Abstract:
Current network switching architectures require communication with a higher level network control plane, which can be slow to reroute communications, resulting in unacceptable losses of communications for customers. Examples embodiments of the present invention reroute communications faster detecting optical power of an optical signal at optical switches coupled via optical communication paths, and causing at least one optical communication path between a first optical switch and second optical switch to switch to an alternative optical communication path, in part, through physical layer triggering in an event optical power at at least one of the first or second optical switches falls below a threshold level. Switching in response to physical layer triggering may result in reduced switching times and, consequently, faster restoration of communications to customers after a network fault interruption.

Description:
RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application No. 60/876,348, filed on Dec. 20, 2006. The entire teachings of the above application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Fiber cuts, equipment faults and degradations cause a significant number of disruptions and outages. Often, fault recovery times are slow as the network components (e.g., switches and network administrators) communicate to switch around equipment failures. As businesses and consumers become increasingly intolerant of network failures, downtime can be very expensive to carriers due to both lost revenue and tarnished reputations. As a result, carriers continually search for better ways to protect networks against such fiber faults and reduce costs by more efficient use of protection bandwidth. 
     SUMMARY OF THE INVENTION 
     An example embodiment of the present invention is a method, and corresponding apparatus, for switching optical communications paths by detecting optical power of an optical signal at optical switches coupled via optical communications paths. The example embodiment further includes causing at least one optical communications path between a first optical switch and second optical switch to switch to an alternative optical communications path, in part, through physical layer triggering in an event optical power at at least one of the first or second optical switches falls below a threshold level. Switches autonomously switching in response to a physical layer trigger results in reduced network fault recovery times. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
         FIG. 1  is a schematic view of optical switches optically coupled by traffic path fibers and protection path fibers, and with optical power detectors at input optical ports; 
         FIG. 2  is a process flow diagram for the switches of  FIG. 1  detecting a traffic path fault and switching to the protection path fibers; 
         FIG. 3  is a schematic view of optical switches optically coupled by a traffic path and a protection path, and with optical power detectors at output (egress) optical ports; 
         FIG. 4  is a process flow diagram for the switches of  FIG. 3  detecting a traffic path or switch matrix fault and switching to the protection path fibers; 
         FIG. 5  is a schematic view of optical switches optically coupled by a traffic path and a protection path, and with optical power detectors at both output (egress) optical ports and input (ingress) optical ports; 
         FIG. 6A  is a process flow diagram for the switches of  FIG. 5  detecting a traffic path fault and switching to the protection path fibers; 
         FIG. 6B  is a process flow diagram for the switches of  FIG. 5  detecting a switching matrix fault and switching to the protection path fibers; 
         FIG. 7  is a schematic view of optical switches optically coupled by a traffic path bi-direction fiber and a protection path bi-direction fiber, and with optical power detectors detecting optical power at each port in the output (egress) direction; 
         FIG. 8  is a process flow diagram for the switches of  FIG. 7  detecting a traffic path or switch matrix fault and switching to the protection path fiber; 
         FIG. 9  is a schematic view of optical switches optically coupled by a traffic path bi-directional fiber and a protection path bi-directional fiber, and with optical power detectors detecting optical power at each port in both the output (egress) direction and input (ingress) direction; 
         FIG. 10A  is a process flow diagram for the switches of  FIG. 9  detecting a traffic path fault and switching to the protection path fiber; 
         FIG. 10B  is a process flow diagram for the switches of  FIG. 9  detecting a switch matrix fault and switching to the protection path fiber; and 
         FIG. 11  is a schematic view of optical switches optically coupled to provide a traffic path and multiple protection paths between two transmitter/receivers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of example embodiments of the invention follows. 
     Integrating fault detection into optical switching at the physical layer can greatly increase the speed of protection switching. Existing protection switching systems usually involves higher level communications or signaling between nodes in the network using complex framed overhead channels or packet communications. Embodiments of this invention increase the speed of fault detection and network protection switching by having intelligent optical switches with optical power detectors that can locally detect faults in the optical line. A network fault typically results in the loss or reduction of optical power. The intelligent optical switch autonomously switches from a traffic fiber pair to protection fiber pairs when a fault is detected. The switching results in other intelligent optical switches also detecting a loss or reduction in optical power, causing those switches to autonomously switch to protection fiber pairs. Loss of optical power caused by a network fault or by a switching event are each a physical layer trigger that causes an intelligent optical switch according to embodiments of this invention to autonomously switch from traffic fibers to protection fibers. Embodiments can also improve fiber utilization by allowing working lines to share a pool of protection paths. 
     Designing networks that are automatically protected against multiple worst-case fiber breaks can be difficult and expensive. As a result, many network protection configurations typically only provide automatic protection against single-fiber faults. The reasoning behind this is that a repair crew will be dispatched immediately after a single fault and hopefully fix the problem before another fault occurs. Many of the overall transport line and network availability calculations are dominated by a probability of a second fault occurring before the first fault is repaired. 
     Major disasters, like earthquakes and hurricanes, can often cause multiple fiber breaks in a network. The “shared pool” concept can be extended to the difficult task of protecting a network against multiple fiber breaks by simply monitoring the protection paths in the same way as the working paths after they are provisioned. This allows the traffic-carrying protection paths to be protected by the remaining resources of the shared pool. If the network experiences a second fiber break on either a provisioned protection path or regular working line, the traffic is automatically switched to another protection path from the pool. The “shared pool” configuration can also enhance network availability by providing protection against multiple fiber faults. 
     The number of faults the network can tolerate is determined by the size of the spare fiber pool. While no system can protect against every contingency, having a network that can automatically reconfigure and recover from multiple fiber faults can greatly improve overall availability. 
     Embodiments of the present invention can be used to replace or supplement existing protection-switching methods in traditional systems like SONET. In practice, an embodiment can be integrated into existing systems as an enhancement for handling fiber faults. For example, the optical switches can communicate faults via physical layer triggering, as described above, but may also interface with the higher-level network control planes through a standard communication channel. In the event of a fiber break, a switch according to an example embodiment of the present invention automatically reconfigures communications traffic flow around the fault according to predefined rules and then informs the higher-level control plane via an upstream interface. Conversely, the higher-level control planes can command the switch to reconfigure in the event of non-fiber faults or turn off the automatic protection switching feature for maintenance operations. Embodiments of the present invention allow the working traffic lines to share efficiently several protection paths without the need for intervention from a higher-level network control layer. 
     An embodiment of the invention includes a pair of optical switches at each end of a communications path. Each switch is equipped with optical power detectors to protect against network faults that result in a reduction or loss of optical power without the need for intervention of higher-level network control planes or complex signaling. 
       FIG. 1  is a network diagram of one example embodiment  100 . A fault  122  is detected using optical power detectors  106 ,  108 ,  110 ,  112 ,  114 ,  116  locally in each switch  102 ,  104 , and each switch  102 ,  104  is configured to detect and react locally to the fault  122 . The switches operate independently to detect faults  122  and initiate protection switches. 
     The switch detects the fiber fault  122  by using an intelligent switch controller  118 ,  120  within each switch  102 ,  104  to monitor loss of optical power at the receiver port  124 ,  126 ,  128 ,  130 ,  132 ,  134  of the optical switch  102 ,  104 . In this example embodiment, the intelligent switch controller  118 ,  120  coordinates the reading of the line optical powers, the switching function, the storage of predefined rules for the switching, and communicating with external higher level network components. 
     Example embodiments of the invention described herein enable efficient use of protection fiber paths because the local switching control allows multiple working paths to share a pool of protection paths. The exact protection path for each working path does not need to be defined before a fiber fault occurs. Because the optical switches know which protection paths are in use at any time, they simply select the next available protection path and then report the network reconfiguration to the higher network control layers. These higher layers can download updated protection switching criteria at any time. 
     An example method of how to select the next available protection path can be determined by a variety of means. For example, one simple method is to pre-provision dark fiber protection paths and predetermine the order in which they are to be assigned to mitigate faults. This protocol enables multiple working paths connected to the switch to share efficiently a common pool of protection fibers and paths. 
     If both fibers in a line pair experience a fault or power degradation, then both switches detect the loss of power at the input ports and automatically switch. 
     Two useful optical switch characteristics for this application are low loss and fast switching times. The low loss minimizes the impact on the transmission line impairment budget; the fast switch time ensures the switching is completed before higher-level control plane layers intervene. 
     The protection switching may be independent of the number of intermediate switches or node hops. In the case where many optical switches are included in the path only the switches at the end of the path are needed to perform a protection switch. However, as the number of intermediate switches or node hops increases, it becomes advantageous to use switches capable of switching in the absence of light, i.e., “dark fiber switches.” In a network configured with dark fiber switches, the switches in a network pathway can align themselves simultaneously when a switching event occurs, resulting in a total switching time approximately equal to the time required for a single switch to perform a switching event, regardless of the number of switches in the network pathway. By contrast, if a network is not configured with dark fiber switches, the switches can only align themselves after an optical signal is present, so each switch must wait for preceding switches in a network pathway to align before being able to align. Consequently, the total switching time increases to approximately the time required for a single switch to perform a switching event multiplied by the number of switches in the network pathway. 
     The physical layer triggering, thus far, has been described as a loss or reduction of optical power caused by a network fault or a switch diverting an optical signal to a protection pathway. However, the physical layer triggering between switches at the ends of the network may utilize a more sophisticated system using amplitude, phase or frequency modulation of the traffic signal. The physical layer triggering between switches at the end of the network can also be accomplished by a non-communication signal. 
     Example Embodiment Using Optical Switches with Input Power Detectors 
     Referring now to  FIG. 1  in more detail, the two optical switches  102 ,  104  are shown with each switch  102 ,  104  including ports  124 ,  126 ,  128 ,  130 ,  132 ,  134 ,  136 ,  138 ,  140 ,  142 ,  144 ,  146 , optical power detectors  106 ,  108 ,  110 ,  112 ,  114 ,  116 , a switch matrix  148 ,  150  and intelligent optical switch controller  118 ,  120 . The ports  124 ,  126 ,  128 ,  130 ,  132 ,  134 ,  136 ,  138 ,  140 ,  142 ,  144 ,  146  define the fiber connection points to each switch  102 ,  104 . The optical detectors  106 ,  108 ,  110 ,  112 ,  114 ,  116  are positioned at the switch input ports  124 ,  126 ,  128 ,  130 ,  132 ,  134  and have directionality where they detect the optical power in the direction of the arrow. The switch matrices  148 ,  150  are fully non-blocking switch matrices. A symmetric N×N or asymmetric N×M switch matrix  148 ,  150  may be used. The intelligent switch controller  118 ,  120  of each switch coordinates the reading of optical power within the switch, the switching function, the storage of the predefined rules for the switching, and the communicating with external higher level network components. The predefined protection paths  156 ,  158  can be downloaded or changed at any time via a communications interface (not shown) and the switch can report all protection configurations changes, switch settings and diagnostics via any form of communications channel. The switch can be configured manually or automatically, to respond to the network fault  122 . The intelligent switch controllers  118 ,  120  can operate autonomously in each node in terms of identifying a loss of optical power in one or two directions and cause the respective switch matrices  148 ,  150  to switch optical signals from a working path  152 ,  154  to a protection path  156 ,  158 . 
     The switches  102 ,  104  are configured into a network by interconnecting them with a primary traffic line pair  152 ,  154  and a protection line pair  156 ,  158 . The primary traffic line pair  152 ,  154  and protection line pair  156 ,  158  each includes, for example, a westbound fiber  154 ,  158  and eastbound fiber  152 ,  156 . The two optical switches  102 ,  104  are connected via a westbound and eastbound fiber pair. A west transponder  164  is connected to optical switch  102  with the transmitter (TX) connected to port  124  and the receiver (RX) connected to port  136 . Likewise, an east transponder  166  is connected to ports  134  and  146  of optical switch  104  with the TX and receiver connected to ports  134  and  146 , respectively. The predefined protection path  156 ,  158  between the switches  102 ,  104  is connected with the eastbound protection fiber  156  connected between switch  102  at port  140  and switch  104  at port  132  and the westbound fiber  158  connected between switch  102  at port  128  and switch  104  at port  144 . 
     While the detection of fiber faults at the Tx and Rx nodes  164 ,  166  at the ends of an optical transmission path and control plane techniques for changing traffic flow to avoid fiber faults are well documented in the literature, a combination of local detection and protection switch initiation and physical layer triggering between the switches by moving multiple fibers in combination is being distinguished herein from traditional detection and fault correction. 
     Prior to network fault  122  in network  100 , switches  102 ,  104  are routing communications traffic signals from transponders  164 ,  1666  via primary traffic line pair  152 ,  154 . After fault  122  occurs and is detected by switches  102 ,  104 , the switches autonomously switch the communications traffic signals to protection line pairs  156 ,  158 . 
       FIG. 2  is a flow diagram that shows how a fault  122  is detected and how the network protection switch is performed in reference to the network configuration of  FIG. 1 . A fault is defined as any fiber or equipment failure or degradation that results in loss of optical power in the transmission line. For example, if the fault  122  (or degradation) occurs that changes the optical line power of westbound fiber  154 , it is detected by optical power detector  108 . The fault  122  or degradation can be caused by any usual fault-causing condition in the traffic line optical path prior to the detector, such as optical switch  104  path failure, an optical line failure, or an input port failure in switch  102 . In this example, the optical power loss is caused by a fiber fault  122  on westbound traffic fiber  154  and the loss of power is detected in at power detector  108 . 
     There are many criteria that can be used to detect faults  122 . One criterion can be an absolute reference level where a predetermined power level is selected and a fault is declared when the power drops below a predetermined level. Another criterion can be a relative reference level in which a predetermined power drop is selected and a fault is declared when the power drops by the predetermined amount. Many other techniques can be used, such as delaying the declaration of a fault until the level or change in power threshold has been exceeded for a predetermined amount of time or comparing optical powers over time. 
     After detecting the fault at  202 , the switch  102  that detects the fault and then switches both the eastbound and westbound traffic paths  152 ,  154  to the corresponding eastbound and westbound protection paths  156 ,  158  at  204 . Switching from the eastbound traffic line  152  to the eastbound protection path  156  provides a physical layer trigger that causes a loss of power that is detected at the corresponding optical power detector  112  at  214 . Switch  104  then switches both the eastbound and westbound traffic fibers to the predetermined protection path  156 ,  158 , reestablishing the traffic connection at  216 . 
     The integrity of the protection switch can be checked by monitoring the power at optical power detectors  110  and  114 . At  206 ,  218 , both switch  102  and switch  104  wait a pre-determined amount of time for both switches  102 ,  104  to complete the protection switch  204 ,  216 . If power is not detected at optical power detectors  110 ,  114  after the wait period at  210  and  220 , then the protection switch was not successfully completed and each switch sets a protection switch error flag at  208 ,  222  for this protection switch. The protection switch event and status can be sent via the communications channel to the higher network layers at  212 ,  224 . 
     The switch can also have a pre-programmed list of actions in the event of the protection switch errors, such as switching through multiple protection paths at a given rate. Finally, the intelligent switch controllers  118 ,  120  in switches  102 ,  104  then report the results to the higher level network control plane  160 ,  162 . The switches  102 ,  104  may report information that includes the change of status of the network connections, the optical power readings, the protection switch error flags, other switch status flags and any other pertinent information. 
     Since the switches  102 ,  104  also have input power detectors  106 ,  116  that are monitoring the power of the TX  164 ,  166 , the switches  102 ,  104  can determine if their respective local TX  164 ,  166  lasers have failed and inform the higher level network control plane  160 ,  162  so an appropriate equipment repair or higher level network protection switch can be made. In this embodiment, input optical power detector  106  and input optical power detector  116  can detect if the source laser in the respective TXs  164 ,  166  have failed. The optical switches  102 ,  104  can also be programmed to switch automatically to backup transponders (not shown) when a TX  164 ,  166  failure is identified. Again, that switch may inform the higher level network control layers of the configuration change. 
     Another way to handle this special case of a TX failure is to have the switches  102 ,  104  reset themselves back to the original configurations if the protection switch error is raised or if another pre-determined action occurs. Another way is to simply let them be reset by the higher level control planes  160 ,  162  when appropriate. 
     This mechanism is fast because the detection of fiber faults  122  and protection switching control are done locally within the switching hardware at each node. Failures are detected in one of the switches at the end of the fiber path. The switching order depends on whether the fault is located in the eastbound fiber paths  152 ,  156  or westbound fiber paths  154 ,  158 . 
     An alternative example embodiment is where the intelligent switch controller  118 ,  120  can also directly communicate between other optical switches to coordinate further optical switching among many other optical switches. 
     Example Embodiment Using Optical Switches with Output Power Detectors 
     Monitoring with output (egress) port optical power detectors allows for detecting faults in a greater portion of the connection path between the transponders than using input (ingress) optical power detectors. With only input (ingress) port optical power detectors on the switches, it is impossible to detect failures in the switch matrix before the RX since the detector is on the input port just before the switch matrix. 
       FIG. 3  shows the network connections with switches  302  and  304  configured with output power detectors  306 ,  308 ,  310 ,  312 ,  314 ,  316 . The switches  302 ,  304  are interconnected with traffic fiber pairs  152 ,  154  and protection fiber pairs  156 ,  158 . 
     The operation for detecting faults  122 ,  322  and protection switching is similar to the input power detecting case of  FIG. 1  and is described in the  FIG. 4  flow diagram, which is similar to the flow diagram of  FIG. 2  and not described in detail for the sake of brevity but should be understood from the flow diagram directly. In this case, if a switch fault occurs in the switch  302  matrix, it is detected by output optical power detector  306 . Thus, output optical power detectors  308  and  312  can detect faults in the switching matrices  348 ,  350  or the TXs  164 ,  166 . 
     Example Embodiment Using Switches with Both Input and Output Power Detectors 
     Monitoring with both input and output optical power detectors in the optical switches allows for both detecting faults over the same portion of the optical path as the case with output detectors and in addition being able to determine the location of the fault. 
       FIG. 5  shows an embodiment of the invention with both input optical power detectors  506 ,  509 ,  511 ,  512 ,  514 ,  517  and output optical power detectors  507 ,  508 ,  510 ,  513 ,  515 ,  516  in optical switches  502 ,  504 . Using both input optical power detectors  506 ,  509 ,  511 ,  512 ,  514 ,  517  and output optical power detectors  507 ,  508 ,  510 ,  513 ,  515 ,  516 , the fault location can be determined to be in the TX  164 ,  166 , the line pair  152 ,  154  or  156 ,  158  or one of the switch matrices  548 ,  550  by comparing optical power readings in different locations along the connection path. 
     The location of the fault is determined by reading and comparing the detected optical powers at the input ports  524 ,  526 ,  528 ,  530 ,  532 ,  534  and output ports  536 ,  538 ,  540 ,  542 ,  544 ,  546  of the switches  502  and  504  connecting to traffic lines  152 ,  154 . This is accomplished, for example, by sequentially comparing detected optical power at optical power detectors  517 ,  513 ,  509 ,  507 , starting from the transmitter end of the optical path and reading the optical detectors sequentially along the line until a drop in power is found. The fault  122 ,  522  is located between the detector with the power drop and the previous detector with no power drop. The operation for protection switching using both input optical power detectors  506 ,  509 ,  511 ,  512 ,  514 ,  517  and output optical power detectors  507 ,  508 ,  510 ,  513 ,  515 ,  516  is shown in  FIG. 6A  flow diagram for detecting a plant fault  122  and  FIG. 6B  shows the flow diagram for detecting a switch path fault  522 . In each case, protection switching is similar to the flow diagram of  FIG. 2  ad is not described in detail for the sake of brevity. The protection switching should be understood from the flow diagrams in  FIGS. 6A and 6B  directly. 
     Controllers  518 ,  520  can determine the location of a fault when using both input optical power detectors and output optical power detectors by comparing detector optical powers: 
     1. If optical power detector  517  detects a reduction in power, then the fault is located in the connection path before optical power detector  517 . The fault is located in TX  166  or in connection  174  between TX  166  and switch  504 . 
     2. If a reduction in power is detected by optical power detector  513  and detected optical power at optical power detector  517  is unchanged, then the fault is located within switch  504 . 
     3. If a reduction in power is detected at optical power detector  509  and detected optical power at optical power detector  513  is unchanged, then the fault  122  is located in traffic line  154  between switches  502  and  504 . 
     4. If a reduction in optical power is detected at optical power detector  507  and detected optical power at optical power detector  509  is unchanged, then the fault  522  is located within switch  502 . 
     5. The higher level control plane  160 ,  162  can determine if the RX  164  has failed by a query of the optical power detectors along the line. If no faults are detected anywhere in the line, then the fault is located in the RX  164  or on line  170  between switch  502  port  536  and the RX  164 . 
     The fault determination method just described also applies in the opposite direction using optical power detectors  506 ,  508 ,  512 ,  516 . 
     Example Embodiment on Bi-Directional Fiber Systems Using Optical Switch with One Optical Detector per Switch Port 
       FIG. 7  shows the operation of switches  702 ,  704  with bi-directional traffic on a single fiber optical line  752 ,  756  where each switch has a single directional optical power detector  706 ,  708 ,  710 ,  712 ,  714 ,  716  per switch port  724 ,  726 ,  728 ,  730 ,  732 ,  734 . In bi-direction fiber systems, both the east and westbound traffic, for example, share the same optical fiber  752 ,  756 . The faults  722 ,  723  are detected by monitoring the optical powers locally in each switch  702 ,  704 . A fiber plant fault  722  on traffic fiber  752  causes detected power to drop at both optical power detector  706  and optical power detector  716 . In this case, controllers  718 ,  720  cause switches  702 ,  704  to switch to ports  728 ,  732 , respectively, to use protection path  756 . The flow diagram for this protection switch is given in  FIG. 8 , which is similar to the flow diagram of  FIG. 2  and is not described in detail for the sake of brevity, but should be understood from the flow diagram in  FIG. 8 . 
     The optical power detectors  708 ,  712  in switches  702 ,  704  can determine whether the input power from the TXs  764 ,  766  has dropped. 
     In an alternative embodiment the detectors directivity could be reversed, i.e., the output optical power detectors can be replaced by input optical power detectors. 
     Invention Embodiment on Bi-Directional Fiber Systems Using Optical Switch with Multiple Optical Detectors per Switch Port 
       FIG. 9  shows the operation of switches  902 ,  904  with bidirectional traffic on a single fiber optical line  752 ,  756  where each port  924 ,  926 ,  928 ,  930 ,  932 ,  934  has two directional optical power detectors  906 ,  907 ,  908 ,  910 ,  911 ,  912 ,  913 ,  914 ,  915 ,  916 ,  917  with the directivity of the two detectors in any port having opposite directions. The extra detectors allow for both fault detection and determination of the location of the fault. Using both input and output optical power detectors, the fault location  722 ,  923  can be determined to be in the TX  764 ,  766 , the line  752 , or one of the switch matrices  948 ,  950  by comparing detected optical power at different ports along the connection path. 
     A fiber fault  722  on the primary fiber  752  or a fault  923  in one of switch matrices  948 ,  950  causes detected optical power to drop at both optical power detector  907  and optical power detector  916 . In this case, both switches  902 ,  904  would switch to ports  928 ,  932 , respectively, to use protection path  756 . The flow diagram for this protection switch is shown in  FIGS. 10A and 10B . In each case, protection switching is similar to the flow diagram of  FIG. 2  and is not described in detail for the sake of brevity. The protection switching should be understood from the flow diagrams in  FIGS. 10A and 10B . 
     The location of the fault is determined by reading and comparing the optical powers from the input and output ports of both switches  902  and  904  on traffic line  752 . This is achieved by comparing the optical power detector measurements starting from the transmitter end  764 ,  766  of the optical path and reading the optical power detectors  906 ,  908 ,  912 ,  916  or  917 ,  913 ,  909 ,  907  sequentially along the line until a drop in power is found. The fault is located between the detector with the power drop and the previous detector with no loss in power. 
     Determining the Location of a Fault When Using Two Detectors per Switch Port on Bi-Directional Fiber Systems by Comparing Detector Optical Powers: 
     Controllers  918 ,  920  determine the location of a fault by comparing detector optical powers: 
     1. If optical power detector  917  detects a reduction in optical power, then the fault is located in the connection path before optical port  934 . The fault is located in TX  766  or in the connection  770  between TX  766  and port  934 . 
     2. If a reduction in power is detected at optical power detector  913  and detected optical power at optical power detector  917  is unchanged, then the fault (not shown) is located within switch  904 . 
     3. If a reduction in power is detected at optical power detector  909  and detected optical power at optical power detector  913  is unchanged, then the fault  722  is located in the fiber  752  between switches  902  and  904 . 
     4. If a reduction in power is detected at optical power detector  907  and detected optical power at optical power detector  909  is unchanged, then the fault  923  is located within switch  902 . 
     Alternatively, implementation of the two optical detectors at each port could be combined into a single bi-directional detector. 
     Protecting Against Multiple Faults 
     Embodiments of the invention can be applied to the difficult task of protection against multiple network faults. This is achieved by monitoring the protection paths the same way as the primary traffic path after a protection path is provisioned and allowing the protection paths to use the remaining pool of spare fiber paths. The communication traffic automatically switches to another protection path in the pool if a subsequent fiber failure occurs in the protection path. This feature can greatly enhance overall network reliability and availability. 
     This allows for efficient use of protection fibers because the local switching control allows the working fiber paths to share multiple protection fiber paths based on predetermined criteria. The exact protection path does not need to be predetermined before a fiber fault occurs. The mechanism simply selects the next available fiber path out of each node path based on a predetermined hierarchy. Because the local protection switch knows which protection paths are in use at any time it simply selects the next available path and can report when all the protection paths are in use to the higher network control layers. Information on the state of the protection switching can be relayed by the switching element to the higher level network control plans and protection switching criteria can be downloaded from higher level network control layers. 
     The exact protection fiber used for a particular working fiber failure does not need to be predetermined. The method of how to select the next available protection path can be determined by a variety of means. For example, one simple method comprises pre-provisioning protection paths and then pre-determining the order in which they will be assigned to mitigate network faults. This allows multiple working paths connected to the switch to efficiently share a common pool of protection fibers and paths. 
     An illustration of protecting against multiple network faults is shown in  FIG. 11 .  FIG. 11  shows a network  1100  with five switches  1102 ,  1104 ,  1106 ,  1108 ,  1110  equipped with input optical power detectors  1112 ,  1114 ,  1116 ,  1118 ,  1120 ,  1122 ,  1124 ,  1126 ,  1128 . Transponders  1164 ,  1166  are initially connected via a primary traffic fiber pair  1130  and have a pool of three pre-defined protection paths  1134 ,  1136 ,  1138  that may be shared among many other transponder pairs (not shown). The pre-defined protection paths  1134 ,  1136 ,  1138  are pre-provisioned to enable fast protection switching. 
     Traffic and Protection Path Fiber Pair Connections: 
     Primary Traffic Path  1130 : Switch  1102  to switch  1104   
     Protection Path  1   1134 : Switch  1102  to switch  1106  to switch  1104   
     Protection Path  2   1136 : Switch  1102  to switch  1108  to switch  1106  to switch  1110  to switch  1104   
     Protection Path  3   1138 : Switch  1102  to switch  1108  to switch  1110  to switch  1104   
     When fault  1140  occurs on the primary traffic path  1130  between switch  1102  and switch  1104 , it is detected in Switch  1102  at optical power detector  1114  and an automatic protection switch is performed to protection path  1134 . After protection path  1134  is established, it is monitored and protected by the remaining un-provisioned protection path pairs  1136 ,  1138  in the pool. When fault  1142  occurs on protection path  1134  between switch  1102  and switch  1106 , it is detected in Switch  1102  at optical power detector  1116  and an automatic protection switch is performed to protection path  1136 . Finally, when fault  1144  occurs on protection path  1136  between switches  1110  and  1104 , it is detected at switch  1104  by optical power detector  1128  and an automatic protection switch is performed to protection path  1138 . 
     The three network faults  1140 ,  1142 ,  1144  could also occur at the same time instead of in sequential order. The switches can be pre-programmed to automatically move to the next available protection path if a protection path error is raised. In this case, if the three faults occurred at the same time, the switches  1102  and  1104  would first switch to protection path  1134 . When the protection switch error is raised on protection path  1134 , switches  1102  and  1104  would automatically switch to protection path  1136 . Finally, when the protection path error is raised on protection path  1136 , switches  1102  and  1104  would successfully protection switch to protection path  1138 . In this case, where protection switch errors are present, the switch waiting times could be adjusted to accommodate differences in switching times. 
     The higher level control system could combine the power readings from the detectors in the optical switches with other network performance monitoring and fault detection criteria to determine network reconfigurations. In a network, switches with different detector configurations could be combined in any manner for protection switching. 
     The method of protecting against multiple faults can also be applied to networks with optical switches using any combination if input and output detectors or networks with mixes of switches with different detector configurations and switching characteristics. 
     The method of protecting against multiple faults can also be applied to networks with bi-directional fiber systems with optical switches using any combination of single or dual detectors per switch port or networks with mixes of switches with different detector combinations and switching characteristics. 
     While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.