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 having higher signal quality. Interface protection is provided through a protection transceiver thus implementing M:N equipment protection and 1+1 optical protection.

Description:
This application claims priority on provisional Application No. 60/398,276 filed on Jul. 24, 2002, the entire contents of which are hereby incorporated 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 event 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 having a ring architecture in an alternate embodiment of the invention; 
       FIG. 2  is a block diagram of a portion of a transceiver at a hub network element; 
       FIG. 3  is a block diagram of a remote network element of the communication network of  FIG. 1 ; 
       FIG. 4  is a block diagram of a remote network element of the communication network of  FIG. 1 ; and, 
       FIG. 5  is a block diagram of a communication network having a ring architecture in an alternate 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. 
     FIG. 1  is block diagram of a communication network  104  having a ring architecture. Shown in  FIG. 1  is a hub network element  110  and a plurality of remote network elements  120  arranged in a ring configuration. Both the hub network element  110  and the remote network elements  120  transmit and receive signals in both the clockwise and counter-clockwise directions around the ring. The ring network may use wavelength division multiplexing (WDM) in which distinct wavelengths are used to define multiple channels on one path. 
     FIG. 2  illustrates transmit and receive paths in transceivers  32  and  34 . As described previously, an optical splitter  40  divides the transmitted signal into two diverse communication paths (clockwise and counter-clockwise). An optical selector  50  selects the higher quality signal received on the diverse communication paths. The clockwise and counter-clockwise paths provide the 1+1 optical path protection. 
     FIG. 3  is block diagram of one remote network element  120 . The remote network element includes a service component  130  and a protection component  140 . The service component  130  includes a first receiver  132  for receiving signals on one of the diverse communication paths (e.g., clockwise). A second receiver  134  receives signals on the other diverse communication path (e.g., counter-clockwise). The received signal having the higher quality is selected by optical selector  50  and directed to a service interface  136 . The service interface  136  provides an ingress and egress point to the ring network for users. 
   The protection component  140  is used when a transceiver  32  is not operational and protection transceiver  34  is activated. In an exemplary embodiment each transceiver  32  sends and receives signals on a separate wavelength. If one of transceiver  32  fails, protection transceiver  34  is activated to replace the missing wavelength. In one embodiment of the invention the protection transceiver  34  generates a signal around 1300 nm, such as 1310 nm. 
   The protection component  140  of remote node  120  includes an optical add/drop multiplexer  142  (OADM), an optical-to-electrical (O/E) converter  143  and an electrical-to-optical (O/E) converter  144 . The OADM  142  selects a protection signal having the protection wavelength (e.g., 1310 nm) and directs the protection signal to the O/E converter  143 . Switches  145  and  146  can couple the O/E converter  143  to the E/O converter  144  to place the OADM in loop-back mode. Alternatively, switches  145  and  147  may be configured to couple the O/E converter  145  to first and second transceivers  132  and  134  in the service component  130 . Similarly, switches  146  and  148  may be configured to coupled the E/O converter  144  to first and second transceivers  132  and  134  in the service component  130 . Thus, the protection component  140  serves as a protection transceiver. Operation of switches  145 – 148  are described in detail herein. 
   During normal operation, network element  110  receives traffic for distribution on the ring through switch  26 . Switch  26  is programmed to distribute traffic to transceivers  32  as established through provisioning. The signals provided to the transceivers  32  are directed around the ring in both the clockwise and counter-clockwise directions. Transceivers  132  and  134  at the remote network elements  120  receive signals and either route the signals off the ring through optical selector  50  and service interface  136  or regenerate the signal and redirect the signal back on the ring. 
     FIG. 3  illustrates the status of the protection component  140  when the ring is in normal operation. Switches  145  and  146  are configured so that the output of O/E converter  143  is coupled directly to the input of E/O converter  144 . This mode is referenced as loop-back mode. Any signal on the protection wavelength may be enhanced (e.g., subject to 3R regeneration of re-amplification, reshaping and retiming) and placed back on the ring for distribution to the next network element. 
   If one of the diverse communication paths (i.e., the clockwise or counter-clockwise) fails, the optical selector  50  in hub network element  110  and selector  50  in remote network elements  120  will select the signal having the higher signal quality. The diverse communication paths provide redundant signals in order to provide the optical path protection. 
   If a transceiver  32  in the hub network element fails, optical interface protection is enacted as follows. For illustration, assume that transceiver  32   1  directing traffic to remote network element  120   1  fails. The service component  130  detects a transceiver failure as both transceiver  132  and transceiver  134  experiencing a loss of signal (LOS). The service component  130  signals protection component  140  to enter a protection mode. As shown in  FIG. 4 , switch  145  is configured to coupled the O/E converter  143  to the receive input of transceivers  132  or  134 . Switch  146  is configured to connect the E/O converter  144  to the transmit output of transceivers  132  or  134 . 
   Similarly, hub network element  110  detects failure of transceiver  32   1  and activates protection transceiver  34 . As noted previously, protection transceiver  34  operates at a wavelength (e.g., 1310 nm) that is selected by OADM  142 . Switch  26  directs incoming traffic destined for the failed transceiver  32   1  to the protection transceiver  34 . The protection transceiver  34  then transmits the signal on the protection wavelength in both directions around the ring. 
   The OADM  142  retrieves the protection wavelength from the ring and directs the received signal to clockwise transceiver  132  or counter-clockwise transceiver  134 . Selector  50  selects the correct transceiver (CW or CCW) based on the configuration of switches  147  and  148  for distribution to the service interface  136 . Incoming signals from the service interface  136  are directed to either the transceiver  132  or transceiver  134 . The incoming signal is directed to the E/O converter  144  through switches  148  and  146 . The E/O converter  144  puts the signal on the protection wavelength and the OADM  142  then places the signal on the ring. The remaining remote nodes  120   2  and  120   3  have the protection component  140  in loop-back mode and direct the protection wavelength back to hub network element  110 . Once the protection wavelength is activated to carry active traffic, the system may be configured such that the protection wavelength is not available to any other remote node on the network or priorities may be established to ensure that the service with the highest priority always has protection available. 
     FIG. 5  depicts an alternate embodiment in which signals transmitted by transceivers  132  and  134  may be multiplexed by a multiplexer  152 . For example, multiplexer  152  may combine multiple signals through time division multiplexing (TDM). This provides the ability to protect multiple wavelengths using the single protection wavelength. 
   The protection transceiver  34 , OADM  142 , O/E converter  143  and E/O converter  144  may operate at a protection wavelength around 1300 nm (e.g., 1310 nm). Such components are widely available and relatively inexpensive. Thus, effective 1:N protection may be achieved without substantial cost. 
   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.