Abstract:
An optical signaling header technique applicable to optical networks wherein packet routing information is embedded in the same channel or wavelength as the data payload so that both the single-sideband modulated header and data payload propagate through network elements with the same path and the associated delays. The header routing information has sufficiently different characteristics from the data payload so that the signaling header can be detected without being affected by the data payload, and that the signaling header can also be removed without affecting the data payload. The signal routing technique can be overlaid onto the conventional network elements in a modular manner using two types of applique modules. The first type effects header encoding and decoding at the entry and exit points of the data payload into and out of the network; the second type effects header detection at each of~the network elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 09/118,437 filed Jul. 17, 1998, now U.S. Pat. No. 6,111,673. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     This invention relates to optical communication systems and, more particularly, to an optical system which accommodates network traffic with high throughput and low latency and effects high-speed header detection and generation. 
     2. Description of the Background Art 
     Recent research advances in optical Wavelength Division Multiplexing (WDM) technology have fostered the development of networks that are orders of magnitude higher in transmission bandwidth than existing commercial networks. While such an increase in throughput is impressive on its own, a corresponding decrease in network latency must also be achieved in order to realize the Next Generation Internet (NGI) vision of providing the next generation of ultra high speed networks that can meet the requirements for supporting new applications, including national initiatives. Towards this end, current research efforts have focused on developing an ultra-low latency Internet Protocol (IP) over WDM optical packet switching technology that promises to deliver the two-fold goal of both high throughput with low latency. Such efforts, while promising, have yet to fully realize this two-fold goal. 
     There are a number of challenging requirements in realizing such IP/WDM networks. First, the NGI network must inter-operate with the existing Internet and avoid protocol conflicts. Second, the NGI network must provide not only ultra low-latency, but must take advantage of both packet-switched (that is, bursty) IP traffic and circuit-switched WDM networks. Third, it is advantageous if the NGI network does not depend upon precise synchronization between signaling and data payload. Finally, a desired objective is that the NGI network accommodates data traffic of various protocols and formats so that it is possible to transmit and receive IP as well as non-IP signals without the need for complicated synchronization or format conversion. 
     Comparison With Other Work 
     The Multi-Wavelength-Optical Network (MONET) system, as reported in the article “MONET: Multi-Wavelength Optical Networking” by R. E. Wagner, et al. and published in the Journal of Lightwave Technology, Vol. 14, No. 6, June 1996, demonstrated a number of key milestones in optical networks including transparent transmission of multi-wavelength through more than 12 reconfigurable network elements spread over the national scale fiber distance. The network, however, is circuit-switched and suffers inefficiency in accommodating bursty traffic. The typical connection setup time from request to switching is a few seconds, limited by capabilities of both Network Control &amp; Management (NC&amp;M) and hardware. Recent efforts within the MONET program to improve on the efficiency concentrated on the “Just-in-Time signaling” scheme. This method utilizes embedded 1510 nm NC&amp;M signaling which precedes the data payload by an estimated delay time. This estimation must be accurately made for each network configuration for every wavelength in order to synchronize the signaling header and switching of the payload. 
     In accordance with the present invention, the optical packet header is carried over the same wavelength as the packet payload data. This approach mitigates the issue of header and payload synchronization. Furthermore, with a suitable use of optical delay at each intermediate optical switch, it eliminates the need to estimate the initial burst delay by incorporating the optical delay directly at the local switches. This makes a striking difference with Just-In-Time signaling in which the delay at each switch along the path needs to be known ahead of time and must be entered in the calculation for the total delay. Lastly, there is little time wasted in requesting a connection time and actually achieving a connection. In comparison to a few second delays seen in MONET, the present inventive subject matter reduces the delay to minimal, only limited by the actual hardware switching delays at each switch. The current switching technology realizes delays of only several microseconds, and shorter delays will be possible in the future. Such a short delay can be incorporated by using an optical fiber delay line at each network element utilizing switches. The present inventive subject matter achieves the lowest possible latency down to the fundamental limit of the hardware, and no lower latency can be achieved by any other technique. 
     The Optical Networks Technology Consortium (ONTC) results were reported in the article “Multiwavelength Reconfigurable WDM/ATM/SONET Network Testbed” by Chang et al. and published in the Journal of Lightwave Technology, Vol. 14, No. 6, June 1996. Both Phase I (155 Mb/s, 4-wavelength) and Phase II (2.5 Gb/s, 8-wavelength) of the ONTC program were configured on a Multihop ATM-based network. While such an ATM based architecture added a large overhead and excluded the possibility of a single-hop network, the packet/header signaling was made possible by utilizing the isochronous ATM cell itself. This communication of NC&amp;M information is made through the same optical wavelength, potentially offering similar benefits as with the technique of the present invention However, the inventive technique offers a number of significant advantages over the ATM-based signaling. First, the inventive technique offers a single hop connection for the payload without the need to convert to electrical signals and buffer the packets. Second, it offers far more efficient utilization of the bandwidth by eliminating excessive overheads. Third, it allows strictly transparent and ultra-low latency connections. 
     The DARPA sponsored All-Optical-Network (AON) Consortium results were reported in an article entitled “A Wideband All-Optical WDM Network”, by I. P. Kaminow et al. and published in the IEEE Journal on Selected Areas of Communication, Vol. 14, No. 5, June, 1996. There were actually two parts of the AON program: WDM as reported in the aforementioned article, and TDM reported in a companion paper in the same issue. First the WDM part of the AON program is first discussed, followed by the TDM part. 
     The AON architecture is a three-level hierarchy of subnetworks, and resembles that of LANs, MANs, and WANs seen in computer networks. The AON provides three basic services between Optical Terminals (OTs): A, B, and C services. A is a transparent circuit-switched service, B is a transparent time-scheduled TDM/WDM service, and C is a non-transparent datagram service used for signaling. The B service uses a structure where a 250 microsecond frame is used with 128 slots per frame. Within a slot or group of slots, a user is free to choose the modulation rate and format. The B-service implemented on the AON architecture is closest to the IP over WDM, which is the subject matter of the present invention. However, the separation of NC&amp;M signaling in the C-service with the payload in the B-service requires careful synchronization between the signaling header and the payload. This requirement becomes far more stringent as the 250 microsecond frame is used with 128 slots per frame with arbitrary bit rates. Not only the synchronization has to occur at the bit level, but this synchronization has to be achieved across the entire network. The scalability and interoperability are extremely difficult since these do not go in steps with the network synchronization requirement. The present inventive subject matter requires only that the payload and the header are transmitted and received simultaneously, inter-operates with existing IP and non-IP traffic, and offers scalability. 
     TDM efforts are aimed at 100 Gb/s bit rates. In principle, such ultrafast TDM networks have the potential to provide truly flexible bandwidth on demand at burst rates of 100 Gb/s. However, there are significant technological challenges behind such high bit rate systems mainly related to nonlinearities, dispersion, and polarization degradations in the fiber. While the soliton technologies can alleviate some of the difficulties, it still requires extremely accurate synchronization of the network—down to a few picoseconds. In addition, the header and the payload must have the identical bit rates, and as a consequence, bit-rate transparent services are difficult to provide. The subject matter in accordance with the present invention does not depend on precise synchronization, relies on no 100 Gb/s technologies, and offers transparent services. 
     The Cisco Corporation recently announced a product based on Tag-Switching and the general description of Cisco&#39;s Tag-Switching is available at the world-wide-web site, (http://www.cisco.com/warp/public/732/tag/). Cisco&#39;s (electronic) Tag Switching assigns a label or “tag” to packets traversing a network of routers and switches. In a conventional router network, each packet must be processed by each router to determine the next hop of the packet toward its final destination. In an (electronic) Tag Switching network, tags are assigned to destination networks or hosts. Packets then are switched through the network with each node simply-swapping tags rather than processing each packet. An (electronic) Tag Switching network will consist of a core of (electronic) tag switches (either conventional routers or switches), which connect to (electronic) tag edge routers on the network&#39;s periphery. (Electronic) Tag edge routers and tag switches use standard routing protocols to identify routes through the network. These systems then use the tables generated by the routing protocols to assign and distribute tag information via a Tag Distribution Protocol. Tag switches and tag edge routers receive the Tag Distribution Protocol information and build a forwarding database. The database maps particular destinations to the tags associated with those destinations and the ports through which they are reachable. 
     When a tag edge router receives a packet for forwarding across the tag network, it analyzes the network-layer header and performs applicable network layer services. It then selects a route for the packet from its routing tables, applies a tag and forwards the packet to the next-hop tag switch. 
     The tag switch receives the tagged packet and switches the packet based solely on the tag, without re-analyzing the network-layer header. The packet reaches the tag edge router at the egress point of the network, where the tag is stripped off and the packet delivered. After Cisco made its announcement about (Electronic) Tag Switching, the IETF (Internet Engineering Task Force) has recommended a MPLS (Multi-protocol Label Switching) to implement standardized, vendor-neutral (electronic) tag-switching function in routers and switches, including ATM switches. 
     A number of features in the Cisco&#39;s (electronic) Tag Switching is similar to the Optical Tag Switching which is the subject matter of the present invention, with the features aimed at the similar goals of simplifying the processing required for packet routing. The key differences are as follows. First, the optical tag switching is purely optical in the sense that both tag and data payload are in an optical form. While each plug-and-play module (a component of the present inventive system) senses the optical tag, the actual packet does not undergo optical-to-electrical conversion until it comes out of the network. The Cisco&#39;s (electronic) Tag Switching will be all electrical, and applies electronic detection, processing, and retransmission to each packet at each router. Secondly, the Optical Tag Switching of the present invention achieves lowest possible latency and does not rely on utilizing buffers. Electronic tag switching will have far greater latency due to electronic processing and electronic buffering. Thirdly, the Optical Tag Switching of the present invention utilizes path deflection and/or wavelength conversion to resolve blocking due to contention of the packets, whereas the Electronic Tag Switching will only utilize electronic buffering as a means to achieve contention resolution at the cost of increased latency, and the performance is strongly dependent on packet size. The present invention covers packets of any length. Lastly, the Optical Tag Switching of the present invention achieves a strictly transparent network in which data of any format and protocol can be routed so long as it has a proper optical tag. Hence the data can be digital of any bit rate and modulation formats. The Electronic Tag Switching requires that data payload to have the given digital bit rate identical to the electronic tag since the routers must buffer them electronically. 
     Another representative technology that serves as background to the present invention is the so-called Session Deflection Virtual Circuit Protocol (SDVC), which is based on a deflection routing method. The paper entitled “The Manhattan Street Network”, by N. F. Maxemchuk” as published in the Proceedings on IEEE Globecom &#39;85, pp 255-261, December 1985, discusses that when two packets attempt to go to the same destination, one packet can be randomly chosen for the preferred output link and the other packet is “deflected” to the non-preferred link. This means that packets will occasionally take paths that are not shortest paths. The deflection method utilized by the present invention does not ‘randomly’ select the packet to go to the most preferred path, rather, it attempts to look into the priorities of the packets, and send the higher priority packet to be routed to the preferred path. The packets will be deflected if they have lower priorities; however, both ‘path deflection’ and ‘wavelength deflection’ are utilized. The path deflection is similar to conventional SDVC in that the optical packet will be simply routed to the path of the next preference at the same wavelength. The wavelength deflection allows the optical packet to be routed to the most preferred path but at a different wavelength. This wavelength deflection is achieved by wavelength conversion at the network elements. Partially limited wavelength conversion is utilized, meaning not all wavelengths will be available as destination wavelengths for a given originating wavelength. The wavelength deflection allows resolution of blocking due to wavelength contentions without increasing the path delay. The combination of path and wavelength deflections offers sufficiently large additional connectivities for resolving packet contentions; however, the degree of partial wavelength conversion can be increased when the blocking rate starts to rise. Such scalability and flexibility of the network are not addressed by conventional SDVC. 
     Besides the foregoing overall system considerations elucidated above, there is also the issue of how to effectively detect and/or re-insert a header which is combined with a data payload for propagation over the network using the same optical wavelength. The primary focus in the literature has been on a technique for combining sub-carrier headers with a baseband data payload. The very first two articles addressing this issued were published in 1992 by A. Bidman et. al., who combined a 2.56 Gb/s data payload with a 40 Mb/s header on 3 GHz carrier [A. Budman, E. Eichen, J. Schalafer, R. Olshansky, and F. McAleavey, “Multigigabit optical packet switch for self-routing networks with subcarrier addressing,” Techical Digest, paper TuO4, pp. 90-91, OFC&#39;92], and W. I. Way et al., who combined a 2.488 Gb/s data payload with a tunable microwave pilot tone (tuned between 2.520 and 2.690 GHz) to route SONET packet in a WDM ring network via acousto-optical tunable filter [W. I. Way, D. A. Smith, J. J. Johnson, H. Izadpanah, and H. Johnson, “Self-routing WDM high-capacity SONET ring network,” Technical Digest, paper TuO2, pp. 86-87, OFC&#39;92, and W. I. Way, D. A. Smith, J. J. Johnson, and H. Izadpanah, “A self-routing WDM high-capacity SONET ring network,” IEEE Photonics Technology Letters, vol. 4, pp. 402-404, April 1992.2,3]. Both of these articles used a single laser diode to carry the data payload and sub-carrier header. This technique has also been extensively studied in a local-area DWDM optical packet-switched network [R. T. Hofmeister, L. G. Katzovsky, C. L. Lu, P. Poggiolini, and F. Yang, “CORD: optical packet-switched network testbed,” Fiber and Integrated Optics, vol. 16, pp. 199-219, 1997], and several other all-optical networks [E. Park and A, E. Wiltner, “Network demonstration of self-routing wavelength packets using an all-optical wavelength shifter and QPSK subcarrier routing control,” IEEE Photonics Technology Letters, vol. 8, pp. 938-940, 1996; and M. Shell, M. Vaughn, A. Wang, D. J. Blumenthal, P. J. Rigole, and S. Nilsson, “Experimental demonstration of an all-optical routing node for multihop wavelength routed networks,” IEEE Photonic Technology Letters, vol. 8, pp. 1391-1393, 1996]. 
     Instead of combing a sub-carrier header with the data payload in the electrical domain, they have also been combined in the optical domain by using two laser diodes at different wavelengths [B. H. Wang, K. Y. Yen, and W. I. Way, “Demonstration of gigabit WDMA networks using parallel-processed sub-carrier hopping pilot-tone (p 3 ) signaling technique,” IEEE Photonics Technology Letters, vol. 8, pp. 933-934, July 1996]. 
     However, using two wavelengths to transport data payload and header separately may not be practical in the following sense: in an all-optical DWDM network, it is preferred that the header, which may contain network operations information, travels along the same routes as data payload so that it can truthfully report the updated status of the data payload. If the header and the data payload were carried by different wavelengths, they could be routed in the network with entirely different paths, and the header may not report what the data payload has really experienced. Therefore, although it is preferred that the sub-carrier header and the data payload be carried by the same wavelength, the art is devoid of such teachings and suggestions. 
     The sub-carrier pilot-tone concept presented in Wang et al. was extended to multiple pilot tones by Shieh et al. [W. Shieh and A. E. Wiliner, “A wavelength routing node using multifunctional semiconductor optical amplifiers and multiple-pilot-tone-coded subcarrier control headers,” IEEE Photonics Technology Letters, vol. 9, pp. 1268-1270, September 1997.], mainly for the purpose of increasing the number of network addresses. 
     Recently, consideration has been given to ‘header replacement’ for the high-throughput operation in a packet-switched network in which data paths change due to link outages, output-port contention, and variable traffic patterns. Moreover, header replacement could be useful for maintaining protocol compatibility at gateways between different networks. However, the only method which has been reported is for time-division-multiplexed header and data payload requires an extremely high accuracy of timing synchronization among network nodes [X. Jiang, X. P. Chen, and A. E. Willner, “All optical wavelength independent packet header replacement using a long CW region generated directly from the packet flag,” IEEE Photonics Technology Letters, vol. 9, pp. 1638-1640, November 1998]. 
     Most recently, Blumenthal et al., in an article entitled “WDM Optical Tag Switching with Packet-Rate Wavelength Conversion and Subcarrier Multiplexed Addressing”, OFC 1999, Conference Digest, pages 162-164, report experimental results of all-optical IP tag switching for WDM switched networks. However, the experimental system is a non-burst system and, moreover, no propagation of the resultant signal over actual fiber is discussed. It is anticipated that the propagation distance will be substantially limited whenever the system is deployed with optical fiber because of phase dispersion effects in the optical fiber. 
     From this overview of the art pertaining to details of header generation and detection, it is readily understood that the art is devoid of teachings and suggestions wherein sub-carrier multiplexed packet data payload and multiple sub-carrier headers (including old and new ones) are deployed so that a &gt;2.5 Gbps IP packet can be routed through a national all-optical DWDM network by the (successive) guidance of these sub-carrier headers, with the total number of sub-carrier headers that can be written is in the range of forty or more. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes a unique optical signaling header technique applicable to optical networks. Packet routing information is embedded in the same channel or wavelength as the data payload so that both the header and data information propagate through the network with the same path and the associated delays. However, the header routing information has sufficiently different characteristics from the data payload so that the signaling header can be detected without being affected by the data payload and that the signaling header can also be stripped off without affecting the data payload. The inventive subject matter allows such a unique signal routing method to be overlaid onto the conventional network elements in a modular manner, including the insertion, detection and processing of the optical header. 
     In accordance with one broad method aspect of the present invention commensurate with the overall NGI system, a method for propagating a data payload from an input network element to an output network element in a wavelength division multiplexing system composed of a plurality of network elements, given that the data payload has a given format and protocol, includes the following steps: (a) adding a single-sideband header to the data payload prior to inputting the data payload to the input network element to produce an optical signal, the header having a format and protocol and being indicative of the local route through each of the network elements for the data payload and the header, the format and protocol of the data payload being independent of the format and protocol of the header; and (b) detecting the header at each of the network elements as the data payload and header propagate through the WDM network, wherein the header is conveyed by a distinct carrier frequency such that the single-sideband spectrum of the header occupies a frequency band above the data payload, such that the step of detecting includes (i) opto-electrically converting the optical signal to detect the header, (ii) processing the header to produce a switch control signal to route the incoming optical signal, (iii) optically filtering the optical signal with a reflective part of a notch filter to delete the header and recover the data payload, and (iv) inserting a new single-sideband header at the given frequency band into the optical signal in place of the deleted header. 
     In accordance with a broad system aspect of the present invention commensurate with the overall NGI system, a system for propagating a data payload from an input network element to an output network element in a wavelength division multiplexing system composed of a plurality of network elements, given that the data payload has a given format and protocol, includes: (a) an adder for adding a single-sideband header to the data payload prior to inputting the data payload to the input network element to produce an optical signal, the header having a format and protocol and being indicative of the local route through each of the network elements for the data payload and the header, the format and protocol of the data payload being independent of the format and protocol of the header; and (b) a detector for detecting the header at each of the network elements as the data payload and header propagate through the WDM network, wherein the header is conveyed by a distinct carrier frequency such that the single-sideband spectrum of the header occupies a frequency band above the data payload, such that system further includes (i) an opto-electrical converter for detecting the optical signal to produce the header, (ii) a processor for processing the detected header to produce a switch control signal to route the incoming optical signal, (iii) an optical filter for filtering the optical signal with a reflective part of the notch filter to delete the header and recover the data payload, and (iv) means for inserting a new single-sideband header at the given frequency band into the optical signal in place of the deleted header. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a pictorial representation of a general network illustrating the coupling between the optical and electrical layers of the network; 
     FIG. 2 illustrates the optical layer of the network of FIG. 1 showing the relationship between the optical signal header and data payload, and the use of the header/payload in network setup; 
     FIG. 3 is a high-level block diagram an optical transmitter in accordance with the present invention for header encoding; 
     FIG. 4 is a high-level block diagram an optical receiver in accordance with the present invention for header decoding; 
     FIG. 5 is illustrative of a WDM circuit-switched backbone network; 
     FIG. 6 illustrates a network element of FIG. 1 with its embedded switch and the use of local routing tables; 
     FIG. 7 depicts a block diagram of an illustrative embodiment of a header encoder circuit for the Plug-&amp;-Play module of FIG. 3; 
     FIG. 8 depicts a block diagram of an illustrative embodiment of a header remover circuit for the Plug-&amp;-Play module of FIG. 3; 
     FIG. 9 depicts a block diagram of an illustrative embodiment of a header detector circuit for the Plug-&amp;-Play module of FIG. 4; 
     FIG. 10 depicts a block diagram for a more detailed embodiment of FIG. 4 wherein the label-switch controller includes interposed demultiplexers, and header detectors and fast memory; 
     FIG. 11 is a flow diagram for the processing effected by each label-switch controller of FIG. 10; 
     FIG. 12 is a block diagram of circuitry for removing the header with the reflective part of a notch filter and for inserting a new single-sideband modulated header; and 
     FIG. 13 is a block diagram of circuitry for the detecting the header with the transmission part of the notch filter, for removing the header with the reflective part of a notch filter, and for inserting a new single-sideband modulated header. 
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
     In order to gain an insight into the fundamental principles in accordance with the present invention as well as to introduce terminology useful in the sequel, an overview is first presented, followed by an elucidation of an illustrative embodiment. 
     Overview 
     The present invention relates to a network for realizing low latency, high throughput, and cost-effective bandwidth-on-demand for large blocks of data for NGI applications. Cost-effective and interoperable upgrades to the network are realized by interposing portable ‘Plug-and-Play’ modules on the existing WDM network elements to effect so-called “WDM optical label switching” or, synonymously, “optical label switching”. The invention impacts primarily the hardware for the NGI network from the network element design perspective. 
     As alluded to, the methodology carried out by the network and concomitant circuitry for implementing the network are engendered by a technique called WDM optical label-switching—defined as the dynamic generation of a routing path for a burst duration by an in-band optical signaling header. Data packets are routed through the WDM network using an in-band WDM signaling header for each packet. At a switching node, the signaling header is processed and the header and the data payload (1) may be immediately forwarded through an already existing flow state connection, or (2) a path can be setup for a burst duration to handle the header and the data payload. WDM label-switching enables highly efficient routing and throughput, and reduces the number of IP-level hops required by keeping the packets routing at the optical level to one hop as managed by the Network Control and Management (NC&amp;M) which creates and maintains routing information. 
     The depiction of FIG. 1 shows the inter-relation between optical layer  120  and electrical layer  110  of generic network  100  as provided by intermediate layer  130  coupling the optical layer and the electrical layer. Electrical layer  110  is shown, for simplicity, as being composed of two conventional IP routers  111  and  112 . Optical layer  120  is shown as being composed of network elements or nodes  121 - 125 . Intermediate layer  130  depicts conventional ATM/SONET system  131  coupling IP router  112  to network element  122 . Also shown as part of layer  130  is header network  132 , which in accordance with the present invention, couples IP router  111  to network element  121 . FIG. 1 pictorially illustrates the location of network  132  on a national-scale, transparent WDM-based backbone network with full interoperability and reconfigurability. It is important to emphasize at this point that the elements of FIG. 1 are illustrative of one embodiment in accordance with the present invention; thus, for example, element  111  may, in another embodiment, be an ATM router or even a switch. 
     Now with reference to FIG. 2, optical layer  120  of FIG. 1 is shown in more detail including the basic technique, in accordance with the present invention, for setting up a fast connection in optical network  201 , composed of network elements  121 - 125 ; the setup uses optical signaling header  210  for the accompanying data payload  211 . This technique combines the advantages of circuit-switched based WDM and packet-switched based IP technologies. New signaling information is added in the form of an optical signal header  210 , which is carried in-band within each wavelength in the multi-wavelength transport environment. Optical signaling header  210  is a label containing routing and control information such as the source, destination, priority, and the length of the packet, propagates and through optical network  201  preceding data payload  211 . Each WDM network element  121 - 125  senses optical signaling header  210 , looks-up a connection table (discussed later), and takes necessary steps such as cross-connections, add, drop, or drop-and-continue. The connection table is constantly updated by continuous communication between NC&amp;M  220  and WDM network elements  121 - 125  through logical connections, such as channel  221 . Data payload  211 , which follows optical signaling header  210 , is routed through a path in each network element (discussed later) as established by the connection. With the arrangement of FIG. 2, there is no need to manage the time delay between optical signaling header  210  and data payload  211 , shown by T in FIG. 2, because each network element provides the optical delay needed for the short time required for connection set-up within each network element via delay on an interposed fiber. Moreover, the format and protocol of the data payload is independent of that of the header, that is, for a given network whereas the format and protocol of the header are predetermined, the format and the protocol of the data payload can be the same as or different from those of the header. 
     Each destination is associated with a preferred path which would minimize ‘the cost’—in FIG. 2, the overall path from source  123  to destination  122  includes paths  201  and  202  in cascade, both utilizing wavelength WP. This cost is computed based on the total propagation distance, the number of hops, and the traffic load. The preferred wavelength is defaulted to the original wavelength. For example, the preferred wavelength on path  202  is WP. If this preferred path at the default wavelength is already occupied by another packet, then network element  121  quickly decides if there is an available alternate wavelength WA through the same preferred-path. This alternate wavelength must be one of the choices offered by the limited wavelength conversion in network element  121 . If there is no choice of wavelengths which allows transport of the packet through the most preferred path, the next preferred path is selected (path deflection). For example, in FIG. 2, paths  203  and  204  in cascade may represent the alternative path. At this point, the preferred wavelength will default back to the original wavelength WP. The identical process of looking for an alternate wavelength can proceed if this default wavelength is again already occupied. In FIG. 2, path  203  is an alternative path with the same wavelength WP, and path  204  is an alternate path using alternate wavelength WA. In an unlikely case where there is no combination of path and wavelength deflection that can offer transport of the packet, network element  121  will decide to drop the packet of lower priority. In other words, the new packet transport through the preferred path at the originating wavelength takes place by dropping the other packet of the lower priority which is already occupying the preferred path. 
     Network elements  121 - 125  are augmented with two types of so-called ‘Plug-and-Play’ modules to efficiently handle bursty traffic by providing packet switching capabilities to conventional circuit-switched WDM network elements  121 - 125  whereby signaling headers are encoded onto IP packets and are removed when necessary. 
     The first type of ‘Plug-and-Play’ module, represented by electro-optical element  132  of FIG. 1, is-now shown in block diagram form in FIG.  3 . Whereas conceptually module  132  is a stand-alone element, in practice, module  132  is integrated with network element  121  as is shown in FIG. 3; module  132  is interposed between compliant client interface (CCI)  310  of network element  121  and IP router  111  to encode optical signaling header  210  onto the packets added into the network via header encoder  321 , and to remove optical signaling header  210  from the packets dropping out of the network via header remover  322 . 
     Generally, encoding/removing module  132  is placed where the IP traffic is interfaced into and out of the WDM network, which is between the client interface of the network element and the IP routers. The client interfaces can be either a CCI-type or a non-compliant client interfaces (NCI)-type. At these interfaces, header encoder  321  puts optical header  210  carrying the destination and other information in front of data payload  211  as the IP signal is transported into network  201  is based on the IP signal&#39;s original IP address, which is obtained from IP router  111  through interface  311 , and. Optical header  210  is encoded in the optical domain by an optical modulator (discussed later). Signaling header remover  322  deletes header  210  from the optical signal dropped via a client interface, and provides an electrical IP packet to IP router  111 . 
     More specifically, module  132  accepts the electrical signal from IP router  111 , converts the electrical signal to a desired compliant wavelength optical signal, and places optical header  210  in front of the entire packet. Module  132  communicates with NC&amp;M  220  and buffers the data before optically converting the data if requested by NC&amp;M  220 . Module  132  employs an optical transmitter (discussed later) with the wavelength matched to the client interface wavelength. (As indicated later but instructive to mention here, module  132  is also compatible with NCI  404  of FIG. 4 since the wavelength adaptation occurs in the NCI; however, the bit-rate-compatibility of NCI wavelength adaption and the IP signal with optical headers must be established in advance.) 
     FIG. 4 depicts a second type of ‘Plug-and-Play’ module, optical element  410 , which is associated with each WDM network element  121 - 125 , say element  121  for discussion purposes. Module  410  is interposed between conventional network element switch-controller  420  and conventional switching device  430 . Module  410  detects information from each signaling header  210  propagating over any fiber  401 - 403 , as provided to module  410  by tapped fiber paths  404 - 406 . Module  410  functions to achieve very rapid table look-up and fast signaling to switching device  430 . Switch controller  420  is functionally equivalent to the conventional “craft interface” used for controlling the network elements; however, in this case, the purpose of this switch controller  420  is to accept the circuit-switched signaling from NC&amp;M  220  and determine which control commands are to be sent to label switch controller  410  based on the priority. Thus, label switch controller  410  receives circuit-switched control signals from network element circuit switch controller  420 , as well as information as derived from each signaling header  210 , and intelligently chooses between the circuit-switched and the label-switched control schemes. The switches (discussed later) comprising switching device  430  also achieve rapid switching. The delay imposed by fibers  415 ,  416 , or  417 , which are placed in input paths  401 - 403  to switching device  430 , are such that the delay is larger than the total time it takes to read signaling header  210 , to complete a table look-up, and to effect switching. Approximately, a 2 km fiber provides 10 microsecond processing time. The types of WDM network elements represented by elements  121 - 125  and which encompass switching device  430  include: Wavelength Add-Drop Multiplexers (WADMs); Wavelength Selective Crossconnects (WSXCs); and Wavelength Interchanging Crossconnects (WIXCs) with limited wavelength conversion capabilities. 
     In operation, module  410  taps a small fraction of the optical signals appearing on paths  401 - 403  in order to detect information in each signaling header  210 , and determine the appropriate commands for switching device  430  after looking up the connection table stored in module  410 . The fiber delay is placed in paths  401 - 403  so that the packet having header  210  and payload  211  reaches switching device  430  only after the actual switching occurs. This fiber delay is specific to the delay associated with header detection, table look-up, and switching, and can typically be accomplished in about 10 microseconds with about 2 km fiber delay in fibers  415 - 417 . 
     Packets are routed through network  201  using the information in signaling header  210  of each packet. When a packet arrives at a network element, signaling header  210  is read and either the packet (a) is routed to a new appropriate outbound port chosen according to the label routing look-up table, or (b) is immediately forwarded through an already existing label-switching originated connection within the network element. The latter case is referred to as “flow switching” and is supported as part of optical label-switching; flow switching is used for large volume bursty mode traffic. 
     Label-switched routing look-up tables are included in network elements  121 - 125  in order to rapidly route the optical packet through the network element whenever a flow switching state is not set-up. The connection set-up request conveyed by optical signaling header  210  is rapidly compared against the label-switch routing look-up table within each network element. In some cases, the optimal connections for the most efficient signal routing may already be occupied. The possible connection look up table is also configured to already provide an alternate wavelength assignment or an alternate path to route the signal. Providing a limited number of (at least one) alternative wavelength significantly reduces the blocking probability. The alternative wavelength routing also achieves the same propagation delay and number of hops as the optimal case, and eliminates the difficulties in sequencing multiple packets. The alternate path routing can potentially increase the delay and the number of hops, and the signal-to noise-ratio of the packets are optically monitored to eliminate any possibility of packets being routed through a large number of hops. In the case where a second path or wavelength is not available, contention at an outbound link can be settled on a first-come, first-serve basis or on a priority basis. The information is presented to a regular IP router and then is reviewed by higher layer protocols, using retransmission when necessary. 
     Routing Example 
     An illustrative WDM circuit-switched backbone network  500  for communicating packets among end-users in certain large cities in the United States is shown in pictorial form in FIG.  5 —network  500  is first discussed in terms of its conventional operation, that is, before the overlay of WDM optical label switching in accordance with the present invention is presented. 
     With reference to FIG. 5, it is supposed that New York City is served by network element  501 , Chicago is served by network element  502 , . . . Los Angeles is served by network element  504 , . . . , and Minneapolis by network element  507 . (Network elements may also be referred to as nodes in the sequel.) Moreover, NC&amp;M  220  has logical connections (shown by dashed lines, such as channel  221  to network element  501  and channel  222  to network element  507 ) to all network elements  501 - 507  via physical layer optical supervisory channels; there is continuous communication among NC&amp;M  220  and network elements  501 - 507 . NC&amp;M  220  periodically requests and receives information about: (a) the general state of each network element (e.g., whether it is operational or shut down for an emergency); (b) the optical wavelengths provided by each network element (e.g., network element  501  is shown as being served by optical fiber medium  531  having wavelength W 1  and optical fiber medium  532  having wavelength W 2  which connect to network elements  502  (Chicago) and  505  (Boston), respectively); and (c) the ports which are served by the wavelengths (e.g., port  510  of element  501  is associated with an incoming client interface conveying packet  520 , port  511  is associated with W 1  and port  512  is associated with W 2 , whereas port  513  of element  502  is associated with W 1 ). 
     Thus, NC&amp;M  220  has stored at any instant the global information necessary to formulate routes to carry the incoming packet traffic by the network elements. Accordingly, periodically NC&amp;M  220  determines the routing information in the form of, for example, global routing tables, and downloads the global routing tables to each of the elements using supervisory channels  221 ,  222 , . . . The global routing tables configure the ports of the network elements to create certain communication links. For example, NC&amp;M  220  may determine, based upon traffic demand and statistics, that a fiber optic link from New York City to Los Angeles (network elements  501  and  504 , respectively) is presently required, and the link will be composed, in series, of: WI coupling port  511  of element  501  to port  513  in network element  502 ; W 1  coupling port  514  of element  502  to port  515  of element  503 ; and W 2  coupling port  516  of element  503  to port  517  of element  504 . Then, input packet  520  incoming to network element  501  (New York City) and having a destination of network element  504  (Los Angeles) is immediately routed over this established link. At network element  504 , the propagated packet is delivered as output packet  521  via client interface port  518 . 
     In a similar manner, a dedicated path between elements  506  and  507  (St. Louis and Minneapolis, respectively) is shown as established using W 3  between network elements  506  and  502 , and W 2  between elements  502  and  507 . 
     Links generated in this manner—as based upon the global routing tables—are characterized by their rigidity, that is, it takes several seconds for NC&amp;M  220  to determine the connections to establish the links, to download the connectivity information for the links, and establish the input and output ports for each network element. Each link has characteristics of a circuit-switched connection, that is, it is basically a permanent connection or a dedicated path or “pipe” for long intervals, and only NC&amp;M  220  can tear down and re-establish a link in normal operation. The benefit of such a dedicated path is that traffic having an origin and a destination that maps into an already-established dedicated path can be immediately routed without the need for any set-up. On the other hand, the dedicated path can be, and most often is, inefficient in the sense that the dedicated path may be only used a small percentage of the time (e.g., 20%-50% over the set-up period). Moreover, switching device  430  (see FIG.  4 ), embedded in each network element which interconnects input and output ports, has only a finite number of input/output ports. If the above scenario is changed so that link from St. Louis to Minneapolis is required and a port already assigned to the New York to Los Angeles link is to be used (e.g., port  514  of network element  502 ), then there is a time delay until NC&amp;M  220  can respond and alter the global routing tables accordingly. 
     Now the example is expanded so that the subject matter in accordance with the principles of the present invention is overlaid on the above description. First, a parameter called the “label-switched state” is introduced and its use in routing is discussed; then, in the next paragraph, the manner of generating the label-switch state is elucidated. The label-switch state engenders optical label switching. 
     NC&amp;M  220  is further arranged so that it may assign the label-switch state to each packet incoming to a network element from a client interface—the label-switch state is appended by Plug &amp; Play module  132  and, for the purposes of the present discussion, the label-switch state is commensurate with header  210 , (see FIG.  2 ). The label-switch state is computed by NC&amp;M  220  and downloaded to each network element  501 - 507  in the form of a local routing table. With reference to FIG. 6, there is shown network element  501  and its embedded switch  601  in pictorial form. Also shown is incoming optical fiber  602 , with delay loop  603 , carrying packet  620  composed of header  210  and payload  211 —payload  211  in this case is packet  520  from FIG.  5 . Fiber  6022  delivers a delayed version of packet  620  to network element  501 . Also, a portion of the light energy appearing on fiber  602  is tapped via fiber  6021  and inputted to optical module  410  which processes the incoming packet  620  to detect header  210 —header  210  for packet  620  is shown as being composed of the label-switch state ‘11101011000’, identified by reference numeral  615 . Also shown in FIG. 6 is local look-up table  610 , being composed of two columns, namely, “Label-switch State” (column  611 ), and “Local Address” (column  612 ). The particular label-switch state for packet  620  is cross-referenced in look-up table  610  to determine the routing of the incoming packet. In this case, the label-switch state for packet  620  is the entry in the fourth row of look-up table  610 . The local switch address corresponding to this label-switch state is “0111”, which is interpreted as follows: the first two binary digits indicate the incoming port, and the second two binary digits indicate the output port. In this case, for the exemplary four-input, four-output switch, the incoming packet is to be routed from input port “0” to output port “11”, so switch  601  is switched accordingly (as shown). After the delay provided by fiber delay  603 , the incoming packet on fiber  6022  is propagated onto fiber  604  via switch  601 . 
     The foregoing description of label-switch state indicates how it is used. The manner of generating the label-switch state is now considered. NC&amp;M  220 , again on a periodic basis, compiles a set of local look-up tables for routing/switching the packet through each corresponding network element (such as table  610  for network element  501 ), and each look-up table is then downloaded to the corresponding network element. The generation of each look-up table takes into account NC&amp;M  220 &#39;s global knowledge of the network  500 . For instance, if incoming packet  620  to network  501  is destined for network  504  (again, New York to Los Angeles), if port  510  is associated with incoming port “01,” and serves fiber  602 , and if outgoing port  511  is associated with outgoing port “11” and serves fiber  604 , then NC&amp;M  220  is able to generate the appropriate entry in look-up table  610  (namely, the fourth row) and download table  610  to network element  510 . Now, when packet  520  is processed by electro-optical module  132 so as to add header  210  to packet  520  to create augmented packet  620 , NC&amp;M  220 &#39;s knowledge of the downloaded local routing tables as well as the knowledge of the destination address embedded in packet  520  as obtained via module  132  enables NC&amp;M  220  to instruct module  132  to add the appropriate label-switch state as header  210 —in this case ‘11101011000’. 
     It can be readily appreciated that processing a packet using the label-switch state parameter is bursty in nature, that is, after switch  601  is set-up to handle the incoming label-switch state, switch  601  may be returned to its state prior to processing the flow state. For example, switch  601  may have interconnected input port ‘01’ to output port ‘10’ prior to the arrival of packet  620 , and it may be returned to the ‘0110’ state after processing (as determined, for example, by a packet trailer). Of course, it may be that the circuit-switched path is identical to the label-switch state path, in which case there is no need to even modify the local route through switch  601  for processing the label-switch state. However, if it is necessary to temporarily alter switch  601 , the underlying circuit-switched traffic, if any, can be re-routed or re-sent. 
     As discussed so far, label switching allows destination oriented routing of packets without a need for the network elements to examine the entire data packets. New signaling information—the label—is added in the form of optical signal header  210  which is carried in-band within each wavelength in the multi-wavelength transport environment. This label switching normally occurs on a packet-by-packet basis. Typically, however, a large number of packets will be sequentially transported towards the same destination. This is especially true for bursty data where a large block of data is segmented in many packets for transport. In such cases, it is inefficient for each particular network element to carefully examine each label and decide on the routing path. Rather, it is more effective to set up a “virtual circuit” from the source to the destination. Header  210  of each packet will only inform continuation or ending of the virtual circuit, referred to as a flow state connection. Such an end-to-end flow state path is established, and the plug-and-play modules in the network elements will not disrupt such flow state connections until disconnection is needed. The disconnection will take place if such a sequence of packets have come to an end or another packet of much higher priority requests disruption of this flow state connection. 
     The priority aspect of the present invention is also shown with respect to FIG.  6 . The local look-up table has a “priority level” (column  613 ) which sets forth the priority assigned to the label-switching state. Also, header  210  has appended priority data shown as the number ‘2’ (reference numeral  616 ). Both the fourth and fifth row in the “label-switch state” column  611  of table  610  have a local address of ‘0111.’ If an earlier data packet used the entry in the fifth row to establish, for example, a virtual circuit or flow switching state, and the now another packet is processed as per the fourth row of column  611 , the higher priority data (‘2’ versus ‘4’, with ‘1’ being the highest) has precedent, and the virtual circuit would be terminated. 
     Detailed Illustrative Embodiments 
     In order to achieve ultra-low latency IP over WDM label switching, processing of the optical header at each optical switch must be kept to a minimum during the actual transmission of the optical packet. To achieve this end, a new signaling architecture and packet transmission protocol for performing optical WDM label switching is introduced. 
     The signaling and packet transmission protocols decouple the slow and complex IP routing functions from the ultra-fast WDM switching and forwarding functions. This decoupling is achieved via the setup of an end-to-end routing path which needs to be performed very infrequently. To send IP packets from a source to a destination, the following step is executed in accordance with the present invention: optical packet transmission, where the arrival of the optical packet triggers the local header processing which among other things looks up the output port for forwarding the packet on to the next hop based on the optical label inside the optical header. 
     Although routing path setup involves invoking the routing function which is generally a slow and complicated procedure, it is performed prior to packet transmission handling, and hence it is not in the critical path that determines transmission latency. 
     Routing Path Setup 
     During routing path setup, the internal connection table of a WDM packet switch will be augmented with a label-switch look-up table, and contains the pertinent packet forwarding information. In particular, in the interest of achieving ultra-low latency and hardware simplicity, the inventive scheme produces label-switch states that remain constant along the flow path For example, label-switch assignments include the following techniques: 
     (1) Destination-based flow label assignment—In this scheme the destination, e.g. a suitable destination IP address prefix, can be used as the label-switch state in next hop look-up. In addition to having no need to modify the optical header, the same header can be used in the event of deflection routing. 
     (2) Route-based flow label assignment—In this scheme the label-switch state assigned refers to the end-to-end route that is computed dynamically at the label-switch state setup phase. The advantage of this scheme is that it can be specialized to meet the Quality-of-Service requirements for each individual label-switched states. 
     Switching Conflict Resolution 
     The present-;day lack of a viable optical buffer technology implies that conventional buffering techniques cannot be used to handle switching conflicts. As previously described, the invention embodiment utilizes fixed delay implemented by an optical fiber to allow switching to occur during this time delay, but not to achieve contention resolution as electrical buffers do in conventional IP routers. To resolve switching contentions, in accordance with the present invention, the following three methods are used: 
     (a) Limited wavelength interchange—where a packet is routed through the same path but at a different wavelength. Since this wavelength conversion is utilized just to avoid the contention, it is not necessary that the network elements must possess the capability of converting to any of the entire wavelength channels. Rather, it is sufficient if they can convert some of the entire wavelength channels. This wavelength conversion converts both the signaling header and the data payload. Care must be taken to prevent a packet from undergoing too many wavelength conversions which will result in poor signal fidelity. A possible policy is to allow only one conversion, which and can easily be enforced by encoding the original wavelength in the optical header. This way an intermediate WDM switch will allow conversion if and only if it is carried on its original wavelength. 
     (b) Limited deflection routing—where a packet may be deflected to a neighboring switching node from which it can be forwarded towards its destination. Care again must be taken to prevent a packet from being repeatedly deflected, thereby causing signal degradation, as well as wasting network bandwidth. A solution scheme is to record a “timestamp” field in the optical header, and allow deflections to proceed if and only if the recorded timestamp is no older than a maximum limit. 
     (c) Prioritized packet preemption—where a newly arrived packet may preempt a currently transmitting packet if the arriving packet has a higher priority. The objective is to guarantee fairness to all packets so that eventually a retransmitted packet can be guaranteed delivery. In this scheme, each packet again has a timestamp field recorded in its optical header, and older packets have higher priority compared to newer packets. Furthermore a retransmitted packet assumes the timestamp of the original packet. This way, as a packet “ages,” it increases in priority, and will eventually be able to preempt its way towards its destination if necessary. 
     It is noted that in all these schemes the optical header can remain constant as it moves around in the network. This is consistent with the desire to keep the optical switching hardware fast and simple. It is also possible to consider combinations of these schemes. 
     Routing Protocol 
     For a network the size of the NGI, centralized routing decisions are quite unfeasible, so the approach needs to be generalized to distributed decision making. Hierarchical addressing and routing are used as in the case of IP routing. When a new connection is requested, NC&amp;M  220  decides whether a WDM path is provisioned for this (source, destination) pair within the WDM-based network. If it is, the packets are immediately sent out on that (one-hop IP-level) path. If no such path is provisioned, NC&amp;M  220  decides on an initial outbound link for the first WDM network element and a wavelength to carry the new traffic. This decision is based on the rest of the connections in the network at the time the new connection was requested. NC&amp;M  220  then uses signaling, through an appropriate protocol, to transfer the relevant information to the initial WDM network element to be placed in the signaling header. After the initial outbound link is determined, the rest of the routing decisions are taken at the individual network element (NE&#39;s) according to the optical signaling header information. This method ensures that the routing tables at each switching node and the signaling header processing requirements are kept relatively small. It also enables the network to scale easily in terms of switching nodes and network users. It is noted, too, that multiple WDM subnetworks can be interconnected together and each subnetwork will have its own NC&amp;M. 
     When a path is decided upon, within a WDM NE, the optical switches can be set in that state (i) for the duration of each packet through the node and then revert back to the default state (called optical label-switching), or (ii) for a finite, small amount of time (called flow switching). The former case performs routing on a regular packet-by-packet basis. The system resources are dedicated only when there is information to be sent and at the conclusion of the packet, these resources are available for assignment to another packet. The latter case is used for large volume bursty mode traffic. In this case, the WDM NE only has to read a flow state label from the optical signaling header of subsequent packets arriving at the NE to be sure such a packet is bound for the same destination, without the need to switch the switching device, and forward the payload through the already existing connection through the NE as previously established by the optical label-switching. 
     The packets are self-routed through the network using the information in the signaling header of each packet. When a packet arrives at a switching node, the signaling header is read and either the packet is forwarded immediately through an already existing flow state connection or a new appropriate outbound port is chosen according to the routing table. Routing tables in each node exist for each wavelength. If the packet cannot follow the selected outbound port because of contention with another packet (the selected outbound fiber is not free), the routing scheme will try to allocate a different wavelength for the same outbound port (and consequently the signal will undergo wavelength translation within the switching node). If no other eligible wavelength can be used for the chosen outbound port, a different outbound port may be chosen from another table, which lists secondary (in terms of preference) outbound links. 
     This routing protocol of the inventive technique is similar to the deflection routing scheme (recall the Background Section), where the session is deflected to some other outbound link (in terms of preference) if the preferred path cannot be followed. The packet is not allowed to be continuously deflected. In traditional routing protocols, a hop count is used to block a session after a specified number of hops. In the new scheme, in case no header regeneration is allowed at the switching nodes, then the hop count technique cannot be used. Alternatively, the optical signaling header characteristics (i.e., the signaling header&#39;s signaling to noise ratio can be looked upon to decide whether a packet should be dropped. 
     IP Routing Algorithm in WDM layer 
     The technique used by NC&amp;M  220  to determine the routing tables is based upon shortest path algorithms that route the packets from source to destination over the path of least cost. Specific cost criteria on each route, such as length, capacity utilization, hop count, or average packet delay can be used for different networks. The objective of the routing function is to have good performance (for example in terms of low average delay through the network) while maintaining high throughput. Minimum cost spanning trees are generated having a different node as a root at each time, and the information obtained by these trees can then be used to set-up the routing tables at each switching node. If deflection routing as outlined above is implemented, the k-shortest path approach can be used to exploit the multiplicity of potential routing paths. This technique finds more than one shortest path, with the paths ranked in order of cost. This information can be inputted into the switching node routing tables, so that the outbound link corresponding to the minimum cost path is considered first, and the links corresponding to larger cost paths are inputted in secondary routing tables that are used to implement deflection routing. 
     Description of Plug-and-Play Modules 
     The present invention is based upon two types of Plug-and-Play modules to be attached to the WDM network elements. Introduction of these Plug-and-Play modules add optical label switching capability to the existing circuit-switched network elements. 
     In FIG. 3, both header encoder  321  and header remover  322  were shown in high-level block diagram form; FIGS. 7 and 8 show, respectively, a more detailed schematic for both encoder  321  and remover  322 . 
     In FIG. 7, IP packets or datagrams are processed in microprocessor  710  which generates each optical signaling header  210  for label switching. Optical signaling header  210  and the original IP packet  211  are emitted from microprocessor  710  at baseband. Signaling header  210  is mixed in RF mixer  720  utilizing local oscillator  730 . Both the mixed header from mixer  720  and the original packet  211  are combined in combiner  740  and, in turn, the output of combiner  740  is encoded to an optical wavelength channel via optical modulator  760  having laser  750  as a source of modulation. 
     In FIG. 8, the optical channel dropping out of a network element is detected by photodetector  81 . 0  and is electrically amplified by amplifier  820 . Normally, both photodetector  810  and the amplifier  820  have a frequency response covering only the data payload but not the optical signaling header RF carrier frequency provided by local oscillator  730 . Low-pass-filter  830  further filters out any residual RF carriers. The output of filter  830  is essentially the original IP packet sent out by the originating IP router from the originating network element which has been transported through the network and is received by another IP router at another network element. 
     Block diagram  900  of FIG. 9 depicts the elements for the detection process effected by Plug-and-Play module  410  of FIG. 4 to convert optical signal  901 , which carries both label-switching signaling header  210  and the data payload  211 , into baseband electrical signaling header  902 . Initially, optical signal  901  is detected by photodetector  910 ; the output of photodetector  910  is amplified by amplifier  920  and filtered by high-pass filter  930  to retain only the high frequency components which carry optical signaling header  210 . RF splitter  940  provides a signal to local oscillator  950 , which includes feedback locking. The signal from local oscillator  950  and the signal from splitter  940  are mixed in mixer  960 , that is, the high frequency carrier is subtracted from the output of filter  920  to leave only the information on label-switching signaling header  210 . In this process, local oscillator  950  with feedback locking is utilized to produce the local oscillation with the exact frequency, phase, and amplitude, so that the high frequency component is nulled during the mixing of this local oscillator signal and the label-switching signaling header with a high-frequency carrier. Low-pass filter  970 , which is coupled to the output of mixer  960 , delivers baseband signaling header  210  as electrical output signal  902 . 
     The circuit diagram of FIG. 10 shows an example of a more detailed embodiment of FIG.  4 . In FIG. 10, each header detector  1010 ,  1020 , . . . ,  1050 , . . . , or  1080  processes information from each wavelength composing the optical inputs arriving on paths  1001 ,  1002 ,  1003 , and  1004  as processed by demultiplexers  1005 ,  1006 ,  1007 , and  1008 , respectively; each header detector is exemplified by the circuit  900  of FIG.  9 . The processed information is grouped for each wavelength. Thus, for example, fast memory  1021  receives as inputs, for a given wavelength, the signals appearing on lead  1011  from header detector  1010 , . . . , the signals appearing on lead  1012  from header detector  1030 , the signals appearing on lead  1013  from header detector  1050 , and the signals appearing on lead  1014  from header detector  1070 . Each fast memory  1021 - 1024 , such as a content-addressable memory, serves as an input to a corresponding label switch controller  1031 - 1034 . Each label switch controller  1031 - 1034  also receives circuit-switched control signals from network element switch controller  420  of FIG.  4 . Each label switch controller intelligently chooses between the circuit switched control as provided by controller  420  and the label switched information supplied by its corresponding fast memory to provide appropriate control signals the switching device  430  of FIG.  4 . 
     Flow diagram  1100  of FIG. 11 is representative of the processing effected by each label-switch controller  1031 - 1034 . Using label-switch controller  1031  as exemplary, inputs from circuit-switched controller  420  and inputs from fast memory  1021  are monitored, as carried out by processing block  1110 . If no inputs are received from fast memory  1021 , then incoming packets are circuit-switched via circuit-switched controller  420 . Decision block  1120  is used to determine if there are any inputs from fast memory  1021 . If there are inputs, then processing block  1130  is invoked so that label-switch controller  1031  can determine from the fast memory inputs the required state of switching device  430 . Then processing block  1160  is invoked to transmit control signals from label-switch controller  1031  to control switching device  430 . If there are no fast memory inputs, then the decision block  1140  is invoked to determine if there are any inputs from circuit-switched controller  1140 . If there are inputs from circuit-switched controller  420 , then processing by block  1150  is carried out so that label-switch controller  1031  determines from the inputs of circuit-switched controller  420  the required state of switching device  430 . Processing block  1160  is again invoked by the results of processing block  1150 . If there are no present inputs from circuit-switched controller  1140  or upon completion of procession block  1160 , control is returned to processing block  1110 . 
     By way of reiteration, optical label-switching flexibly handles all types of traffic: high volume burst, low volume burst, and circuit switched traffic. This occurs by interworking of two-layer protocols of the label-switched network control. Thus, the distributed switching control rapidly senses signaling headers and routes packets to appropriate destinations. When a long stream of packets reach the network element with the same destination, the distributed switching control establishes a flow switching connection and the entire stream of the packets are forwarded through the newly established connections. 
     A label switching method scales graciously with the number of wavelengths and the number of nodes. This results from the fact that the distributed nodes process multi-wavelength signaling information in parallel and that these nodes incorporate predicted switching delay in the form of fiber delay line. Moreover, the label switching utilizes path deflection and wavelength conversion for contention resolution. 
     Optical Header Processing 
     The foregoing description focused on optical header processing at a level commensurate with the description of the overall NGI system configured with the overlaid Plug-and-Play modules. Discussion of header processing at a more detailed level is now appropriate so as to exemplify how low-latency can be achieved at the circuit-detail level. 
     To this end, opto-electrical circuitry  1200  of FIG. 12, which is a more detailed block diagram elucidating certain aspects of prior figures, especially FIGS. 9 and 10, is considered. By way of a heuristic overview, the processing carried out by the opto-electrical circuitry  1200  is such that a header signal (e.g., 155 Mb/s on a microwave carrier) is frequency-division multiplexed with a baseband data payload (e.g., 2.5 Gb/s). The header signal is processed by a single-sideband (SSB) modulator, so only the upper sideband representation of the header signal is present in the frequency-division multiplexed signal. Moreover, the technique effected by circuitry  1200  is one of label replacement wherein the original header signal at the given carrier frequency is first removed in the optical domain, and then a new header signal is inserted at the same carrier frequency in the optical domain. A notch filter is used to remove the original header signal; the notch filter is realized, for example, by the reflective part of a Fabry-Perot filter. 
     In particular, circuitry  1200  has as its input the optical signal at optical wavelength λ 1  on path  1001  as received and processed by demux  1005 , both of which are re-drawn from FIG.  10 . Circuitry  1200  is composed of: a lower path to process optical signal  1201  emanating from demux  1005 ; and an upper path to process optical signal  1202  emanating from demux  1005 . The lower path derives the label, conveyed by the incoming SSB header in optical signal  1001 , to control optical switch  1203 ; switch  1203  is a multi-component element encompassing components already described, including fast memory  1021  and label switch controller  1031  of FIG. 10 as well as switching device  430  of FIG.  4 . The upper path is used to delete the old header signal, including the label, at the sub-carrier frequency and then insert a new header label, in a manner to be described below after the lower path is first described. 
     The lower path is an illustrative embodiment of header detector  1010  originally shown in high-level block diagram form in FIG.  10 . In particular, header detector  1010  includes, in cascade: (a) opto-electrical converter  1210  (e.g.,;a photodetector) for producing electrical output signal  1211 ; (b) multiplier  1215  to convert electrical signal  1211  to intermediate frequency signal  1217 —to accomplish this, multiplier  1215  is coupled to local oscillator  1218  which provides a sinusoid  1216  at a frequency to down-convert the incoming sub-carrier conveying the header label, designated for discussion purposes as ƒ c , to an intermediate frequency ƒ I ; (c) intermediate frequency bandpass filter  1220  having signal  1217  as its input; (e) demodulator  1225  to convert the intermediate frequency to baseband; (e) detector  1230  responsive to demodulator  1225 ; and (f) read circuit  1235  which outputs signal on lead  1011  of FIG.  10 . Elements  1211 ,  1215 ,  1216 ,  1217 ,  1220 ,  1225 , and  1230  can all be replaced by a simple envelope detector if the subcarrier header was transmitted using an incoherent modulator such as ASK (amplitude-shift keying). It is not always required to use a coherent demodulator as shown in FIG.  12 . (In fact, FIG. 13 will depict the case for an incoherent modulation). 
     The operation of header detector  1010  of FIG. 12 is as follows. It is assumed that the second type of ‘Plug-and-Play’ module of FIG. 4 injects a 2.5 Gbps IP data packet (e.g., with QPSK/QAM modulation) which is sub-carrier multiplexed with a 155 Mbps single-sideband header packet (e.g., with SSB modulation) at the modulation frequency ƒ c ; as before, the header precedes the data payload in time and both are carried by the optical wavelength λ 1 . In each network node that receives the combined header and payload at wavelength λ 1 , the sub-carrier header at ƒ c  is multiplied by multiplier  1215 , is band-pass filtered by intermediate filter  1220 , and is demodulated to baseband by demodulator  1225 . Then, the demodulated baseband data burst is detected by detector  1230  (e.g., a 155 Mbps burst-mode receiver), and read by circuit  1293  (e.g., a microprocessor). 
     This foregoing operational description has focused only on the detection of the optical header to control the routing path through switch  1203 . As alluded to in the Background Section, header replacement is now considered important to present-day NGI technology so as to accomplish high-throughput operation in a packet switched network in which data paths change due to, for example, link outages and variable traffic patterns. Moreover, header replacement is useful to maintain protocol compatibility. The upper path components of FIG. 12 that have heretofore not been described play a central role in header replacement. Actually, the notion of header replacement has a broader connotation in that the header may be composed of various fields, such as a “label” field and a “time-to-leave” field. The description to this point has used the header and label interchangeably; however, it is now clear that the header may actually have a plurality of fields, and as such any or all may be replaced at any node. 
     Now continuing with the description of FIG. 12, the upper processing path which processes the optical signal on path  1202  includes: (a) circulator  1240 ; (b) Fabry-Perot (FFP) filter  1245 , coupled to circulator  1240  via path  1241 , with filter  1245  being arranged so that one notch in its free spectral range (FSR) falls at ƒ c ; and (c) attenuator  1250  coupled to the reflective port (R) of FFP  1245 . An exemplary FFP  1245  is available from The Micron Optics, Inc. as model No. FFP-TF (“Fiber Fabry-Perot Tunable Filter”). The combination of these latter three elements, shown by reference numeral  1251 , produces a notch filter centered at ƒ c  which removes the SSB header signal propagating with ƒ c  as its center frequency, as shown pictorially by the spectra in the upper portion of FIG.  12 . As illustrated, spectrum  1242  of signal  1202  includes both a baseband data spectrum and the header signal spectrum centered at ƒ c . After processing by notch filter  1251 , spectrum  1243  obtains wherein only the baseband data spectrum remains. 
     The output of notch filer  1251 , appearing on path  1244  of circulator  1240 , serves as one input to Mach-Zender modulator (MZM)  1270 . Two other inputs to MZM  1270  are provided, namely, via path  1271  emanating from multiplier  1290  and via path  1272  emanating from phase shift device  1295 . As discussed in the next paragraph, the signal appearing on lead  1271  is the new header signal which is double-sideband in nature. The signal on path  1272  is phase-shifted by π/ 2  relative to the signal on path  1271 . MZM  1270  produces at its output the upper-sideband version of the signal appearing on path  1271 , that is, the new header signal. The single-sideband processing effected by MZM  1270  is described in detail in the paper entitled “Overcoming Chromatic-Dispersion Effects in Fiber-Wireless Systems Incorporating External Modulators”, authored by Graham H. Smith et al., as published in the IEEE Transactions on Microwave Theory and Techniques, Vol. 45, No. 8, August 1997, pages 1410-1415, which is incorporated herein by reference. Moreover, besides converting the new header signal to an optical single-sideband signal (OSSB), MZM  1270  also adds this OSSB signal to the incoming optical baseband signal on path  1244  to produce the desired frequency-multiplexed signal of baseband plus SSB header on output path  1273  from MZM  1270 . 
     The new header signal delivered by path  1271  is derived as follows. Write circuit  1275  is responsible for providing data representative of a new header signal, such as a new label represented in binary. The header signal that arrives at the input to demux  1005  is referred to as the active header signal. The replacement header signal is called the new header signal. Write circuit  1275  has as its input the output of read device  1235 , so write circuit  1275  can reference or use information from the active header signal to derive the new header signal, if necessary. The new header signal, as provided at the output of write circuit  1275 , is delivered to pulse generator  1280 , which performs the operation of converting the new header signal data to, as exemplary, a 155 Mb/s signal on a microwave carrier. The signal from generator  1280  is filtered by low-pass filter  1285  to remove spurious high-frequency energy. Then the signal from filter  1285  is delivered to modulator  1290 ; modulator  1290  also has as a sinusoidal input at frequency ƒ c  provided by local oscillator  1218 . The output of modulator  1290 , which appears on path  1271 , is the new header signal centered at a frequency of the local oscillator, namely ƒ c ; also, the output of modulator  1290  serves as the only input to phase-shift device  1295 . 
     MZM  1270  produces a spectrum that includes both the original baseband data spectrum as well as the spectrum of the new header signal at ƒ c . This is shown in frequency domain visualization  1274  in the top right-hand comer of FIG. 12, which is counterpart of the visualization in the top left-hand comer. 
     The new optical signal on path  1273  is switched via optical switch  1203 , as controlled by the active or original incoming header signal, under control of the label on lead  1011   
     It is noted that, in terms of presently available components, the processing time of the header removal and insertion technique should take less than 30 ns. On the other hand, if it is assumed that there are 15 bits in each packet header signal, then the time to read 15 bits, write 15 bits, and add 10 preamble bits can take about 260 ns for a 155 Mbps burst. Therefore, allowing for some variations, each header signal is about 300 ns. This means that it may be necessary to insert a delay line in the main optical path between circulator  1240  and MZM  1270  of 300 ns, so the length of delay line would be around 60 meters. To save processing time, the data rate of the subcarrier header can be increased to, for example, 622 Mb/s or higher, depending upon the future network environment. 
     Another Illustrative Embodiment of a Header Removal and Insertion Technique 
     The circuit arrangement of FIG. 12 is realized using the so-called reflective port of FFP  1245 . FFP  1245  also has a transmission port which may be utilized wherein the characteristics of the optical signal emanating from the transmission port are the converse of the optical signal from the reflective port. So whereas the reflective port provides an attenuation notch at ƒ c , the transmission port attenuates frequencies relative to ƒ c , so that only frequencies in the vicinity of ƒ c  are passed by the transmission port. An alternative to circuitry  1200  of FIG. 12 is shown by circuitry  1300  of FIG.  13 . The main difference between FIGS.  12  and FIG. 13 is the manner in which the lower processing path now derives its input signal via path  1301  (as compared to input signal on path  1201  of FIG.  12 ). 
     In particular, FFP  1325  now has a transmission (T) port in addition to the reflective (R) port. The output from transmission port, on path  1301 , now serves as the input to opto-electrical converter  1210 . Because the signal on path  1301  conveys only frequencies centered about ƒ c , that is, the baseband data information has been attenuated by FFP notch filter  1345 , and can be processed directly by detector  1230  via LPF  1320 . The remainder of circuitry  1300  is essentially the same as circuitry  1200  of FIG.  12 . 
     Optical Technology 
     Optical technologies span a number of important aspects realizing the present invention. These include optical header technology, optical multiplexing technology, optical switching technology, and wavelength conversion technology. 
     (a) Optical Header Technology 
     Optical header technology includes optical header encoding and optical header removal as discussed with respect to FIGS. 3 and 4. In effect, optical header  210  serves as a signaling messenger to the network elements informing the network elements of the destination, the source, and the length of the packet. Header  210  is displaced in time compared to the actual data payload. This allows the data payload to have any data rates/protocols or formats. 
     (b) Optical Multiplexing Technology 
     Optical multiplexing may illustratively be implemented using the known silica arrayed waveguide grating structure. This waveguide grating structure has a number of unique advantages including: low cost, scalability, low loss, uniformity, and compactness. 
     (c) Optical Switching Technology 
     Fast optical switches are essential to achieving packet routing without requiring excessively long fiber delay as a buffer. 
     Micromachined Electro Mechanical Switches offer the best combination of the desirable characteristics: scalability, low loss, polarization insensitivity, fast switching, and robust operation. Recently reported result on the MEM based Optical Add-Drop Switch achieved 9 microsecond switching time 
     (d) Wavelength Conversion Technology 
     Wavelength conversion resolves packet contention without requiring path deflection or packet buffering. Both path deflection and packet buffering cast the danger of skewing the sequences of a series of packets. In addition, the packet buffering is limited in duration as well as in capacity, and often requires non-transparent methods. Wavelength conversion, on the other hand, resolves the blocking by transmitting at an alternate wavelength through the same path, resulting in the identical delay. Illustratively, a WSXC with a limited wavelength conversion capability is deployed. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.