Abstract:
A switch for optical signals, including a plurality of external inputs, a plurality of external outputs, a wavelength conversion entity and a plurality of core switching entities. Each core switching entity is associated to a respective set of at least two wavelengths. The approach is based on switching groups of at least two wavelengths in each core switching entity, while still maintaining per-wavelength switching granularity but sharing the provided capacity for wavelength conversion connections across the group. Thus, wavelength conversion resources assigned to a group of wavelengths are usable by any wavelength in that group. In this way, the blocking statistics in the node as a whole are improved with respect to a single-wavelength-plane configuration. In addition, the resulting switch is modular as it can be upgraded by adding or removing one or more switching modules as required.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    The present invention is related in subject matter to the following U.S Patent Applications, hereby incorporated by reference herein in their entirety:  
         [0002]    Ser. No. 09/511,065, entitled “Switch for Optical Signals” to Graves et. al, filed on Feb. 23, 2000;  
         [0003]    Ser. No. 09/580,495, entitled “Optical Switch with Power Equalization” to Graves et al., filed on May 30, 2000;  
         [0004]    Serial No. 60/207,292, entitled “Optical Switch with Connection Verification” to Graves et al., filed on May 30, 2000;  
         [0005]    Ser. No. 09/726,027, entitled “Protection Switching Arrangement for an Optical Switching System” to Graves et. al., filed on Nov. 30, 2000;  
         [0006]    Ser. No. 09/742,232, entitled “Gain Determination for Correlation Processes” to Andre Van Schyndel, filed on Dec. 22, 2000;  
         [0007]    Ser. No. 09/893,493, entitled “Communications Network for a Metropolitan Area” to Graves et. al, filed on Jun. 29, 2001;  
         [0008]    Ser. No. 09/893,498, entitled “Metropolitan Photonic Switch” to Graves et. al, filed on Jun. 29, 2001;  
         [0009]    Ser. No. 09/972,989, entitled “Optical Wavelength Plan for Metropolitan Photonic Network” to Graves et. al, filed on Oct. 10, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0010]    The present invention relates generally to modular photonic switches with wavelength conversion and, more particularly, to techniques for implementing such switches such as to exhibit desirable blocking performance even when heavily loaded and even when a significant percentage of incoming carriers are required to undergo wavelength conversion.  
         BACKGROUND OF THE INVENTION  
         [0011]    [0011]FIG. 1 shows a metropolitan network  100  with modular photonic switching, with a level of lambda conversion, including a set of service aggregation devices  120 , a set of edge photonic switch nodes  130 , a set of tandem photonic switch nodes  140  and a core  100  comprising a set of core photonic switch nodes  150 . An architecture similar to that of FIG. 1 has been described in aforementioned U.S. patent application Ser. No. 09/893,493.  
           [0012]    In the metropolitan network  100  shown in FIG. 1, the core photonic switch nodes  150  can provide transport level grooming of sub-lambda-level (and lambda-level) services and can also provide direct service-level switching of lambda-level services. Furthermore, the core photonic switch nodes  150  can include sub-lambda-granular service-level switches  160  such as core routers (or packet switches), core ATM switches and core TDM switches to provide centralized switching of services at less than an entire lambda capacity. The service aggregation devices  120  are adapted to aggregate packet traffic (destined for a core router or packet switch), ATM traffic (destined for a core ATM switch), and TDM traffic (destined for a core TDM switch) into separate optical carriers, in order to simplify the core  110  and prevent core node capacity explosion. This segregation also allows the optical carriers to be fed directly into the appropriate sub-lambda-granular service-level switch  160  by the core nodes. Alternatively, multiple service types may share a common optical carrier, in which case a stage of electronic sub-wavelength switching should be interposed between the core nodes  150  and the sub-lambda-granular service-level switches  160  in order to route different components of the common optical carrier to different ones of the sub-lambda-granular service-level switches  160 .  
           [0013]    Each of the photonic switch nodes  130 ,  140 ,  150 , includes a photonic switch and an optical carrier conditioning/validation sub-system (not shown in FIG. 1) for providing wavelength-level switching of optical signals. Thus, the metropolitan network  100  provides switchable express photonic pipes between the service aggregation systems  120  and the core  110 . By using this photonic lambda switching architecture, in conjunction with electro-optical sub-lambda switching nodes at the core locations, the complexity of the electronic and electro-optic components of the network  100  is fundamentally minimized, requiring only one stage of electronic multiplexing, one stage of electro-optic conversion, one stage of opto-electronic conversion on the way to the service-level switch  160 , one stage of service-level switching within the metro area, and then a return path to the far end metro customer. This leads to a much lower cost and complexity for the electro-optic and electronic parts of the network  100  but introduces the requirement for a photonic infrastructure. As such, the electronic and electro-optic complexity of the network  100  is fundamentally minimized through the use of photonic functionality to provide the appropriate connections.  
           [0014]    In addition, the use of photonic paths that completely bypass the electronic and electro-optic components of the core nodes will permit the establishment of all-photonic end-to-end switched connections. Initially, it is expected that such end-to-end photonic paths will be very uncommon, due to the extreme bandwidth requirements to make them economically practical, the prove-in currently being situated at about 3 to 4 gigabits per second (Gb/s) per connection. However, as time progresses and optical integration becomes practicable, the prove-in is expected to drop to around 150 Mb/s, at which point many end-to-end transport pipes can be directly provisioned in a photonic fashion. The provisioning of end-to-end optical transport pipes will require an ability to change the wavelengths of optical carriers in the network, i.e., to move optical carriers from one wavelength slot to another, analogous to moving timeslots in a TDM switched network. This process, often called wavelength (or lambda) conversion, is required at some, but not necessarily all network nodes.  
           [0015]    With continued reference to the network  100  in FIG. 1, each core node  150  includes a wavelength-level switch  155  and one or more (electrical) service-level switches  160  such as IP/packet switches, TDM/SONET switches and STS cross-connects. In addition, the core photonic switch nodes  150  may include or otherwise be connected to legacy equipment such as the TDM telephony network, and provide connectivity to long haul (LH) gateways. At least two core photonic switch nodes  150  are typically required in the network  100  and usually a greater number are provided. Exactly two core photonic switch nodes  150  give survivability, while more than two give scalability and offer protection savings.  
           [0016]    The core photonic switch node  150  serves not only to connect each incoming wavelength to the appropriate sub-lambda-granular service-level switch  160  for a given wavelength payload, but also to select the correct capacity port on that sub-lambda-granular service-level switch  160  so as to avoid stranding core resources. As such, the wavelength-level switch  155  of the core photonic switch node  150  provides wavelength-level connectivity between buildings and electronic protocol-specific or service-specific boxes. Otherwise, the provisioning granularity would be at the fiber level, precluding the advantageous use of dense wavelength division multiplexing and demultiplexing. By the same token, exploitation of the wavelength-level switch  155  to its full potential requires some level of segregation with respect to the wavelengths traveled by IP and TDM traffic within the network  100 . It is also noted that the wavelength-level switch  155  may provide dynamic network load balancing and protection in case of network failures.  
           [0017]    Regarding the edge photonic switch nodes  130 , these provide the ingress and egress points into the metropolitan network  100 . The edge photonic switch nodes  130  are typically located in office buildings, although they may appear elsewhere. The optical signals migrate from sparse DWDM (S-DWDM) into DWDM by an interleaving process and continue their path across the network  100  (see above-mentioned U.S. patent application Ser. No. 09/893,498 and U.S. patent application Ser. No. 09/972,989). The location where the access (S-DWDM) plant meets the inter-office plant is the edge Central Office. The edge photonic switch nodes  130  can be planar in nature, since there is no substantial need for wavelength conversion anywhere but in the core  110 . Wavelength conversion is only applied in the case of intra-metro end-to-end wavelength circuits and this can be done in the tandem photonic switch nodes  140  or in the core photonic switch nodes  150 . Initially, only rarely will the lack of wavelength conversion capability at the edge node  130  result in a wavelength that could have been locally switched being sent to a wavelength-conversion-equipped node instead, although the prevalence of this occurrence will increase somewhat over time. However, the changed community of interest statistics of the evolved data network, relative to the old telephony network (with its preponderance of local calling), means that the lack of local lambda conversion to complete a local photonic connection and the consequent need for back-haul to a node that does have lambda conversion does not become a problem, since only a relatively small percentage of traffic will be back-hauled when it could have been locally converted.  
           [0018]    For its part, a tandem photonic switch node  140  provides a number of functions including a further point of partial fill consolidation before reaching the core  110 , establishing end-to-end wavelength paths with wavelength conversion where required and providing a flexibility point for the addition of more core photonic switch nodes  150  or edge photonic switch nodes  130  without having to add dedicated core-edge paths. In addition, the tandem photonic switch nodes  140  may also operate in concert with the edge photonic switch nodes  130  and core photonic switch nodes  150  to provide dynamic traffic load balancing, protection and restoration functions against equipment failure or cable cuts in the core network. A level of wavelength conversion in the tandem photonic switch nodes  140  is beneficial in order to accommodate back-hauled intra-metro wavelength services without routing them back to the core photonic switch nodes  150 . All other services/circuits travel to the core photonic switch nodes  150  since this is where the long haul gateways and the sub-wavelength switching functions are located.  
           [0019]    As can be appreciated from the above, there is little need for wavelength conversion anywhere but in the tandem photonic switch nodes  140  and core photonic switch nodes  150 . It has been estimated that once the metropolitan network  100  is used exclusively for photonic end-to-end connections, only in the case of about 5-10% of wavelengths will the lack of wavelength conversion capability at an edge photonic switch node  130  result in a wavelength that could have been locally switched being sent to a wavelength-conversion-equipped node instead. Stated differently, an edge photonic switch node  130  would ideally be required to provide about 5-10% wavelength conversion under expected future traffic conditions. On the other hand, analyses have shown that the tandem photonic switch nodes  140  and core photonic switch nodes  150  will need to be able to convert as much as 30% and 70% of their incoming wavelengths, respectively, in order to provide satisfactory performance under expected future traffic conditions, once the network  100  has transitioned to the provision of photonic end-to-end paths. In the meantime, the numbers will be somewhat lower, due to the need to terminate most optical carriers in the electro-optic structure of the core photonic switch node  150 .  
           [0020]    Furthermore, as the photonic switched network shown as  100  in FIG. 1 evolves and thereby grows to accommodate increased traffic, both the number of nodes and the size of those nodes will have to evolve. Thus, the nodes of a practical network have to be sized both initially and in terms of growth, to the actual network traffic levels of provisioned traffic through each individual node, precluding a “one size fits all” approach or approaches which do not scale well, both up and down in size.  
           [0021]    These requirements place a fundamental demand on the nodes to be flexible in terms of overall capacity and to permit both various initial node throughputs and various different growth rates. This can only be achieved with a scalable, modular node since for non-scalable nodes size increase requires a “fork-lift” upgrade with the attendant massive disruption to the network at that node site.  
           [0022]    Thus, there exists a need in the industry to provide a modular, scalable photonic switch that exhibits desirable blocking performance on both its through paths and its wavelength conversion paths even when heavily loaded and even when a significant percentage of incoming wavelengths need to be converted.  
           [0023]    Some conventional photonic switches can be highly modular but exhibit poor blocking performance, as is the case with the switch described in aforementioned U.S. patent application Ser. No. 09/511,065. Such switches are based on a per-wavelength switching structure and hence are highly modular. However, they only provide sufficient wavelength conversion capability to handle the pure edge photonic switch applications and some hybrid edge-tandem photonic switch nodes up to the case where a few percent of incoming carriers must undergo wavelength conversion. However, the longer-term (and in some cases near-term) tandem photonic switch node and core photonic switch node wavelength conversion capability requirements are beyond the reach of the per-wavelength switch, since the latter exhibits a significant blocking probability, especially when heavily loaded.  
           [0024]    Other conventional switches can exhibit superior blocking performance but are highly non-modular and non-scalable. Such is the case with the LambdaRouter™ all-optical switch from Lucent Technologies, which is an any-to-any switch based upon a large 3-D MEMS mirror chamber. The any-to-any property exhibits low blocking. However, such switches generally do not provide a wide range of sizes combined with scalability and modularity, generally requiring either a massively over-provisioned initial switch core or a “fork-lift” upgrade once the core runs out of capacity. These factors, combined with the complexity and expense of achieving a functional solution, have prevented any-to-any switches from achieving practicality.  
           [0025]    Clearly, there still exists a need in the industry to provide a modular photonic switch with wavelength conversion that exhibits desirable blocking performance even when heavily loaded and even when a significant percentage of incoming carriers need to undergo wavelength conversion.  
         SUMMARY OF THE INVENTION  
         [0026]    The present invention provides a switch structure which permits all three node types (edge, tandem, core) to be addressed with the same basic technology. The present invention recognizes that a per-wavelength switch structure limits the amount of wavelength conversion that can be performed per input signal of a particular wavelength, which causes blocking to occur at a low switch load.  
           [0027]    The approach used by the present invention is based on switching of a few wavelengths as a group within a fundamentally planar, but not wavelength-planar switch, while still maintaining per-wavelength switching granularity but sharing the provided capacity for wavelength conversion connections across the group. Therefore, according to a first broad aspect of the present invention, there is provided a switch for optical signals, including a plurality of external inputs, each external input carrying light that occupies a wavelength associated to that external input. The switch also includes a plurality of external outputs, each external output carrying light that occupies a wavelength associated to that external output. This switch further includes a wavelength conversion entity having a plurality of inputs and a plurality of groups of outputs, each output in each particular one of the groups of outputs carrying light that occupies a wavelength in a group of wavelengths associated to the particular one of the groups of outputs.  
           [0028]    In addition, the switch includes a plurality of core switching entities, each core switching entity associated to a respective set of at least two wavelengths. Still in accordance with the first broad aspect of the present invention, each core switching entity is equipped with a plurality of first core inputs respectively connected to those external inputs for which the associated wavelength belongs to the respective set of at least two wavelengths, a plurality of second core inputs respectively connected to those outputs of the wavelength conversion entity belonging to the group of outputs for which the associated group of wavelengths belongs to the respective set of at least two wavelengths, a plurality of first core outputs respectively connected to those external outputs for which the associated wavelength belongs to the respective set of at least two wavelengths; and a plurality of second core outputs respectively connected to individual ones of the inputs of the wavelength conversion entity. In addition, each core switching entity is capable of selectably transferring optical signals from any of its first core inputs to any of its second core outputs and each core switching entity is further capable of selectably transferring optical signals from any of its second core inputs to any of its first core outputs.  
           [0029]    By assigning multiple wavelengths to each core switching entity, the blocking statistics in the node as a whole are improved, as demonstrated by computer simulation/modeling results. For a given overall node size, this improvement can be ascribed to the increased number of optical carrier connections in each switch module, combined with the reduction in the number of switch modules as compared to a single-wavelength-plane configuration. Advantageously, the resulting switch is modular because it can be upgraded by adding or removing one or more switching modules as required.  
           [0030]    In accordance with a second broad aspect of the present invention, there is provided a method of switching a plurality of incoming optical signals occupying individual wavelengths of light. The method includes grouping the signals into signal groups, each signal group including optical signals occupying at least two wavelengths associated with that signal group; selectably switching each signal in each given signal group towards either an output or a set of wavelength conversion resources associated with the given signal group; and using the set of wavelength conversion resources associated with each given signal group for wavelength conversion of a subset of the incoming signals occupying wavelengths associated with the given signal group. These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    In the accompanying drawings:  
         [0032]    [0032]FIG. 1 shows an optical transport network including a plurality of optical switches;  
         [0033]    [0033]FIG. 2 shows an optical switch in accordance with an embodiment of the present invention, including a photonic switch core and a wavelength conversion module;  
         [0034]    [0034]FIG. 3A illustrates a wavelength-group photonic switching module used in the optical switch of FIG. 2;  
         [0035]    [0035]FIG. 3B illustrates the concept of a three-stage switching structure existing within the optical switch of FIG. 2;  
         [0036]    FIGS.  4 A- 4 E show various embodiments of the wavelength conversion module of FIG. 2;  
         [0037]    FIGS.  5 A- 5 C are graphs illustrating the performance of the optical switch of FIG. 2, as modeled using computer simulations; and  
         [0038]    FIGS.  6 - 8  show alternative embodiments of the optical switch of FIG. 2.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    With reference to FIG. 2, there is shown an optical switch  200  in accordance with an embodiment of the present invention. The optical switch  200  may be implemented as part of an edge node, a tandem node or a core node. The optical switch  200  includes a photonic switch core  210 , which has an arrangement of parallel photonic switching modules  250   1,2, . . . ,G . In a manner to be described later on, each of the switch modules  250   1,2, . . . ,G  handles a subset of the total number of optical carriers passing through the node, with the partitioning among the photonic switching modules  250   1,2, . . . ,G  being made on the basis of groups of optical carrier frequencies, each at a specific wavelength associated with that frequency, which is in turn precisely defined on the network DWDM grid plan. In this disclosure, these will be referred to as “wavelength groups”, with the understanding that this refers to groups of optical carriers at frequencies associated with a standard frequency plan, with groups of those carriers falling within each wavelength group. Additionally, the optical switch  200  includes a wavelength conversion module  220 , which is fed with optical carrier paths to and from each of the photonic switching modules  250   1,2, . . . ,G  that make up the overall switch core  210 .  
         [0040]    Also included in the illustrated embodiment of the optical switch  200  is a bank of M wavelength division demultiplexing (WDD) devices  230   1,2, . . . ,M  connected at an input end of the optical switch  200  and which provide an optical demultiplexing function on an incoming DWDM stream so as to permit individual optical carriers to be individually switched by the photonic switch core  210 . The individual WDD devices  230   1,2, . . . ,M  are typically distributed amongst a plurality of line cards. The optical switch  200  further includes a bank of M wavelength division multiplexing (WDM) devices  240   1,2, . . . ,M connected at an output end of the optical switch 200, each of which serves to recombine the switched traffic into a number of DWDM streams for onward transmission. The individual WDM devices 240   1,2, . . . ,M  are also typically distributed amongst a plurality of line cards, which may be the same line cards containing the WDD devices  230   1,2, . . . ,M . The reader will appreciate that the number of WDD devices  230   1,2, . . . ,M , the number of WDM devices  240   1,2, . . . ,M  and the number of line cards may vary depending on the operational requirements of the invention. In fact, it is envisaged that in some embodiments of the invention, one or more of the WDD devices  230   1,2, . . . ,M  and WDM devices  240  can be bypassed.  
         [0041]    Each m th  WDD device  230   m , 1≦m≦M, has a WDD input port  232   m  for accepting an incoming multi-wavelength optical signal and a total of N WDD output ports  234   m,1 ,  234   m,2 , . . . ,  234   m,N . The incoming multi-wavelength optical signal includes a plurality of incoming optical signals occupying individual distinct wavelengths. The WDD device  230   m  is operative to separate the incoming multi-wavelength optical signal at its WDD input port  232   m  into its single-wavelength constituent signals. Each of the incoming single-wavelength optical signals so produced is provided at a respective one of the output ports  234   m,1 ,  234   m,2 , . . . ,  234   m,N  of the WDD device  230   m .  
         [0042]    For the purposes of this example, the total number of wavelengths occupied by the incoming single-wavelength optical signals in the incoming multi-wavelength optical signal is equal to N (viz. the number of WDD output ports  234   m,1 ,  234   m,2 , . . . ,  234   m,N  of the WDD device  230   m ), while the specific wavelength occupied by the incoming single-wavelength optical signal emerging at WDD output port  234   m,n  is denoted λ n , for 1≦n≦N. Thus, it can be said that each WDD output port  234   m,n  of WDD device  230   m  is associated with a distinct wavelength (or color) λ n .  
         [0043]    The N output ports  234   m,1 ,  234   m,2 , . . . ,  234   m,N  of the WDD device  230   m  are divided into G output port groups, each of size N/G (requiring an integer harmonic relationship between N and G), and collectively denoted  260 . In particular, output port group  260   m,g  refers to the g th  output port group (1≦g≦G) on the m th  WDD device  230   m  (1≦m≦M). This grouping effectively classifies the WDD output ports of the WDD devices  230   1,2, . . . ,M  as a function of wavelength, such that each output port group corresponds to a distinct set of wavelengths, or “wavelength group”. For the sake of convenience, the wavelength groups will hereinafter be referred to as group 1 (for those wavelengths corresponding to output port groups  260   1,1 ,  260   2,1 , . . . ,  260   M,1 ), group 2 (for those wavelengths corresponding to output port groups  260   1,2 ,  260   2,2 , . . . ,  260   M,2 ), and so on, up to group “G” (for those wavelengths corresponding to output port groups  260   1,G ,  260   2,G , . . . ,  260   M,G ).  
         [0044]    The WDD output ports corresponding to a given wavelength group, regardless of the WDD device on which they are located, are connected to respective input ports of a common one of a plurality of G wavelength-group photonic switching modules  250   1,2, . . . ,G  in the photonic switch core  210 . This means that, for example, the output ports in output port groups  260   1,g ,  260   2,g , . . . ,  260   M,g  are connected to respective input ports of photonic switching module  250   g , where (1≦g≦G). In an example embodiment, there are between 3 and 8 wavelength-group photonic switching modules  250   1,2, . . . ,G  although the present invention is in no way limited to this range.  
         [0045]    In some embodiments, (N/G=) two, three or four output ports of each WDD device will lead to the same one of the wavelength-group photonic switching modules  250   1,2, . . . ,G . In the example case where there are four carriers per group and a total of 32 channels in the DWDM grid, this will lead to a requirement for 8 wavelength-group photonic switching modules  250   1,2, . . . ,8 . In other embodiments, a greater number of the output ports of each WDD device will lead to the same wavelength-group switching module. In all cases, however, each of the wavelength-group photonic switching modules  250   1,2, . . . ,G  is associated with a group of wavelengths that includes at least two distinct wavelengths of light, i.e., at least two colors (when the light is visible).  
         [0046]    It should be appreciated that different numbers of wavelengths may be associated with each group and, within a given group of wavelengths, the associated wavelengths may or may not be neighbouring wavelengths in the optical transmission spectrum. Moreover, the set of all wavelengths in the optical transmission spectrum may be re-assigned to different groups, either dynamically or upon halting operation of the switch  200 . It should also be appreciated that the complete absence of the WDD devices  230   1,2, . . . ,M  would not impact the functionality of the present invention, as it would be possible to feed the incoming single-wavelength optical signals directly into the wavelength-group photonic switching modules  250   1,2, . . . ,G  (e.g., from an array of DWDM transponder sources).  
         [0047]    It should further be appreciated that the photonic switch  200  is modular on a per line card basis (single fiber in or out carrying multiplexed DWDM traffic) and on a per wavelength group basis (for the photonic switch core  210 ).  
         [0048]    With additional reference to FIG. 3A, each wavelength-group photonic switching module  250   g  includes a first set of input ports  310   g  for receiving at least two incoming single-wavelength signals on each of at least two wavelengths in the associated group, i.e., group “g”. The input ports  310   g  may be termed “external” input ports, as they carry incoming single-wavelength optical signals received from outside the optical switch  200 , in this case via WDM devices  230   1-M . In addition, the wavelength-group photonic switching module  250   g  further includes a second set of input ports  320   g  that do not carry signals received from outside the optical switch. Rather, the single-wavelength optical signals received at the second set of input ports  320   g  are supplied by a respective set of output ports  330   g  of the wavelength conversion module  220 . As such, the input ports  320   g  may be termed “internal” input ports. In practice, the internal and external ports are identical to one another, with the partitioning of “internal” or “external” being effected according to how they are used, which is determined by whether they are connected to WDM devices, WDD devices or to the wavelength conversion module  220 .  
         [0049]    One difference between the internal input ports  320   g  and the external input ports  310   g  is in the wavelength occupied by the optical signal expected to arrive via each of these ports. Specifically, each of the external input ports  310   g  will generally carry light only of a fixed wavelength associated with the port of the WDD device that it is connected to (one of N/G different values, in this case constrained to belonging to group “g”), while the signal arriving via one of the internal input ports  320   g  will have undergone wavelength conversion and may occupy any of the wavelengths in group “g” at any given time. As will be described herein below, this characteristic allows the design of the wavelength-group switching module  250   g  to be simplified, to be made more modular and to provide better blocking characteristics than is the case for the basic single-wavelength-plane switch.  
         [0050]    Each wavelength-group photonic switching module  250   g  further has a plurality of output ports, including internal output ports  340   g  and external output ports  350   g . The external output ports  350   g  of the g th  photonic switching module  250   g  carry switched single-wavelength optical signals occupying pre-determined wavelengths (within group “g”) for onward transmission outside of the optical switch  200 , optionally via the WDM devices  240   1,2, . . . ,M . As for the internal output ports  340   g  of each wavelength-group photonic switching module  250   g , these carry switched single-wavelength optical signal towards a respective set of input ports  360   g  of the wavelength conversion module  220 . It is noted that any given one of the internal output ports  340   g  of a particular wavelength-group photonic switching module  250   g  may, at any given time, be used by any one of the wavelengths belonging to group “g”.  
         [0051]    Each of the WDM devices  240   1,2, . . . ,M , if used, combines light from a subset of the totality of external output ports  350   1,2, . . . ,G  on each of the wavelength-group photonic switching modules  250   1,2, . . . ,G . More specifically, each of the WDM devices  240   1,2, . . . ,M  combines light from a subset of the external output ports which carries switched single-wavelength optical signals occupying distinct wavelengths. The output of each of the WDM devices  240   1,2, . . . ,M  is an outgoing multi-wavelength optical signal, which is routed to other parts of the optical transport network. Of course, it is within the scope of the present invention for the switched single-wavelength optical signals received via the external output ports  350   g  of the wavelength-group optical switching units  250   1,2, . . . , G  to lead directly out of the optical switch  200  without undergoing optical multiplexing.  
         [0052]    For the purposes of this description, it will be assumed that the aggregate number of external input ports  310   1,2, . . . ,G  is evenly distributed amongst all the wavelength-group photonic switching modules  250   1,2, . . . ,G . Moreover, it will be assumed that the number of external output ports  350   g  on each wavelength-group photonic switching module  250   g  is equal to the number of external input ports  310   g  on that module. Thus, each of the wavelength-group photonic switching modules  250   1,2, . . . ,G  has ((M×N)/G) external input ports  310   g  and ((M×N)/G) external output ports  350   g . However, it should be understood that the totality of the external input ports  310   1,2, . . . ,G  need not be evenly distributed amongst the wavelength-group photonic switching modules  250   1,2, . . . ,M  and that the number of external output ports  350   g  on a particular wavelength-group photonic switching module  250   g  need not equal the number of external input ports  310   g  on that module. In fact, in the case where dark lambda concentration is being effected at an edge photonic switch node, there may be more inputs than outputs in the access-to-core direction and more outputs than inputs in the core-to-access direction.  
         [0053]    It should be apparent that the relative amount of switching resources devoted to wavelength-converted signals is a parameter of interest when evaluating the performance of the optical switch  200 . This can be quantified by a ratio that defines the number of external input ports  310   g  per internal input port  320   g  for a given wavelength-group photonic switching module  250   g . In other words, if R g  represents this ratio, referred to as a wavelength conversion resource factor, then there will be R g  times as many external input ports  310   g  than internal input ports  320   g  for wavelength-group optical switching unit  250   g . Of course, if each of the wavelength-group photonic switching modules  250   1,2, . . . ,G  has the same number of internal and external input ports, then clearly there will be R g  (=R) times as many external input ports  310   1,2, . . . ,G , in total, than there will be total internal input ports  320   1,2, . . . ,G . Moreover, the fact that there is one output port per input port means that there will also be R times as many external output ports  350   g  as there are internal output ports  340   g  for each wavelength-group photonic switching module  250   g .  
         [0054]    Thus, each wavelength-group photonic switching module  250   g  in the example of FIG. 2 has (M×N/G) external input ports  310   g , (M×N/(G×R)) internal input ports  320   g , (M×N/G) external output ports  350   g  and (M×N/(G×R)) internal output ports  340   g . This gives a total port count of (M×N/G)×(1+1/R) input ports and as many output ports per wavelength-group photonic switching module. It should further be appreciated that R, the wavelength conversion resource factor, may be less than unity. In other words, there may be fewer external input (or output) ports than internal input (or output) ports on any or all of the wavelength-group photonic switching modules  250   1,2, . . . ,G . However, normally R will be greater than unity with, as an example, 33% wavelength conversion creating a value of R=3.  
         [0055]    [0055]FIG. 3A provides further detail regarding the internal structure of an example wavelength-group photonic switching module  250   g  suitable for use within the photonic switch core  210  of the present invention. In one embodiment, the wavelength-group photonic switching module  250   g  is equipped with the capability to switch any of its input ports to any of its output ports. Specifically, implementation of the wavelength-group switching module  250   g  may be by way of providing one large ((M×N/G)×(1+1/R))-square cross-point optical switch. This allows each of the external input ports  310   g  and each of the internal input ports  320   g  to be switched to any of the external output ports  350   g  or any of the internal output ports  340   g .  
         [0056]    However, in many practical applications, the full ((M×N/G)×(1+1/R))-square switching capability of the wavelength-group photonic switching module  250   g  is not required and hence the design of the module can be simplified.  
         [0057]    Specifically, recall that each of the external input ports  310   g  carries an incoming single-wavelength optical signal occupying a pre-determined wavelength of light. In some cases, it will be necessary to convert this wavelength (CASE I) and in other cases, in will not be necessary to convert this wavelength (CASE II).  
         [0058]    CASE I  
         [0059]    The incoming single-wavelength optical signal arriving at the wavelength-group switching module  250   g  via one of its external input ports  310   g  is redirected towards one of the internal output ports  340   g  connected to the input ports  360   g  of the wavelength conversion module  220 . Individual ones of the internal output ports  340   g  are not associated with any particular wavelength and are capable of receiving any of the incoming single-wavelength optical signals.  
         [0060]    CASE II  
         [0061]    The incoming single-wavelength optical signal arriving at the wavelength-group switching module  250   g  via one of its external input ports  310   g  can directly exit the optical switch  200  via one of the WDM devices  240   1,2, . . . ,M . However, only a limited number of input ports of each WDM device  240   m  (1&lt;m&lt;M) are associated with the exact wavelength of the signal in question. Thus, when switching from the external input ports  310   g  to the external output ports  350   g , the wavelength-group photonic switching module  250   g  only need to provide the capability of switching each of the external input ports  310   g  to the limited subset of the external output ports  350   g  associated with the same wavelength of light.  
         [0062]    In view of the above, it is desirable for each wavelength-group photonic switching module  250   g  to be configured so as to allow any of its external input ports  310   g  to be switched to any of its internal output ports  340   g , while the switching of those external input ports  310   g  associated with a given wavelength can be limited to only those external output ports  350   g  that are associated with the same wavelength. Moreover, because a signal output by the wavelength conversion module  220  may occupy any wavelength within group “g”, the wavelength-group photonic switching module  250   g  will need to be able to switch any of its internal input ports  320   g  to any one of its external output ports  350   g . However, since there is typically no reason for a signal output by the wavelength conversion module  220  to re-enter the latter, the wavelength-group photonic switching module  250   g  need not be equipped with the ability to switch its internal input ports  320   g  to its internal output ports  340   g .  
         [0063]    Accordingly, FIG. 3B shows that the desired switching functionality can be achieved by providing a first set of switch cross-points  370  that are dedicated to switching the external input ports  310   g  of the wavelength-group photonic switching module  250   g  and a second set of cross-points  380  that are dedicated to switching the internal input ports  320   g  of that module. The cross-points  370 ,  380  may be implemented by placing, at each cross-point, a mechanically controlled micro-mirror, such as a micro-electro-mechanical switch (MEMS). Control instructions regarding the desired state (raised or lowered) of a particular mirror are provided by a switch controller  290 , which maintains a connection map. The switch controller  290  may either be a part of, or external to but in communication with, the photonic switch core  210 . In some embodiments, the switch controller  290  may be embedded as a microprocessor on a control card.  
         [0064]    For its part, the wavelength conversion module  220  represents the second stage of a CLOS switching architecture. In particular, it is noted that the wavelength conversion module  220  may be broken down into multiple parallel centre stage switches of size (K×G)×(K×G), K being an integer above 0. In the illustrated example, the centre stage switches are denoted  222   1 ,  222   2  and the total number of centre stage switches is equal to two. This means that for a total of (M×N/(G×R)) input ports of the wavelength conversion unit are divided amongst both centre stage switches  222   1 ,  222   2 , which means that in this example, K=(M×N)/(R×2). In general, where Y denotes the total number of centre stage switches in the second stage of the CLOS switching architecture, the dimensions of each switch will be (M×N×G)/(R×Y) by (M×N×G)/(R×Y).  
         [0065]    Thus, the number of CLOS centre stage switches can be made a function of the level of lambda conversion required and thus for low levels of lambda conversion, the number of CLOS centre stages is reduced and the virtual CLOS first and third stages will automatically reduce in size, as ports are diverted back into being external, not internal ports. Thus, there is modularity due to better scaling for the CLOS architecture. Advantageously, given the architecture of the optical switch  200 , additional wavelength conversion resources can be provided on an as-needed basis.  
         [0066]    For its part, the wavelength conversion module  220  includes the G sets of input ports  360   1,2, . . . ,G  and the G sets of output ports  330   1,2, . . . ,G . Each of the input ports in the set of input ports  360   g  carries a single-wavelength optical signal occupying any wavelength in group “g” as provided by the g th  wavelength-group photonic switching module  250   g . Each received signal in group “g” is converted into one of the wavelengths belonging to, say, group “h”. The converted signal is provided on the appropriate one of the output ports  340   h , which leads to a respective one of the internal input ports  320   h  of the wavelength-group photonic switching module  250   h . For each received signal in group “g”, the switch controller  290 , which maintains the connection map for the photonic switch core  210  and the wavelength conversion module  220 , provides the identity of the wavelength group “h”, as well as the precise wavelength to which the received signal is being converted and the output port to which it is being sent.  
         [0067]    Those skilled in the art will observe that incoming single-wavelength optical signals not requiring wavelength conversion will be switched once by the cross-points  370 , while incoming single-wavelength optical signals requiring wavelength conversion will be switched once by the cross-points  370 , once by the wavelength conversion module  220  and once by the cross-points  380 . With reference now to FIG. 3A, a conceptual view of the switch as providing three-stage CLOS switching functionality for wavelength-converted signals is presented. Specifically, it is possible to identify cross-points denoting the first stage of switching as well as cross-points denoting the third stage of switching. Thus, the fist and third stages are embedded within the wavelength-group photonic switching modules  250   1,2, . . . ,G . The second stage of switching occurs in the wavelength conversion module  220 .  
         [0068]    Greater detail regarding possible configurations for the wavelength conversion module are provided in FIGS. 4A through 4E. It will be appreciated that each of the options  4 A through  4 E can be used as one of several parallel CLOS center stage switches or, by making the switches much larger, can provide a single switching operation. However, the use of these designs as CLOS center stages permits scalability and the use of smaller converter second stage switches, which is more compatible with existing photonic technology.  
         [0069]    Specifically, with reference to FIG. 4A, there is shown a wavelength conversion module  410  with a bank of opto-electronic converters  411 , one converter for each of the inputs  360   1,2, . . . ,G  to the wavelength conversion module  410 . Each of the opto-electronic converters  411  converts light to an electrical signal. The outputs of the opto-electronic converters  411  are connected to inputs of an electrical switch  412 , for providing switching such as SONET, ATM, IP or Ethernet switching, for example. The outputs of the electrical switch  412  are connected to inputs of electro-optical converters  413 . Each of the electro-optical converters  413  converts an electrical signal to light of a particular wavelength. The outputs of the electro-optical converters  413  represent the complete set of outputs  330   1,2, . . . ,G  of the wavelength conversion module  410 .  
         [0070]    Another embodiment is presented in FIG. 4B, wherein is shown a wavelength conversion module  420  with a bank of opto-electronic converters  421 , one converter for each of the inputs  360   1,2, . . . ,G  to the wavelength conversion module  420 . Each of the opto-electronic converters  421  converts light to an electrical signal. The outputs of the opto-electronic converters  421  are connected to inputs of a corresponding bank of electro-optical converters  422 . Each of the electro-optical converters  422  converts an electrical signal to light of a particular wavelength. The outputs of the electro-optical converters  422  are fed to a photonic switch  423 , for providing purely photonic switching. The outputs of the photonic switch  423  represent the complete set of outputs  330   1,2, . . . ,G  of the wavelength conversion module  420 .  
         [0071]    Yet another embodiment is presented in FIG. 4C, wherein is shown a wavelength conversion module  430  with a photonic switch  431 , for providing purely photonic switching. The photonic switch  431  has one input for each of the inputs  360   1,2, . . . ,G  to the wavelength conversion module  430 . The outputs of the photonic switch  431  are provided to a bank of opto-electronic converters  432 . Each of the opto-electronic converters  432  converts light to an electrical signal. The outputs of the opto-electronic converters  432  are connected to inputs of a corresponding bank of electro-optical converters  433 . Each of the electro-optical converters  433  converts an electrical signal back to light of a specified wavelength. The outputs of the electro-optical converters  433  represent the complete set of outputs  330   1,2, . . . ,G  of the wavelength conversion module  430 .  
         [0072]    The above-described embodiments of the wavelength conversion module  410 ,  420  and  430  provide dedicated electro-optical and opto-electronic conversion resources for each signal arriving at the wavelength conversion module or each signal leaving the wavelength conversion module. This has the effect of guaranteeing that a signal that can be switched by the underlying photonic or electrical switch will emerge at the appropriate wavelength. At the same time, however, this results in a wastage of resources when the level of wavelength conversion is expected to be relatively small. In order to permit scalability from very low levels of wavelength conversion to wavelength conversion for all inputs, the embodiments of FIGS. 4D and 4E may be used.  
         [0073]    Specifically, FIG. 4D shows a wavelength conversion module  440  in which dual photonic switches  441  and  442  are used. The inputs of photonic switch  441  correspond to the inputs  360   1,2, . . . ,G  of the wavelength conversion module  440  and the outputs of photonic switch  442  correspond to the outputs  330   1,2, . . . ,G  of the wavelength conversion module  440 . In between the two photonic switches, there is provided a bank of tandem opto-electronic/electro-optical conversion units  443  that grows in accordance with the switching requirements of the wavelength conversion unit  440 .  
         [0074]    In a similar fashion, FIG. 4E shows a wavelength conversion module  450  in which there is provided a series combination of an electrical switch  451  and a photonic switch  452 . The electrical switch  451  is preceded by a full bank of opto-electrical converters  453  connected to the inputs  360   1,2, . . . ,G  of the wavelength conversion module  450 , while a subset of the outputs of the electrical switch  451  are connected to a corresponding subset of the inputs of the photonic switch  452  via a provisioned-as-needed bank of electro-optical converters  454 . Clearly, as the wavelength conversion requirements grow, a greater number of electro-optical converters  454  may be added.  
         [0075]    In each of the embodiments  4 A through  4 E, it should be understood that the wavelength conversion unit  220  actually consists of parallel “slices”, each slice consisting of an individual module as described above and illustrated at  410 ,  420 ,  430 ,  440  and  450 . Of course, in such instances, the inputs and outputs of the wavelength conversion module would be distributed amongst the individual slices.  
         [0076]    In operation, it is of interest to evaluate the blocking performance of the optical switch  200 . Those skilled in the art will appreciate that the load of the optical switch  200  is a parameter of interest when evaluating the performance of the optical switch  200 . Specifically, the load of the switch  200  at a given time may be defined as the percentage of the total number of external input ports  310   1,2, . . . ,G  on the set of G wavelength-group photonic switching modules  250   1,2, . . . ,G  that carry an incoming single-wavelength optical signal at the given time. The load of the optical switch  200 , expressed as a percentage, will thus range from 0 (representing no load) to 100 (representing full load).  
         [0077]    As has been previously mentioned with reference to FIG. 3A, the optical switch  200  functions as a single-stage switch for incoming single-wavelength optical signals not requiring wavelength conversion and as a three-stage CLOS switch for input signals that do require wavelength conversion. Due to the fact that each wavelength-group photonic switching module  250   g  in the optical switch  200  handles N/G times as many wavelengths as in a single-wavelength-plane switch, the blocking performance of the optical switch  200  for most traffic mixes, and particularly under conditions of heavy load, is considerably improved with respect to its single-wavelength-plane counterpart. At the same time, the optical switch  200  is not nearly as complex or mechanically fragile as a three-dimensional fully non-blocking architecture. A computer-based simulation was used to confirm the superior blocking performance of the optical switch  200  relative to its conventional single-wavelength-plane counterpart. This is now described with reference to FIGS. 5A through 5C.  
         [0078]    Specifically, FIGS. 5A through 5C show various curves of the blocking probability versus the total number of external input ports  310   1,2, . . . ,G  that carry an optical signal. The blocking probability, denoted “P block ”, is defined as the probability that an incoming single-wavelength optical signal (received by the photonic switch core  210  along one of the external input ports  310   1,2, . . . ,G  of the wavelength-group photonic switch modules  250   1,2, . . . ,G ) will not exit the optical switch  200  at the desired wavelength. The blocking probability P block  is a benchmark by which the performance of the switch  200  may be evaluated. While the criteria used to evaluate whether a blocking probability is considered desirable or acceptable depend on the network design and carrier requirements, it is generally the case that a blocking probability of 0.1% (or more) at a load of 60% (or less) would be considered inadequate and a blocking probability of 0.1% (or less) at a load of 85% (or more) would be considered adequate, dependent upon the actual network application. Note that, once a switch node reaches a utilization level of about 85%, the sheer unpredictability of traffic forecasting demands that the node be reinforced by increasing switching capacity, so a graceful further increase in blocking beyond 0.1% at greater than 85% load is generally not overly problematic.  
         [0079]    It should be apparent that an incoming single-wavelength optical signal not requiring wavelength conversion will be blocked when all the like-wavelength external output ports of the corresponding photonic switching module are already occupied. In addition, an incoming single-wavelength optical signal occupying a wavelength λ 1  in group “g” and requiring wavelength conversion to a wavelength λ 2  in group “h” will be blocked when either (1) all the internal output ports  340   g  of photonic switching module  250   g  are already occupied or (2) all the external output ports  350   h  of photonic switching module  250   h , which are associated with wavelength λ 2 , are occupied.  
         [0080]    The various simulations in FIGS. 5A through 5C relate the blocking performance of the optical switch  200  for different values of the wavelength conversion resource factor (described previously and denoted R) and the requirement for wavelength conversion (denoted B). Regarding the wavelength conversion requirement “B”, it can be assumed that the incoming single-wavelength optical signals have a probability “B” of requiring wavelength conversion, where B % wavelength conversion requirement in the traffic mix signifies that one out of every 100/B incoming single-wavelength optical signals will require wavelength conversion by the optical switch  200 .  
         [0081]    For the purposes of the simulation in FIGS. 5A through 5C, the aggregate number of input ports (i.e., internal input ports  320   1,2, . . . ,G  and external input ports  310   1,2, . . . ,G ) for the photonic switch core  210  is equal to 512. Thus, the sum of (M×N) external input ports and ((M×N)/R)) internal input ports equals 512. Hence, for different values of R, the number of external and internal input ports will vary, although the sum total will remain constant. In the case where the sum total equals to 512, the total number of available external input ports is equal to (512*R/(1+R)) It should be noted that when R is equal to unity, this means that there are as many external input ports to the wavelength-group photonic switch modules as there are internal input ports. When R is greater than one, then there are more external input ports than internal input ports and when R is less than one, then the balance is in favour of the internal input ports. It should be noted that it is intrinsic to the nature of CLOS switches that some over-provisioning of lambda conversion capacity is required for optimal dynamic blocking characteristics. This over-provisioning allows R to drop below unity.  
         [0082]    The blocking behavior of the switch was tracked as the switch was increasingly loaded with random traffic over the full range from 0-100%, twenty times in each configuration and then the results were averaged and fitted to a curve (see curves  510 ,  520 ,  530 ,  540  and  550 ). These results have been contrasted with those for a single-wavelength plane switch (see curves  511 ,  521 ,  531 ,  541  and  551 ) under the same conditions of the wavelength conversion resource factor (“R”) and the requirement for wavelength conversion (“B”).  
                                                                                                                   Available       # of external ports (and corresponding           external       % load) at which . . .                Curve   R   ports   B   P block  = 1%   P block  = 0.1%                            510   3   384   25%   Never reached   330/384 (86%)           511   3   384   25%   218/384 (57%)   112/384 (29%)           520   3   384   33%   310/384 (81%)   230/384 (60%)           521   3   384   33%   154/384 (40%)    74/384 (19%)                  
 
         [0083]    In both of curves  510  and  520 , R=3, i.e., the photonic switch core has 3 times as many available external input ports as there are internal input ports. Between curves  510  and  520 , the wavelength conversion requirement (B) has been increased. As seen from the above table, a blocking probability of 1% is never reached (i.e., regardless of switch load) and a blocking probability of 0.1% is reached at 86% load, for the case where there is a 25% wavelength conversion requirement. Meanwhile, a blocking probability of 1% is reached at 81% load and a blocking probability of 0.1% is reached at 60% load, for the case where there is a 33% wavelength conversion requirement. Thus, FIG. 5B shows that a combination of controlled over-provisioning and wavelength-group switching allows highly desirable blocking performance to be achieved.  
         [0084]    This compares favourably to the single-wavelength plane switch (curves  511  and  521 ), in which a blocking probability of 1% is reached at only 57% load and a blocking probability of 0.1% is reached at only 29% load, for the case where there is a 25% wavelength conversion requirement, while a blocking probability of 1% is reached at only 40% load and a blocking probability of 0.1% is reached at only 19% load, for the case where there is a 33% wavelength conversion requirement.  
                                                                                                                   Available       # of external ports (and corresponding           external       % load) at which . . .                Curve   R   ports   B   P block  = 1%   P block  = 0.1%                            520   3   384   33%   310/384 (81%)   230/384 (60%)           521   3   384   33%   154/384 (40%)    74/384 (19%)           530   11/5    352   33%   Never reached   342/352 (97%)           531   11/5    352   33%   278/352 (79%)   138/352 (39%)                  
 
         [0085]    In both of curves  520  and  530 , the wavelength conversion requirement (B) is kept constant at 33%, i.e., one out of every three incoming single-wavelength optical signals will require wavelength conversion. Between curves  520  and  530 , the wavelength conversion resources have been enhanced, for example by adding a line card. As seen from the above table, a blocking probability of 1% is reached at a switch load of 81% load and a blocking probability of 0.1% is reached at 60% load, for the case where there are three times as many external input ports as internal input ports. Meanwhile, a blocking probability of 1% is never reached (regardless of switch load) and a blocking probability of 0.1% is reached at 97% load, for the case where there are 2.2 times as many external input ports as internal input ports.  
         [0086]    This compares favourably to the single-wavelength-plane switch (curves  521  and  531 ), in which a blocking probability of 1% is reached at a switch load of only 40% load and a blocking probability of 0.1% is reached at only 19% load, for the case where there are three times as many external input ports as internal input ports, while a blocking probability of 1% is reached at only 79% load and a blocking probability of 0.1% is reached at only 39% load, for the case where there are 2.2 times as many external input ports as internal input ports.  
                                                                                                                   Available       # of external ports (and corresponding           external       % load) at which . . .                Curve   R   ports   B   P block  = 1%   P block  = 0.1%                            540   1   256   100%   226/256 (88%)   164/256 (64%)           541   1   256   100%   146/256 (57%)   100/256 (39%)           550   7/9   224   100%   Never reached   218/224 (97%)           551   7/9   224   100%   170/224 (76%)   122/224 (54%)                  
 
         [0087]    In both of curves  540  and  550 , the wavelength conversion requirement (B) is kept constant at 100%, i.e., every incoming single-wavelength optical signal requires conversion of its wavelength. Between curves  520  and  530 , the wavelength conversion resources have been enhanced to the point where, in curve  550 , the number of internal input ports exceeds the number of external input ports. As seen from the above table, a blocking probability of 1% is reached at a switch load of 88% load and a blocking probability of 0.1% is reached at 64% load, for the case where there are as many external input ports as internal input ports. Meanwhile, a blocking probability of 1% is never reached (reached regardless of switch load) and a blocking probability of 0.1% is reached at 97% load, for the case where there are 0.78 times as many external input ports as internal input ports.  
         [0088]    This compares favourably to the single-wavelength-plane switch (curves  541  and  551 ), in which a blocking probability of 1% is reached at a switch load of only 57% load and a blocking probability of 0.1% is reached at only 39% load, for the case where there are as many external input ports as internal input ports, while a blocking probability of 1% is reached at a load of only 76% and a blocking probability of 0.1% is reached at only 54% load, for the case where there are 0.78 times as many external input ports as internal input ports.  
         [0089]    From the above, it is clear that the wavelength-group photonic switch module architecture provides substantially improved performance in comparison to the single-wavelength-plane switch. Specifically, configurations of the optical switch  200  of the present invention exist for which 0.1% blocking probability is achieved even when the load of the switch is as high as 97%, and even when each of the incoming single-wavelength optical signals is required to undergo wavelength conversion. This will clearly satisfy the needs of tandem and core switches in existing and future metropolitan networks. The superior performance is achieved bat least in part because wavelength conversion resources are shared amongst a group of wavelengths, which means that wavelength conversion of a signal at a particular wavelength is possible, as long as there remains one available path to the wavelength converter for that group, and not on an individual per-wavelength basis.  
         [0090]    Moreover, the switch remains modular in that additional wavelength-group switching modules can be added as needed to satisfy the requirements of a particular application. This exemplifies the superiority of the design of the optical switch  200  vis-à-vis a three-dimensional fully non-blocking device, which suffers from an inability to scale up or down with an increase or decrease in the number of wavelengths and or optical signals to be switched.  
         [0091]    The optical switch  200  can be provided with various enhancements and optional features as now described with reference to FIGS. 6, 7 and  8 , although they need not be applied progressively in that order. For example, FIG. 6 shows an optical switch  600  that is provided with protection switching capability in addition to per-wavelength-group optical switching. A description of the protection switching capability has been provided in the context of a single-wavelength-plane switch in above-mentioned U.S. patent application Ser. No. 09/726,027.  
         [0092]    Specifically, in order to provide protection against the possibility of a failure of one of the wavelength-group photonic switching modules  250   1,2, . . . ,G , an additional photonic switching module  650  can be provided. This would allow for a 1:G protection switching architecture. In order to implement this scheme, the m th  line card of the switch  200 , which already contains the m th  WDD device  230   m  and the m th  WDM device  240   m , is now also equipped with a respective input protection switch  630   m  and a respective output protection switch  640 .  
         [0093]    The input protection switch  630   m  serves to intercept any of the G groups of carriers leading to the G wavelength-group photonic switching modules  250   1,2, . . . ,G  and to route the intercepted group to the protection photonic switching module  650 . The output protection switches  640   1,2, . . . ,M  serve to inject the switched carriers arriving from the protection photonic switching module  650  into the various paths that the intercepted carriers would have followed, had they not been intercepted by the input protection switch  630   m .  
         [0094]    The protection switches  630   1,2, . . . ,M  can be implemented in many ways, such as through the use of a multi-port MEMS device as described in above-mentioned U.S. patent application Ser. No. 09/726,027. In normal operation, all carriers transit the MEMS from left to right and all MEMS mirrors do not obstruct the path of the incoming single-wavelength optical signals. In the event of a failure of one of the G wavelength-group photonic switching modules  250   1,2, . . . ,G , a group of MEMS mirrors associated with the carriers leading to the failed wavelength-group photonic switching module are raised into the optical paths transiting the protection switch  630  and thus deflect the intercepted incoming single-wavelength optical signals towards the protection photonic switching module  650 . In order to enable the deflection, a sufficient number of mirrors is required in the m th  protection switch  630   m  to permit all of the optical carriers leading from the m th  WDD device  230   m  to any one of the wavelength-group photonic switch modules  250   1,2, . . . ,G  to be diverted towards the protection photonic switching module  650 . Additional mirrors may be provided so as to permit test signals to be injected into the failed wavelength-group photonic switch module in order to confirm that it has failed, and to test its replacement before returning to service. Control of the protection photonic switching module  650  and the protection switches  630   1,2, . . . ,G ,  640   1,2, . . . ,G , is provided by the switch controller  290 .  
         [0095]    In addition, it is desirable that the ongoing (densely) wavelength division multiplexed carriers leaving the switch  200  do so at approximately equal powers per optical carrier. It is especially important to do so in situations where the carriers in any given outgoing multiplexed optical signal have completely different ancestries coming into the switch  200 . FIG. 7 shows a switch  700  that is similar to the optical switch  600  of FIG. 6 but which has been enhanced with optical carrier power flattening functionality, which ensures that each optical carrier in a DWDM group is transmitted with the same power level. A description of the optical carrier power flattening capability has been provided in the context of a single-wavelength-plane switch in aforementioned U.S. patent application Ser. No. 09/580,495.  
         [0096]    Specifically, the switch  700  in FIG. 7 includes a set of variable optical amplifiers/attenuators (VOAs, collectively denoted  710 ) are placed in series with each of the external output ports of the wavelength-group photonic switching modules  250   1,2, . . . ,G . The VOAs  710  may be located on the switch fabric cards or trib cards. The attenuation or amplification of each of the VOAs  710  is controlled by a feedback loop, based on tapping the output multi-wavelength optical signal in an asymmetric (e.g. 5%) tap and feeding a spectrum analysis block. The outputs of power monitors in the spectrum analysis block are used to adjust the amplification or attenuation of each of the VOAs  710  to a level that results in the required optical power in each of the carriers in the output multi-wavelength optical signals and hence in a flat output spectrum.  
         [0097]    Yet another variation of the present invention, an embodiment of which is shown in FIG. 8, provides an optical switch  800  that has the per-wavelength-group switching functionality of switch  200 , the protection switching functionality of switch  600  and the optical carrier power flattening functionality of switch  700 . In addition, the optical switch  800  features connection verification capabilities, as described in above-mentioned U.S. Patent Application Serial No. 60/207,292 in the context of a single-wavelength plane switch. Specifically, the switch  800  is equipped with a set of optical splitters at an input end, each associated with a corresponding optical fiber transporting an individual incoming multi-wavelength optical signal to the photonic switch core  210 .  
         [0098]    The switch  800  also utilizes optical splitters at an output end in order to recover a portion of the power of each outgoing multi-wavelength optical signal optical signal, which can then be optically demultiplexed, thus affording visibility into the set of single-wavelength optical signals which exit the external output ports of the photonic switch core  210 . Moreover, additional optical splitters (not shown) may be provided at the input to the wavelength conversion module  220 , thereby to provide full visibility of all of the single-wavelength optical signals having been switched by the photonic switch core  210 .  
         [0099]    The switch  800  is further equipped with a path integrity analyzer  830  connected to the splitters and to the splitters. The path integrity analyzer  830  can thus ascertain the integrity of the connection involving each individual single-wavelength optical signal at the output of the switch  800  by comparing it to the incoming single-wavelength optical signal from which it is expected to be derived, as determined from the connection map received from the switch controller  290  (not shown in FIG. 8). Those skilled in the art will appreciate that the details of the path integrity analyzer  830  are of little significance to this embodiment, as path integrity may be assessed in a number of ways such as by performing a comparison of detected and expected test signals (see above-mentioned U.S. Patent Application Serial No. 60/207,292) or by evaluating the strength of a correlation existing between pairs of input and output signals (see above-mentioned U.S. patent application Ser. No. 09/742,232).  
         [0100]    Of course, those skilled in the art will appreciate that an optical switch having any combination of the individual additional features described with reference to FIGS. 6, 7 and  8  is within the scope of the present invention.  
         [0101]    While specific embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that numerous modifications and variations can be made without departing from the scope of the invention as defined in the appended claims.