Abstract:
A method and system for providing tandem protection in a communication system. Path protection is provided using at least two redundant communication paths and selecting the communication path having a higher signal quality. Interface protection is provided through a protection transceiver. The interface protection may be delayed while the path protection attempts to restore communication.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional application ser. No. 60/398,276 filed Jul. 24, 2002, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates generally to optical communication networks and in particular to methods and systems for providing protection in an optical communication network. 
     2. Description of Related Art 
     Communication networks often include protection mechanisms to reroute signals in the even of a service interruption. Primary causes of service interruption are link failures and networking equipment failures. Link failures may be caused by failure of the transmission medium, such as the cut of an optical fiber cable, or by failure of an active component that affects all the optical channels on a dense wavelength division multiplexed (DWDM) link, such as an optical amplifier. With carrier-class optical networking equipment, the most likely cause of equipment failure is failure of an isolated optical channel interface. 
     A variety of protection techniques exist in order to provide protection against service interruption. For example, a 1+1 protection scheme provides a redundant protection path for each working path. A switch at the receiving end of the working path can switch to receive the redundant signal on the protection path if signal quality is deteriorated on the working path. Another known protection scheme is 1:1 protection in which a protection path is associated with each working path, but the protection path is not utilized until signal quality is deteriorated on the working path. Another known protection scheme is 1:N protection in which a protection path is associated with multiple working paths. If signal deterioration is detected on one of the working paths, traffic is redirected to the protection path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a block diagram of a communication network in an embodiment of the invention; 
         FIG. 2  is a block diagram of a communication network in an alternate embodiment of the invention having optical switch control feedback; 
         FIG. 3  is a block diagram of a communication network in an alternate embodiment of the invention having WDM transmission with multiple switches; 
         FIG. 4  is a block diagram of a communication network in an alternate embodiment of the invention having WDM transmission with a single switch; 
         FIG. 5  is a block diagram of a communication network in an alternate embodiment of the invention having alternate channel routing; 
         FIG. 6  is a block diagram of a communication network in an alternate embodiment of the invention having intermediate network elements between source and destination network elements; 
         FIG. 7  is a flowchart of an exemplary process for providing tandem protection; and 
         FIG. 8  is a block diagram of a communication network having a mesh architecture in an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The following detailed description of embodiments of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof. 
     The invention may be used in a variety of communications networks, including electrical and optical networks, and combination electrical/optical networks. The expression “communicates” as used herein refers to any connection, coupling, link or the like by which signals carried by one system element are imparted to the “communicating” element. Such “communicating” devices are not necessarily directly connected to one another and may be separated by intermediate components or devices. Likewise, the expressions “connected” and “coupled” as used herein are relative terms and do not require a direct physical connection. This invention may be implemented over a physical linear, ring or mesh topology. 
       FIG. 1  is block diagram of a communication system  100  in an embodiment of the invention. The system  100  includes two network elements  12  and  14 . The network elements may be any known network element such as a switch, router, etc. In one embodiment, network elements  12  and  14  may be a CoreDirector® switch available from CIENA Corporation. 
     Network element  12  includes a number of transceivers  22  and at least one protection transceiver  24 . As described in further detail herein, the protection transceiver  24  provides 1:N optical interface protection for transceivers  22 . SONET 1:N APS is an example of one type of 1:N optical interface protection. Additionally, it is understood that any number of protection transceivers (M) may be utilized to protect any number of transceivers (N). Thus, M:N optical interface protection may be provided. A switch  26  directs incoming traffic to an appropriate transceiver. 
     Similarly, the network element  14  includes transceivers  32 , at least one protection transceiver  34  and a switch  36 . For simplicity, communication is described as transmission from network element  12  and reception at network element  14 . It understood that bi-directional communication may exist between the network elements. 
     At the output of transceivers  22  and protection transceiver  24  is a splitter  40  that divides the output into at least two diverse communication paths  42  and  44 . For the purpose of this description, 2 diverse paths are shown but it is understood that more than two diverse communication paths may be used for higher redundancy. The diverse communication paths  42  and  44  represent separate communication paths between the network elements. These paths may be physically isolated such as separate optical fibers geographically separated to reduce the likelihood that both paths will be disrupted simultaneously. 
     At the receiver side at network element  14 , an optical selector  50  monitors the signal on each communication path  42  and  44  and selects the diverse communication path having the better signal quality. The optical selector may be a simple, low cost device that monitors the first communication path  42  and switches to the second communication path  44  (or additional path, if available) in the event of a loss of signal (LOS) or loss of modulation (LOM), for example, on the first communication path  42 . More complex techniques may be used to detect deterioration of signal quality as described herein. The redundant diverse communication paths  42  and  44  and the optical selector  50  provide optical path protection. 
     Optical interface protection is not constrained to using SONET 1:N protection. In the same way that SONET 1:N optical interface protection provides service survivability of the SONET signal, aggregated link interfaces on a router could provide survivable paths for IP packets. 
       FIG. 2  is a block diagram of another communication system  101  in which the network element  14  provides a control signal  52  to optical selector  50  to cause the optical selector  50  to select a different communication path. The network element  14  may be able to detect more sophisticated signal deterioration than the optical selector  50  (e.g. using performance monitoring capabilities located on transceiver  32 ). This allows the optical selector to be a low cost device that monitors for simple signal deterioration (e.g. LOS). Network element  14  can detect signal quality such as bit error rate, eye pattern, signal-to-noise ratio, etc. If the network element  14  detects deteriorated signal quality, control signal  52  from network element  14  causes the optical selector  50  to select an alternate communication path. 
     Table 1 illustrates how selector  50  may be programmed to choose when to connect to diverse optical path  42  or  44 . 
     If control signal  52  does not exist (or is not activated), then selector  50  will connect to the diverse optical path with better quality signal. Without control signal  52 , if the quality of signal on each diverse optical path is the same, then the selector  50  will not change state. 
     If control signal  52  does exist, then network element  14  will send switch status instructions to selector  50 . Selector  50  will follow those instructions if conditions at the detectors for optical path  42  and optical path  44  are normal. Under all other conditions, selector  50  will ignore said switch status instructions from network element  14  and perform as if control signal  52  does not exist. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Detector at 
                 Detector at 
                   
                   
               
               
                 Path 42 
                 Path 44 
                 Control Signal 52 
                 Selector 50 Action 
               
               
                   
               
             
             
               
                 Normal 
                 Normal 
                 None 
                 No Change 
               
               
                   
                   
                 Instruction from NE 
                 Follow NE 14 
               
               
                   
                   
                 14 
                 Instruction 
               
               
                 Failure/ 
                 Failure/ 
                 None 
                 No Change 
               
               
                 Degraded 
                 Degraded 
                 Instruction from NE 
                 No Change 
               
               
                 Signal 
                 Signal 
                 14 
                 Ignore NE 14 
               
               
                   
                   
                   
                 Instructions 
               
               
                 Normal 
                 Failure/ 
                 None 
                 Connect to Path 42 
               
               
                   
                 Degraded 
                 Instruction from NE 
                 Connect to Path 42 
               
               
                   
                 Signal 
                 14 
                 Ignore NE 14 
               
               
                   
                   
                   
                 Instructions 
               
               
                 Failure/ 
                 Normal 
                 None 
                 Connect to Path 44 
               
               
                 Degraded 
                   
                 Instruction from NE 
                 Connect to Path 44 
               
               
                 Signal 
                   
                 14 
                 Ignore NE 14 
               
               
                   
                   
                   
                 Instructions 
               
               
                   
               
             
          
         
       
     
       FIG. 3  shows an alternate communication system  102  in which the diverse communication paths are implemented using wavelength division multiplexing (WDM). The outputs of transceivers  22  and protection transceiver  24  are divided at splitters  40  and directed into diverse communication paths  42  and  44 . At each diverse communication path, the outputs from the transceivers  22  and protection transceiver  24  are combined into a WDM signal by WDM equipment (not shown). Prior to arriving at the selectors  50 , the WDM signal is demultiplexed. This provides for monitoring of each WDM channel. As discussed above, the selectors  50  direct the higher quality signal to network element  14 . 
       FIG. 4  shows an alternate communication system  103  in which the diverse communication paths are implemented using wavelength division multiplexing (WDM). The outputs of transceivers  22  and protection transceiver  24  are combined into a WDM signal by WDM multiplexing equipment  60 . At the output of the WDM multiplexing equipment  60 , the combined multiplex of signals is divided at splitter  40  and directed into diverse communication paths  42  and  44 . Prior to arriving at WDM demultiplexing equipment  61 , the selector  50  chooses the best quality multiplex of signals. This provides for monitoring of many WDM channels simultaneously. The selector  50  directs the higher quality signals to WDM demultiplexing equipment  61 . Each WDM signal is then passed to its appropriate transceiver  32  and protection transceiver  34  in network element  14 . 
       FIG. 5  is a block diagram of another communication system  104  in which the selectors  50  and  51  are configured to receive optical signals from different diverse optical paths  42  and  44  respectively. This configuration provides network element  14  with information about the health of each diverse communication path  42  and  44  by receiving performance information from transceivers  32  and protection transceiver  34  (e.g. SONET PM) simultaneously. 
       FIG. 6  is a block diagram of a communication system  105  in another embodiment in which the tandem optical path protection and optical interface protection are provided over a communications network with intermediate network elements located between source and destination network elements. 
     For sake of illustration, this example describes optical path protection and optical interface protection for a signal transmitted from network element  12  to network element  14 . During provisioning of communication paths, two diverse paths ( 42  and  44 ) are established between network element  12  and network element  14 . The intermediate network element  70  is provisioned to pass signals along to network element  14  using diverse communication path  42  and intermediate network element  80  is provisioned to pass signals along to network element  14  using diverse communication path  44 . 
     For the purpose of this example, network element  70  is defined to terminate the optical signal using an optical to optical (OO) interface. It receives the signal optically and re-transmits an optical signal towards network element  14  without electrical conversion. An example of such a network element may be an optical line amplifier (e.g. Erbium Doped Fiber Amplifier) or an all-optical switch. If WDM transmission is used then network element  70  may be an Optical Add Drop Multiplexer or a Wavelength Selective Optical Switch. 
     For the purpose of this example, network element  80  is defined to terminate the optical signal using an optical to electronic to optical (OEO) interface. It receives the signal optically, converts the optical signal to an electronic signal and re-transmits a regenerated optical signal towards network element  14 . An example of such a network element may be a SONET regenerator (e.g. Section Terminating Equipment or Line Terminating Equipment). 
       FIG. 7  is a flowchart of a method for providing tandem protection services for one embodiment of this communication system. The method is described with reference to  FIG. 2 , but may be implemented in a variety of network architecture configurations, including liner point-to-point, ring and mesh. 
     At step  310  the process begins. At this point, the communications system is operating under normal conditions. For example, under normal conditions, the primary communication path may be defined as diverse communication path  42  and the backup communication path may be defined as diverse communication path  44 . In this step, the primary communication path  42  supports communication signals between network elements  12  and  14 . Also, under normal conditions, the transceivers  22  and  32  are defined as the working interfaces and the protect transceivers  24  and  34  are unused. 
     The impact of signal degradation upon the primary communication path  42  will be described first followed by the impact of signal degradation upon a working interface (e.g. transceiver  22  or  32 ). 
     At step  321  signal quality on the primary communication path, for example communication path  42 , is monitored. In one embodiment, the optical selector  50  monitors signal quality. The associated transceiver  32  may also detect signal quality as discussed herein. 
     At step  322 , it is determined whether the signal quality has deteriorated. The signal deterioration may be detected as a loss of signal (LOS) or Loss of Modulation (LOM), for example, which a low cost optical selector  50  is capable of detecting. More sophisticated signal quality (bit error rate, eye pattern, SONET alarms etc.) detection may be performed by the optical selector  50  or by transceiver  32 . The process loops back to step  321  if the signal quality has not been recognized as deteriorated. If signal quality deterioration is detected, the process continues to step  323 . 
     At step  323 , the optical selector  50  switches to select the backup communication path  44 . 
     At step  324  signal quality on the backup communication path, for example communication path  44 , is monitored and it is determined whether signal quality has been restored. If signal quality deterioration continues to be detected, the process moves to step  325 . The process advances to step  340  if signal quality is restored to a normal state. 
     At step  325 , it is recognized that all diverse communication paths (e.g. communication paths  42  and  44 ) have deteriorated and the communication system has failed. 
     At step  340 , the communications system is operating under alternative conditions. For example, under the alternative conditions of this step, the backup communication path  44  supports communication signals between network elements  12  and  14 . Also, as under normal conditions, the transceivers  22  and  32  continue as the working interfaces and the protect transceivers  24  and  34  are unused. 
     At step  341 , an optional reversion routine may be implemented to determine when the failed primary communication path  42  is repaired. If it is determined that repair of primary communication path  42  has not occurred, the communications system will continue to operate under the alternative conditions of step  340 . If it is determined that repair of primary communication path  42  has occurred and it is able to resume its role supporting communication signals between network elements  12  and  14 , the process advances to step  342 . 
     At step  342 , the optical selector  50  switches to select the primary communication path  44 . The communication signals between network elements  12  and  14  revert to primary communication path  44  and the communication system resumes operation under the normal conditions of step  310 . 
     The impact of signal degradation upon a working interface (e.g. transceiver  22  or  32 ) is now described. At step  331  signal quality at the working interfaces, for example transceivers  22  and  32 , is monitored. At step  332 , it is determined whether the signal quality at the interface has deteriorated. Because signal quality is being monitored at an electrical interface, the signal deterioration may be detected using sophisticated measurements such as bit error rate, eye pattern, SONET PM or alarms (e.g. AIS). The process loops back to step  331  if the signal quality has not been recognized as deteriorated. If signal quality deterioration is detected, the process continues to step  333 . 
     At step  333 , a protection delay is initiated after the interface (e.g. transceiver  32 ) detects a deterioration of signal quality. The network elements  12  and  14  initiate a hold-off timer and they wait for a pre-determined time described as protection delay (e.g., 10 milliseconds) to give time for the optical selector  50  to select the best diverse communication path per steps  321 ,  322  and  323 . Once the protection delay has expired, the process advances to step  334 . 
     At step  334 , it is determined whether the signal quality at all interfaces has been corrected or if multiple interfaces continue to be deteriorated. If it is recognized that multiple interfaces continue to be in a degraded condition, the process advances to step  335 . Otherwise, the process moves to step  336 . 
     At step  335 , it is recognized that steps  321  and  322  were unable to identify signal degradation on the primary communication path because multiple signal degradations still exist after the interface protection delay (step  333 ). To correct the multiple signal degradations, control signal  52  is used to request that selector  50  switches from receiving signals from the primary communication path  42  to the backup communication path  44  (per step  323 ). 
     At step  336 , it is determined whether the quality of a single signal continues to be deteriorated. If it is recognized that all interfaces have been corrected from the degraded condition, then the process advances to step  340 . If a single signal is not restored, the process advances to step  337 . 
     At step  337 , the network elements  12  and  14  initiate the optical interface protection for the single degraded signal. In the embodiment shown, the use of SONET 1:N APS optical interface protection results in traffic being directed from a working transceiver pair  22  and  32  to protection transceiver pair  24  and  34 . Using SONET signaling standards switching time is deterministic and complete connection restoration can occur within 50 milliseconds of detecting the signal deterioration. Upon completion, the process moves to step  350 . 
     At step  350 , the communications system is operating under alternative conditions. For example, under the alternative conditions of this step, the primary communication path  42  supports communication signals between network elements  12  and  14 . However, a signal from one of the transceiver pairs  22  and  32  is now communicated between the protect transceivers  24  and  34 . 
     At step  351 , an optional reversion routine may be implemented to determine when the failed transceiver interface  22  or  32  is repaired. If it is determined that repair of transceiver interface  22  or  32  has not occurred, the communications system will continue to operate under the alternative conditions of step  350 . If it is determined that repair of transceiver interface  22  or  32  has occurred and it is able to resume its role supporting communication signals between network elements  12  and  14 , the process advances to step  352 . 
     At step  352 , the network elements  12  and  14  revert the optical interface protection switch and the communication system resumes operation under the normal conditions of step  310 . 
       FIG. 8  is a block diagram of a communication system  106  in another embodiment in which the tandem optical path protection and optical interface protection are provided over a mesh communications network with more than two diverse optical paths between source and destination network elements. 
     For sake of illustration, this example describes optical path protection and optical interface protection for a signal transmitted from network element  12  to network element  14 . During provisioning of communication paths, three diverse paths ( 42 ,  44  and  46 ) are established between network element  12  and network element  14 . An intermediate network element  70  is provisioned to pass signals along to network element  14  using diverse communication path  42  and network element  80  is provisioned to pass signals along to network element  14  using diverse communication path  44 . Diverse communication path  46  connects network element  12  to network element  14  directly. 
     At the output of transceivers  22  and protection transceiver  24  is a splitter  43  that divides the output into more than two diverse communication paths. This example illustrates three communication paths  42 ,  44  and  46 . The diverse communication paths  42 ,  44  and  46  represent separate communication paths between the network elements. These paths may be physically isolated such as separate optical fibers geographically separated to reduce the likelihood that all paths will be disrupted simultaneously. 
     At the receiver side at network element  14 , an optical selector  53  monitors the signal on each of the many communication paths (in this example paths  42 ,  44  and  46 ) and selects the diverse communication path having the better signal quality. The optical selector may be a simple, low cost device that monitors the first communication path  42  and switches to the second communication path  44  or third communication path  46  in the event of a loss of signal (LOS) or Loss of Modulation (LOM). The redundant diverse communication paths  42 ,  44  and  46  and the optical selector  50  provide optical path protection. 
     For the purpose of this example, network element  70  is defined to terminate the optical signal using an optical to optical (OO) interface. It receives the signal optically and re-transmits an optical signal towards network element  14  without electrical conversion. An example of such a network element may be an optical line amplifier (e.g. Erbium Doped Fiber Amplifier) or an all-optical switch. If WDM transmission is used then network element  70  may be an Optical Add Drop Multiplexer or a Wavelength Selective Optical Switch. In a mesh network, network element  70  may terminate more than two optical paths. Optical paths  71  and  72  represent alternative optical paths in this mesh network scenario. 
     For the purpose of this example, network element  80  is defined to terminate the optical signal using an optical to electronic to optical (OEO) interface. It receives the signal optically, converts the optical signal to an electronic signal and re-transmits a regenerated optical signal towards network element  14 . An example of such a network element may be a SONET regenerator (e.g. Section Terminating Equipment or Line Terminating Equipment). In a mesh network, network element  80  may terminate more than two optical paths. Optical paths  81  and  82  represent alternative optical paths in this mesh network scenario. 
     In the mesh embodiment, new communication paths may be provisioned when signal deterioration is detected on an existing communication path. For example, if comunication path  42  experiences signal degradation, selector  53  swtiches to communcation path  44 . Another communication path may be provisioned to provide an optical protection path for the newly selected communcation path. 
     Embodiments of the invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.