Abstract:
An optical cross-connect system and method allows service providers to economically and efficiently handle capacity upgrades to meet future demands. The optical cross-connect can be embodied as a three-stage switch having a first, a middle, and a last stage. Capacity upgrades may be accomplished by adding additional first and last stage switches to meet increased demand and by replacing the middle stage switches. Accordingly, the original first and last stage switches may be retained in the upgraded optical cross-connect. The resulting optical cross-connect may include both optical and electronic components and the upgrade may be performed without interrupting service.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a system and method for a scalable optical cross-connect in an optical telecommunications system and, in particular, an optical cross-connect system and method that efficiently and economically handle capacity upgrades. 
     2. Description of the Related Art 
     Advances in computer and network technology have made it simple and convenient to send and receive information throughout the United States, and indeed throughout the world. Internet usage has expanded rapidly within the past few years, and the information available and the number of people able to access that information has grown exponentially. It is now common to collect a variety of information through the Internet, including educational, consumer, recreational, and commercial information. More transactions are being conducted through the Internet and more business, medical, and government transactions are becoming paperless. Currently, huge volumes of information must be transferred to meet Internet and business communication demands. 
     As computer technology advances and today&#39;s possibilities become tomorrow&#39;s reality, the demand for information is expected to increase. For example, graphics and images require a significantly greater volume of data than does simple text. As quality graphics and real-time image processing applications become commonplace, additional huge volumes of data will need to be transferred rapidly. Moreover, video-on-demand services, video telephone and teleconferencing services, and medical image archiving and retrieval, to name just a few, are expected to expand in the coming years. 
     The telecommunications network serves as the pipeline through which the bulk of information is transferred. Network service providers have begun to turn to new types of optical equipment ideally suited to meet current and fixture demands for information. One such type of optical equipment is the optical cross-connect switch. Optical cross-connects (OXCs) perform switching operations in networks, such as ring and mesh networks, so that information can travel to its intended recipient. Optical cross-connects enable network service providers to switch high-speed optical signals efficiently. For example, an OXC stationed in Chicago may receive incoming information from New York and strip off the received information destined for Chicago, switch a portion of the received information to Houston, and switch another portion of the information to San Francisco. 
     However, optical switching equipment is expensive. There remains a need for systems and methods that can handle the information volumes anticipated in the near-term and that are capable of economically expanding to meet long term demands. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above circumstances and has as an object to provide a simple and economical system and method for scaling optical communications equipment. 
     A further object of the invention is to provide a versatile optical cross-connect design capable of economically scaling to meet future needs. 
     A further object is to provide an efficient and economical method for upgrading the capacity of an optical cross-connect. 
     Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises an optical cross-connect in an optical telecommunications network comprising a plurality of first stage switch matrices, first and second cabinets, a plurality of middle stage switch matrices having input ports and output ports, and a plurality of last stage switch matrices having input ports and output ports. Each of the first stage switch matrices have a plurality of input ports, each input port receiving an input communication signal, and a larger number of output ports, where the first stage switch matrices switch the input communication signals to selected output ports. Each of the first and second cabinets have a predetermined number of bays, with at least one of the first stage switch matrices housed in a bay of the first cabinet and at least one of the first stage switches housed in a bay of the second cabinet. The input ports of the middle stage switch matrices are coupled to the output ports of the first stage switch matrices for receiving communication signals output from the first stage switch matrices. The middle stage switch matrices switch communications signals received at their input ports to their output ports. The input ports of the last stage switch matrices are coupled to the output ports of the middle stage switch matrices for receiving communication signals output from said middle stage switch matrices. The last stage switch matrices switch communications signals received at their input ports to their output ports. In addition, the middle stage itself can be recursively a multistage switch. 
     The invention further comprises an optical telecommunications network comprising a plurality of network nodes interconnected via fiber optic links, wherein at least one of the network nodes includes an optical cross-connect. The optical cross-connect includes a plurality of first stage switch matrices, first and second cabinets, a plurality of middle stage switch matrices having input ports and output ports, and a plurality of last stage switch matrices having input ports and output ports. Each of the first stage switch matrices have a plurality of input ports, each input port receiving an input communication signal, and a larger number of output ports, where the first stage switch matrices switch the input communication signals to selected output ports. Each of the first and second cabinets have a predetermined number of bays, with at least one of the first stage switch matrices housed in a bay of the first cabinet and at least one of the first stage switches housed in a bay of the second cabinet. The input ports of the middle stage switch matrices are coupled to the output ports of the first stage switch matrices for receiving communication signals output from the first stage switch matrices. The middle stage switch matrices switch communications signals received at their input ports to their output ports. The input ports of the last stage switch matrices are coupled to the output ports of the middle stage switch matrices for receiving communication signals output from said middle stage switch matrices. The last stage switch matrices switch communications signals received at their input ports to their output ports. 
     The present invention further includes a method for scaling an optical cross-connect to a larger capacity, where the optical cross-connect includes first stage working switches, middle stage working switches having inputs coupled to outputs of the first stage working switches, and last stage working switches having inputs coupled to outputs of the middle stage working switches. The first, middle, and/or last stage switches can be formed by multistage switches themselves. The method comprising the steps of coupling the outputs of the first stage working switches and outputs of additional first stage working switches to inputs of replacement middle stage working switches, and coupling the inputs of the last stage working switches and inputs of additional last stage working switches to outputs of the replacement second stage working switches. 
     In addition, the present invention allows the complete replacement of the entire switch with an all-optical fabric that may have a smaller number of ports, but that allows for greater scalability because of its photonic nature. The individual ports can be run at higher bit rates. 
     The present invention further provides a communications switching apparatus for an optical telecommunications network including a plurality of first stage switch matrix cards, each having at least one first stage switch matrix with a plurality of inputs, each input receiving an input communication signal, and a larger number of outputs, where said first stage switch matrices switch the input communication signals to selected outputs; a plurality of last stage switch matrix cards, each having at least one last stage switch matrix with inputs and outputs, wherein said last stage switch matrices switch communications signals received at their input ports to selected output ports thereof, a frame having a first, second, and third groups of slots, wherein the first stage switch matrix cards are received in the first group of slots and the last stage switch matrix cards are received in the third group of slots, the second group of slots configured to receive middle stage switch matrix cards and optical extender module cards, the middle stage switch matrix cards each having at least one middle stage switch matrix having inputs and outputs, wherein the middle stage switch matrices switch communications signals received at their input ports to selected output ports; and a backplane coupled to the first stage switch matrix cards and the last stage switch matrix cards (1) for coupling the outputs of the first stage switch matrices to the inputs of the middle stage switch matrices and for coupling the outputs of the middle stage switch matrices to the inputs of the last stage switch matrices when the middle stage switch matrix cards are received in the second slots, and (2) for coupling communication signals from the outputs of the first stage switch matrices to the optical extender module cards and for coupling external communication signals received by the optical extender module cards to the inputs of the last stage switch when the optical extended module cards are received in the second slots. 
     The present invention further includes an optical communications switching apparatus for an optical communications network, comprising opto-electronic receivers for receiving optical signals on a plurality of optical fibers; an electronic switch matrix for switching electronic signals received from the first opto-electronic receivers, the electronic signals derived from the optical signals; and an optical switch matrix for switching signals from the electronic switch and optical signals from at least one optical fiber. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiment(s) of the invention and together with the description, serve to explain the principles of the invention. 
     FIG. 1A provides a schematic of an optical cross-connect (OXC) coupled to wavelength division multiplexing/demultiplexing (WDM) equipment through transponders in accordance with a first embodiment of the invention. 
     FIG. 1B illustrates a second embodiment of the invention in which the transport interface is built into the OXC. 
     FIG. 1C illustrates an embodiment of an OXC in accordance with the present invention embodied as a three-stage Clos matrix having a first stage, a middle stage, and a last stage. 
     FIG. 1D illustrates an embodiment of an OXC scaled from that shown in FIG.  1 C. 
     FIG. 2 illustrates a cabinet architecture that may be used in connection with the present invention. 
     FIGS. 3 and 4 illustrate an arrangement and technique for scaling the optical switch formed in the cabinet architecture of FIG. 2 to a higher capacity. 
     FIGS. 5A,  5 B, and  5 C illustrate possible switch arrangements for an OXC for the first, middle, and last stage switches in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope or spirit of the invention. 
     FIG. 1A provides a schematic of an optical cross-connect (OXC)  10  coupled to wavelength division multiplexing/demultiplexing (WDM) equipment  20 ,  30  and  40  through transponders  50 . Generally, the OXC  10  may be located at a network node. The OXC  10  may be connected in a mesh network, a ring network, a combination mesh/ring network, or another network architecture. OXC  10  serves. to switch incoming signals to selected output paths. The incoming signals to OXC  10  may have been received from other nodes in the network or from equipment of the same node as OXC  10 . More particularly, OXC  10  may receive input signals from and/or transmit output signals to one or more of an IP router, an ATM switch, a synchronous optical network (SONET) add-drop multiplexer or other SONET equipment, equipment from another or the same node, local equipment, or other equipment. OXC  10  may switch inputs to outputs in accordance with internal control signal(s), an external control signal(s), and/or control signals received via the input signals. 
     For example, as shown in FIG. 1A, WDM optical signals may be transmitted to and received from other network nodes using WDM equipment  20 ,  30 , and  40 . The WDM equipment  20 ,  30 , and  40  multiplex multiple optical wavelengths into WDM signals that may be transmitted to another node and demultiplex WDM signals received from other nodes into multiple optical wavelengths. OXC  10  may receive demultiplexed signals from one or more of WDM equipment  20 ,  30  and  40  and switch the received signals as appropriate to one or more of WDM equipment  20 ,  30 , and  40  for multiplexing and transmission. While three WDM equipment are shown in FIG. 1A, any number of WDM equipment may be used consistent with the present invention. 
     OXC  10  may be configured to operate in point-to-point, multicast, and/or drop-and-continue modes. In a point-to-point mode, a single input signal to the OXC  10  is applied to a single output of OXC  10 . For example, a demultiplexed signal received from WDM equipment  20  may be switched to WDM equipment  30  for transmission. Multicast mode involves the connection of a single input to multiple outputs. For example, a demultiplexed signal received from WDM equipment  20  may be switched to multiple channels of WDM equipment  30  or to both WDM equipment  30  and WDM equipment  40 . Drop-and-continue mode allows a signal to be split for connection to a drop port for a local connection and also continue to another network destination. For example, assuming that WDM equipment  30  corresponds to a local connection, a demultiplexed signal received from WDM equipment  20  may be switched to WDM equipment  30  and to WDM equipment  40 . The OXC  10  may be configured to operate in one or more of these modes simultaneously. For example, some input signals may be directed as point-to-point connections, others may be multicast, and yet others handled in drop-and-continue mode. 
     FIG. 1B illustrates an alternate embodiment in which the transport interface is built into the OXC  10 . Accordingly, separate transponders  50  are not required for WDM equipment  20 ,  30 , and  40 . This reduces the number of optical-to-electrical and electrical-to-optical conversions and reduces equipment costs. 
     FIG. 1C illustrates an embodiment of an OXC embodied as a three-stage matrix  100  having a first stage  110 , a middle stage  120 , and a last stage  130 . The first stage  110  can be connected to the middle stage  120  through a backplane  115  and the middle stage  120  can be coupled to the last stage  130  via abackplane  125 . The backplanes  115  and  125  may be high-speed subsystems with embedded traces to carry signals (e.g., control signals and data signals) between the first stage  110  and middle stage  120  and between the middle stage  120  and last stage  130 , respectively. The backplanes  115  and  125  may, for example, include plugs to receive cards on which the first, middle, and last stages  110 ,  120 , and  130  are mounted. The backplanes may carry electrical and/or optical signals. They may simply comprise electrical cable or optical fiber. The backplanes  115  and  125  may also couple to optical extenders, as discussed in more detail below. Additional backplanes may be provided if the switch stages themselves include a multi-stage arrangement. 
     In general, the first stage  110  includes K switches  111 - 1  through  111 -K (referred to collectively as first stage switches  111 ). Each of first stage switches  111  may receive N input signals and may produce  2 N output signals. When the number of outputs is  2 N- 1 , the result is a strictly non-blocking Clos architecture. The middle stage  120  includes  2 N switches  121 - 1  through  121 - 2 N (referred to collectively as middle stage switches  121 ). Each of the middle stage switches  121  receives an input signal from the K first stage switches  111 . Accordingly, each of middle stage switches  121  receives at least K input signals. Each of the middle stage switches  121  produces at least K output signals. The last stage  130  includes K switches  131 - 1  through  131 -K (referred to collectively as last stage switches  131 ). Each of the last stage switches  131  receives at least  2 N input signals, one from each of middle stage switches  121 . Last stage switches  131  produce at least N output signals. The matrix  10  is an K*N×K*N matrix because it includes K*N inputs and K*N outputs. 
     While each of switches  111  is shown to be of the same size, this need not be the case. Similarly, each of switches  121  and each of switches  131  need not be of the same size. Some or all of switches  111 ,  121 , and/or  131  may be formed of smaller switch matrices. Further, some or all of switches  111  and some or all of switches  121  may be implemented by larger switches. For example, the N× 2 N switches  111  may be implemented using  2 N× 2 N switches, for example. Switches  111 ,  121 , and/or  131  may have additional input and/or output ports (not shown) that may be used for other purposes, such as testing, service channels, local connections, or other purposes. 
     Each switch of the first, middle, and last stage switches  111 ,  121 ,  131  may be formed on its own card, for example, a printed circuit board, or may be combined with other switches on the same card. The switches  111 ,  121 ,  131  may be electronic switches, optical switches, or hybrid switches including both electronic and optical components. For example, in an embodiment of FIG. 1C in which N=16 and K=32, the thirty-two (32) first and last stage switches  111  and  131  are 16×32 switches and 32×16 switches, respectively. The thirty-two (32) middle stage switches  121  are 32×32 switches. In this example, the three-stage Clos switch matrix is capable of switching  512  inputs to any of  512  outputs. 
     In accordance with the present invention, the three-stage switch matrix may be scaled without replacing the first and last stage switches  111  and  131 . The middle stage switches  121  may be replaced to achieve scalability to arbitrary sizes. FIG. 1D illustrates the three-stage Clos switch matrix  100  scaled from an N×K capacity to a N× 2 K capacity by (1) adding K first stage switches  112 - 1  to  11   2 -K, each having N inputs and  2 N outputs and adding K last stage switches  132 - 1  to  132 -K, each having  2 N inputs and N outputs, and (2) replacing the middle stage switches  121  with  2 N middle stage switches  122 , each having  2 K inputs and  2 K outputs. Accordingly, the scaled matrix  100  has a total of  2 K first stage switches  111 ,  112  with N× 2 N capacity;  2 N middle stage switches  122  with  2 K× 2 K capacity; and  2 K last stage switches  131 ,  132  with  2 N×N capacity. 
     Using the example described in connection with FIG. 1C, the three stage Clos matrix  100  of FIG. 1D may be scaled from a 512×512 matrix to a 1024×1024 matrix by (1) adding thirty-two (32) new first stage switches  112  and thirty-two (32) new last stage switches  132  to the existing first stage switches  111  and last stage switches  131 , and (2) replacing the thirty-two (32) middle stage switches  121  with thirty-two (32) new 64×64 middle stage switches  122 . Accordingly, the first stage  110  includes thirty-two (32) original first stage switches  111  and thirty-two (32) new first stage switches  112 ; the middle stage includes thirty-two (32) new middle stage switches  122 ; and the last stage  130  includes thirty-two (32) original last stage  131  and thirty-two (32) new last stage switches  132 . Each of the new 64×64 middle stage switches  122  receives an input from the first stage switches  111  and  112  and supplies an output to each of the last stage switches  131  and  132 . While this embodiment illustrates an arrangement in which the new first and last stage switches  112  and  132  are all the same size and the same size as the original first and last stage switches  111  and  131 , it should be understood that the invention is not so limited. For example, some or all of the new first stage switches  112  may be multiples of the first stage switches  111 , e.g., 32×64 switches and/or 8×16 switches, which, of course, will affect the number of switches needed. Of course, the same is true of the second stage switches  122  and the last stage switches  132 . 
     FIG. 1D illustrates connections between the first stage  110  and the middle stage  120  are made via backplane  115  and that connections between the middles stage  120  and the last stage  130  are made via backplane  125 . Of course, additional backplane capacity may be added when scaling the matrix. Further, additional equipment, such as electrical or optical extenders, may be used to facilitate the connections. 
     FIG. 2 illustrates a cabinet architecture  200  that may be used in connection with the present invention. The cabinet architecture  200  includes four bays  210 ,  220 ,  240 , and  260 . The bays  210 ,  220 ,  230 , and  240  may each comprise a frame structure having slots. Of course a frame structure may be provided absent a cabinet architecture and even a bay. The first bay  210  and the fourth bay  260  may be transmitter/receiver bays. For example, as shown in FIG. 2, the upper and lower portions of the first bay  210  and fourth bay  260  may each house  128  transmitter/receiver (transceiver) cards  212 ,  262  for a total of  512  bi-directional ports that support 2.5 Gb/s signals, such as standard OC-48 or STM-16 signals. In particular, the cards  212 ,  262  may be received in slots provided in the first and fourth bays  212 ,  262 . Accordingly, the cabinet architecture  200  supports a 512×512 optical switch. 
     The second bay  220  may house master control equipment  222  and working switches  224 . The working switches  224  may be arranged with eight (8) middle stage switch cards  232  sandwiched between four (4) first and last stage switch cards  231  and four (4) first and last stage switch cards  233 , with the cards being received in slots. Each of the first/last stage switch cards  231  and the first/last stage switch cards  233  may hold, for example, four (4) first stage switches  111  and four (4) last stage switches  131 . Each first stage switch  111  may be a 16×32 switch and each last stage  131  switch may be a 32×16 switch. Each of the middle stage switch cards  232  may hold, for example, four (4) middle stage switches  121 . The middle stage switches  121  may be 32×32 switches. The first, middle, and last stage switches  111 ,  121 ,  131  may be interconnected as described above and as shown in FIG. 1C, for example, using backplanes (not shown) into which the switch cards plug. 
     The third bay  240  may house synchronization control equipment  242  and protection switches  244 . In general, the protection switches  244  may be identical to the working switches  224 . The working switches  224  are active during normal operations, with the protection switches  244  serving as back-ups to prevent or restrict service outage in the event that one or more working switches  224  malfunction. The protection switches  244  may be arranged similar to the working switches  224 , with eight (8) middle stage protection switch cards  252  sandwiched between four (4) first/last stage protection switch cards  251  and four (4) first/last stage protection switch cards  253 . Each of the first/last stage protection switch cards  251  and the first/last stage protection switch cards  253  may hold, for example, four (4) first stage protection switches  111  and four (4) last stage protection switches  131 . Each of the middle stage protection switch cards  252  may hold, for example, four (4) middle stage protection switches  121 . The cards may be inserted into slots. As above, the first stage switches  111  may be 16×32 switches, the middle stage switches  121  may be 32×32 switches, and the last stage switches  131  may be 32×16 switches. The first stage switches, the middle stage switches, and the last stage switches may be interconnected using one or more backplanes, as discussed above. 
     In some arrangements, there may be fewer protection switches  244  than working switches  224 . It should be understood that any arrangement of first and last stage switch cards and/or switches may be used. The second and third bays  220  and  240  may also include shelf controller modules (SCM) for managing each shelf individually. A master controller module (not shown) integrates all SCMs for overall system control. 
     FIGS. 3 and 4 illustrate an arrangement and technique for scaling the 512×512 optical switch formed by cabinet  200  to a higher capacity. FIG. 3 illustrates cabinet  200  scaled to a higher switching capacity. In this case, the first and fourth bays  210  and  260  are unchanged. Further, the first and last stage switch cards  231 ,  233 ,  251 ,  253  are unchanged. However, optical extender modules (OEMs)  281 ,  282  have replaced middle stage switches  232 ,  252 . For example, the OEMs  281 ,  282  may be provided on cards that are received in the slots previously occupied by the middle stage switch cards to interconnect with the backplane. 
     OEMs  281 ,  282  serve to optically interconnect the first and last stage switch cards  231 ,  233 ,  251 ,  253  to a larger middle stage matrix  500  via optical fibers, as discussed in greater detail below. The OEMs may include electro-optical converters to convert electrical signals to optical signals for transmission and/or opto-electrical converters to convert transmitted optical signals to electrical signals. Of course, one or both of electro-optical converters and opto-electrical converters may not be used, depending on the desirability of electrical or optical signals at a particular stage. OEMs are useful if electrical cables and/or backplane of the switching equipment make scaling difficult. Moreover, optical transmission may be used to improve signal quality over longer distances. Although the FIG. 3 illustrates optical extenders, the extenders may be electrical if the distances between the expanded middle stage matrix  500  and the existing matrix bays  220 ,  240  are short enough. 
     FIG. 4 shows the scaling of the 512×512 optical switch of cabinet  200  to a 1024×1024 optical switch. As shown in FIG. 4, a second cabinet  400  is deployed. The second cabinet  400  may be arranged in a similar way to that of first cabinet  200 . In particular, the second cabinet  400  includes four bays  410 ,  420 ,  440 , and  460 . The first bay  410  and fourth bay  460  may be transmitter/receiver bays, which may each have upper and lower portions that house, for example,  128  transceiver cards  412 ,  462 . For example, the 128 transceiver cards  412 ,  462  may accommodate a total of  512  bidirectional ports that support 2.5 Gb/s signals, such as standard OC-48 or STM-16 signals. 
     The second bay  420  of the second cabinet  440  may house equipment including working switches  424 . The working switches  424  may be arranged such that eight (8) OEMs  481  are sandwiched between four (4) first and last stage switch cards  431  and four (4) first and last stage switch cards  433  in provided slots. Each of the first/last stage switch cards  431  and the first/last stage switch cards  433  may hold, for example, four (4) first stage switches  112  and four (4) last stage switches  132 . The third bay  440  may house equipment including protection switches  444 , which may be identical to the working switches  424 . The working switches  424  are active during normal operations, with the protection switches  424  serving as back-ups to prevent or reduce service outage in the event that one or more working switches  424  malfunction. The protection switches  444  may be arranged similar to the working switches  424 , with eight (8) OEMs  482  sandwiched between four (4) first and last stage protection switch cards  451  and four (4) first and last stage protection switch cards  453 . Each of the first/last stage protection switch cards  451  and the first/last stage protection switch cards  453  may hold, for example, four (4) first stage protection switches  112  and four (4) last stage protection switches  132 . In both the working switches  424  and the protection switches  444 , the first stage switches  112  may be 16×32 switches and the last stage switches  132  may be 32×16 switches. The switches may be interconnected using one or more backplanes, for example, into which the switch cards plug. 
     FIG. 4 further includes a switching matrix  500 , including working switch matrix  524 , a protection switch matrix  544 , and OEMs  581  and  582 . A first bay  520  may house working switch matrix  524  and OEMs  581  and a second bay  540  may house the protection switch matrix  544  and OEMs  582 . Working switch matrix  524  may include sixteen (16) working switch cards  532 . Protection switch matrix may include sixteen (16) protection switch cards  552 . Each of the working and protection switch cards  532 ,  552  may include two (2) middle stage switches  122 , for a total of thirty-two (32) middle stage working switches and thirty-two (32) middle stage protection switches. The middle stage switches  122  in this case may be 64×64 switches. As above, the switch cards may be received in slots. 
     OEMs  581  interconnect switch matrix  524  between OEMs  381  of cabinet  200  and OEMs  481  of cabinet  400 . Similarly, OEMs  582  interconnect protection switch matrix  544  between OEMs  282  of cabinet  200  and OEMs  482  of cabinet  400 . Specifically, some of OEMs  581  are coupled to receive incoming signals from those OEMs  281  and  481  that couple to first stage switches on cards  231 ,  431 . Incoming signals received by OEMs  581  are coupled as input signals to working switches  524 . Output signals from working switches  524  are supplied to OEMs  581 , which connect those outputs signals to the last stage switches on cards  233 ,  433  via OEMs  281 ,  481 . 
     Similarly, some of OEMs  582  are coupled to receive incoming signals from those OEMs  282  and  482  that couple to first stage protection switches on cards  251 ,  451 . The incoming signals received by OEMs  582  are coupled as input signals to protection switches  544 . Output signals from protection switches  544  are supplied to OEMs  582 , which connect those outputs signals to the last stage protection switches on cards  253 ,  453  via OEMs  282 , 482 . 
     The scaled optical cross-connect arrangement illustrated in FIG. 4 includes four (4) bays  210 ,  260 ,  410 , and  460  of optical input/output ports, two (2) bays of working first and last stage switches  220  and  420 , two (2) bays of first and last stage protection switches  240  and  440 , one (1) bay for the middle stage working switches  520 , and one (1) bay for the middle stage protection switches  540 . 
     The scaling of the switching matrix in FIG. 2 to the switching matrix shown in FIG. 4 can be made in-service. More particularly, the scaling may be achieved in an exclusively nonblocking manner, without requiring an interruption in service through the switching matrix. This may be accomplished as follows. Service is provided on cabinet  200  through one of the working switches  224  or the protection switches  244 . For purposes of illustration, assume that service is provided on the working switches  224 . The middle stage protection switches  252  may be swapped out of the third bay  240  in favor of OEMs  282 . The OEMs  282  are connected to OEMs  582  so as to establish a connection path through middle stage protection switches  544  of switching matrix  500 . At this point, the protection switches  244  of cabinet  200  can be filly interconnected with protection switches  544  of switching matrix  500  and protection switches  444  of cabinet  400 . Accordingly, service on cabinet  200  can be switched over from working switches  224  to protection switches  244  without dropping service. 
     With service off of working switches  224 , the middle stage switch cards  232  can be swapped out in favor of OEMs  281 . As noted above, OEMs  281  may be connected to OEMs  581  to establish a connection path through the working switches  524  of switching matrix. In this way, the second bay  220  of cabinet  200  may be fully interconnected to the switching matrix  500  and the second cabinet  400 . Service may be maintained on the protection switches of cabinets  200 , cabinet  400 , and switching matrix  500 , reserving the working switches of cabinet  200 , cabinet  400 , and switching matrix  500  for protection. Accordingly, the protection switches would be effectively become the working switches, and the working switches would become the protection switches. Alternatively, service may be switched back over to the working switches, reserving the protection switches for protection. 
     It should be noted that although cabinet  200  was doubled in scale without a loss of service, only the working and protection middle stage switches  232  and  252  were replaced from the original arrangement of cabinet  200 . Accordingly, the scaling arrangement and technique described herein is capable of reducing the equipment replaced in the changeover. 
     Moreover, while the above example illustrates an arrangement in which scaling is accomplished without loss of service where each working switch has a corresponding protection switch, scaling may also be accomplished without loss of service even in arrangements in which there are fewer protection switches than working switches. In such a case, the protection switches of cabinets  200  and  400  may be transitioned first to switching matrix  500  as described above, and then the working switches  232 ,  432  may be transitioned to switching matrix  500  in groups, for example of one or more cards at a time, with the protection switches providing service for each group of working switches while that group is being transitioned. 
     The example provided in FIGS. 2-4 is based upon the transition of a 512 port OXC to a 1024 port OXC. The following table illustrates the physical size of the OXC as a function of the number of cabinet bays it may occupy. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                 # I/O 
                 # 1st/Last SW Bays 
                 # Middle SW Bays 
                 Total # 
               
             
          
           
               
                 # Ports 
                 Bays 
                 Working 
                 Protect 
                 Working 
                 Protect 
                 of Bays 
               
               
                   
               
             
          
           
               
                  512 
                 2 
                 1 Bay for working 1st, Mid, Lst SW 
                 4 
               
               
                   
                   
                 1 Bay for protection 1st, Mid, Lst SW 
               
             
          
           
               
                 1024 
                 4 
                 2 
                 2 
                 1 
                 1 
                 10 
               
               
                 2048 
                 8 
                 4 
                 4 
                 2 
                 2 
                 20 
               
               
                 8192 
                 32 
                 16 
                 16 
                 32 
                 32 
                 128 
               
               
                   
               
             
          
         
       
     
     FIGS. 5A,  5 B, and  5 C illustrate possible switch arrangements for an OXC  10  for the first middle, and last stage switches  110 ,  120 , and  130 . FIG. 5A illustrates an arrangement in which each of the first stage switches  110 , the middle stage switches  120 , and the last stage switches  130  are each formed by an electronic switch fabric. Accordingly, incoming optical signals on transmission optical fibers  102  are received by opt-electrical receivers  140 , which convert the incoming optical signals into electrical signals. The electrical signals are applied to the first stage switches  110 . Electrical extensions  103  couple the output of the first stage  110  to the middle stage switches  120 . Similarly, electrical extensions  105  couple the output of the middle stage switches  120  to the last stage switches  130 . The output of the last stage switches  130  is received by electro-optical transmitters  150 , converted into optical signals, and transmitted on transmission optical fibers  108 . Transmission optical fibers  102  and  108  may be coupled to WDM equipment  20 ,  30 , and  40 , as illustrated in FIGS. 1A and 1B. 
     FIG. 5B illustrates an arrangement of OXC  10  in which the first, middle, and last stage switches are each formed by an electronic switch fabric. Incoming optical signals on optical fibers  102  are received by opto-electrical receivers  140 , which convert the incoming optical signals into electrical signals. The electrical signals are applied to the first stage switches  110 . The output of first stage switches  110  is converted into optical signals by electro-optical transmitters  141  and transmitted over optical fibers  104 . The optical signals are received by opto-electrical receivers  142  and converted back into electrical signals. The electrical signals are applied to middle stage switches  120 . The output of middle stage switches  120  is converted into optical signals by electro-optical transmitters  143  and transmitted over optical fibers  106 . The optical signals are received by opto-electrical receivers  144 , converted back into electrical signals, and applied to last stage switches  130 . The output of the last stage switches  130  is received by electro-optical transmitters  150 , converted into optical signals, and transmitted on optical fibers  108 . 
     FIG. 5C illustrates a hybrid arrangement in which the first and last stage switches  110  and  130  are each formed by an electronic switch fabric and the middle stage switches  120  are formed by an optical switch fabric. Optical fibers  102  are coupled either to opto-electrical receivers  140  or to optical transmitter/receivers (TRs)  146 . Moreover, if optical termination is not needed, optical fibers  102  may couple directly to the optical switch fabric  120 . Opto-electrical receivers  140  receive incoming signals from optical fibers  102  and convert the incoming optical signals into electrical signals. The electrical signals are applied to the first stage switches  110 . The output of first stage switches  110  is converted into optical signals by electro-optical transmitters  141  and transmitted over optical fibers  104 . Additional equipment may be provided between the opto-electronic receivers  140  and the electro-optical transmitters  141 , for example, to condition or groom the electrical signal. The optical signals from electro-optical transmitters  141  and from TRs  146  are applied to middle stage switches  120 . The output of middle stage switches  120  is transmitted over optical fibers  106  to either opto-electrical receivers  144 , to optical transmitter/receivers  148 , or to optical fibers  108 . Opto-electrical receivers  144  convert received optical signals into electrical signals and apply the electrical signals to last stage  130 . The output of the last stage switches  130  is received by electro-optical transmitters  150 , converted into optical signals, and transmitted on optical fibers  108 . Additional equipment may be provided between the opto-electronic receivers  144  and the electro-optical transmitters  150 , for example, to condition or groom the electrical signal. The optical signals received by optical TRs  148  are also transmitted on optical fibers  108 . TRs  146 ,  148  may connect directly to transport fibers, for example, fibers  102  and  108 , when those fibers are implemented as transport fibers. It should be noted that optical signals on a fiber  102  may be applied directly to the middle stage  120 , which may switch the optical signals to an optical fiber  108  for transmission. 
     The hybrid electrical/optical architecture shown in FIG. 5C has a number of advantages. For example, the optical middle stage matrix  120  permits the electronic first and last stage matrices  110 ,  130  to be bypassed if necessary so that the TRs can connect directly to the optical middle stage matrix  120 . This permits the format and/or bit rate of signals applied to middle stage  120  to be independent. For example, the TRs can be operated at 40 Gb/s without any demultiplexing down to signal sub-rates (such as OC-48). The entire signal received on the TRs can be switched if necessary. 
     Table 2 below illustrates exemplary scenarios for scaling an OXC to very large port counts where the size of the first and last stage switches remains constant, in this example 16×32 for the first stage switches and 32×16 for the last stage switches. Table 2 further assumes a constant data rate of 2.5 Gigabits/sec as an example. Of course the data rate may be different for some or all of the switches. Note that to achieve large port counts, the middle stage switches may be formed of multiple switch stages. For example, as shown in table 2, the middle stage switches may implemented using 32×32 electronic switch chips, a middle stage switch having  512  inputs and  512  outputs may be built using three stages of the 32×32 electronic switch chips. Table 2 additionally illustrates hybrid arrangements in which the first and last stages are composed of electrical switching components and the middle stage is composed of optical switching components. 
     
       
         
               
             
               
               
             
               
               
               
               
               
             
               
             
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 Electrical Switch Fabric 
               
             
          
           
               
                 Size of 1st/Lst stage 
                 16 × 32/32 × 16 
               
             
          
           
               
                 Size of middle stage 
                 32 × 32 
                 64 × 64 
                 512 × 512 
                 2048 × 2048 
               
               
                 No. of middle stages 
                 1 
                 1 
                 3 
                 3 
               
               
                   
                   
                   
                 (32 × 32 chips) 
                 (64 × 64 chips) 
               
               
                 Total number of stages 
                 3 
                 3 
                 5 
                 5 
               
               
                 Fabric data rate 
                 2.5 Gb/s 
                 2.5 Gb/s 
                 2.5 Gb/s 
                 2.5 Gb/s 
               
               
                 input/output ports 
                 512 
                 1024 
                 8192 
                 32K 
               
               
                 Total capacity 
                 1.28 Tb/s 
                 2.5 Tb/s 
                 20 Tb/s 
                 80 Tb/s 
               
               
                   
               
             
          
           
               
                 Hybrid-Electrical 1st/Last, Optical Middle Stages 
               
             
          
           
               
                 Size of 1st/Lst stage 
                 16 × 32/32 × 16 
               
             
          
           
               
                 Size of middle stage 
                 256 × 256 
                 1024 × 1024 
                 2048 × 2048 
                 8192 × 8192 
                 32768 × 32768 
               
               
                 No. of middle stages 
                 1 
                 1 
                 3 
                 3 
                 3 
               
               
                   
                   
                   
                 (64 × 64 Mod.) 
                 (128 × 128 Mod.) 
                 (256 × 256 Mod.) 
               
               
                 Total number of stages 
                 3 
                 3 
                 5 
                 5 
                 5 
               
               
                 Fabric data rate 
                 2.5 Gb/s 
                 2.5 Gb/s 
                 2.5 Gb/s 
                 2.5 Gb/s 
                 2.5 Gb/s 
               
               
                 Input/output ports 
                 4096 
                 16K 
                 32K 
                 131K 
                 524K 
               
               
                 Total capacity 
                 10 Tb/s 
                 40 Tb/s 
                 80 Tb/s 
                 328 Tb/s 
                 1310 Tb/s 
               
               
                   
               
             
          
         
       
     
     The electronic switch fabrics may be formed using gallium arsenide (GaAs), silicon bipolar, silicon germanium (SiGe), BiCMOS, or other semiconductor technologies. For example, the first, middle, and last stage switches  111 ,  121 ,  131  may be implemented using switches from several vendors. Moreover, switches having 10 Gigabits/sec. capacity are now becoming available. 
     Several different optical fabric switch designs may be used. For example, two-dimensional microelectromechanical systems (MEMS) switches, two-dimensional waveguided switches (such as a bubble switch), three-dimensional MEMS switches, or another optical switch fabric. For example, a two-dimensional MEMS switch may include an N×N array of movable mirrors, each of which can be positioned to reflect an incident beam or be moved out of the way to allow the beam to pass. 
     A two-dimensional waveguided switch may be, for example, Hewlett-Packard&#39;s bubble switch, which reduces beam diffraction by confining the light in two sets of intersecting waveguides. This is accomplished using a narrow trench etched across each waveguide intersection and that is filled with an index-matching fluid. A slot can be formed in a mirror, which will reflect light from one waveguide into the other, by vaporizing some of the fluid to form a bubble. 
     A three-dimensional MEMS switch, for example an Astarte switch, collimates inputs using a two-dimensional array of collimators, and each collimator is equipped with a multi-position, two-axis angular deflector mirror using MEMS-type technology. Each mirror can deflect its input beam to any output port. An additional set of collimators with angular deflectors is used to couple the output signals to output fibers. The signal beams are accompanied by alignment beams, which are used with active servo systems to hold the mirrors in position for the desired connections. It is preferable to use optical switch fabrics in which the physical size and the optical loss are as small as possible, and within practical limits. 
     One of the major advantages of the present invention is that the cross-connect capacity is scalable to arbitrary sizes in a modular fashion. In addition, as shown in FIG. 5C, a hybrid electrical/optical architecture using an optical middle stage matrix permits the electronic first and last stage matrices to be bypassed if necessary so that the TRs can connect directly to the optical matrix. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.