Expanding the coverage of a time-shared network comprising electronic edge nodes interconnected by bufferless fast-switching optical nodes is enabled by combining spatial switching with temporal switching. The output side of each edge node preferably connects to a large number of core switches through individual time-locked channels and the input side preferably connects to each of a small number of core switches through a channel band having a sufficiently large number of channels. Wavelength routers may be used to aggregate individually-routed channels into WDM links.

FIELD OF THE INVENTION

The present invention relates to time-shared wide-coverage networks employing fast-switching optical nodes. In particular, the invention relates to burst switching in an optical-core network.

BACKGROUND

In a prior-art burst-switching scheme, a source node sends a burst transfer request to a core node to indicate that a burst of data follows the request. The request indicates the size of the accompanying burst and its destination. Responsive to this burst transfer request, the core node configures a space switch to connect a specific channel in a link on which the burst will be received to any channel in a link directed towards the requested burst destination. The burst follows the burst transfer request after a predetermined time period sufficient to configure the switch and it is expected that, when the burst arrives at the core node, the space switch would have been configured by the core-node controller to accommodate the burst. The core node may fail to schedule the transfer of the burst from input to output at the respective arrival time, and, if the core node does not include a buffer, the burst may be lost. Naturally, the probability of burst loss decreases with the number of channels per link and increases with the number of bufferless core nodes traversed. This switching method is hereinafter called “spatial switching”.

An alternative burst switching method, hereinafter called “temporal switching”, and a mechanism for burst transfer in a composite-star network having optical core nodes is described in Applicant's U.S. patent application Ser. No. 09/750,071, filed on Dec. 29, 2000, and titled “Burst Switching in a High-Capacity Network”, the specification of which is incorporated herein by reference. According to the method, a burst-transfer request is sent to a controller of a selected core node after a burst has been formed at a source node. High network efficiency is realized by pipelining burst scheduling and burst-transfer. The transfer of bursts across the optical core nodes is loss-free.

To realize low-latency temporal switching, Applicant developed a technique according to which burst schedules may be initiated by any of a plurality of bufferless core nodes and distributed to respective edge nodes. A burst size is determined by a core node according to an allocated flow rate of a burst stream to which the burst belongs. The technique is described in applicant's U.S. patent application Ser. No. 10/054,512, filed on Nov. 13, 2001 and titled “Rate-Controlled Optical Burst Switching”, the specification of which is incorporated herein by reference. Burst formation takes place at source nodes, according to information accompanying the burst schedules. An allocated flow rate of a burst stream may be modified according to observed usage of scheduled bursts of a burst stream. A method of control-burst exchange between each of a plurality of edge nodes and each of a plurality of bufferless core nodes enables time coordination, burst scheduling, and loss-free burst switching. Both the load bursts and control bursts are carried by optical channels connecting the edge nodes and the core nodes. The burst descriptors are generated by a master controller of an optical switch in a core node. The switching times of the bursts corresponding to the generated descriptors are scheduled, and the schedules are distributed to the respective edge nodes. The burst-descriptor generation is based on burst-stream flow-rate-allocation defined by the source nodes. A method and an apparatus for allocating an appropriate flow rate to a data stream is described in U.S. Pat. No. 6,580,721, issued to Beshai on Jun. 17, 2003 and titled “Routing and rate control in a universal transfer mode network, the specification of which is incorporated herein by reference.

The burst-switching methods of U.S. patent applications Ser. Nos. 09/750,071 and 10/054,512 require time-coordination where each path from a source edge node to a core node adjacent to a destination edge node is time locked. The network coverage, in terms of the number of edge nodes, is determined primarily by the dimensions of the core nodes.

Extending the network coverage to a global scale may be realized by using a cascade of channel switching and time-slot switching (channel switching establishes a connection from an input port to an output port of a switch and holds the connection for an extended period of time). Extending the network coverage may also be realized by partial use of random-access buffers at selected core channels, as described in Applicant's U.S. patent application titled “Hybrid fine-coarse carrier switching”, filed on Nov. 8, 2002, and assigned Ser. No. 10/290,314, the specification of which is incorporated herein by reference. Such buffers may require optical-to-electrical conversion, electronic storage, and electrical to-optical conversion. The selected core channels are provided with buffers to enable temporal alignment of signals arriving at any core node from several other core nodes. A buffer may be provided at either end of a channel connecting two core nodes. Otherwise, all other ports of the core nodes may be bufferless. Providing such buffers enables the construction of a high-capacity wide-coverage network comprising a large number of core nodes shared by numerous edge nodes. Time-division-multiplexing and burst transfer can coexist in the network disclosed in the aforementioned U.S. patent application Ser. No. 10/290,314 and both are enabled by time-locking each edge node to an adjacent core node (or to a core node reached through channel switching) and by time-alignment at the random-access buffers.

Thus, as described above, extending the coverage of a network based on temporal switching may require an intermediate electronic-switching step either at an intermediate edge node or, preferably, at a random-access buffer placed at selected core channels. It may be desirable, however, to avoid electronic buffering, except at the source edge node and the destination edge node, and explore means for providing fast-switched paths in an entirely optical core in a wide-coverage network.

SUMMARY OF THE INVENTION

The present invention provides a method for extending the coverage of a time-shared fast-switching bufferless-core network using temporal switching in conjunction with spatial switching. The method significantly reduces the mean number of hops in a large-diameter core network by providing individually routed temporally-switched upstream channels from each edge node to numerous core switches. Signals are transferred from a core switch to another core switch or to a destination edge node through a process of spatial switching from an input channel to an output channel selected from a channel band during a specified time interval.

In accordance with one aspect, the present invention provides a method of switching a burst at a specific node in a network comprising a plurality of nodes. The method comprises steps of: receiving from a control channel invariant information of the burst; assigning as a candidate channel any available output channel from among a band of output channels connecting the specific node to a succeeding node according to the invariant information; sending to the succeeding node an identifier of the candidate channel; receiving from the control channel an identifier of an incoming channel assigned to carry the burst to the specific node; and scheduling an internal path from the incoming channel to the candidate channel if the identifier is a valid number. The invariant information includes a destination of the burst, duration of the burst, and a delay time after which the burst follows the invariant information.

In accordance with another aspect, the present invention provides a method of transmitting a signal originating from a first edge node to a second edge node in a network of edge nodes interconnected by bufferless core switches. The method comprises steps of: temporal switching of the signal in the first core switch; and spatial switching of the signal in the second core switch. The step of temporal switching includes steps of: allocating any time-interval during which a specific output channel of the first core switch is free; determining a transmission time of the signal from the first edge node so that the signal arrives at the first core switch at the start of the any time interval; and transmitting the signal at the determined transmission time. The step of spatial switching includes a step of allocating, from among a specific band of output channels of the second core switch, any output channel that is free during a specific time interval.

In accordance with a further aspect, the present invention provides a network comprising: a network core having a plurality of core switches interconnected by multiple-channel links; a plurality of shell switches each having a plurality of outward channels individually routed to selected core switches from among the plurality of core switches; and a plurality of edge nodes each edge node having at least one time-locked upstream channel to at least one of the shell switches and a downstream channel band from each of at least one core switch. Each of the core switches has at least one multiple-channel path to at least one of the edge nodes.

In accordance with another aspect, the present invention provides an asymmetrical edge node having outbound channels individually routed to selected core switches from among a plurality of core switches, and inbound channels arranged in channel bands each channel band emanating from a core switch from the plurality of core switches. The asymmetrical edge node is operable to: time lock each of the outbound channels to a corresponding core switch from among the selected core switches; receive bursts of a burst downstream over at least one inbound channel of the each channel band; and collate the bursts of the burst downstream according to a predetermined order.

In one mode of burst scheduling, the asymmetrical edge node is operable to: formulate burst-transfer requests, each burst-transfer request specifying a destination and a burst duration of a corresponding burst; send the each burst-transfer request over one of the outbound channels to a specific core switch from among the selected core switches; receive from the specific core switch an indication of a required transmission time for the corresponding burst; and transmit the corresponding burst at the required transmission time.

In an alternative mode of burst scheduling, the asymmetrical edge node is further operable to: formulate flow-rate-allocation requests, each flow-rate-allocation request specifying a destination and a required flow rate for a burst upstream; send the each flow-rate-allocation request over one of the outbound channels to a specific core switch from among the selected core switches; receive from the specific core switch burst-transfer permits for the burst upstream, each permit including an indication of a permissible size and a transmission time of an upstream burst; formulate an upstream burst within the permissible size; and transmit the upstream burst at the required transmission time.

In accordance with yet another aspect, the present invention provides a controller associated with a switching node, the switching node having input ports, output ports, and a switching fabric. The controller comprises: a temporal scheduler operable to allocate a path from a specific input port to a specific output port during any time interval of a specified duration; and a spatial scheduler operable to allocate a path from a specific input port to any output port from within a band of output ports during a specified time interval. The controller further includes a timing circuit for exchanging time locking signals with other controllers. The controller also includes a selector operable to arbitrate between the temporal scheduler and the spatial scheduler when the specific output port is within the band of output ports.

DETAILED DESCRIPTION

The terminology used in describing the embodiments of the invention is listed below.

Edge node: A switching node having subtending (directly connected) information sources and sinks (also called subtending traffic sources and sinks) and connecting to other nodes is called an edge node.Source node: An edge node transmitting signals, received from subtending traffic sources, to other nodes is called a source edge node, or a source node.Sink node: An edge node receiving signals from other nodes, for delivery to subtending traffic sinks, is called a sink edge node, or a sink node.Core switch: A switching node connecting only to other switching nodes, which may be edge nodes or core switches, is called a core switch. A core switch does not connect directly to traffic sources and sinks.Shell switch: A core switch receiving signals directly from an edge node is called a shell switch.Composite-star network: A network comprising a set of electronic edge nodes interconnected through independent core switches that are not interconnected is called a composite-star network. The composite-star network used in this disclosure has fast-switching optical switches.Native edge node: Edge nodes of the same composite-star network are called native edge nodes.Native shell switch: Shell switches of the same composite-star network are called native shell switches.Network coverage: Network coverage is defined herein as the number of edge nodes in a network, regardless of the number of core nodes.Time-shared network: A network employing fast switching in the core so that successive data blocks (such as bursts) in an inter-nodal channel may be destined to different nodes is a time-shared network.Input port: A port of a switching node receiving information signals from either a subtending information source or from an external node is called an input port.Output port: A port of a switching node transmitting information signals to either a subtending information sink or an external node is called an output port.Outer port: In an edge-node, an input port receiving signals from a traffic source, or an output port transmitting signals to a traffic sink, is called an outer port.Inner port: In an edge-node, an input port receiving signals from another switching node, or an output port transmitting signals to another switching node, is called an inner port (outer ports interface with access devices while inner ports interface with other switching nodes).Ingress port: An input port of an edge node receiving information signals from subtending information sources (traffic sources) is referenced as an ingress port. An ingress port is an outer port.Egress port: An output port of an edge node transmitting information signals to subtending information sinks (traffic sinks) is referenced as an egress port. An egress port is an outer port.Inbound port: An input port of an edge node receiving information signals from external switching nodes is referenced as an inbound port. An inbound port is an inner port.Outbound port: An output port of an edge node transmitting information signals to external switching nodes is referenced as an outbound port. An outbound port is an inner port.Inbound channel: An inbound channel is a communication channel, usually a wavelength channel in a fiber-optic link, connecting an external node to an inbound port of an edge node.Outbound channel: An outbound channel is a communication channel, usually a wavelength channel in a fiber-optic link, connecting an outbound port of an edge node to an external node.Inlet port: An input port, of a shell switch or a core switch, receiving an outbound channel of an edge node, is herein called an inlet port.Outlet port: An output port, of a shell switch or a core switch, connecting to an inbound channel of an edge node, is herein called an outlet port.Inward port: An input port, of a shell switch or a core switch, receiving a wavelength channel from another core switch, is called an inward port.Outward port: An output port, of a shell switch or a core switch, having a wavelength channel to another shell switch or core switch, is called an outward port.Inward and outward channels: A channel connecting a first core switch (or shell switch) to a second core switch (or shell switch) is an inward channel with respect to the second core switch (or shell switch) and an outward channel with respect to the first core switch (or shell switch).Upstream: The adjective ‘upstream’ refers to a flow in the direction from an edge node to a shell switch or core switch.Downstream: The adjective ‘downstream’ refers to a flow in the direction from a shell switch or a core switch to an edge node.Link: A channel band within a fiber transmission line forms a link. A fiber transmission line may include two or more channel bands, i.e., two or more links, which may be demultiplexed at a wavelength router and directed to different destinations.Uplink: An uplink is a communication link, usually a multiple-channel link, from an edge node to a shell switch or a core switch.Downlink: A downlink is a communication link, usually a multiple-channel link, from a shell switch or a core switch to an edge node.Up-channel: An up-channel is a channel, usually a wavelength channel, within an uplink.Down-channel: A down-channel is a channel, usually a wavelength channel, within a downlink.Core link: A core link is a link connecting two core switches. A core link is preferably a wavelength-division-multiplexed (WDM) fiber link.Core channel: A channel in a core link is a core channel.Adjacency (degree): The adjacency of a specific switching node is the number of switching nodes to which the specific switching node connects through at least one channel. This is often called the ‘degree’ of the switching node.Hop: A direct connection from one switching node to another is called a hop. A route from one edge node to another edge node may include several hops. The number of hops in a route is the number of switching nodes traversed by the route minus one (i.e., the number of intermediate switching nodes plus one).Topological length of a route: The number of hops along a route from one edge node to another is herein called the topological length of the route.Network diameter: The topological length of a route from a source edge node to a sink edge node in a switched network may be used as an indication of “network diameter”. The network diameter may be the largest topological length of a route, or may be chosen to be a traffic-weighted topological length for all designated routes.Sparse connectivity: A switched network where the network diameter is large (8 for example) is considered a sparsely connected network. A ring network having a large number of nodes (16 or more, for example) is a sparsely connected network. A full mesh of switching nodes forms a densely connected network. A partially-meshed network lies between these two extremes.Node dimension: The dimension of a switching node is determined by both the number of input ports and the number of output ports where each input port connects to an input channel and each output port connects to an output channel. A switching node having 256 input ports and 320 output ports, for example, is said to be of dimension 256×320.Data packet: It is a conventional data block of arbitrary size and having an identifying header.Data burst: A data burst may be an aggregation of data packets having a burst header in addition to the individual packet headers; a data burst may contain only one packet of a large size, in which case only the burst header is required. According to an embodiment of the present invention, a data burst may have a split header the parts of which may be communicated during noncontiguous time intervals.Time-limited signal: A signal occupying a channel during a relatively short period of time, 16 microseconds for example, is called a time-limited signal. Successive time-limited signals carried by a channel may be directed to different destinations. Packets and bursts are examples of time-limited signals.Load burst: A data burst formed at an edge node is a load burst, herein also referenced as a ‘burst’ for brevity. When the data burst modulates an optical carrier of a given central wavelength, the corresponding portion of the modulated carrier is also called a load burst (or simply a burst). In other words, the term burst refers to a baseband time-limited signal or a time-slice of a carrier wave modulated by the baseband signal.Data stream: A stream of signals originating from a first edge node and destined to a second edge node is called a data stream. Signals of a data stream are normally assigned the same route towards the second edge node.Packet stream: A packet stream is a data stream where the data units are data packets; generally of variable and arbitrary sizes.Burst stream: A burst stream is a data stream in which data units are aggregated into data bursts. Where distinction is not required, the terms data stream, packet stream, and burst stream may be used interchangeably.Traffic signal: A signal containing information generated by a source to be sent to a sink is a traffic signal. A traffic signal may have a fixed size or a variable size.Control signal: A signal containing control information generated by a switching node in a network is called a control signal.Flow rate: The mean rate, usually expressed in bits per second, of a data stream of any data format is the flow rate of the data stream.Time counter: A time counter is a device that measures time. A digital watch is a time counter. The time counter used hereinafter is a conventional clock-driven cyclical counter having a specified wordlength (32 bits, for example).Time Locking: It is a technique for time coordination using time-counters to enable time alignment of signals received at connecting nodes. A first node is time-locked to a second node if a signal transmitted at an instant of time indicated by a time counter at the first node arrives at the second node at the same instant of time as indicated by an identical time counter at the second node. When a first node is time-locked to a second node along a given path, the given path is said to be time-locked. The path may include multiple wavelength channels in which case, due to dispersion, each of the channels may be time locked separately.Time reference: A first time counter provides a time reference to several other time counters if each of the other time counters is time-locked to the first time counter. Hereinafter, a reading of a time counter collocated with a bufferless switch (such as an optical switch) may serve as a time reference for several time counters associated with electronic edge nodes.Temporal degree of freedom: The ability to select an arbitrary interval in the time domain to transfer a signal is a temporal degree of freedom.Spatial degree of freedom: The ability to select an arbitrary channel within a channel band to transfer a signal is a spatial degree of freedom.Temporal switching: A process of finding a time interval during which a specific input channel and a specific output channel are free and subsequently connecting the specific input channel to the specific output channel during the sought time interval is called “temporal switching”. The name reflects the fact that the only degree of freedom is the time-domain search. Temporal switching may be characterized as coherent switching.Spatial switching: A process of finding an output channel, within a bundle of output channels, during which a specific input channel and the sought output channel are free during a specified interval of time, and subsequently connecting the specific input channel to the sought output channel during the specified time interval is called “spatial switching”. The name reflects the fact that the only degree of freedom is the output-channel search.Spectral switching: Spatial switching from an input port of an optical switch to any wavelength channel in an output WDM link is also called spectral switching. Spectral switching is a form of spatial switching.Channel switching: Channel switching establishes a connection from an input port to an output port of a switch and holds the connection for an extended period of time.Burst switching: Burst switching establishes a connection from an input port to an output port of a switch and holds the connection for a predetermined duration of the burst.Single-channel data stream: A data stream transmitted along a path from a first edge node to a second edge node where successive data units (bursts for example) of the data stream are confined to a designated single channel along each hop between successive nodes along the path is called a single-channel data stream. The single channel may be centered on different wavelengths in different hops. Temporally-switched bursts form a single-channel data stream.Multiple-channel data stream: A data stream transmitted along a path from a first edge node to a second edge node where successive data units (bursts for example) of the data stream may occupy different channels in any hop between successive nodes along the path is called a multi-channel data stream. Spatially-switched bursts may form a multiple-channel data stream.Wavelength router: A wavelength router is an optical device, well-known in the art, which has a set of wavelength-division multiplexed (WDM) input links and a set of WDM output links, and which connects wavelength channels received at an input link to corresponding output links. The connection pattern is static; typically based on a spatial cyclic mapping of input wavelengths to output port numbers.
Spatial Versus Temporal Switching

FIG. 1-Aillustrates a connection established according to a spatial switching process in a space switch110A having input ports122connecting to output ports142through a bufferless time-shared fabric112A. Each input port122may receive input signals from an input channel120and each output port may transmit output signals over an output channel140connecting to an output port142. The output channels140are routed in channel bands150where all the channels of each channel band are directed to a subsequent switching node. The input signals are switched instantly because of the absence of input buffers in space switch110A. As defined earlier, a process of establishing a connection from an input port122to any output port142in a specific group of output ports during a specified time interval is spatial switching process. A signal arriving at an input port122is either switched immediately to an appropriate output port142or discarded. The signal cannot be queued because the input port122does not have a signal buffer. The appropriate output port is determined by a spatial scheduler (not illustrated inFIG. 1-A).

FIG. 1-Billustrates a connection established according to a temporal-switching process in a space switch110B comprising input ports162, a switch fabric112B, and output ports182. Each input port162is provided with an input buffer164(only one is illustrated) and may receive input signals from an input channel160. Each output port182may transmit output signals over an output channel180. The output channels180may be routed individually to different adjacent nodes. As defined above, a process of establishing a connection from a specific input port162to a specific output port182during any selectable time interval is a temporal switching process (coherent switching process). An input signal arriving at an input port162may wait in a buffer164until a pre-selected output port182becomes free. The instant of time at which the input signal is switched is determined by a temporal scheduler (not illustrated inFIG. 1-B).FIG. 1-Billustrates a connection to an output port182connecting to an output wavelength channel of central wavelength λ9.

FIG. 2illustrates the basic processes of spatial and temporal switching. The figure illustrates an output-channel band250comprising eight channels240. In a spatial-switching process, the eight channels240, labeled Coto C7, are routed together to a subsequent switching node, and a burst routed to the subsequent switching node, which may be an intermediate switching node or the destination edge node, may be switched to any of the eight output channels240. In a temporal-switching process, the eight channels240may, however, be routed individually towards up to eight adjacent nodes. Individual-channel routing may be realized using channel routers as will be described with reference toFIGS. 34 and 35. If the eight channels240are routed individually to adjacent nodes, then an input burst would be switched to a particular output channel leading to a specific destination.

As indicated inFIG. 2, a request to transmit a forthcoming burst to a specific destination is received at an input port of a switching node at time t0. The request indicates a duration, D, of the forthcoming burst. Additionally, if the receiving input port is bufferless, the request specifies an instant of time t1at which the forthcoming burst associated with the request is to arrive at the switching node. A spatial scheduler associated with the switching node examines the occupancy states of the eight channels240, determines that channel c6, for example, is free during the time interval {t1, t1+D} and allocates channel C6for this time interval. If the switching node has input buffers and if the specific destination is reached through a particular output channel (channel C4in this example) a temporal scheduler associated with the switching node attempts to allocate an interval of time during which the particular output channel is free. In this example, the temporal scheduler allocates channel c4for the period {t2, t2+D}.

The probability of finding a specific output port to be in a busy state at the instant of a new burst arrival (instant t1inFIG. 2) may be quite high even at a moderate occupancy. For example, if the mean occupancy of an output port is only 0.5 and under the assumption that the arrival process is independent of (uncorrelated to) the state of any of the output channels, then the probability that the new burst finds a specific channel occupied with (or reserved for) a prior burst, i.e., the probability of rejecting the new burst, is approximately 0.5. Such a high rejection probability cannot be tolerated. The incoming burst, however, may experience a very low probability of rejection, under low occupancy, if it can select any channel from a channel band having a sufficiently large number of channels connecting a current node to a subsequent node towards destination. For example, if a current node has a 32-channel link to a subsequent node, the probability that all the 32 channels are busy, when the mean occupancy is 0.5, is negligibly small. Under pure-randomness conditions, this probability would be of the order of 10−5. As illustrated inFIG. 1-A, a burst arriving at an un-buffered input port may select one of many candidate output ports142each connecting to a wavelength channel140.

FIG. 3illustrates spatial switching to an output channel band350having six wavelength channels340occupying wavelength bands centered on wavelengths λ0, λ1, λ2, λ3, λ4, and λ5. The wavelength channels340are labeled λ0, λ1, λ2, λ3, λ4, and λ5. Five time-limited signals (bursts) of arbitrary durations and labeled a, b, c, d, and e (referenced as311,312,313,314, and315, respectively) arrive at input ports on wavelength channels having central wavelengths of λ4, λ1, λ2, λ5, and λ4, respectively. The signals are received during overlapping time intervals at respective input ports122of the bufferless switch110-A ofFIG. 1-A. The signals are switched to output channels340of an output channel band350leading to a subsequent node determined according to a route-selection process.FIG. 3illustrates time intervals321,322,323, and324, in wavelength channels centered on wavelengths λ1, λ2, λ3, and λ4, which were reserved for previous bursts. Burst ‘a’ (311) of duration Dais expected to arrive at time taover a channel of wavelength λ4and only output channels of wavelengths λ0and λ5were determined to be available during the interval {ta, ta+Da}. In the illustrated example, the switch scheduler arbitrarily selected the output channel of wavelength λ0. A spectral translation process shifts the input channel band centered around λ4to a channel band centered around λ0. Burst ‘b’ (312) is received over wavelength λ1but had to be shifted to another wavelength λ5because the λ1output channel was reserved for burst321at the instant of arrival of burst ‘b’. Burst ‘c’ (313) is switched without spectral translation because wavelength channel λ2is determined to be free, burst ‘d’ (314), arriving over a λ5-input-channel is switched to a λ4-output-channel, and burst ‘e’ (315), arriving over a λ4-input-channel is switched to a λ3-output channel. Thus, in this example, bursts a, b, d, and e had to undergo spectral translations.

FIG. 4further illustrates temporal switching where the same signals labeled ‘a’ to ‘e’ ofFIG. 3are received during overlapping time intervals at respective input ports162of the bufferless switch110-B ofFIG. 1-Band are switched to a specific output channel482(corresponding to one of output channels182ofFIG. 1-B) leading to a subsequent node determined according to a route-selection process. The signals are appropriately delayed at input buffers of respective input ports162to enable conflict-free switching to the specific output channel482.

FIG. 5further illustrates spatial and temporal switching. A new burst512arriving over an input channel may be scheduled to be switched to a specific output channel540(of wavelength λ0in this example) after an appropriate waiting period, or may be switched instantly to any one of free output channels540in a channel band550having sixteen wavelength channels occupying spectral bands centered around wavelengths λ0to λ15. A temporal scheduler selected an interval starting at time τ in a specified output channel540centered on wavelength λ0. A spatial scheduler determined, in this example, that channels having central wavelengths λ4, λ7, λ9, and λ14are free and the channel of central wavelength λ7is selected. Control signals and load signals may be interleaved in a single channel when temporal switching is used as described in Applicant's U.S. patent application Ser. No. 10/054,509, filed on Nov. 13, 2001 and titled “Time-Coordination in a Burst-Switching Network”, the specification of which is incorporated herein by reference. With spatial switching, a selected channel, herein called a control channel, in a channel band connecting two nodes may be dedicated to carry control signals.

Although the example ofFIG. 5illustrates instantaneous spatial switching, deferred switching for a fixed period of time called ‘offset time’ (the period {t1-t0} inFIG. 2) would be needed to allow a spatial scheduler sufficient time to allocate a free output channel540. Deferred spatial switching with a constant delay for each burst yields the same overall blocking probability as instantaneous spatial switching. However, the blocking probability for different burst streams may differ if a burst is given a stream-dependent offset time.

Network Diameter Versus Nodal Degree

FIG. 6illustrates a known relationship between network diameter and nodal degree in a hypothetical network. The network diameter is an indication of the number of intermediate nodes traversed by a path from a source edge node to a sink edge node. A nodal degree (also called node adjacency) of a specific node is an indication of a number of other switching nodes to which the specific node directly connects. As the nodal degree increases, between one and128in this example, the network diameter decreases rapidly. An increase in network diameter increases network complexity and worsens network performance. A network diameter may be defined in terms of a traffic weighted mean number of hops per route for selected routes. The minimum value of a network diameter is one, which corresponds to a hypothetical fully meshed network providing a direct route of sufficient capacity for each pair of originating and terminating nodes.

The relationship between nodal degree and network diameter may be illustrated using a canonical network structure. Consider, for example, a network core having a classical multi-stage structure which has a predetermined number of hops for each input-output pair. Using core switches each having n links, a ν-stage network, where ν is an odd number, would have a number of input links equal to n(ν+1)/2, with an equal number of output links. With n=4, a network of relatively small coverage, having 4096 input links (and 4096 output links), requires an 11-stage structure (ν=11). A medium-capacity medium-coverage network having 65536 input links (and 65536 output links) requires a 15-stage structure. It is well known in the art that the efficiency of a network decreases with increasing the number of switching stages along each of the end-to-end paths. It is, therefore, an objective of the present invention to devise means for reducing the number of hops while allowing high network coverage despite the limitation of nodal-degree.

FIG. 7illustrates a network with large diameter. The network has numerous switching nodes each having a small degree, i.e., is adjacent to a small number of other nodes. A path from a source edge node710A to a sink edge node710B traverses 15 intermediate nodes720.

A time-shared network may comprise electronic edge nodes interconnected by a bufferless network core having interconnected fast-switching optical nodes. A path from a source edge node to destination edge node may traverse more than one optical node. Due to the absence of random-access buffers in an optical node, a signal received at each optical node along the path is switched instantly to either an output port connecting to a subsequent optical node along the path, or the destination edge node. Signals arriving at the input ports of an optical node and originating from several edge nodes, are not necessarily time coordinated, and two or more signals of different originating edge nodes may follow a common path from the optical node to a common destination. This may lead to output-port contention, at each node along the common path, which cannot be resolved due to the lack of buffers. To reduce the incidence of signal loss due to contention, each link in the network should have a sufficient number of channels to render the probability that all channels of a link being busy simultaneously negligibly small at the expected traffic load. Each optical node, therefore, should have a large number of dual ports (a dual port comprises an input port and an output port), where each dual port supports an incoming channel and an outgoing channel. For example, under the optimistic assumption of purely random traffic, a link having 64 channels would experience a blocking of 0.0001 when its mean occupancy is approximately 0.6. In order to reduce the mean number of hops from source to destination, the nodal degree, i.e., the number of adjacent nodes, should be sufficiently high. In a large scale network having several thousand edge nodes, the nodal degree is preferably of the order of 16 or so. With a requirement of 64 channels per link, the dimension of an optical node would be 1024×1024.

The number of links of a core switch is limited by the dimension of the node and the need to provide a sufficiently large number of channels per link to yield a low probability of signal loss given the absence of buffers at the core switches. The dimension of a fast-switching (low-latency) optical switch may be quite limited and each optical core node would have a small number of links connecting to edge nodes and to other core switches.

FIG. 8illustrates a network800having a fast-switching optical core840where edge nodes connect to the core through multi-channel links and the core is adapted to handle uncoordinated signal arrivals. Spatial burst switching in such a network has been extensively studied in several publications.

FIG. 9illustrates a network900where the optical core840ofFIG. 8is implemented as a partial-mesh940interconnecting a plurality of optical core switches930. The channels of each core switch930are divided into channel bands each band having a sufficiently large number of channels in order to facilitate spatial switching. Several routes may be found to connect an edge node910A to an edge-node910B. The number of channels of an optical core switch may be quite limited, thus leading to a small number of channel bands per core switch930. This, in turn, results in a network of large ‘diameter’, as described earlier with reference toFIG. 7, where the mean number of hops for core-switch pairs may be significantly large.

FIG. 10illustrates a path from a first edge-node910A to a second edge node910B in optical-core network900. The path traverses six core switches. Core switch j, 0≦j<6, has a processing latency of Δj. A burst ‘offset time’ at a core switch j, defined as the time difference between the arrival of a control signal and the arrival of an associated burst, should at least equal processing latency Δj.

Spatial Switching with ‘Offset Time’

When a time-limited signal (a burst) arrives at an input channel of an optical core switch, a controller of the optical core switch identifies a free channel in an outgoing multi-channel link along a selected route leading to a specified signal destination. Failing to find a free channel, the signal would be discarded due to the absence of storage capability at the optical core switch. To account for processing delay at the controller, a time-limited signal may be preceded by a header containing specifics of the signal. The header may be sent ahead of the signal itself by an interval of time, often called an “offset time interval” as mentioned earlier, to allow the controller a sufficient time to select a channel in an appropriate outgoing link. In order to permit reasonable link utilization (occupancy), given a specified signal-loss-probability tolerance, the number of channels per link should be sufficiently high; 64 for example. With a 256×256 optical node, the number of input links would be four and the number of output links would be four. This limits the adjacency of the core switch to four, i.e., the core switch may directly connect to at most four other switching nodes. A core switch may connect to edge nodes and other core switches or it may connect exclusively to other core switches. With a network having several-thousand edge nodes, a path between two edge nodes may have to traverse several core switches, thus resulting in low network efficiency in addition to low performance. The signal-loss probability naturally increases with the increase of the number of traversed core switches.

The offset time interval associated with a burst in a spatial-burst-switching network is selected at the electronic edge node that originates the burst. The value of the offset time may decrease as the burst and its header traverse bufferless switching nodes along the path to destination. The burst itself propagates along the path as a modulated optical carrier and experiences only propagation delay, with no processing delay at any intermediate switch. However, the burst may undergo spectral translation at any of the optical nodes it traverses. The header undergoes optical-to-electrical conversion at an optical core switch and is processed electronically to determine a destination and other attributes of the burst. The header may be modified, at least to indicate to a subsequent optical core switch, along the path to destination, the wavelength channel on which the current node has sent the burst. The header is then retransmitted, after electronic-to-optical conversion, to the subsequent switch. The offset time is then reduced by the value of the electronic processing time at the current switch. In an alternative control scheme, according to an embodiment of the present invention, the optical control signals, containing burst headers, received over the control wavelength channel are split at each optical switch along the path. The headers are retransmitted without delay to a subsequent switch, if any, and a copy of each of the optical control signals is directed to the controller of the optical switch for processing.

Header Content in a Spatially-Switched Burst

In a network employing spatial switching, a burst header is transmitted over a dedicated control channel within a multi-channel link while the burst itself (the load burst) may be transmitted over any channel in the multi-channel link. A burst header contains invariant information about the burst duration, its destination, and other attributes that do not change as the burst traverses intermediate switching nodes en route. The invariant information is also referenced herein as ‘state-independent information’ because the information remains unchanged as the burst traverses the intermediate switching nodes regardless of the allocated output channel at each intermediate switching nodes. A burst header also contains essential hop-dependent (hence variable) information, mainly an identifier of a channel in the multi-channel link over which the burst is transmitted by an immediately preceding switching node and the time-lag between the header and the forthcoming burst. As will be described below, the invariant information and the hop-dependent variable information of a header may be communicated independently and during different time intervals.

Header Processing in Spatial Switching

The propagation of a header along the path ofFIG. 10is illustrated for two header-management schemes. Edge node910A transmits the header τ1seconds ahead of the load signal, i.e., the ‘burst offset interval’ mentioned earlier is τ1seconds at a first intermediate switching node (node-0).

In a prior art header-propagation scheme (illustrated by line1120), each core switch may forward the header to a subsequent node along a selected path only when a channel to the subsequent node is successfully allocated. The cumulative processing delay along the path is the sum of Δ0to Δ5. Consequently, the temporal separation between the header and the load signal diminishes along the path and the processing-time allowance shrinks gradually from its initial value, τ1, as illustrated inFIG. 11, with the last core switch having the smallest offset time τ* determined as τ*=τ1-Δ0-Δ1-Δ2-Δ3-Δ4.

A sufficient processing-time allowance, at least equal to the sum of the processing delays in the optical core switches along the path (the sum of Δ0to Δ5in the example ofFIG. 10) is therefore required at the first core switch. If each core switch along a path requires 20 microseconds, for example, to schedule and set an internal connection, then the first core switch along a path of 12 hops, for example, must be given a burst-offset interval of at least 240 microseconds. The edge node transmitting the burst should therefore have a large storage capacity to hold each burst until it is transmitted when its offset interval expires. A method, according to the present invention, for reducing the burst delay and consequently reducing the required storage at the edge node is to send a burst header while the burst is still under formation. The header would then contain an estimate of an upper bound of the size of the burst at the instant of transmitting the burst. A burst is formed by aggregating individual packets of a common data stream, where all the constituent packets of the data stream have the same destination edge node. The burst size would be limited by the upper bound indicated in the header. A burst may contain less data than indicated by its size upper bound resulting in a capacity waste. However, the waste may be reduced using biased estimates of expected burst sizes.

In an alternative header-propagation scheme, according to the present invention (illustrated by the dashed lines1140), a first header for each burst is transmitted by a source edge node and may continue to propagate along a predetermined path regardless of the outcome of scheduling processes at the traversed nodes. The initial offset time interval at the source edge node is set at τ2seconds. If the transmission is successful at each intermediate switch, the separation between the first header and the signal would always be τ2seconds at the input of each subsequent node. This scheme is attractive because each node has the same offset time, hence the offset time need not be excessive and the storage requirement at the transmitting edge nodes may be reduced. Each intermediate node along a designated path may reserve a channel within a multi-channel link leading to a subsequent node to accommodate a forthcoming burst. However, an intermediate node would not be aware of the output channel selected by a previous node. A second header may then be sent by the source edge node and by each intermediate node along the path to indicate an identifier of the selected channel to the subsequent node. The second header would be sent shortly before the burst is transmitted by a time interval, δ, just sufficient for the receiving node to set a connection from the input port receiving the second header to an output port scheduled based on the first header. Thus, the second header is sent after the first header by a period of τ2-δ. It is noted that the process of finding an output channel that is free during the required time interval may be more time consuming than the process of directing an input channel to the selected output channel, i.e., Δj>>δ, Δjbeing the required scheduling time at a node j, as defined earlier with reference toFIG. 10.

If an intermediate node fails to find a free output channel at the desired time interval, the intermediate node may either not send a second header or, preferably, send a second header that includes an indication of burst loss. A null entry in place of an identifier of a selected channel may be used. The subsequent node may then cancel any output channel reservation made on the basis of information of a corresponding first header in order to reduce potential capacity waste resulting from blocking at the preceding intermediate node. A node may mark a reserved output channel as free and available for other requests if it does not receive a second header at the anticipated time or if a second header indicating burst rejection is received from a preceding node. In a properly provisioned network, capacity waste due to blocking would be negligible.

It may be desirable to use the above double-header scheme, where a first header containing invariant information is sent first then followed by a second header carrying variant information, under normal traffic-load conditions and use the prior-art single-header method described above under heavy traffic load, however detected.

The following steps are executed by a controller of a spatial-switching node regardless of the control scheme used:(1) determining burst destination and identifying a channel band leading to the destination according to a routing rule;(2) identifying an incoming channel carrying a forthcoming burst;(3) reserving a channel in the channel band for a period at least equal to the duration of the burst and starting before the arrival instant of the burst;(4) selecting an internal path within the switching node;(5) transmitting invariant burst information (duration, destination, etc.) to a subsequent switching node along the route;(6) transmitting variable burst information (channel identifier, time-lag of succeeding load burst, etc.); and(7) transmitting the load burst when it is received from the incoming channel.

FIG. 11illustrates the two schemes that may be considered in managing the headers, hence controlling the offset-interval.

The first scheme is the prior art scheme outlined above, where a single header is used and the header is transmitted from a core switch only if a channel to a subsequent switching node along the path (the subsequent node being either another optical core switch or the destination edge node910B) is successfully allocated for a forthcoming burst. As illustrated onFIG. 11, node-0transmits a full header including the invariant and variable burst information to node-1along the route to destination edge node910B, thus allowing node-1to execute steps 1 and 2 (1121and1122) described above. Node-1has all the information needed to execute steps 3 and 4 described above. Step 3 (1123) allocates a path to node-2, i.e., reserves a specified time-interval on a selected outgoing channel in a channel band leading to subsequent node-2, and step 4 (1124) establishes an internal path from the incoming channel to the selected outgoing channel. The time interval is determined from the burst duration (part of the invariant information) and the offset time interval (part of the variable information). Steps 3 and 4 (1123and1124) together are executed in Δ1seconds. In steps 5 and 6 (1125and1126respectively), node-1transmits to node-2the invariant header information and the variable header information which includes an identifier of the selected outgoing channel and the value of the offset time reduced by Δ1. Node-1then switches the burst along the internal path to node-2(step 7,1127).

In the alternative scheme outlined above, according to the present invention, the header is divided into a first header containing the invariant burst information and a second header containing the variable header information. The scheme is advantageous for paths traversing more than one intermediate switching node where the accumulation of processing delays reduces the offset time. The first header travels along a multi-hop path regardless of the result of channel-allocation at each traversed core switch. As illustrated inFIG. 11, node-1transmits a first header to node-2, thus enabling node-2to execute step 1 (1131), and node-2instantly transmits the same first header to node-3(step 5,1135). Node-2then attempts to reserve an output channel in a channel band directed to node-3(step 3,1133). If successful, node-2includes an identifier of the reserved channel in a second header to be sent to node-3(step 6,1136). Meanwhile, node-2receives an identifier of the channel selected by node-1to carry the burst to node-2(step-2,1132) and uses this information, together with the outcome of step 3 (1133) to establish an internal connection (step 4,1134). The burst is then switched along the internal path (step 7,1137). Thus, the order of executing the above seven steps differs significantly in the two schemes. If node-2fails to reserve an output channel, the identifier of the reserved channel, included in the second header, may be replaced by a null entry (an invalid identifier).

In spatial switching, a burst may be transmitted over any wavelength channel (a load channel) in a multi-channel link but a header of the burst is transmitted over a dedicated wavelength channel (control channel) in the multi-channel link. Due to the dependence of the propagation speed on the spectral band occupied by a wavelength channel in a fiber link, the temporal separation between an optical burst and its header at the receiving end of the multi-channel link may differ appreciably from the temporal separation at the sending end. The separation difference increases with both the length of the multi-channel link and with the spectral separation of the ‘load channel’ and the control channel. Therefore a guard time may be needed between successive bursts and a means for determining the propagation-delay differential along a link may be used to reduce the guard time.

Traffic Efficiency

FIG. 12illustrates a known dependence of the mean occupancy of a link on the number of channels in the link at a given specified service-quality index defined in terms of signal-loss probability under given traffic characteristics. For example, if an output channel band directed to a subsequent node has 16 channels to be used for spatial switching, the mean occupancy of the channel band has to be kept below 0.275 to realize a low burst-loss probability of 0.00001. The corresponding permissible occupancies if the channel band has 32, 64, 128, or 256 channels would be 0.428, 0.569, 0.684, and 0.776, respectively. These occupancies are based on the assumption of traffic pure randomness and would change for different traffic characteristics. Thus, as the number of channels per link increases, the link's traffic capacity may increase appreciably; the traffic capacity being the permissible mean occupancy of the link under a specified service-quality index. However, as the number of links per core switch decreases the mean number of hops increases, as illustrated inFIG. 6. The network efficiency may be determined as the ratio of the capacity of the access ports to the total capacity of all ports including the access ports the inner ports of the network. The link efficiency increases with increased number of channels per link and the network efficiency decreases with increased number of hops.

FIG. 13illustrates the combined effect of increasing link efficiency and increasing number of hops as the adjacency (also called ‘degree’) of a core switch decreases, the core switch having a predetermined dimension and capacity. Herein, the term ‘link’ generally refers to any channel band in a fiber transmission line. The core switch supports 256 channels that may arranged in 128 links of two channels each, 64 links of 4 channels each, 32 links of 8 channels each, and so on. The adjacency is determined by the dimension of the core switch. Given a limited number of dual channels in an optical core switch (a dual channel includes an input channel and an output channel), the larger the number of channels per link the smaller the number of links, hence the smaller the adjacency. The larger the number of channels per link the higher the link efficiency and the larger the network diameter. A large network diameter results in low network efficiency and performance degradation. Line1310illustrates the reduced link efficiency with the decrease of the number of channels per link, the link efficiency being the link occupancy at a specified blocking probability (burst-loss probability). Line1320illustrates the reduced mean number of hops with the decrease of the number of channels per link in a hypothetical network using identical core switches each of dimension 256×256. Line1330illustrates the variation of network traffic capacity with different channel-band sizes (link sizes) in the hypothetical network. The number of channels per link may be selected to maximize the network efficiency as indicated by line1340.

The connectivity pattern of input channels and output channels of an optical switch (generally any bufferless switch) to adjacent nodes depends on the extent of temporal and spatial switching. In one extreme, all paths may be time locked and only loss-free temporal switching need be used, as will be described with reference toFIG. 14. The ports of the optical switch may then connect to individually routed channels. In another extreme, none of the paths may be time locked and only spatial switching is used as illustrated in the prior-art networks ofFIGS. 8 and 9. The ports of the optical node would then be arranged to connect to channel bands with each channel band directed to the same adjacent node.

Temporal Switching in a Composite-Star Network

If the path from one edge node to another traverses only one optical node, the path can be time locked and, hence time-limited signals (signal bursts) may be transferred along the path. An example of a network proving time-locked paths is the composite-star network described in Applicant's U.S. patent application Ser. No. 10/180,050, filed on Jun. 27, 2002 and titled “High-Capacity Optical Node”, the specification of which is incorporated herein by reference. The composite-star network comprises a multiplicity of electronic edge nodes interconnecting through independent optical nodes, where the optical nodes are not connected to each other. An optical node may comprise several optical-switch planes. Each edge node is time-locked to each optical node to which it is connected in order to enable temporal switching. Edge nodes of different capacities and dimensions, and optical nodes of different capacities and dimensions may be used. Time-locking enables scheduling the transfer of time-limited signals, of any form, from one edge node to another through the optical core, despite the unavailability of buffers at the optical nodes. The composite-star structure may be used for loss-free burst switching as described in the aforementioned U.S. patent application Ser. No. 09/750,071.

The capacity of a composite-star network can grow to several petabits per second. The number of edge nodes is determined by the dimensions of the optical nodes and the number of optical nodes is determined by the dimensions of the edge nodes. Therefore, a geographic coverage of a continental scale is realizable. A larger number of core switches, each having fewer switch planes, may be realized using wavelength routers as will be described with reference toFIGS. 34 and 35.

FIG. 14illustrates a prior-art composite-star network1400described in the aforementioned Applicant's U.S. patent application Ser. No. 10/180,050. The network structure permits time-locked paths, and therefore, temporal switching, for each edge-node pair. The number of edge nodes1410in the composite-star network is determined by the feasible dimension of a core switch. This would be of the order of 256 to 1024 edge nodes. Each core switch1430may be configured as parallel optical switches. Each edge node1410may have a capacity of several terabits per second and the total capacity of network1400may be in the order of several petabits per second.

Optical Node Using Spatial and Temporal Switching

FIG. 15illustrates a space switch1500, according to the present invention, having several input ports1522, a switch fabric1510, and several output ports1542where at least one input port is provided with an input buffer1564. Such a buffer may be realized electronically, thus requiring optical-to-electrical and electrical-to-optical conversions. Two internal paths in the space switch are illustrated, one path1552established according to a spatial-switching process (as illustrated inFIG. 1-A) and the other path1554established according to a temporal-switching process (as illustrated inFIG. 1-B). The first path1552relates to a signal arriving at an input port1522that does not have a signal buffer. The signal may be switched immediately to an appropriate output channel1540of a channel band or be discarded. The second path1554relates to a signal arriving at an input port1522provided with a buffer1564. The signal can be scheduled for switching to a specific output port1542when the specific output port1542becomes free.

FIG. 16illustrates an optical switch1600having a bufferless switch fabric1610and bufferless input ports. Some input ports1622receive signals from distant edge nodes, each of the distant edge nodes having a buffer1664. Through a time-locking process, the distant edge nodes can be made to appear as if they were collocated with the optical switch1600. A time-locking process is described in U.S. Pat. No. 6,570,872, issued on May 27, 2003 to Beshai et al. and titled “Self-configuring distributed switch”, the specification of which is incorporated herein by reference. Thus, each signal can be switched to a specific output port1642in switch1600in a manner similar to that of the buffered signal inFIG. 15.

Consider the five time-limited signals (bursts) labeled a, b, c, d, and e, used inFIGS. 3 and 4. The signals arrive at input ports1622of the bufferless switch1600on wavelength channels having central wavelengths of λ4, λ1, λ2, λ5, and λ4. The signals originate from edge nodes each having a signal buffer1664. The signals are appropriately delayed at their originating edge nodes to avoid temporal overlapping at the input side of the bufferless switch1600. Applying appropriate delays at the edge nodes, enabled by time locking the edge-nodes buffers1664to the bufferless switch fabric1610, results in non-overlapping arrival at the respective input ports1622of the optical switch1600so that the signals can be successively switched to a designated output port1642.

FIG. 17illustrates the use of combined temporal and spatial switching, in accordance with an embodiment of the present invention, to reduce the mean number of hops and, hence, extend the network coverage and capacity upper bound. A time-shared network exclusively using temporal switching is suitable for intermediate coverage of the order of 256 to 1024 edge nodes as described above with reference to the composite-star network ofFIG. 14. A time-shared network exclusively using spatial switching has coverage limitations due to the limited nodal degree. A network according to the present invention, conceptually illustrated inFIG. 17, uses temporal switching to reach a core switch that is topologically close (having a small number of hops) to a specified destination edge node. The signals are then transferred from the core switch to the destination node under spatial switching. Thus, bursts may be transferred over a path under both temporal and spatial switching (path1720). Edge nodes subtending to the same optical node have the privilege of exchanging signals directly under temporal switching (paths1722and1724).

FIG. 18is a conceptual representation of a network having edge nodes, bufferless shell switches, and bufferless core switches. The shell switches are optical core switches that receive signals directly from edge nodes. The use of intermediate shell switches provides shorter paths for a subset of edge-node pairs and helps in extending the use of loss-free temporal switching. A subset of edge nodes may exchange signals through the shell switches using temporal switching, as in path1810. In general, edge nodes may exchange signals through temporal switching in the shell switches followed by spatial switching in the core switches, as in path1820.

Asymmetrical Core Access

FIG. 19illustrates a network1900realizing the concept ofFIG. 17. An edge node1910may connect to each of several fast core switches1930of an optical core1940in the upstream direction through a small number, possibly one, of wavelength channels. Each edge node1910has signal buffers and may, therefore, time-lock to each core switch1930to which it is connected and use temporal switching of signals over a single channel. The optical core1940, however, may not be able to maintain time-locking for connections among core switches or in the downstream direction towards the edge nodes. Thus, a core switch may not be able to temporally schedule signals arriving from two or more other core switches. Signals are then switched using spatial switching, which requires that a respective switching node have a channel band connecting to a subsequent node along the route to destination. An edge node1910, therefore, receives signals from the optical core1940through a channel band having a sufficient number of channels to render the probability of burst loss negligibly small under specified load conditions. For example, an edge node may have 32 upstream time-locked channels connecting to 32 core switches1930and 32 downstream channels received from a single core switch1930to permit downstream spatial switching at negligible burst-loss probability.

FIG. 20illustrates a network2000further illustrating fast-switching optical core1940ofFIG. 19. The fast-switching optical core includes fast optical switches2030. An edge node2010has individually routed output channels2012connecting to different optical switches. However, an edge node2010receives signals from the core through channel bands2014each originating from a single optical core switch2030. The optical core switches2030of the network are labeled as2030A1to2030A18. An edge node2010A has individual upstream channels2012to core switches2030A2,2030A3,2030A10,2030A14, and2030A17and a downstream multi-channel link2014from core switch2030A1. An edge node2010B has individual upstream channels to nodes2030A2,2030A3,2030A4,2030A11, and2030A12, and a downstream multi-channel link2014from core switch2030A13. A path from edge node2010A to edge node2010D may traverse only nodes2030A17and2030A6. The selection of core switches2030to which a source edge node2010connects may be based on an estimate of the distribution of the traffic volume from the source edge node2010to destination edge nodes.

An edge node2010time locks to each optical core switch2030to which it connects in the upstream direction. Edge nodes2010subtending to a common core switch2030may communicate using temporal switching through the common core switch while edge nodes that do not connect to a common core switch may connect through temporal and spatial switching. For example, a path from edge node2010A to2010D inFIG. 20may be effected through a time-locked path to core switch2030A6then through spatial switching from core switch2020A6to destination edge node2010D. Upstream channel2012X from edge node2010A is time locked to core switches2030A17. Core switch2030A17is then able to schedule bursts received from upstream channel2012X for loss-free switching to a channel from core switch2030A17to core switch2030A6from which the bursts may be spatially switched to channels in channel band2014D to the destination edge node2010D.

FIG. 21further extends the structure ofFIG. 19. A network2100has edge nodes2110connecting to fast-switching optical nodes2120called ‘shell switches’. The shell switches2120may be interconnected through fast-switching core switches2130collectively forming a fast-switching optical core2140. The shell switches2120and core switches2130may be structurally similar, and both may be bufferless. The main difference is in their connectivity and their role in the overall network2100. The shell switches2120may directly interconnect edge nodes2110through time-locked connections, i.e., through temporal switching. The shell switches2120may also aggregate traffic to be switched in core switches2130towards topologically-distant destination edge nodes2110. A shell switch2120preferably has several upstream channels2122that may be individually routed to several core switches2130and downstream channels that may be individually routed to several edge nodes. For example, a shell switch may have 128 upstream channels connecting to a number of core switches not exceeding 128 and 32 downstream channels connecting to at most 32 edge nodes.

Each edge node preferably has many output channels individually routed to shell switches and core switches. Shell-switch connectivity will be described with reference toFIG. 29.

A shell switch2120interfaces with edge nodes2110. The output channels2112of the edge node may be routed individually to other edge nodes, and through time-locked paths to shell switches2120, or to core switches2130. To receive signals from optical nodes, the inbound ports of the edge nodes may be arranged into groups, with each group receiving a channel band (a multi-channel link)2114from an optical switch2130. The optical switch2130applies spatial switching over the channel band2114to communicate signals to the edge node.

A core switch2130interfaces with shell switches2120and other code switches2130. Output channels2112of a shell switch2120may be routed individually to core switches2130. Thus, the input ports of a core switch may receive signals from independently routed channels as well as channel bands. However, all the output channels of a core switch2130are grouped into channel bands to enable spatial switching because the input channels of the core switch may not be time coordinated.

FIG. 22illustrates a network2200where the optical nodes are identified as shell switches2220A1to2220A10and core switches2230B1to2230B8. A shell switch may have several subtending edge nodes. A shell switch2220has an asymmetrical connectivity as in network2100. A shell switch transmits to other optical nodes through individually routed output channels2222. For example, shell switch2220A1has output channels connecting to core switches2230B2,2230B4,2230B5,2230B6, and2230B8. The selection of core switches to which a shell switch connects may be based on an estimate of the distribution of the aggregate traffic volume from subtending edge nodes to destination edge nodes.

FIG. 23illustrates a network2300that combines the asymmetrical edge-node connectivity ofFIG. 20and the asymmetrical shell-switch connectivity ofFIG. 22. An edge node2310may have output channels2312to several shell switches2320and core switches2330, and input channels arranged in a small number of channel bands2314each channel band originating from a single core switch2330. A shell switch2320may have output channels2322individually routed to several core switches2330and input channels2312received from edge nodes2130.

A bufferless core switch2030(FIG. 20) having input ports each supporting an input channel, a fast-switching fabric, and output ports each supporting an output channel, may receive individually routed time-locked input channels as well as input channel bands carrying temporally uncoordinated signals. An input channel band may also carry temporally-coordinated (time-locked) signals. A core controller associated with the bufferless node may then include a composite scheduler having a temporal scheduler, operable to allocate a path from a specific input port to a specific output port during any time interval of a specified duration, as well as a spatial scheduler operable to allocate a path from a specific input port to any output port from within a band of output ports during a specified time interval. The core controller includes a timing circuit for exchanging time locking signals with edge controllers associated with subtending edge nodes to enable temporal switching in the bufferless core switch. The time-locking signals generated by a core switch2030may be communicated to a subtending edge node through any path.

The spatial scheduler of a composite scheduler associated with a specific bufferless core switch2030is likely to have a limited interval of time (within an offset-time) to schedule a path within the bufferless core switch for a forthcoming (already propagating) burst while the temporal scheduler of the composite scheduler has the luxury of dictating the time at which a burst waiting in a buffer at an edge node arrives at the bufferless core switch. Therefore, according to one policy, the spatial scheduler of the composite scheduler may be given priority over the temporal scheduler.

A selector may be provided at the composite scheduler of a core controller to arbitrate between the temporal scheduler and the spatial scheduler when a specific output port designated for temporal switching is one of the output ports of the band of output ports used for spatial switching. Thus, when the core controller receives a first request to schedule the transfer of a burst, waiting in a buffer at an edge node, over a specific output channel and a second request to schedule an already propagating burst, the selector may either activate the temporal scheduler, if the first request has been waiting for a period of time exceeding a predetermined delay tolerance, or activate the spatial scheduler otherwise. A time-space channel-allocation map (basically a calendar) may be used to indicate channel availability (port availability).

FIG. 24illustrates an implementation of network1900ofFIG. 19. Network2400comprises a plurality of edge nodes2410interconnected by a network core2440. Each edge node2410has an upstream module and a downstream module (not illustrated) which may be integrated to share memory and control. Each edge node2410is represented by a block2410-1in the left side and a block2410-2in the right side ofFIG. 24for clarity. In other words, a block2410-1on the left side is paired with a block2410-2on the right side; both blocks forming the same edge node2410as will be further described with reference toFIG. 25which illustrates an asymmetrical edge node2410.

The network core2440comprises a number of core planes2460, each core plane2460interconnecting some or all of the edge nodes2410. A core plane2460comprises fast-switching core switches2430interconnected by multi-channel links2432. Each edge node2410receives traffic from a core switch2430in each core plane2460through a channel band2414. Consider a specific edge node2410. The upstream module of the specific edge node has input channels (ingress channels) receiving data from traffic sources (not illustrated) and outbound channels2411connecting to different core switches2430.FIG. 24illustrates individually routed outbound channels2411and individually routed inbound channels2412for receiving temporally-scheduled bursts from core nodes2430. In bound channels2412may be provided to enable direct temporal switching for edge nodes2410subtending to common core switches2430. The downstream module has output channels (egress channels) transmitting data to subtending traffic sinks (not illustrated) and inbound channels2414receiving traffic from the core planes2460. The inbound channels2414are preferably divided into channels bands, each channel band connecting to one core switch2430in one of the core planes2460.

Temporal switching, which observes a time-reference as described earlier, is feasible from an edge node2410to an output channel of any core switch2430to which the edge node connects. However, a core switch may also receive traffic over channel bands originating from different core switches which may have different time references. Thus, spatial switching which does not require inter-nodal time coordination is used within each core plane2460. Spatial switching requires a spatial degree of freedom, where any of outgoing channels may be used during a time-interval dictated by a previous node.

FIG. 25illustrates an electronic edge node2410ofFIG. 24having several input ports and output ports with asymmetrical input-side connectivity and output-side connectivity. The asymmetrical edge node2410has ingress channels2506for receiving data from data sources, egress channels2516for transmitting data to data sinks, inbound channels2412for receiving temporally switched signals from core nodes, inbound channels2508, arranged in inbound channel bands2414, for receiving spatially switched signals from optical switches (core switches), and outbound channels2411for transmitting signals to optical switches (core switches). The egress channels2516may individually connect to traffic sinks. Other channels may connect directly to other edge nodes2410, if so desired, but are not illustrated inFIG. 25.

The ingress channels2506are individually connecting to traffic sources but the (downstream) inbound channels are received in channel bands2414, each channel band originating from a core switch2430. The edge node egress channels2516are individually connecting to traffic sinks and the (upstream) outbound channels2411are individually routed to core switches2430. Because the edge nodes include buffers, the upstream outbound channels2411may be time locked to corresponding bufferless core switches2430. Time locking permits scheduling the transmission of signals over the outbound channels; each signal is timed to arrive at a corresponding optical core switch at an instant determined by a scheduler of the corresponding optical node. With spatial switching, an optical core switch may transmit a signal to an edge node2410through any inbound channel2508of a given channel band2414. Thus, successive signals belonging to a given data stream destined to an edge node2410may be transmitted to the edge node2410over different inbound channels2508of an inbound channel band2414(FIGS. 24 and 25). As described earlier, an optical node may resort to spatial switching when time reference is not maintained along an entire path. An edge controller2550, associated with edge node2410, may then schedule the signals of the same stream so that they may be delivered to data sinks in proper temporal order. The process of collating the signals (bursts) of each data stream will be described below with reference toFIG. 28.

Due to the availability of a buffer, temporal-switching may result in significantly higher channel efficiency in comparison with spatial switching. Hence, in an edge node2410, the input capacity may generally be higher than the output capacity as indicated inFIG. 25.

It is noted that block2410-1ofFIG. 24includes egress ports2506, inbound ports2412, egress ports2516, and outbound ports2411while block2410-2includes only inbound channel bands2414. The two blocks2410-1and2410-2are drawn separately to simplifyFIG. 24.

Edge controller2550(FIG. 25) of asymmetrical edge node2410schedules the transfer of packets received from ingress channels2506to outbound channels2411. An outbound port associated with each outbound channel2411may aggregate packets it receives from ingress channels2506into data bursts and formulate burst-transfer requests, each burst-transfer request specifying a destination and duration of a corresponding burst. The outbound port then sends each burst-transfer request over an outbound channel2411to a selected core switch2430. A controller of selected core switch2430determines a schedule for transmitting the burst associated with each request indicating a required transmission time of the burst, and communicates the schedule to edge controller2550. Controller2550receives the schedule, associates the schedule with a corresponding outbound channel, and communicates the indicated transmission time to the outbound port associated with the corresponding output channel2411. The outbound port transmits the corresponding burst at the indicated transmission time. Naturally, edge controller2550may also schedule internal paths from ingress channels2506to egress channels2516.

In an alternative mode, each packet is associated with a burst stream and each burst stream is allocated a flow rate. An outbound port associated with an outbound channel2411formulates a flow-rate-allocation request for each burst stream, each flow-rate-allocation request specifying a destination and a flow rate. The outbound port sends each flow-rate-allocation request to a selected core switch. The selected core switch formulates burst-transfer permits for each burst stream and communicates the permits to edge controller2550; each permit specifies a burst size and transmission time. Edge controller2550distributes the permits to respective outbound ports, and the outbound ports formulate bursts accordingly, and transmit each formed burst at the time instant indicated in a corresponding permit.

FIG. 26illustrates a connection of an edge node2410to a core switch2430in network2400(FIG. 24). The core switch2430may be asymmetrical, having more output ports than input ports. Recall that a core switch may receive traffic from subtending edge nodes through individually routed channels. The core switch, however, sends traffic to the subtending edge nodes through channel bands and also to other core switches through channel bands. Spatial switching with-low blocking probability requires that the channel bands be kept at a relatively low occupancy. Hence, more output capacity than input capacity is needed in the core switch. In contrast, as described earlier, the input capacity of an edge node exceeds its output capacity because the edge node receives traffic from core switches using spatial switching over low-occupancy channel bands.

In the configuration ofFIG. 26, the illustrated core switch2430may have 80 input channels possibly originating from 80 different edge nodes2410, and 96 output channels2628arranged into channel bands2414, for example two channel bands of 48 channels each, each channel band2414terminating in an edge node2410. The illustrated core switch2430also has 160 input channels2638arranged in channel bands2639each incoming from another core switch2430, and 160 output channels2658arranged in channel bands2659each terminating on another core switch2430. Channel bands2639and2659correspond to channel bands2432ofFIG. 24. The illustrated edge node2410has 200 ingress channels2606(from traffic sources), 200 egress channels2616(to traffic sinks), 200 output channels2412individually routed to core switches2430, and 240 inbound channels2608, grouped in channel bands2414each channel band2414emanating from a core switch2430. With channel bands of 60 channels each, for example, the inbound channels may receive traffic from four core switches2430.

FIG. 27illustrates an implementation of network2100ofFIG. 21. Network2700comprises a plurality of composite-star networks2702interconnected by a network core2740. Each composite star network2702has at least one shell switch2720connecting a set of edge nodes2710. Each edge node2710has an upstream module and a downstream module which may be integrated to share memory and control. Each edge node2710is represented by a block2710-1on the left side and a block2710-2on the right side ofFIG. 27for clarity. In other words, a block2710-1on the left side is paired with a block2710-2on the right side, both blocks forming the same edge node2710(to be further illustrated inFIG. 28). As will be described below, with reference toFIG. 28, an edge node2710may be asymmetrical. In a uniform composite-star network2702, the edge nodes2710are of equal dimension and the shell switches2720are of equal dimension. The number of edge nodes in a uniform composite-star network is determined by the dimension of each shell switch. For example if the dimension of a shell switch is 160×200, then the maximum number of edge nodes in a uniform composite-star network2702is 160. The number of shell switches2720is determined by the dimension of the edge nodes. If each edge node has a dimension of 450×400, for example, thus supporting 450 input channels and 400 output channels, then 200 of the output channels may be used to interconnect an edge node to at most 200 shell switches, and 250 of the input channels may receive traffic from shell switches2720and core switches2730. A composite-star network2702may, however, be non-uniform as described with reference toFIG. 14.

An edge node2710time locks to each shell switch2720to which it connects. Edge nodes2710subtending to a common shell switch2720may communicate through temporal switching at the common shell switch while edge nodes2710that do not connect to a common shell switch may connect through temporal switching followed by spatial switching. For example, a burst from edge node2710A to2710B (FIG. 27) may be transferred through a time-locked path traversing shell switch2720A towards core switch2730A. Core switch2730A then switches the burst over any available channel in a channel band2714leading to destination edge node2710B. Note that the two blocks inFIG. 27identified by the reference numerals2710B-1and2710B-2belong to the same edge node2710B.

To extend network coverage, each shell switch2720may also have output channels2722individually connecting to the network core2740. The network core2740may comprise a number of core planes2760; each core plane2760comprising fast-switching core switches2730interconnected by multi-channel links2732. The shell switches2720are organized into a plurality of shell-switch sets each set comprising at least one shell switch from each composite star network2702, and the shell switches of each set are interconnected through a core plane2760. Each edge node2710receives traffic from other native edge nodes2710of the same composite-star network2702through shell switches2720and receives traffic from edge nodes2710of other composite-star networks2702through the network core2740. Each edge node2710is preferably connected to a core switch2730in each core plane2760through a downstream channel band2714. Consider a specific edge node2710of a specific composite-star network2702. The upstream module of the specific edge node2710has input channels (ingress channels) receiving data from traffic sources and outbound channels2711connecting to different shell switches of the specific composite-star network2702. The downstream module has output channels (egress channels) transmitting data to subtending traffic sinks and input channels divided into two groups, a first group of inbound channels2712receiving traffic from the shell switches of the specific composite-star network2702and a second group of input channels receiving traffic from core planes2760. The second group of inbound channels is preferably divided into channels bands2714, each channel band2714connecting to one core switch in one of the core planes2760.

Temporal switching is feasible from an edge node2710to an output channel of any shell switch to which the edge node connects. However, a core switch2730receives traffic over channels originating from different shell switches2720which may have different time references. The time reference of a shell switch is determined by an associated time counter. The time counters of all shell nodes may be clocked at the same rate and have equal cyclical periods. However, the starting times of the cyclical periods may not be coordinated. The core switches2730may not, therefore, be able to maintain time coordination. Thus, spatial switching that requires no inter-nodal time coordination is used within each core plane.

Thus, the structure of network2700allows edge nodes2710of each composite star network2702to communicate through loss-free temporal switching. The structure also allows each shell switch2720to connect to multiple core switches2730in corresponding core planes2760, thus reducing the mean number of hops in the spatially switched core. Several structures may be used to realize a core plane2760, as illustrated inFIGS. 30-33below.

FIG. 28illustrates an electronic edge node2710(FIG. 27), having several input ports, several output ports, and a switching fabric2810, with asymmetrical input-side connectivity and output-side connectivity. Notably, edge node2710has three groups of input channels. One group includes ingress channels2806individually connecting to traffic sources. A second group includes inbound channels2712individually routed from shell switches to carry time-coordinated signals transmitted over time-locked paths. The third group includes downstream inbound channels2808arranged in channel bands2714, each channel band2714originating from a core switch2730. Edge node2710has two groups of output channels. One group includes egress channels2816individually connecting to traffic sinks and the other group includes upstream outbound channels2711individually routed to shell switches2720or core switches2730. Because the edge nodes2710include buffers, the upstream channels2711may be time locked and, hence, may employ temporal switching.

It is noted that block2710-1ofFIG. 27includes ingress ports2806, inbound ports2712, egress ports2816, and outbound ports2711while block2710-2includes only inbound channel bands2714. The two blocks2710-1and2710-2are drawn separately to simplifyFIG. 27.

In a network where each path from a source edge node to a destination edge node is time locked, any output port of the source edge node may individually connect to any bufferless switch. Such a network is described in the aforementioned U.S. patent application Ser. No. 10/180,050. The present disclosure covers a network where some paths between edge nodes may not be time locked and the network, therefore, employs a mixture of temporal switching and spatial switching to extend the network coverage while realizing high network utilization, given the absence of time coordination in the core.

Collating Inbound Signals at an Edge Node

The asymmetrical edge node2710is operable to time lock each of the outbound channels2711to a corresponding optical node. Time locking permits temporal scheduling the transmission of signals (bursts) over an outbound channel; each signal is timed to arrive at a corresponding optical node at an instant determined by a temporal scheduler of the corresponding optical node. As described earlier, an optical node may resort to spatial switching when time reference is not maintained along an entire path. An optical core switch may spatially switch a signal to an edge node through any inbound channel2808within a channel band2714. Thus, successive signals belonging to a given data stream destined to an edge node2710may be transmitted to the edge node2710over different inbound channels2808of an inbound channel band2714.

At a destination edge node2710, an inbound channel band2714occupying a WDM link is demultiplexed into its constituent wavelength channels2808and the optical signal carried by each channel2808is demodulated to extract its modulating digital data (the baseband signal). The digital data of each channel2808is presented to an inbound port for switching through the edge-node's switching fabric2810to destination egress ports. An inbound channel band2714may carry both temporally-switched bursts and spatially-switched (spectrally switched) bursts. Temporally-switched bursts of a specific data stream form a single-channel data stream terminating on a single inbound port of the destination edge node. Spatially-switched bursts are likely to form a multiple-channel data stream and successive bursts of a specific multiple-channel data stream may appear at different inbound ports of the destination edge node.

Bursts received at an inbound port of an edge node are placed in an input buffer and are switched to respective egress ports according to a schedule determined by an edge controller2850of the edge node. The edge controller2850determines a transfer time of each burst according to competing demands for read-access at the inbound port and write-access at the egress port. Only one burst can be transferred from an inbound port at a given instant and only one burst can be received by an egress port at any given instant. When there are several bursts waiting at an inbound port for transfer to at least one egress port, the edge controller2850schedules the waiting bursts for transfer over successive time intervals. When there are many bursts waiting at different inbound ports and destined to a specific egress port, the bursts will be scheduled for transfer to the specific egress ports over successive time intervals. Thus, the queueing delays of bursts of the same data stream (destined to the same egress port) but waiting at different inbound ports are influenced by uncoordinated arrival processes and the bursts of a multi-channel data stream may be received at the corresponding egress port in a temporal order which may differ from the temporal order in which the bursts were sent from their source edge node.

Two methods may be adopted in order to deliver bursts to their corresponding sinks in proper order, noting that each burst of a specific data stream naturally carries an identifier of the specific data stream and each burst is processed electronically at an edge node. Each inbound port of an edge node announces the arrival of a burst to an edge controller of the edge node by sending a scheduling request specifying the size and the data-stream identifier (or a destination) of each burst.

An obvious method to properly collate bursts is to assign a cyclical identifier to each burst in a data stream and sort the bursts accordingly at each egress port. This approach may incur additional delay at the egress ports. An alternative method, according to the present invention, schedules burst transfer at the receiving edge node according to a desired temporal order, as described below.

Edge controller2850, associated with edge node2710, collates the signals of a specific stream so that they may be delivered to data sinks in proper temporal order. This may be realized by using a common cyclic time reference (a time counter) for all inbound ports of the edge node2710and time stamping each inbound signal according to its arrival time. The time reference is preferably generated by the edge controller2850. The time reference is cyclical with a period significantly exceeding the maximum expected queueing delay at the inbound buffers of the edge node. A scheduler associated with the edge controller2850then schedules the transfer of the bursts to respective traffic sinks according to the time stamp. It is noted that bursts of the same stream are routed along the same path, traversing the same sequence of bufferless switches. However, the bursts may be sent to the destination edge node on different wavelength channels2808of a channel band2714. The duration of each burst is therefore required to exceed the maximum differential propagation delay along different wavelength channels of any end-to-end multi-channel path. For example, if the maximum differential propagation delay along two wavelength channels of a WDM path is 2 microseconds, the minimum burst duration may be specified to substantially exceed this value (8 microseconds for example) thus eliminating temporal out-of-order arrival of bursts at the destination edge node. Consequently, although successive signals (bursts) of a multi-channel stream may be received at different inbound ports of an edge node, they would not be received simultaneously or out of proper temporal order. However, this does not guarantee that the signals (bursts) will be delivered to the intended egress port in proper order because of the varying queueing delays at the inbound buffers as described earlier.

Although the process of collating spatially switched inbound signals is described above with reference to edge node2710, the description is applicable to edge node2410by replacing reference to temporally switched signals received from shell nodes2720by temporally switched signals, if any, received from core nodes2430.

FIG. 29illustrates a shell switch2720in network2700, and its connection to one of subtending edge nodes2710. The shell switch2720may be asymmetrical, having more output ports than input ports. Shell switch2720receives traffic only from subtending edge nodes2710. Shell switch2720, however, sends traffic to subtending edge nodes2710as well as to core switches2730of a core plane2760. A specific edge node2710in a specific composite-star network2702sends all its outbound traffic to shell switches2720of the specific composite-star network2702(FIG. 27) through individually routed channels2711, and receives inbound traffic from shell switches2720over individually routed channels2712and from core switches2730over channel bands2714. The specific edge node2710receives traffic originating from other native edge nodes2710through selected native shell switches2720using temporal switching but receives traffic originating from foreign edge nodes2710through core switches2730using spatial switching. While each edge node2710has an upstream channel2711to each native shell switch, an edge node may have downstream channels2712from only a subset of its native shell switches2720. The number of downstream channels2712from a specific shell switch2720to its subtending edge nodes2710may, therefore, be less than the number of upstream channels2711from the subtending edge nodes to the specific shell switch. It is noted that all outbound traffic from an edge node2710is directed to shell nodes2720while inbound traffic to the edge node is received from both shell nodes2720and core nodes2730.

In the configuration ofFIG. 29, shell switch2720has 160 input channels2711originating from up to 160 native edge nodes, 40 downstream output channels2712individually routed to up to 40 of the 160 native edge nodes, and 160 output channels2722individually routed to selected core switches2730. The illustrated edge node2710has 200 ingress channels2906(from traffic sources), 200 egress channels2916(to traffic sinks), 50 inbound channels2712from native shell switches, 200 outbound channels2711individually routed to native shell switches, and 200 inbound channels2908, grouped in channel bands2714emanating from core switches2730. With channel bands2714of 50 channels each, for example, the edge node2710may receive signals from four core switches2730in at most four core planes2760. As such, a composite-star network2702(FIG. 27) may have 200 shell switches2720and 160 edge nodes2710, each edge node2710having an ingress (or egress) capacity of 200 channels, thus realizing a composite-star network2702of access capacity (ingress or egress) of 32000 channels. With each channel having a capacity of 10 gigabits/second (Gb/s), for example, the total access capacity of the composite-star network2702would be 320 terabits per second (Tb/s). The entire network2700may comprise numerous composite star networks2702, thus realizing a total access capacity of several petabits per second. It is emphasized here that the actual capacity of a network is determined by the total capacity of its access channels (outer channels).

Implementations of core planes2460or2760are described below. The description relates to core plane2760but its adaptation to core plane2460is straightforward.

FIG. 30illustrates a core plane,2760A, in network2700in which core switches3030are interconnected as a dual ring where successive core switches3030are connected through two-way links3032, each two-way link having a sufficient number of channels in each direction to enable low-loss spatial switching. Upstream channels3022from each shell switch3020are connected to core switches3030that are topologically evenly-spaced in order to reduce the mean number of hops given an unknown spatial traffic distribution. Other connectivity patterns may be selected based on estimated spatial traffic distribution. As described with reference toFIG. 27, an edge node2710preferably connects to at least one core switch2730in each core plane2760by a down-stream channel band. InFIG. 30, the illustrated edge node3010receives traffic from the illustrated core plane2760A through two down-stream channel bands3014A and3014B.FIG. 30illustrates the upstream connectivity of one shell switch3020, but it is understood that other shell switches3020of the set of shell switches connecting to the core plane2760A are likewise connected, each to respectively selected core switches.

FIG. 31illustrates a core plane2760B, similar to core plane2760A ofFIG. 30, having core switches3130interconnected to form a dual ring. The main difference is that some core switches may have a larger degree permitting each to have links3132X to more core switches3130as illustrated. A person skilled in the art is aware that a ring is a sparsely connected structure resulting in routes of a large number of hops and the additional links3132X ofFIG. 31reduces the mean number of hops per route.

FIG. 32illustrates core plane2760C comprising core switches3230in a two-dimensional lattice structure where each core switch3230connects to each of at most four neighboring core switches3230through a multi-channel link3232. The core switches3230would typically be geographically spread over a wide area and the distances between successive nodes may vary significantly. A core switch3230may receive signals from edge nodes3210through time-locked individual channels3212and transmits signals to edge nodes3210through downstream multi-channel links3214. In a conventional lattice structure, having m>1 core switches per row and n>1 core switches per column, the number of intermediate core switches traversed by a path from one core switch to another varies between 1 and (m+n−3). Network efficiency is heavily dependent on the traffic-weighted mean number of hops per path (the number of hops per path equals the number of intermediate nodes plus one) and the conventional lattice structure is therefore practically limited to a very small number of core switches. In network2700of the present invention, edge nodes, shell nodes, or both have individual upstream-channel access to core nodes, thus reducing the mean number of hops and increasing network throughput.

In summary, the capacity of a core plane2760(FIG. 27) having a sparsely connected structure, would have a very limited access capacity due to multi-hop waste. However, with each shell switch2720accessing numerous core switches, and with each edge node switching traffic to the core through several shell switches, the weighted mean number of hops can be significantly reduced.FIG. 30illustrates one extreme structure of a core plane2760having core switches of low dimension, each core switch having only two dual links to adjacent core switches, thus forming a dual-ring structure. In another extreme (not illustrated), core switches of larger dimension may be used, with each core switch providing a multi-channel link to each other core switch to create a fully-meshed core plane.

FIGS. 33illustrate different input-output configurations of an optical space switch. In configuration3310, each input channel and each output channel may be individually routed. In configuration3320some output channels are routed individually and others are grouped into channel bands with the channels of each channel band routed together. In configurations3320and3330, the input channels may be routed individually or in bands. In configuration3330, all output channels are arranged in output channel-bands with the channels of each output channel-band routed together to an adjacent node. In configuration3340, the input channels are arranged in input channel-bands and the output channels are arranged in output channel-bands. The channels of an input channel-band are routed together and the channels of an output channel-band are routed together. Configuration3310is suitable for temporal switching in a shell node2720(FIG. 27). Configurations3320, and3330are suitable for temporal and spatial switching in core nodes2430(FIG. 24) or2730(FIG. 27). Configuration3340is suitable for spatial switching in a core node2430or2730.

Individual Channel Routing

To increase the adjacency of an optical node used for temporal switching, in accordance with an embodiment of the present invention, wavelength routers may be used to distribute the channels of the outbound links of the edge nodes of network2000(FIG. 20),2300(FIG. 23) or network2400(FIG. 24) among multi-channel links leading to core nodes, or the edge nodes of network2700(FIG. 27) among multi-channel links leading to shell nodes2720or core node2730. This allows access to a large number of core switches or shell switches. The core switches may then be distributed geographically over a wide area. For example, in a uniform composite-star network2702(FIG. 27) where each edge node directly connects to each shell switch, if each edge node has 128 outbound channels, then the number of accessed shell switches may be as high as 128. Without the use of wavelength routers, the 128 outbound channels would be grouped into channel bands (to exploit WDM economy) each channel band having several channels, 16 for example, and an edge node would have upstream links to at most eight core switches. The use of wavelength routers to exchange individual channels of different channel bands will be described with reference toFIG. 34which illustrates the use of a conventional wavelength router to enable independent routing of individual channels.

FIG. 34illustrates a number of edge nodes3410connecting to a wavelength router3400through multi-channel links3412. Each of the output links3414of the wavelength router carries a wavelength channel from each of the edge nodes and may be directed to any of the optical switches, including shell switches3420and core switches3430. Likewise, the arrangement ofFIG. 34applies where the input links to the wavelength router originate from shell switches and the output links are directed to core switches or, possibly, other shell switches. The wavelength router may be replaced by an optical switch that may be reconfigured to follow changes in the spatial traffic distribution. A slow-switching optical node may be used for this purpose.

FIG. 35further illustrates the use of wavelength routers where edge nodes3510are grouped into edge-node groups3502and an arrayed-waveguide-grating router (AWGR)3550, well known in the art, shuffles the wavelength channels of each edge node group3502so that individual wavelength channels in WDM links3512are distributed to WDM links3514directed to optical core switches3530. A core switch3530A, for example, would then receive at least one wavelength channel from each edge node in edge-node group3502A through WDM link3514A. Likewise, core switch3530A may receive a WDM link3514B carrying at least one wavelength channel from each edge node in edge-node group3502B, and so on.

Routing in a Network Employing Temporal and Spatial Switching

The process of selecting a route for a burst or a burst stream is greatly facilitated by designating selected candidate routes for each directed pair of edge nodes. The selected candidate routes are then sorted according to some merit criterion. The sorted routes, from an edge node to another edge node, are herein called a route set. An edge node may store a description of a route set to each other edge node.

In a network employing temporal and spatial switching, a first hop in a route from a first edge node to a second edge node may traverse a time-locked channel to a selected optical node, which may be a shell switch or a core switch. The remaining path from the selected optical node to the second edge node may be a direct path, if the selected optical node connects to the second edge node, or may be an indirect path traversing one or more core switches according to a spatial-switching mode. The selected candidate routes from the first edge node to the second edge node may be sorted according to the number of intermediated core switches or according to a composite criterion combining the number of intermediate core switches and the overall distance from the first edge node to the second edge node. An edge node may select a route in a route set according to a local policy.

The invention, therefore, provides means for reducing the number of hops in a spatially-scheduled network core where time coordination is not maintained. Using temporal switching from the edge to selected core switches can significantly reduce the number of hops in a vast network. Temporal switching along a path segment starting from an edge node is feasible due to the availability of buffers at the edge node.

The described embodiments are to be considered as illustrative and not restrictive. It will be apparent to a person skilled in the art that variations and modifications to the embodiments may be made within the scope of the following claims.