Abstract:
A fiber-optic networking system that can survive a break in the fiber and restore full connectivity to all nodes in the network. The network can detect the presence of a cut in the fiber, locate the cut, recover from the cut, and restore the communication capacity that was available before the cut. The network is also capable of automatically determining the existence and identity of the nodes in the network, the distance of the node from the headend, and the executing procedures to handle both planned and unplanned (i.e., during a fault) removal of nodes from the network.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/301,669, filed on Jun. 28, 2001, which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The invention relates generally to communications systems and, more particularly, to fault tolerant networks for the transport of data (computer data, voice, video, or multimedia) over optical fiber. 
     BACKGROUND OF THE INVENTION 
     As is known in the art, the level of fault-tolerance and fault-recovery in a communications network depends upon the network topology and transmission format. Conventional optical networks, i.e., networks that use optical fiber as the transmission medium, can be configured using a variety of known topologies, each of which provides varying levels of fault-protection. For example, a network organized in a star topology offers little or no protection against a cut in the fiber from a node to the central unit (often referred to as a “hub”), because a cut totally isolates a node from the rest of the star. So-called ring topologies are popular because all nodes remain connected to each other in the event of a cut in one of the fibers. However, in these designs optical signals need to be received, processed and sometimes modified, and retransmitted at every node in order to detect a fiber cut and wrap traffic around the cut. In the event of a fiber cut, the affected nodes must detect, locate, and recover from the failure, and the other nodes need to participate in the recovery process. 
     SUMMARY OF THE INVENTION 
     The present invention provides an optical network that provides fault tolerant data communication. With this arrangement, the network can recover from a cable fault, such as a cut in an optical fiber. While the invention is primarily shown and described in conjunction with a network having a loop topology with a particular redundant fiber cable arrangement, it is understood that the invention is applicable to networks in general in which fault tolerance, recovery and location identification is desirable. 
     In one aspect of the invention, an optical network includes fault recovery for protecting the network from loss in connectivity in the presence of a cable fault. In one particular embodiment, a cable interconnecting a loop of nodes and a headend includes a primary transmit fiber and a primary receive fiber along with redundant secondary transmit and receive fibers. In an exemplary embodiment, the respective primary and secondary fibers propagate in opposite directions. With this arrangement, nodes downstream of a cable fault can switch to the secondary fibers to maintain connectivity with the other nodes and the headend in the presence of a cable fault. 
     In another aspect of the invention, a fault tolerant network includes automatic discovery of new (undiscovered) nodes. In one embodiment, a headend sends from time to time a broadcast message for processing by the discovered nodes, i.e., the nodes of which the headend is aware. The discovered nodes reply to the broadcast message, which can include identifications for each of the nodes. The undiscovered nodes also reply to the broadcast message for identification to the headend. The headend can then send a confirmation message to the undiscovered nodes after which the headend can treat this node as a discovered node. The headend can also exchange ranging messages with the undiscovered node to ascertain its location in the network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic depiction of a fault tolerant optical network in accordance the present invention; 
         FIG. 2  is a schematic depiction of a multi-fiber optical cable that can be used in the network of  FIG. 1 ; 
         FIG. 3  is a schematic depiction showing further details of the network of  FIG. 1 ; 
         FIG. 4  is a schematic depiction showing further details of fault tolerant operation in the network of  FIG. 1 ; 
         FIG. 5  is a block diagram of a fault tolerant optical network in accordance with the present invention having a fiber cut; 
         FIG. 6  is a schematic depiction showing a fault tolerant network having node auto discovery in accordance with the present invention; and 
         FIG. 7  is a schematic representation of a fault tolerant network having primary and second headends in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an exemplary optical network  100  having fault tolerant data communication in accordance with the present invention. The network  100  includes a loop of a multifiber cable FC extending serially from a headend HE to each of a plurality of nodes N 1 - 5  in the network. The cable FC can then terminate at the headend HE. 
     In an exemplary embodiment shown in  FIG. 2 , the fiber-cable FC includes four independent optical fibers encased within the same cable. The individual fibers within the cable include a first collection (transmit) fiber C 1 , a second collection fiber C 2 , a first distribution (receive) fiber D 1 , and a second Distribution fiber D 2 . While the invention is shown and described in conjunction with the illustrated four fiber cable in a loop topology, it is understood that a variety of cable structures and network topologies within the scope of the invention will be readily apparent to one of ordinary skill in the art. 
     Referring again to  FIG. 1 , the multifiber cable FC interconnects the nodes in the network in a so-called “daisy chain” arrangement. However, unlike conventional daisy chains, the nodes passively and optically tap each optical fiber C 1 , D 1 , C 2 , D 2  in the multifiber cable FC, as described below. The nodes N 1 - 5  include passive optical connections to the fiber-cable loop. That is, the nodes do not repeat or terminate the previous node&#39;s transmissions, nor do they perform any processing or modification on the transmissions. Thus, the nodes N 1 - 5  do not perform optical-to-electronic-to-optical (O-to-E-to-O) conversions. One consequence of this passive tapping mechanism is that transmissions from the nodes need to be scheduled (e.g., in a Time Division Multiplexed sense) to prevent collisions on the collection and distribution C 1 , C 2 , D 1 , D 2  optical fibers. An exemplary optical network arrangement having passive tapping and scheduled transmission is described in U.S. patent application Ser. No. 09/994,475, filed on Nov. 26, 2001, which is incorporated herein by reference. 
     This is in contrast to known topologies such as Fiber Distributed Data Interface (FDDI), Distributed Queue Dual Bus, and/or SONET in which each node terminates each segment of a two fiber multifiber cable for converting the incoming optical signal to an electrical signal, processing and potentially modifying the signal, and then reconverting the signal back to an optical signal and retransmitting it to the next node in the sequence. 
     It is understood that the network system  100  can use wavelength division multiplexing (WDM), in which multiple distinct wavelengths of light are carried simultaneously within the same optical fiber. As is well known to one skilled in the art, WDM creates virtual fibers within one physical fiber, thus there is a duality between physical fibers and WDM channels. This relationship can be used to provide a variety of alternative embodiments to meet the needs of a particular application. In one particular embodiment described herein, a first wavelength is used for transmitting and receiving data from/to the nodes (the “data channel”), and a second wavelength is used for control signaling (the “control channel”). Alternatively, the system can include an additional fiber to carry the control channel, instead of using an additional wavelength. 
     As shown in  FIG. 3 , in combination with  FIG. 1 , collection fibers C 1   a,b , C 2   a,b  carry data transmitted from the nodes Na, Nb to the headend HE. In the illustrated embodiment, the first collection fiber C 1  runs clockwise from the headend HE and the second collection fiber path C 2  runs counterclockwise. As described above, nodes transmit on the collection fibers during scheduled times. The distribution fibers D 1   a,b , D 2   a,b  are used to transmit data and control from the headend HE to the nodes N 1 - 5 . The first distribution fiber path D 1  runs counterclockwise and the second distribution fiber D 2  runs clockwise. 
     At the endpoint of each optical fiber at the headend HE, there are detectors to determine the presence or absence of the control channels on the Distribution fibers and the pilot tones on the Collection Fibers. In the illustrated embodiment, the headend HE includes a first connector CN 1  having ports for each of the four fibers C 1   a , D 1   a , C 2   a , D 2   a . It is understood that the cable FC extends from the first connector CN 1  to a first node Na (not shown). The headend HE also includes a second connector CN 2  having ports for each of the four fibers C 1   b , D 1   b , C 2   b , D 2   b  extending to a second node Nb (not shown) in the loop. 
     In the particular embodiment shown, a first pilot tone detector  150  is coupled to the first collection fiber C 1   a  in the first connector CN 1  and a second pilot tone detector  152  is coupled to the second collection fiber C 2   b  in the second connector CN 2 . A first control channel detector  154  is coupled to the first distribution fiber D 1   a  in the first connector CN 1  and a second control channel detector  156  is coupled to the second distribution fiber D 2   b  in the second connector CN 2 . A first pilot tone generator  158  is coupled to the second collection fiber C 2   a  in the first connector CN 1  and a second pilot tone generator  160  is coupled to the first collection fiber C 1   b  in the second connector CN 2 . And as described above, a control channel  162   a  and a data channel  162   b  are received by the second distribution fiber D 2   a  of the first connector CN 1  and by the first distribution fiber D 1   b  of the second connector. 
     In general, the distribution fibers D 1 , D 2  also carry control messages from the headend HE to the nodes on a wavelength (the “control channel”) that is separate from the data wavelength (the “data channel”). Again, a separate fiber could be used instead of a separate wavelength. The headend HE sends the control messages on both D 1  and D 2 , and sends a control signal (a “pilot tone”) on C 1  and C 2  over the control channel. These signals are used to determine if a fiber-cut has occurred, and can be ignored by the nodes. 
     For example, the headend HE sends a pilot tone over the control channel on the first collection fiber C 1   b  in the second connector CN 2 . If the headend HE pilot tone detector  150  in the first connector CN 1 , which is at the other end of the fiber, does not receive this signal then it can be deduced that the optical fiber has been cut. 
       FIG. 4  shows an exemplary node  200  input/output connection in accordance with the presence invention. In one embodiment, a network interface card (NIC) can include the illustrated connections. The node  200  includes a first connector  202  having respective ports P 1 C 1 , P 1 D 1 , P 1 C 2 , P 1 C 2  for each of the first collection fiber C 1 , the first distribution fiber D 1 , the second collection fiber C 2 , and the second distribution fiber D 2 . The node  200  also includes a second connector  204  having respective ports P 2 C 1 , P 2 D 1 , P 2 C 2 , P 2 D 2  for the collection and distribution fibers C 1 , D 1 , C 2 , D 2 . In the illustrated embodiment, the node  200  provides a fiber connection from the first port connector  202  to the second connector  204  for each of the fibers. 
     The node  200  includes a first passive optical tap  206  on the first distribution fiber D 1  and a second passive optical tap  208  on the second distribution fiber D 2 . The node  200  further includes a first passive optical coupler  210  on the first collection fiber C 1  and a second passive optical coupler  212  on the second collection fiber C 2 . A transmit optical switch  214  is coupled to the first and second optical couplers  210 , 212  and a receive optical switch  216  is coupled to the first and second optical taps  206 , 208 . As described above, the node  200  transmits on the collection fiber C 1  (or C 2 ) and receives on the distribution fiber D 1  (or D 2 ). 
     Referring to  FIG. 4  in combination with  FIG. 1 , in normal operation, the first collection fiber C 1  collects transmissions from the nodes N 1 - 5  ( FIG. 1 ) and directs them to the headend HE, with the second collection fiber C 2  serving as a standby fiber that is used if the fiber-cable is cut or otherwise faulty. The headend HE receives data from the nodes N 1 - 5  on the first collection fiber C 1  and distributes the data back to the nodes on the first distribution fiber D 1 , with the second distribution fiber D 2  serving as a standby fiber in the event of a fiber cut. Nodes “speak” on the Collection fibers C 1 , C 2 , and “listen” on the Distribution fibers D 1 , D 2 . Note that in the illustrated embodiment, the transmitting node&#39;s message must pass through the headend HE before being heard by the receiving node, even if the receiving node is downstream of the transmitting node on the Collection fiber C 1 , again because nodes do not listen on the Collection fibers. 
     It is understood that the directions of the fibers can be changed, so long as the primary and secondary collection fibers C 1 , C 2  propagate in opposite directions around the loop, and similarly with the distribution fibers D 1 , D 2 . With modifications that will be readily apparent to one of ordinary skill in the art, the network can allow downstream nodes to listen to the Collection fiber instead of the Distribution fiber. For example, such an arrangement can be used for optimization in multi-loop configuration, as described below. 
     During normal operations, the headend HE sends control messages over the control channel on D 1  to all the nodes. These messages are, in effect, permissions for the nodes to transmit to a particular node (or set of nodes in the case of a “multicast” transmission) for a duration specified in the message. All nodes listen to the control channel on the first distribution fiber D 1  with each node ignoring those control messages that are not addressed to that node. That is, in general a node only takes action on those control messages that are addressed to it. When a node receives a control message addressed to it, that node then transmits data on the first collection fiber C 1 , to the node indicated in the permission for the specified duration. The headend HE receives this transmission and then distributes it to the nodes using the distribution fibers D 1 , D 2 . The nodes listen on D 1 , ignoring those messages not intended for it. In the illustrated embodiment, the second distribution fiber D 2  is a standby optical fiber for the first distribution fiber D 1 , and is ignored during normal operations. 
     Referring now to  FIG. 5 , which is similar to  FIG. 1  with the addition of a fiber cut  300 , in combination with  FIG. 3 , another aspect of the invention is shown in which the network  200  includes fiber cable cut localization and recovery. To illustrate cable cut localization and recovery in accordance with the present invention, consider the case where the cable FC has been completely cut in a given location  300  and all individual optical fibers C 1 , D 1 , C 2 , D 2  in the cable FC are severed at that point. When this occurs, the pilot tone on C 1   a  of the first connector CN 1  will not be received at the headend HE, thus indicating the presence of the cut. Upon detecting the fiber-cable cut  300 , the headend HE immediately stops sending permissions to the nodes. The headend HE sends instead control messages indicating a fault condition, such as an “alarm message.” 
     During normal operation of the network, nodes N 1 - 5  receive a continuous stream of control messages from the headend HE on the first distribution fiber D 1 . Due to the fiber-cable cut  300 , nodes downstream of the cut, i.e., nodes N 4 , N 5 , no longer receive the stream of control messages. In response, these nodes N 4 , N 5  switch over to the second distribution fiber D 2  to obtain control messages. These nodes N 4 , N 5  can then also transmit on the secondary collection fiber C 2  with permission. Once the nodes downstream of the cut N 4 , N 5  switch to D 2 , all nodes in the system receive the control messages being sent by the headend HE indicating the presence of the fault. At this point, the headend HE and all the nodes are aware of the fault, and all nodes know which side of the cable-cut  300  they are located, indicated by the Distribution fiber (D 1  or D 2 ) from which the node is receiving control messages. 
     The headend HE then determines the exact location of the cut  300 . To do this, the headend HE sends each node a message asking it which side of the cut it is on, e.g., which Distribution fiber (D 1  or D 2 ) the node is listening to. Upon receiving responses from all nodes N 1 - 5 , the headend HE can determine the location of the cut. That is, the headend HE determines which pair of nodes surrounds the cut. In the illustrated embodiment, the cut  300  is between the third and fourth nodes N 3 , N 4 . 
     The scenario described above assumes that all optical fibers in the cable are cut at the same place. It is understood, however, that it is possible for a subset of the four fibers to be cut, thus resulting in sixteen different combinations of optical fiber cuts. In addition, one of ordinary skill in the art will appreciate that it is possible to optimize the localization and recovery procedure for each particular combination. Alternatively, for the sake of consistency and ease of implementation, a general localization and recovery procedure can be designed that can be used for any fault configuration. 
     Still referring to  FIGS. 3 and 5 , in a generalized procedure for fiber cut localization, since there is a detector at the endpoint of every optical fiber C 1   a,b , D 1   a,b , C 2   a,b , D 2   a,b  in the multifiber cable, as shown and described above, the headend HE can sense the presence of a cut in any of the fibers, but may not know exactly where the cut is located. Upon detecting any cut, the headend HE immediately ceases to send out further permissions to the nodes, and sends a continuous stream of “alarm” messages on the control channels on the first and second distribution fibers D 1 , D 2  to alert the nodes of the presence of the fault. If a node does not receive the control channel on D 1 , it switches over to D 2 . The nodes make note of which Distribution fiber is still functional, such as by setting appropriate status registers present at the node. 
     Next, the headend HE commands all nodes N 1 - 5  to determine if they can obtain the control channel on D 2 . The headend HE sends a control message instructing all nodes to listen to D 2  to test for the presence of the control channel. A node that is already listening on D 2  for the control channel can ignore this step, as the node already knows the state of D 2 . Nodes listening to D 1  for the control channel temporarily listen for the control channel on D 2 , and make note of the presence or absence of the control channel, and then return to listening to D 1 . 
     The headend HE next sends individual commands to each node instructing it to send a test message to itself on C 1 . If the node receives the test message (on the functional Distribution fiber), then C 1  is functional for that node, and the node makes note of this fact. The analogous process is used to determine the state of the C 2  fiber. At this point, each node knows the state of each of the four fibers. 
     Finally, the headend HE commands each node to inform the headend of the status of its fibers. This information is used by the headend to precisely identify and locate the fault, and to inform network management software and administrative personnel. 
     In another aspect of the invention, an optical network includes automatic node discovery and ranging. With this arrangement, new nodes can be discovered and located automatically. 
     Referring now to  FIG. 6 , which is similar to  FIG. 1  with the addition of a new (undiscovered) node N 6 , node auto discovery in accordance with the present invention is described. In an exemplary embodiment, the headed HE sends a “discovery” control message over the control channel from time to time. This message is addressed to a broadcast address so that every node N 1 - 6  will process it, and will contain a list of node identification numbers. It is understood that all nodes N 1 - 5  of which the headend is aware, have a globally unique identification number, which can be similar to the 48-bit addresses used in Ethernet networks. If a node&#39;s identification number is not in the list, e.g. N 6 , the undiscovered node N 6  will send a “hello” packet informing the headend HE of the node&#39;s existence and address. In an exemplary embodiment, the new node N 6  must be discovered before the headend HE will permit this node N 6  to communicate with other nodes N 1 - 5 . 
     However, the new node N 6  cannot send a transmission without a permission message from the headend HE. And since the headend HE is not aware of the undiscovered node, the headend HE will not send the undiscovered node a permission message, resulting in a deadlock situation. In one particular embodiment, the deadlock is resolved when, after sending out the initial “discovery” control message, the headend HE waits an amount of time for the nodes N 1 - 5  to process the message and prepare a “hello” response, and then sends a broadcast message commanding the undiscovered nodes N 6  to transmit their “hello” control messages. If there is only one undiscovered node, e.g., N 6 , its message will be sent to the headend HE, and the headend will send confirmation of the discovery in a later control message. 
     If there is more than one undiscovered node, a collision could result because multiple nodes will send their “hello” response messages simultaneously. A collision results in an invalid message, e.g., the resulting frame check sequence will indicate an error, and the headend will reject the response. The nodes will realize that a collision has occurred because they will not receive confirmation of their presence from the headend. After not receiving a confirmation message after a fixed amount of time, the nodes will wait a random amount of time before attempting to resend the “hello” message. In particular, each node will wait for a random number of transmission opportunities (permissions from the headend) to pass, and then attempt retransmission at the next permission. This process will continue for every undiscovered node until every node receives a confirmation from the headend. 
     In addition to knowledge of the existence of a node e.g., N 6 , the distance of the node from the headend HE may be needed. The process of measuring this distance automatically is known as “ranging.” The headend HE initiates this process by sending a “ranging command” control message over the control channel (on D 1 , during normal operation) to the node N 6 . The node N 6  then sends a message addressed to itself over C 1 , and listens for it on D 1 , measuring the time it takes to return. Dividing this elapsed time by two reveals the downstream delay from the node N 6  to the headend HE. For precision, the propagation time through the headend HE, which is a fixed and known quantity, needs to be subtracted from the round-trip delay. A similar procedure is used to determine the delay from the headend HE to the node N 6  in the other direction: the node N 6  sends a message to the headend on C 2 , and listens for it on D 2 . Subtracting the propagation delay through the headend, and then dividing by two reveals the time-delay from headend to the node. The headend HE then sends a control message to the node N 6 , requesting the findings of the ranging measurements. 
     In a further aspect of the invention, nodes can be inserted and removed from the network with minimal disruption. Inserting a node in the network entails temporarily breaking the loop. In order to minimally disrupt network operations during the addition of the new node, the headend HE can be informed that the loop is going to be intentionally cut. The headend HE then instructs the nodes to use the Collection and Distribution fibers that permit network operations during the planned cut. After this has been done, a new node may be inserted into the loop at the location specified to the network management software, and the headend HE will ignore fiber-cable cut alarms, as it is expecting this to happen. With the new node in place and the fiber-cable reconnected, the headend HE begins the automatic discovery to learn the identity of the new node, followed by a ranging measurement to determine its position. 
     Another way to add a node, e.g., N 6 , is to simply insert it into the loop without informing the headend HE. The headend HE treats the breaking of the loop as a fault, and performs the fiber-cut recovery procedure. When the loop has been restored (with the new node in place), the headend HE conducts the automatic discovery and ranging procedures. The consequence of not informing the headend HE prior to adding a new node is that network operations are disrupted resulting in lost traffic. While the network eventually recovers, it can do so much more gracefully without the loss of any traffic if the headend HE is informed in advance that a new node is about to be added. 
     Removing a node from the network entails breaking the loop, and the headend HE commences recovery procedures until the loop is restored. As in the last case, the system can more gracefully remove a node if it is informed prior to doing so. 
     A node that fails (for example, because of a power failure) presents little problem: since the node&#39;s connection to the network is passive, the network is not dependent on that node (as it is in daisy-chained networks such as FDDI). The headend periodically asks for feedback messages from the nodes. A headend will assume that a node that has not responded to feedback requests is not functioning, and will eventually stop granting it permissions. If the node restarts at some later time, the headend will detect it during one of the periodic automatic discovery procedures. 
     In another aspect of the invention shown in  FIG. 7 , a network  300  includes a headend failover configuration having a primary headend  302  and a secondary heandend  304 . The secondary headend  304  can automatically assume system control operations in the event that the primary headend  302  suffers component failure, power outage, or other hardware or software damage. 
     In general, the fibers are connected such that the primary headend  302  uses only the primary collection and distribution fibers C 1 , D 1  to connect to the nodes N 1 - 3 , while the secondary headend uses the C 2  and D 2  pair. More particularly, the primary headend  302  includes a first connector  306  coupled to a first connector  308  of the third node N 3  so as to connect the first collection and distribution fibers C 1 , D 1 . A second connector  310  of the primary headend  302  connects the first collection and distribution fibers C 1 , D 1  to a second connector  312  of the first node N 1 . A first connector  314  of the secondary headend  304  connects the second collection and distribution fibers C 2 , D 2  to the first connector  308  of the third node N 3  and second connector  316  connects the second collection and distribution fibers C 2 , D 2  to the second connector  312  of the first node N 1 . A first connector  318  of the first node N 1  connects a cable C 1 , D 1 , C 2 , D 2  to a second connector  320  of the second node N 2  and a first connector  322  of the second node N 2  connects to a second connector  324  of the third node N 3 . The primary and secondary headends  302 ,  304  communicate via a connection  326 . 
     During normal network operation, the secondary headend  304  remains idle, waiting for messages from the nodes informing it that the primary headend  302  has failed and monitoring status updates from the primary headend on the inter-headend connection  326 . If one headend fails, the other assumes control, and nodes remain fully connected. If the cable between two nodes is cut, the nodes are separated into two segments. The primary headend  302  services the nodes on one side of the cut (one segment), while the secondary headend  304  services the other side (the other segment). The connection  326  between the headends is used to provide connectivity between the two segments. 
     Both headends  302 ,  304 , in coordination with each other, need to independently perform the automatic discovery and ranging procedures on all the nodes. During a loop cut, the headends  302 ,  304  conduct communication from one segment to the other via the connection between the headends. As in the single headend configuration described above, each headend monitors the endpoints of its Collection and Distribution fiber for the control channel and pilot tone. The loss of these signals indicates a fault. 
     If a node detects that the primary headend  302  is suffering from a failure, the node switches to the secondary headend  304  by switching to C 2  and D 2 , and informs the secondary headend  304  of the switch. Through a process of elimination in which all the alarms and error messages are correlated, the nodes and the headend can determine the nature and location of the failure, and restore full connectivity using the procedures very similar to the ones described for the single-headend case. 
     While the embodiments shown and described above have a single loop, one of ordinary skill in the art will readily recognize that to scale the network to support many nodes, a system can use multiple loops, and use an optical, electronic, or opto-electronic switching subsystem, located in the headend, so that data traffic from one loop can be shunted to another loop. When the switching subsystem is not enabled, all the loops operate independently; the traffic in one loop is isolated from another loop. With the switching system enabled, loops can exchange traffic with one other loop, a subset of other loops (multicast), or to all loops (broadcast), depending on the connections permitted by the switching subsystem. For example, if the switching subsystem is an electronic crosspoint switch, singlecast, multicast, and broadcasts can be supported. Similarly, a switching subsystem composed of a matrix of micro-mechanical optical switches can support one loop to another. 
     Further fault tolerant features are shown and described in U.S. patent application Ser. No. 10/186,303, entitled, “Multiple Switch Protected Architecture” filed on even date herewith, which issued as U.S. Pat. No. 7,200,106 on Apr. 3, 2007, the disclosure of which is incorporated herein by reference. 
     While loop topology is utilized for the embodiment detailed here, it will be readily apparent to one of ordinary skill in the art that the techniques described above are applicable to a variety of topologies including rings, trees of rings, general mesh networks, and the like. 
     One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.