) subsystem 13100. The switching system 13000 presented in FIG. 33 has a plurality of inputs 10010 and outputs 10020, each one comprised of at least one optical link with a plurality of wavelengths. The switching system 13000 in FIG. 33 comprises a switch controller 13030, a plurality of optical alignment subsystems 10900, star couplers 13010, WLC subsystems 13100, and wavelength division multiplexers (WDMs) 10050, and an optical interconnection subsystem 13020 coupling selected ones of the WLC subsystems 13100 to at least one of the wavelength division multiplexers 10050. An optical alignment subsystem 10900 is associated with each respective one of the inputs 10010. The optical alignment subsystem 10900 aligns to the common time reference (CTR) data units transported over the plurality of wavelengths of its respective input 10010. A star coupler 13010 is associated with each input 10010. It forwards the signal received through its input 10010, i.e., all the wavelengths carried by the optical link coupled to its respective input 10010, on all of its output lines 13040 to a plurality of WLC subsystems 13100. In the embodiment presented in FIG. 33, the number of WLC subsystems 13100 connected to each star coupler 13010 is the same as the number of switch outputs 10020. An alternative embodiment comprises a number of WLC subsystems 13100 per star coupler 13010 smaller than the number of outputs 10020. Another alternative embodiment comprises a number of WLC subsystems 13100 per star coupler 13010 larger than the number of outputs 10020. The optical alignment subsystems 10900, star couplers 13010, WLC subsystems 13100, optical interconnection subsystem 13020, and WDM MUXes 10050, are controlled by the switch controller 13030, responsive to the CTR 002, through five bidirectional control lines 13737, 13036, 13041, 13042, and 13034, respectively. Each of the five control lines provides configuration information from the switch controller 13030 to the optical aUgnment subsystems 10900, star couplers 13010, WLC subsystems 13100, optical interconnection subsystem 13020, and WDM MUXes 10050; and via the five bi-directional control lines 13737, 13036, 13041, 13042, and - Ill - 13034, the switch controller 13030 receives various status and control information from the optical alignment subsystems 10900, star couplers 13010, WLC subsystems 13100, optical interconnection subsystem 13020, and WDM MUXes 10050. Each WLC subsystem 13100 converts a specific wavelength, responsive to the CTR 002. FIG. 34A shows a possible embodiment of a WLC subsystem 13100 comprising a wavelength conversion (WLC) scheduling controller 13120 responsive to the CTR 002 and to a wavelength mapping table 13110 downloaded from the switch controller 13030 through the bi-directional control line 13041, and a tunable wavelength conversion subsystem 13150 responsive to a signal 13140 from the WLC scheduling controller 13120. As shown by the switch architecture depicted in FIG. 33, the tunable wavelength conversion subsystem 13150 within the WLC subsystem 13100 in FIG. 34A is connected to a respective one of the output lines 13040 of a respective one of the star couplers 13010 from which the tunable wavelength conversion subsystem 13150 receives an optical signal comprising a plurality of wavelengths. During each time frame of the CTR the tunable wavelength conversion subsystem 13150 is tuned by the Color control signal 13140 to convert a first selected wavelength, i.e., color, into a second selected wavelength sent out on the output line 13510. The second selected wavelength emitted on the output line 13510 of the tunable wavelength conversion subsystem 13150 as a result of the conversion of the first wavelength received on its input line 13040 carries the same information as the first wavelength. In a possible embodiment the color control signal 13140 selects a different first wavelength for conversion by the tunable wavelength conversion subsystem 13150 during each time frame. In a possible embodiment the tunable wavelength conversion subsystem 13150 converts the first selected wavelength into a second fixed wavelength during each time frame. In an alternative embodiment the tunable wavelength conversion subsystem 13150 converts the first selected wavelength into a second wavelength that can be different during each time frame. FIG. 34B is a sample timing diagram describing the operation of the aforementioned alternative embodiment of tunable wavelength conversion subsystem 13150. The timing diagram shows a sequence of CTR time frames TF and for each time frame a first selected wavelength on data line 13040 being converted into a second selected wavelength emitted on data line 13510, wherein each wavelength is identified by the color (green, yellow, red, and blue) of the corresponding light beam. For example, in the leftmost time frame shown in the timing diagram depicted in FIG. 34B the green wavelength is converted into the blue wavelength, i.e., during the leftmost time frame the blue light beam on data line 13510 carries the same information as the green light beam on data line 13040. In an alternative embodiment the color control signal 13140 selects a different wavelength for conversion by the tunable wavelength conversion subsystem 13150 during each sub-time frame. The Color control signal 13140 is generated by the WLC scheduling controller 13120 according to the content of the wavelength mapping table 13110 that indicates the wavelength on which data units should be received during each TF. In a possible embodiment the wavelength mapping has a predefined pattern that is repeated every time cycle and super cycle. When the WLC subsystem 13100 in FIG. 34 is deployed in the architecture shown in FIG. 33, the mapping information contained in the wavelength mapping table 13110 determines the route within the switching system 13000 of the data units carried over the plurality of optical channels during each time frame. In a possible embodiment the content of the wavelength mapping table 13110 is updated whenever a FLP is created or torn down. In a possible embodiment, the above mentioned mapping repeats each time cycle or each super cycle. An alternative embodiment features a centralized WLC scheduling controller that generates the color control signal 13140 for all the tunable wavelength conversion subsystems 13100 in the respective switching system. In a possible implementation of such an embodiment the centralized WLC scheduling controller is within the switch controller 13030 depicted in FIG. 33. The switching system 13000 architecture shown in FIG. 33 provides multicast transmission capability in that it is possible to transmits on more than one output 10020 data units received on the same wavelength of a first input 10010. Multicasting is achieved by tuning the WLC subsystems 13100 coupled to the selected outputs 10020 to convert the same wavelength. In a possible implementation the number of wavelengths of each input 10010 is smaller than the number of inputs 10010. In another possible implementation the number of wavelengths of each input 10010 is smaller than the number of inputs 10010. In a possible embodiment the color control signal 13140 selects a different wavelength for reception of data units by the tunable receiver 13210 during each time frame. FIG. 34B is a sample timing diagram describing the operation of a tunable receiver 13210 in this embodiment. The timing diagram shows a sequence of CTR time frames TF and for each time frame the wavelength on which data units are received by the tunable receiver 13210, wherein each wavelength is identified by the color (green, yellow, red, and blue) of the corresponding light beam. In an alternative embodiment the color control signal 13140 selects a different wavelength for reception of data units by the tunable receiver 13210 during each sub- time frame. In an alternative embodiment the color control signal 13140 selects a different wavelength for being converted by the TWLC 13310 during each sub-time frame. In an alternative embodiment, the TWLC 13330 converts the first selected wavelength signal specified by the color control signal 13140 in a second selected wavelength signal specified by the color control signal 13140. The wavelength to be converted and the wavelength resulting from the conversion can be changed each time frame, as shown in the timing diagram depicted in FIG. 34B. The WLC scheduling controller 13120 depicted in FIG. 34B controls the color signal 13140, and ultimately the TWLC 13310 responsive to the CTR 002 and to a wavelength mapping table 13110. The wavelength mapping table 13110 depicted in FIG. 34A contains the mapping between each time frame and the wavelength to be converted during the time frame. The mapping can be periodic repeating every time cycle and super cycle, as defined by the CTR. An alternative embodiment features a centralized WLC scheduling controller that generates the color control signal 13140 for all the TWLCs 13310 in the respective switching system. In such an embodiment the centralized WLC scheduling controller can be implemented within the switch controller 13030 depicted in FIG. 33. FIG. 35A shows a possible architecture 13400 for an alternative implementation of tunable wavelength conversion subsystem 13150 that can, be used in the WLC subsystem 13100 in FIG. 34 within the switching system 13000 presented in FIG. 33. The tunable wavelength conversion subsystem 13400 in FIG. 35A comprises a tunable receive (TR) 13210 and a tunable laser (TL) 13410, both responsive to the color control signal 13140 from the WLC scheduling controller 13120. The color control signal 13140 indicates the wavelength on which data units are to be received by the TR 13210 and the wavelength to be generated by the TL 13410 for transmitting the data units previously received by the TR 13210. The TR 13210 receives an optical signal on line 13040. Such optical signal possibly comprises a plurality of wavelengths. In an embodiment, the TR 13330 receives data units on the wavelength signal specified by the color control signal 13140 and the TL 13410 transmits them using another wavelength signal specified by the color control signal 13140. The wavelength to be received and the wavelength to be transmitted can be changed each time frame, as shown in the timing diagram depicted in FIG. 35B. The WLC scheduling controller 13120 controls the color signal 13140, and ultimately the TR 13210 and TL 13410 depicted in FIG. 35A responsive to the CTR 002 and to a wavelength mapping table 13110, as shown in FIG. 34. The wavelength mapping table 13110 contains the mapping between each time frame and the wavelength to be received during the time frame. The mapping can be periodic repeating every time cycle and super cycle, as defined by the CTR. An alternative embodiment features a centralized WLC scheduling controller that generates the color control signal 13140 for all the TRs 13310 and TLs 13410 in the respective switching system. In such an embodiment the centralized WLC scheduling controller can be implemented within the switch controller 13030 included in the switch architecture depicted in FIG. 33. In the preferred embodiment of the present invention the color control signal 13140 selects a different wavelength (color) for being received by the TR 13210 and a different wavelength to be generated by the TL 13410 during each time frame. FIG. 35B is a sample timing diagram describing the operation of this embodiment of tunable wavelength conversion subsystem 13400. The timing diagram shows a sequence of CTR time frames TF and for each time frame the wavelength to be received (R:) by the TR 13210 and the wavelength to be generated (T:) by the TL 13410, wherein each wavelength is identified by the color (green, yellow, red, and blue) of the corresponding light beam. In an alternative embodiment the color control signal 13140 selects a different wavelength for reception by the TR 13210 and for transmission by the TL 13410 during each sub-time frame. FIG. 35C depicts the block diagram 13450 of a possible embodiment of tunable wavelength conversion subsystem 13150 comprising an alignment subsystem 10100, a tunable receiver (TR) 13210 and a fixed laser 13220. During each time frame of the CTR the receiver 13210 in FIG. 35C is tuned by the Color control signal 13140 to receive data units carried by a specific wavelength, i.e., color. Received data units are stored in the alignment subsystem 10100 that aligns them to the CTR responsive to the CTR signal 002. Data units are retrieved from the alignment subsystem 10100 to be transmitted by the laser 13220 on a fixed wavelength. FIG. 34B contains a timing diagram showing the operation of the tunable receiver 13210 in FIG. 35C. The timing diagram shows the wavelength on which the receiver 13210 receives data units during each time frame of the CTR, responsive to the Color signal 13140 from the WLC scheduling controller 13120 depicted in FIG. 34. In a possible embodiment the color control signal 13140 selects a different wavelength for reception of data units by the tunable receiver 13210 during each time frame. FIG. 34B is a sample timing diagram describing the operation of a tunable receiver 13210 in this embodiment. The timing diagram shows a sequence of CTR time frames TF and for each time frame the wavelength on which data units are received by the tunable receiver 13210, wherein each wavelength is identified by the color (green, yellow, red, and blue) of the corresponding light beam. In an alternative embodiment the color control signal 13140 selects a different wavelength for reception of data units by the tunable receiver 13210 during each sub- time frame. When the embodiment 13450 of tunable wavelength conversion subsystem 13150 presented in FIG. 35C is deployed in a WLC subsystem 13100 within a switching system, an architecture derived from the one depicted in FIG. 33 is used. The switching system deploying the embodiment 13450 of tunable wavelength conversion subsystem 13150 presented in FIG. 35C does not need the optical alignment subsystems 10900 on the inputs 10010, as shown in FIG. 33, since alignment is performed by the alignment subsystem 10100 within the tunable wavelength conversion subsystem 13450. In a switching system architecture such as the one depicted in FIG. 33, deploying at least one of the embodiments of tunable wavelength conversion subsystem presented in FIG. 35A and FIG. 35C within the WLC subsystems 13100, the lasers 13220 of all the WLC subsystems 13100 connected to the same WDM 10050 transmit on a different wavelength. The WDM 10050 multiplexes on the same output fiber 10020 all the wavelengths received from its respective lasers 13020 through its respective lines 13030. Hence, each wavelength on a switch output 10020 is uniquely associated to a specific one of the switch inputs 10010. Consequently, a tunable receiver 13100 in a first switch 13000 tuned to receive data units on a first wavelength determines that data units that have reached a second upstream switch through a first input 10010 uniquely associated to the first wavelength are going to be forwarded by the first switch through the output 10020 associated to the tunable receiver. According to the architecture depicted in FIG. 33, the wavelength (color) to which a WLC subsystem 13100 is tuned in a first switching system 13000 during a first selected time frame determines the input 10010 of a second upstream switching system 13000 from which the wavelength signal had been forwarded to the first switching system 13000, wherein one of the inputs 10010 of the first switching system 13000 is coupled to one of the outputs 10020 of the second switching system 13000. The wavelength (color) emitted by a WLC subsystem in the first switching system 13000 during the first selected time frame determines the output 10020 of the first switching system 13000 through which the wavelength signal is being forwarded. In other words, routing of the data units traversing one or more switching systems 13000 is determined by the tuning of the WLC subsystem 13100 comprised in each switching system 13000, i.e., by determining the wavelength to be converted and the wavelength generated as a result of the conversion. In the embodiment 13600 of optical interconnection subsystem 13020 for a switching system 13000 such as the one presented in FIG. 33, each of the outputs 13040 of a star coupler 13010 is coupled, through a respective WLC subsystem 13100 and star coupler 13610 to every switch output 10020. Consequently, data units received through any switch input 10010 can be forwarded through any output 10020. Moreover, due to the topology of the connections 13620, the operation of the WDMs 13630, and the filters 13640, during each time frame multiple wavelength signals received through the same input 10010 can be transferred to a selected output 10020. Li other words, it is - lie - possible to transfer to the same output 10020 data units received on two different wavelengths carried by the same input fiber. In a possible embodiment of the switching system 13000 depicted in FIG. 33, all the WLC subsystems 13100 are able to emit the same set of wavelengths, wherein the total number of wavelength each WLC subsystem 13100 is able to generate is equal to the number of outputs 10020. In an alternative embodiment there are at least two different types of WLC subsystems 13100, wherein WLC subsystems 13100 of the first type are able to generate a first set of wavelengths and WLC subsystems 13100 of the second type are able to generate a second set of wavelengths and so on, wherein the total number of wavelength in each set of wavelength is equal to the number of outputs 10020. In an alternative embodiment the number of wavelengths in each set of wavelengths is greater than the number of outputs 10020. In an alternative embodiment, the number of wavelengths in each set of wavelengths is smaller than the number of outputs 10020. In an alternative embodiment the number of wavelengths in at least one set of wavelengths is different than the number of wavelengths in the other sets of wavelengths. Optical Programmable Delay System In the optical domain data units flow at light speed through optical fibers and other transport media. Li the optical domain memory is realized via optical fiber in which optical signals are stored for the time they take to cross the optical fiber. Consequently, the amount of data units stored and the time spent by data units inside the storage medium (i.e., the optical fiber) depend on the length of the fiber. A random access memory in which data units can be stored for any amount of time regardless of the time spent in the memory by other data units, is approximated in the optical domain by using a number of different techniques. Li the following two such techniques known in the art are briefly described: serial optical delay line and parallel optical delay line. A possible embodiment of the programmable delay system 10930 is shown in FIG. 36 and includes a programmable optical switching matrix 2730 with a plurality of input ports, or inlets, and output ports, or outlets (numbered from 1 to N); a programmable delay controller 2720; a pluraUty of optical fibers 2740-2 through 2740- N, each connecting one of the outputs to a respective one of the inputs, and an programmable delay controller 2720. One or more optical fibers can have the same length or have different lengths, wherein fiber I'S length is given by /, = C t_, where C is the speed of light in the fiber and t { is the delay introduced by the coπesponding optical fiber. By properly configuring input output connections 2790 across the programmable optical switching matrix an optical signal entering the programmable delay system from a master inlet 2710 connected to switch inlet 1 is delayed, i.e., buffered, until it exists from switch outlet 1 connected to a master outlet 2715, for a time coπesponding to the sum of the propagation delay through a subset of the said plurality of optical fibers. For example, when the input/output connections 2790-1 through 2790-4 in FIG. 36 are configured, the delay experienced by an optical signal traveling from the input 2710 to the output 2715 of the programmable delay system 10930 is t 2 +t 4 +t N , as expressed by the delay equation in FIG. 36. Li other words, the optical programmable delay system 10930 presented in FIG. 36 provides a delay which is obtained as the sum of the time required by an optical signal to traverse an arbitrary subset of the plurality of optical fibers 2740 connecting the switch outputs to the switch inputs. In the example configuration in FIG. 36, an optical signal entering the programmable delay system 10930 through its input 2710 is switched by input output connection 2790-1 of the programmable optical switching matrix 2730 to output 2 and travels on the corresponding optical fiber 2740-2 to input 2, where it is switched to output port 4 by the input/output connection 2790-2. Then the optical signal enters optical fiber 2740-4 and travels to input 4 where it is switched to output port N by the input output connection 2790-3. After having traveled through optical fiber 2740-N, the optical signal is switched to output port 1, connected to the programmable delay system output 2715, by the input/output connection 2790-4. The programmable delay controller 2720 configures the programmable optical switching matrix 2730 to provide the input/output connections 2790 required to introduce the required delay between input 2710 and output 2715 of the programmable delay system 10930. The programmable delay controller 2720 can receive control and status information from the programmable optical switching matrix 2730. In a possible embodiment, the length of each fiber in the set of fibers 2740 is chosen as a multiple of a base length, wherein the multiple is a power of 2. For example, if C-t 0 is the base length, the length of each fiber in the set of N fibers can be chosen as C-t 0 , 2-C-t 0 , 2 2 -C-t 0 , 2 N"1 -C-t 0 . The choice of this set allows the delay imposed by the programmable delay system 10930 to be varied between 0 and (2 N -l)-t 0 , with a granularity t 0 . The total amount of fiber needed is (2 N -l)-C-t 0 , which is the amount required by a tap-based optical delay line, and much smaller than the amount required by a traditional parallel fiber delay line. Given a maximum delay D, the total number of switch inputs outputs N required to provide a granularity t 0 is ceil[log 2 ceil(D/t 0 +l)], where ceil(x) is a function returning the smallest integer greater than or equal to x. Notice that an optical signal delayed by an optical programmable delay system 10930 according to this embodiment traverses the programmable optical switching matrix at most N times, while the optical signal delayed by a serial optical delay line traverses D/t 0 taps. In other words, assuming that a tap introduces the same attenuation as a programmable optical switching matrix (they are both switches), the attenuation (measured in dB) introduced by an optical programmable delay system 10930 according to this embodiment is roughly the base 2 logarithm of the attenuation (measured in dB) introduced by a serial optical delay line. For example, given a basic delay t 0 =80 ns, provided by 16 meters of fiber, and an 8-by-8 programmable optical switching matrix 2730, an programmable delay system

Claims: can be realized which provides a variable delay between 0 and 10 microseconds with a granularity of 80 ns. The resulting programmable delay system requires a total of 2,032 meters of fiber. The programmable optical switching matrix 2730 in FIG. 36 is traversed at most 8 times by an optical signal, while an equivalent serial optical delay line contains 256 taps. Given that the insertion loss of a tap is the same as the one of a programmable optical switching matrix, the power loss (measured in dB) of an optical signal traversing the presented embodiment of programmable optical delay system is (measured in dB) 8 times lower than the one introduced by a serial optical delay line. From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications that fall within the scope of the claims. WHAT IS CLAIMED IS: 1. A switching system having an input and an output, each of the input and the output comprising at least one channel coupled for communicating data units, the switching system further comprising: a first communications switch and a second communications switch connected by at least one communications link, comprising at least one channel, for transmitting a plurality of data units from said communications link to the output of the switching system; wherein each channel is at least one of an input channel and an output channel; a Common Time Reference (CTR), divided into a plurality of consecutive time frames (TFs); wherein the time frames have at least one of a plurality of predefined time durations; wherein each of the first and the second communications switches is further comprised of a plurality of input ports and a plurality of output ports, each of the input ports connected to and receiving data units from at least one of the channels, and each of the output ports connected and transmitting data units to at least one of the channels; wherein each of the first and the second communications switches has a respective switch controller, coupled to the CTR, the respective input ports, and the respective output ports; wherein each of the first and the second communications switches has a respective switch fabric coupled to the respective switch controller, the respective input ports, and the respective output ports; wherein each of the respective switch controllers is responsive to the CTR and to the respective predefined time durations of respective ones of the time frames, for scheduling a connection via the respective switch fabric from a respective one of the input ports, on a respective one of the input channels during a predefined time interval. 2. The system as in claim 1, wherein each of the respective switch controllers defines the coupling from each one of the respective input ports for data units received during any one of the time frames, on a respective one of the channels, for output during a predefined time frame to at least one selected one of the respective output ports on at least one selected respective one of the channels. 3. The system as in claim 2, wherein the data units that are output through the respective switch fabric during a first predefined time frame on a selected respective one of the channels through the respective output port on the first communications switch are forwarded through the respective output port of the second communications switch during a second predefined time frame on a selected respective one of the channels responsive to the CTR. 4. The system as in claim 1, wherein the data units received at multiple ones of the channels during a first one of the predefined time frames are combined for output during a second one of the predefined time frames responsive to the respective switch controller. 5. The system as in claim 1, wherein the data units received at a single one of the channels during a plurality of the predefined time frames are combined for output to a second one of the channels during a second predefined time frame. 6. The system as in claim 4, wherein the first one and the second one of the predefined time frames have different durations. 7. The system as in claim 1, wherein the time frames are each comprised of at least one sub-time frame. 8. The system as in claim 1, wherein the switch fabric is at least one of the following: a crossbar, a generalized multi-stage cube network, a Clos network, a Benes network, an Omega network, a Delta network, a multi-stage shuffle exchange network, a Banyan network, a combination of optical demultiplexers and optical multiplexers, a passive optical star, a plurality of passive optical stars, a plurality of tunable lasers, a plurality of tunable lasers connected to at least one passive optical star, a plurality of optical tunable receivers connected to at least one passive optical star, an optical switch, an optical cross-bar, an optical Banyan network, a Lithium-Niobate optical switch, an Indium Phosphate optical switch, a 2-D MEMS optical switch, a 3-D MEMS optical switch, a semiconductor optical amplifier (SOA) based optical switch, an holographic optical switch, and Bubble optical switch. 9. The system as in claim 7, wherein a predefined number of at least one consecutive one of the time frames comprise a time cycle (TC), wherein a predefined number of at least one consecutive one of the time cycles comprise a super cycle (SC). 10. The system as in claim 9, wherein the plurality of input ports each receives data units over at least one of a plurality of incoming channels (j), and wherein the plurahty of output ports each sends data units over at least one of a plurality of outgoing channels (i); wherein each of the incoming channels (j) has a unique time reference (UTR-j) that is phase independent of the CTR; wherein the UTR-j is divided into consecutive UTR-j super cycles; wherein the UTR-j super cycles are divided into consecutive UTR-j time cycles; and wherein the UTR-j time cycles are divided into consecutive UTR-j time frames; wherein the UTR-j time frames have a plurahty of predefined time durations. 11. The system as in claim 10, wherein the UTR-j time frames have at least one of the following: same duration as the CTR time frames, and different duration than the CTR time frames. 12. The system as in claim 10, wherein the UTR-j time cycles have at least one of the following: same duration as the CTR time cycles, and different duration than the CTR time cycles. 13. The system as in claim 10, wherein the UTR-j super cycles have at least one of the following: same duration as the CTR super cycles, and different duration than the CTR super cycles. 14. The system as in claim 10, wherein the UTR-j super cycles have a starting time and an ending time that is at least one of: (1) different starting time and ending time than the CTR super cycles; and (2) same starting time and ending time as the respective CTR super cycles. 15. The system as in claim 10, wherein the UTR-j time cycles have a starting time and an ending time that are at least one of: (1) different starting time and ending time than the CTR time cycles; and (2) same starting time and ending time as the respective CTR time cycles. 16. The system as in claim 10, further comprising: a plurality of buffer queues, wherein each of the respective buffer queues is associated, for each of the CTR time frames, with a combination of one of the incoming channels and one of the outgoing channels; a mapping controller within the switch controller system for logically mapping, for each of the (UTR-j) time frames, selected incoming channels (j) to selected buffer queues, and for logically mapping, for each of the CTR time frames, selected ones of the plurality of buffer queues to selected outgoing channels (1); wherein each of the buffer queues is further comprised of an alignment subsystem comprised of a plurahty of time frame queues, wherein each of the time frame queues comprises means to determine that the respective time frame queue is empty, wherein each of the time frame queues further comprises means to determine that the respective time frame queue is not empty; wherein the data units that arrive via the incoming channel (j) are stored in the respective time frame queue of the alignment subsystem responsive to the mapping controller; and wherein the mapping controller further provides for coupling of selected ones of the time frame queues to respective ones of the outgoing channels (1), for transfer of the respective stored data units during the respective associated time CTR time frames. 17. The system as in claim 16, wherein the alignment subsystem, responsive to the mapping controller, transfers the data units associated with a respective first time frame as defined by the UTR-j into an empty first time frame queue from incoming channel (j), during the respective selected first time frame of the UTR-j time frames, wherein the respective time frame queue is designated as full; and wherein the alignment subsystem, responsive to the mapping controller, transfers data units out of a full second time frame queue to respective outgoing channel (1), during at least one of the following: a selected one of the CTR time frames and a selected one of the CTR sub-time frames, wherein the second time frame queue is designated as empty. 18. The system as in claim 1, further comprising: a plurality of channel bit rates; means for mapping for each of the time frames of each of the channels that is connected to the output port a predefined subset of the data units from a respective subset of the time frames for a respective subset of the channels that are connected to the input ports. 19. The system as in claim 18, wherein the data units are at least one of a byte, a word, a packet, an ATM cell. 20. The system as in claim 18, wherein the mapping of the data units for the time frames of the input channels into the respective time frame of the respective output channel is done in a predefined order. 21. The system as in claim 20, wherein the means for mapping maps between at least one of the following: selected ones of four OC-3 channels to selected one of OC- 12 channels, selected ones of sixteen OC-3 channels to selected one of OC-48 channels, selected ones of four OC-12 channels to selected one of OC-48 channels, selected ones of sixteen OC-12 channels to selected one of OC-192 channels, selected ones of four OC-48 channels to selected one of OC-192 channels, and selected ones of sixteen OC- 48 channels to selected one of OC-768 channels. 22. The system as in claim 1, further comprising: a plurahty of channel bit rates; and means for mapping for each respective one of the time frames from the respective input channel to at least one time frame of at least one of the output channels. 23. The system as in claim 22, wherein the time frames for each of the channels are grouped according to a respective common cycle; wherein each of the common cycles is associated with respective ones of the channels, wherein the common cycles have a common duration. 24. The system as in claim 23, wherein the common cycles for each of the channels are time offset relative to the respective common cycles for the other ones of the channels. 25. The system as in claim 23, wherein the mapping reoccurs periodically for each of the common cycles. 26. The system as in claim 22, wherein each of the time frames for the input channel is comprised of at least one sub-time frame; wherein each of the input channel sub-time frames is mapped into at least one of the output channel time frames. 27. The system as in claim 22, wherein each of the time frames for the output channels and the input channels is comprised of at least one sub-time frame; wherein each of the input channel sub-time frames is mapped into one of the output channel sub-time frames. 28. The system as in claim 22, wherein each of the time frames provides a plurality of data units; wherein the mapping of the time frames for the respective ones of the output channels is provided from a respective subset of the input channels; wherein each of the time frames of the output channels receives a subset of data units from the input channel. 29. The system as in claim 1, further comprising: a contiguous plurality of SONET frames each having a predefined time duration; means for separating each of the SONET channels into at least one SONET sub-channel each comprising of a sequence of contiguous SONET frames; wherein each SONET frame consists of at least one data unit; first means for mapping each of the SONET frames to selected ones of the time frames; and means for providing for data transport of the SONET frames responsive to the first means for mapping and the CTR. 30. The system as in Claim 29, wherein each of the SONET frames is comprised of a plurality of parts; wherein the first means for mapping selectively maps each of the parts of one of the SONET frames to each of multiple respective ones of the time frames. 31. The system as in Claim 30, wherein each of the parts is at least one of the following: STS-1, STS-3, STS-12, STS-48, STS-192, and STS-768; 32. The system as in Claim 30, wherein the first means for mapping selectively maps at least one of the following: multiple ones of the SONET frames, and multiple ones of the parts of the SONET frames to a respective one of the time frames. 33. The system as in Claim 32, wherein the means for providing the data transport is a time-driven switching apparatus; the system further comprising: a switching node with a plurality of input ports each having a unique address and a plurahty of output ports each having a unique address; a position logic for determining a relative position for each of said respective incoming parts of the SONET frame within the respective particular time frame; and a forwarding and transmit delineation controller responsive to (1) the unique address of the input port associated with each one of the incoming parts of the SONET frames; (2) the associated time frame of arrival; and (3) the associated relative position for each said respective incoming parts of the SONET frame within said time frame of arrival, to provide a routing to an associated particular one of the output ports at an associated particular position and within an associated second particular time frame. 34. The system as in claim 33, wherein the forwarding and transmit delineation controller provides routing of each of the incoming parts of the SONET frames to a plurality of the output ports, each of the plurahty of the incoming parts of the SONET frames having a respective unique associated particular position within an associated one of the predefined time frames. 35. The system as in claim 33, wherein each of the time frames has a defined duration from start to end, wherein each of the input ports is comprised of a serial receiver that provides position delimiters associated with each of the incoming parts of the SONET frames coupled thereto, wherein the position logic is a position counter, wherein said position counter counts the position delimiters that have occurred since the start of the respective one of the time frames. 36. The system as in claim 35, wherein the time frames are cyclically recurring, wherein a sequence of position delimiters within the cyclically recurring time frames is implicitly defined by a predefined sequence of time units of equal duration. 37. The system as in claim 35, wherein the time frames are cyclically recurring, wherein the sequence of position delimiters within the cyclically recurring time frames is defined by a predefined sequence of time intervals of arbitrarily different duration. 38. The system as in claim 35, wherein the position logic is incremented at predefined time intervals relative to the start of the time frame. 39. The system as in claim 38, wherein each of the positions is of predefined time duration. 40. The system as in Claim 32, wherein the means for providing the data transport is a time-driven priority apparatus; said system further comprising: a pipe comprising at least two switching nodes interconnected via at least one optical channel in a path; a Forwarding and Transmit Delineation Controller for assigning selected predefined time frames for transfer into and out from each of the respective switching nodes responsive to the common time reference; wherein for each switching node within the pipe there is a first predefined time frame within which a respective parts of the SONET frame is transferred into the respective switching node, and a second predefined time frame within which the respective parts of the SONET frame is forwarded out of the respective switching node; and wherein the time frame assignment provides consistent fixed intervals between the time between the input to and output from the pipe. 41. The system as in claim 40, wherein there are a plurahty of the pipes, each of the pipes comprising at least two of the switching nodes interconnected via optical channels in a path. 42. The system as in claim 41, wherein for each of the same predefined time frames, multiple SONET frames can be transfeπed utilizing at least two of the pipes. 43. The system as in Claim 29, further comprising: a second means for mapping the data transport within at least part of one time frame to respective ones of the SONET frames associated with at least one SONET sub-channel; means for multiplexing the respective ones of the SONET frames associated with at least one SONET sub-channel, to form a sequence of SONET frames associated with a second SONET channel. 44. The system as in claim 1, the system further comprising: means for controlling the transport of SONET channels comprising SONET frames; wherein the SONET frames are transported as data units during selected ones of the predefined time frames; the system further comprising: means for mapping the data units from the respective predefined time frame to the respective SONET frames associated with at least one SONET sub-channel; and means for multiplexing the respective SONET frames associated with the at least one SONET sub-channel to form a sequence of SONET frames associated with a respective one of the SONET channels. 45. The system as in Claim 44, wherein the means for mapping maps the data units from multiple ones of the time frames to respective ones of the SONET frames. 46. The system as in Claim 44, wherein each of the time frames consists of at least one sub-time frame; wherein the means for mapping maps data units from each of the sub- time frames to a selected one of the SONET frames. 47. The system as in Claim 46, wherein data units are combined from a plurahty of sub-time frames for transmission as respective SONET frames responsive to the means for multiplexing. 48. The system as in Claim 44, wherein the data transport is provided at a first bit rate, and wherein each of the SONET channels has a second bit rate; wherein the first bit rate is larger that the second bit rate. 49. The system as in Claim 48, wherein each of the SONET channels has an independent second bit rate; wherein the first bit rate is at least the sum of the independent second bit rates. 50. The system as in Claim 49, wherein the second bit rate is at least one of OC-3, OC-12, OC-48, OC-192, and OC-768. 51. The system as in Claim 29, wherein the means for providing for data transport provides for multiple independent data transports, each having a respective first bit rate; wherein each of the SONET channels has a respective second bit rate; wherein the sum of the respective first bit rates is at least equal to the sum of the second bit rates. 52. The system as in Claim 51, wherein the second bit rate is at least one of OC-3, OC-12, OC-48, OC-192, and OC-768. 53. The system as in Claim 1, the system further comprising: means for separating out the input channels, to provide separated wavelength signals, each with a defined connection to at least one of the output ports; means for mapping, comprising a stored wavelength mapping table, said means for mapping providing for a mapping of each of the separated wavelength signals to a tuned wavelength signal for the connection to the at least one of the output ports, responsive to the CTR signal and the stored wavelength mapping table; wherein the tuned wavelength signals are changeable between the time frames. 54. The system as in claim 53, wherein the tuned wavelength signal is provided by at least one of the following: a tunable laser, a tunable filter. 55. The system as in claim 54, wherein a predefined number of at least one time frame comprises a time cycle, wherein a predefined number of at least one time cycle comprises a super cycle. 56. The system as in claim 53, wherein the association of the wavelength signals with respective ones of the time frames reoccurs periodically. 57. The system as in claim 55, wherein the association of the wavelength signals with respective ones of the time frames reoccurs periodically during at least one of each of the time cycles and each of the super cycles. 58. The system as in claim 53, wherein the means for separating is further comprised of a wavelength division multiplexing (WDM) demultiplexer coupled to each communications link. 59. The system as in claim 53, further comprising means for alignment of the separated wavelength signals with the CTR. 60. The system as in claim 53, further comprising means for alignment of all the wavelength signals with a defined connection to each of the input ports with the CTR. 61. The system as in claim 60, wherein alignment is achieved by means of adjustable delay. 62. The system as in claim 53, wherein a plurahty of the tuned wavelength signals is selectively multiplexed to a selected one of the communications links. 63. The system as in claim 54, wherein the switch controller loads the stored wavelength mapping table responsive to the CTR, to coordinate the assignment of the tuned wavelength signals. 64. The system as in claim 53, wherein the switch controller loads the stored wavelength mapping table responsive to the CTR, to coordinate the assignment of the tuned wavelength signals to a respective one of the time frames. 65. The system as in claim 53, wherein the defined connection to the output port is implemented via an optical cross-connect device. 66. The system as in claim 1, wherein each of the chaimels is carried on a defined first wavelength; a plurality of wavelength conversion subsystems each coupled to a respective one of the plurality of communications links; wherein each wavelength conversion subsystem selectively converts from the first wavelength to a second wavelength, responsive to the CTR, to provide a respective output of a second communications link carrying the second wavelength; a plurahty of wavelength division multiplexers (WDM's), each having a plurality of optical channel inputs; an optical interconnection subsystem for coupling the second optical links to selected ones of the optical channel inputs of an associated one of the WDM's; and wherein each of the WDM's multiplexes its respective plurahty of optical channel inputs to at least one respective communications link. 67. The system as in claim 66, wherein the wavelength conversion subsystem is further comprised of: a Wavelength Conversion (WJLC) scheduling controller comprising a wavelength mapping table defining the associative mapping between the first wavelength and the second wavelength for each of the time frames; and a tunable wavelength conversion subsystem for converting, for each of the time frames, the first wavelength into the second wavelength, responsive to the WLC scheduling controller. 68. The system as in claim 67, wherein the tunable wavelength conversion subsystem is further comprised of: a tunable receiver, responsive to the WLC scheduling controller, for providing an electrical signal representative of the first wavelength; and a laser, responsive to the electrical signal output, for transmitting as an output an optical signal of the second wavelength representative of the first wavelength. 69. The system as in claim 68, wherein the laser is a tunable laser, responsive to the WLC scheduling controller, for providing as an output an optical signal of the second wavelength. 70. The system as in claim 67, further comprising: an alignment subsystem coupled between the tunable receiver and the laser, responsive to the CTR, for aligning a beginning of each of the time frames with the CTR. 71. The system as in claim 59, wherein the means for aUgnment is further comprised of: a programmable optical switching matrix comprising a plurality of independent inlets and outlets; a pluraUty of predefined-length fibers each coupling from one outlet to one inlet of the pluraUty of independent inlets and outlets; and a programmable delay controUer for mapping selected ones of the pluraUty of the inlets to selected ones of the pluraUty of outlets such that a subset of the plurahty of fibers are concatenated to act as an effective single fiber of equivalent combined length of the subset. 72. The system as in Claim 71, wherein the programmable optical switching matrix provides for receiving an optical signal at, selected ones of the inlets and for outputting an optical signal at selected ones of the outlets responsive to the mapping. 73. The system as in Claim 71, wherein the mapping defines a switching matrix configuration determining for each of the outlets, a coupling to at most a selected one of the inlets. 74. The system as in Claim 73, wherein there are a pluraUty of the switching matrix configurations, wherein each of the switching matrix configurations is associated with a predefined concatenation of an equivalent combined length. 75. The system as in Claim 73, further comprising: a master inlet coupled to an external incoming optical signal and to a selected one of the inlets of the programmable optical switching matrix; a master outlet for coupling an outgoing optical signal from a selected one of the outlets of the programmable optical switching matrix to an external coupling. 76. The system as in Claim 75, wherein the master inlet is coupled via the respective subset and therefrom provided as the selected one of the outlets of the programmable optical switching matrix coupled to the master outlet. 77. The system as in claim 73, further comprising: a length signal defining a desired length; wherein each of the outlets of the programmable optical switching matrix is associated with a specified predefined fiber length; wherein there are a pluraUty of subsets; wherein each of the pluraUty of the subsets has an equivalent combined length; wherein the programmable delay controUer responsive to the length signal, selects a particular one of the subsets of the fibers which has an equivalent combined length approximately equal to that of the desired length. 78. The system as in Claim 77, wherein the programmable delay controller selects an order of concatenation for the particular one of the subsets to determine and provide the mapping. 79. The system as in Claim 71, further comprising a second programmable optical switching matrix controlling the coupling of each of the fiber lengths. 80. The system as in Claim 71, wherein there is a mapping of fibers between the outlets and the inlets of the programmable optical switching matrix.

Inventors: BALDI MARIO, OFEK YORAM, BALDI, MARIO, OFEK, YORAM
Applicants: SYNCHRODYNE NETWORKS INC, SYNCHRODYNE NETWORKS, INC.
Application year: 2001
Main classification: H04Q11
IPC codes: H04L12, H04Q11