Abstract:
A network element receives frames from multiple ring networks. Each ring network linked to the network element is supported by a designated support program. The received frames are monitored for conditions indicative of a failure in one of the ring networks. Upon detection of a failure condition, the designated support program for the failing ring network is determined and notified. The designated support program then addresses the failure condition by, for example, switching to a backup link. In one example, the multiple ring networks are SONET BLSR networks.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to telecommunications networks and more specifically to network elements in ring networks. 
   2. Description of the Related Art 
   The arrangement of network elements in a telecommunications network is known as “topology”. In Synchronous Optical Network (SONET), for example, network elements can be arranged in a ring or a linear topology. Network elements in a linear topology are arranged along a line, whereas in a ring topology the network elements are arranged in a circular fashion. 
   SONET is well known and described in the following documents: American National Standards Institute (“ANSI”) documents ANSI T1.105, ANSI T1.105.01, ANSI T1.105.02, ANSI T1.105.03, ANSI T1.105.04, ANSI T1.105.05, ANSI T1.105.06, ANSI T1.105.07, ANSI T1.105.08, and ANSI T1.105.09; Bellcore Standards GR-253-CORE (Issue 2, December 1995), GR-1230-CORE (Issue 4, December 1998), GR-1375-ILR (Issue 1A Revision 1, August 1995), GR-1400-CORE (Issue 1, March 1994, Revision 1, October 1995), and TR-NWT-000496 (Issue 3, May 1992); see also, W. J. Goralski, “SONET: A guide to Synchronous Optical Networks,” McGraw-Hill 1997. All of the aforementioned SONET documents are incorporated herein by reference in their entirety. 
   SONET specifications provide for a number of self-healing optical ring topologies including the Unidirectional Path Switched Ring (UPSR) and the Bidirectional Line Switched Ring (BLSR), both of which are well known. In a UPSR ring, the originating network element transmits duplicate SONET frames on two communications links. The receiving network element receives the frames from both links and, depending on the quality of the received signals representing the frames, uses the frame from one of the links. The receiving network element does not have to notify the transmitting network element if one of the links is locally detected to be defective. 
   In a BLSR ring, the SONET frames are transmitted by the originating network element on a working link. When the receiving network element detects that the working link is defective, it so informs the transmitting network element and initiates a switchover to a protect (i.e. back up) link. Coordination between network elements in switching to a protect link is performed using a signaling protocol which uses overhead bytes of the SONET frames. 
   It is desirable to have a single network element that can support multiple rings. The flexibility afforded by such a network element reduces the cost of the network and simplifies the interconnection of rings. 
   SUMMARY 
   The present invention relates to a method and associated apparatus for supporting multiple ring networks in a single network element. 
   In one embodiment, a network element is coupled to receive frames from multiple ring networks. Each ring network linked to the network element is supported by a designated support program (e.g., software task). The support programs are isolated from one another, and run concurrently. The received frames are monitored for conditions indicative of a failure in one of the ring networks. Upon detection of a failure condition, the designated support program for the failing ring network is determined and notified. The designated support program then addresses the failure condition by, for example, switching to a backup link. 
   In one embodiment, the frames are Synchronous Optical Network (SONET) frames. 
   In one embodiment, the ring networks are SONET Bidirectional Line Switched Ring (BLSR) networks. 
   These and other features of the present invention will be apparent to a person of ordinary skill in the art upon reading the following description and figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic diagram of a SONET network in the prior art. 
       FIG. 2  shows a schematic diagram of a SONET network in one embodiment. 
       FIG. 3A  shows a schematic diagram of a network element in one embodiment. 
       FIG. 3B  pictorially illustrates the arrangement of information in a system communications link in one embodiment. 
       FIG. 4  shows a process for supporting multiple ring networks in one embodiment. 
       FIGS. 5A and 5B  show a process for handling switching requests in one embodiment. 
   

   The use of the same reference symbol in different figures indicates the same or like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a schematic diagram of a SONET network  10  in the prior art. Network  10  includes ring networks RING A and RING B. Network elements (NEs)  11 ,  12 , and  13  belong to RING A while NEs  21 ,  22 , and  23  belong to RING B. Because none of the network elements in network  10  is capable of supporting more than one ring network, communications between network elements in different ring networks must past through NE  23  and NE  13 . For example, a SONET Synchronous Transport Signal (STS) from NE  21  has to traverse NE  23  and NE  13 , via link  16 , to reach NE  12 . Typically, link  16  is a SONET 1+1 linear link while the rest of the links coupling the network elements in RING A and RING B are SONET UPSR or BLSR links. 
     FIG. 2  shows a schematic diagram of a SONET network  30 . Network  30  includes an NE  31 , a network element that supports multiple ring networks in accordance with an embodiment of the invention. NE  31  simplifies, speeds up, and reduces the cost of network  30  by eliminating the need to provide a separate link (e.g., link  16 ) between RING A and RING B. Further, NE  31  provides the functionality of two network elements, which are NE  13  and  23  in this example. 
     FIG. 3A  shows a schematic diagram of the pertinent components of NE  31 . In one embodiment, NE  31  is of the same type as the Model ONS 15454 optical transport system from Cisco Systems, Inc. NE  31  can also be of the same type as the network elements disclosed in the following commonly-owned U.S. patent applications which are incorporated herein by reference in their entirety: U.S. patent application Ser. No. 09/343,122, entitled “GENERATION OF DATA USED FOR NETWORK OPERATION,” filed on Jun. 29, 1999; U.S. patent application Ser. No. 09/478,287, entitled “AUTOMATIC PROPAGATION OF CIRCUIT INFORMATION USED IN A COMMUNICATION NETWORK”, filed on Jan. 5, 2000; and U.S. patent application Ser. No. 09/274,078, “FLEXIBLE CROSS-CONNECT WITH DATA PLANE,” filed on Mar. 22, 1999. A person of ordinary skill in the art can appreciate that the present technique for supporting multiple ring networks in a single network element can also be adapted to work with other types of network elements. 
   As illustrated in  FIG. 3A , NE  31 , in one embodiment, includes line interfaces  46 – 49  for sending and receiving SONET STSs (i.e. SONET frames) via conventional SONET links (e.g., two-fiber or four-fiber SONET links; not shown). Interfaces  46  and  47  are linked to ring network RING A while interfaces  48  and  49  are linked to ring network RING B. NE  31  can support additional ring networks by including additional pairs of interfaces. 
   Interfaces  46 – 49 , Timing Communications and Control (TCC) card  42 , and Cross-Connect (XCON) card  44  communicate with each other by way of system communications links (SCLs)  41 , which provide time division multiplexed (TDM) point-to-point connections. Time division multiplexing, in general, is well known.  FIG. 3B  shows a pictorial representation of the arrangement of information in a frame of an SCL  41 . As shown in  FIG. 3B , each frame of an SCL  41  contains 64 time slots (TS 0 , TS 1 , . . . TS 63 ), with each time slot consisting of 32 bits. In one example, each SCL  41  uses an 8 KHZ framing clock, which results in TS 0  through TS 63  lasting for 125 μs (i.e., ⅛ KHz=125 μs). Each time slot carries a single byte of each of four logical buses which are BUS 0 , BUS 1 , BUS 2 , and BUS 3 . For example, the 32 bits of TS 0  consist of Bit  7  of BUS 0 , Bit  7  of BUS 1 , Bit  7  of BUS 2 , Bit  7  of BUS 3 , Bit  6  of BUS 0  . . . Bit  0  of BUS 3 ; TS 1  contains another byte of each of the four logical buses, and so on. Thus, essentially, each logical bus consists of 64 bytes carried in 64 different time slots. Each byte of each logical bus is designated to contain a specific type of information. For example, an overhead byte of a SONET STS received by interface  46  can be sent to TCC card  42  using the byte of logical bus BUS 1  in time slot TS 12  of the SCL  41  between interface  46  and TCC card  42 . 
   TCC card  42  is an electronic printed circuit board containing a processor for running software, memory for storing software and associated data, and a TDM cross-connect (TDM-XC) for relocating time slots from one SCL  41  to another. The TDM-XC uses the well known sequential-write, random-read cross-connect technique. The so-called K1 and K2 bytes (“K-bytes”) from the overhead section of the SONET STSs received on interfaces  46 – 69  are routed to the TDM-XC and then passed to XCON card  44 . XCON card  44  is a full crosspoint, non-blocking cross-connect that supports broadcast switching. SONET cross-connects, in general, are well known. XCON card  44  cross-connects a SONET STS from one line interface to another. Thus, a SONET STS received by NE  31  from a network element in one ring network can be transmitted to another network element in another ring network. However, the capability to cross-connect a SONET STS from one line interface to another is not enough to support multiple ring networks in a single network element. What is further required, and lacking in the prior art, is the capability to process in a single network element switch requests from multiple ring networks. 
     FIG. 4  shows a process for supporting multiple ring networks in a single network element in one embodiment. As can be appreciated by a person of ordinary skill in the art, the process shown in  FIG. 4  and all other processes in this disclosure can be stored in computer-readable media such as floppy disks, hard disks, CD-ROMs, and memory devices. In action  81 , a human user provisions a ring network coupled to NE  31  by assigning, among other parameters, a Ring Id to identify the ring network, a Node ID to identify NE  31  in the ring network, and a pair of interfaces (an east interface and a west interface) linked to the ring network. The aforementioned provisioning information is entered by the user into a computer (not shown). 
   In action  82 , the provisioning information is conventionally downloaded to NE  31 . In one embodiment, the data-entry computer communicates with NE  31  using conventional CORBA (Common Object Request Brokerage Architecture) calls over a TCP/IP connection (e.g., Ethernet). The CORBA calls cause a user provisioning message to be sent to a ring network software task running in TCC card  42 . In one embodiment, ring networks RING A and RING B are both BLSR rings and the ring network software task running in TCC card  42  is a BLSR task (hereinafter “TCC BLSR task”). 
   In action  83 , the TCC BLSR task receives the user provisioning message, which includes a BLSR provisioning table containing the provisioning information entered by the user. 
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               EXAMPLE BLSR PROVISIONING TABLE FOR NE 31 
             
           
        
         
             
               Ring 
               Ring 
               Node 
               West 
               East 
             
             
               Index No. 
               ID 
               ID 
               Interface No. 
               Interface No. 
             
             
                 
             
           
        
         
             
               0 
               0 
               1 
               46 
               47 
             
             
               1 
               1 
               4 
               48 
               49 
             
             
               2 
               x 
               255 
               x 
               x 
             
             
               3 
               x 
               255 
               x 
               x 
             
             
               4 
               x 
               255 
               x 
               x 
             
             
                 
             
           
        
       
     
   
   Table 1 shows an example BLSR provisioning table. In the example of Table 1, ring network RING A is assigned a Ring ID of “0” and is linked to NE  31  via interfaces  46  and  47 . The Node ID of NE  31  in RING A is “1”. Similarly, RING B is assigned a Ring ID of “1” and is linked to NE  31  via interfaces  48  and  49 . The Node ID of NE  31  in RING B is “4”. 
   A Ring Index No., which is internal to NE  31 , is also assigned to each provisioned ring network so that the ring network can be uniquely identified across all software running in NE  31 . In one example, the Ring Index No. is assigned based on the ring network&#39;s row number in the BLSR provisioning table. Thus, the Ring Index No. of RING A is “0” because RING A&#39;s provisioning information is in the first row of Table 1. Similarly, the Ring Index No. of RING B is “1” because RING B&#39;s provisioning information is in the second row. In Table 1, a node ID of 255 indicates that the row is unused, and an “x” in any of the cells indicates a “don&#39;t care.” 
   In action  84 , the TCC BLSR task creates a state machine (hereinafter “TCC state machine”) for each new and valid ring network identified in the BLSR provisioning table (e.g., two ring networks require two TCC state machines). In one example, a valid ring network has a Node ID between 0 and 31. 
   In action  85 , each TCC state machine generates a ring map, a squelch table, and a payload table for its corresponding ring network. An example pseudo-code of the TCC state machine is shown in APPENDIX A, which is an integral part of this disclosure. The ring map, squelch table, and payload table for a ring network can also be generated using the technique described in the incorporated and commonly-owned disclosure U.S. patent application Ser. No. 09/343,122, entitled “GENERATION OF DATA USED FOR NETWORK OPERATION”. 
   The ring map contains the IP (Internet Protocol) address and the Node ID of each network element in the ring network. The topology of the ring network, which includes such information as the Node ID and IP address of each network element in the ring, can be automatically detected using the techniques described in the incorporated and commonly-owned disclosures U.S. patent application Ser. No. 09/478,287, entitled “AUTOMATIC PROPAGATION OF CIRCUIT INFORMATION USED IN A COMMUNICATION NETWORK” and U.S. patent application Ser. No. 09/343,122, entitled “GENERATION OF DATA USED FOR NETWORK OPERATION”. Table 2 shows a ring map for RING A using network  30  ( FIG. 2 ) as an example. Similarly, the ring map for RING B is shown in Table 3. 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               EXAMPLE RING MAP FOR RING A OF NETWORK 30 
             
           
        
         
             
                 
               IP Address 
               Node ID 
             
             
                 
                 
             
             
                 
               10.3.1.5 
               1 
             
             
                 
               10.3.2.5 
               3 
             
             
                 
               10.3.4.5 
               2 
             
             
                 
                 
             
           
        
       
     
   
                                 TABLE 3                   EXAMPLE RING MAP FOR RING B OF NETWORK 30                IP Address   Node ID                       10.4.1.5   2           10.4.3.5   3           10.3.1.5   4                        
As shown in Table 2, NE  31  has an IP address of “10.3.1.5” in both RING A and RING B (see also  FIG. 2 ). While the Node ID of NE  31  is “1” in RING A and “4” in RING B, NE  31  can also have the same Node ID in both RING A and RING B as long as the Node ID is unique in both ring networks.
 
   The squelch table contains information indicating where in the ring network a particular SONET STS is added and dropped. Table 4 and Table 5 show example squelch tables for RING A and RING B of network  30  ( FIG. 2 ), respectively. 
   
     
       
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 4 
             
           
           
             
                 
             
             
               EXAMPLE SQUELCH TABLE FOR RING A OF NETWORK 30 
             
           
        
         
             
                 
               West 
                 
               East 
                 
             
             
                 
               (Intf 46) 
                 
               (Intf 47) 
                 
             
           
        
         
             
               STS 
               Incoming 
               Outgoing 
               Incoming 
               Outgoing 
             
             
                 
             
             
               1 
               Node 3 
               Node 3 
               Node 3 
               Node 2 
             
             
               2 
               Node 3 
               — 
               Node 2 
               Node 3 
             
             
               3 
               — 
               Node 1 
               Node 1 
               — 
             
             
                 
             
           
        
       
     
   
                                                               TABLE 5                   EXAMPLE SQUELCH TABLE FOR RING B OF NETWORK 30                West       East           STS   (Intf 48)       (Intf 49)                No.   Incoming   Outgoing   Incoming   Outgoing               1   Node 4   —   —   Node 4       2   Node 2   —   Node 3   —       3   Node 3   Node 2   —   —                    
In the example of Table 4, STS No. 1 received on interface  46  of NE  31  is added on Node  3  of RING A (i.e., NE  12 ) while the STS No. 1 leaving interface  46  is dropped on Node  3  of RING A. Thus, the STS No. 1 on interface  46  is a bi-directional STS between NE  12  and NE  31 . Table 4 also shows that the STS No. 2 received on interface  47  is added on Node  2  (i.e., NE  11 ) while the STS No. 2 leaving interface  47  is dropped on Node  3 . Further, Table 4 shows that the STS No. 3 leaving interface  46  is dropped on Node  1  (i.e., NE  31 ). This is an example of a unidirectional STS. Correspondingly, the STS No. 3 received on interface  47  is added on Node  1  (i.e., NE  31 ). In Tables 4 and 5, blank cells indicate an unequipped STS.
 
   The payload table contains information indicating the type of each SONET STS (e.g., STS-1, STS-3C, STS-12C or UNEQUIPPED) in the ring network. Table 6 shows an example payload table for NE  31  in RING A. In Table 6, the columns “West” and “East” refer to the pair of line interfaces used by each network element in the ring network. Each interface supports three SONET STSs in this example. 
                                         TABLE 6                   EXAMPLE PAYLOAD TABLE OF NE 31 ON RING A                Node ID   STS No.   West   East                       1   1   STS-1   STS-3C           1   2   STS-1   STS-3C           1   3   STS-1   STS-3C           2   1   STS-1   STS-1           2   2   STS-1   STS-1           2   3   STS-1   STS-1           3   1   STS-3C   STS-3C           3   2   STS-3C   STS-3C           3   3   STS-3C   STS-3C                        
As shown in Table 6, STS No. 1 of Node  1  (i.e., NE  31 ) on the west interface (i.e., interface  46 ) is an STS- 1 , STS No. 1 of Node  1  on the east interface (i.e., interface  47 ) is an STS-3C, and so on. Similarly, NE  31  has a payload table (not shown) indicating the type of each SONET STS in RING B.
 
   The ring map, squelch table, and payload table describe the interconnection of network elements and flow of SONET STSs in a particular ring network. Thus, in the event of a link failure, the SONET STSs can be re-routed to protection links in accordance with the well known Automatic Protection Switching protocol (APS) (see also, Bellcore document Generic Requirements GR-1230-CORE (Issue 4, December 1998), incorporated herein by reference). 
   Every time a user provisioning message is received by the TCC BLSR task, the accompanying BLSR provisioning table is compared against those previously received. This allows the TCC BLSR task to determine if a new ring network is being provisioned, if an existing ring network is being modified, or if an existing ring network is being deprovisioned (i.e., removed). To simplify the comparison process, a ring network always appears in the same row of the BLSR provisioning table. In one example, the following algorithm is followed when a new provisioning table is received:
         a) If row i of the new provisioning table is invalid (e.g., has a Node ID of “255”) and row i of the old provisioning table is also invalid, then nothing has to be done.   b) If row i of the new provisioning table is invalid but row i of the old provisioning table is valid (i.e., has a Node ID between 0 and 31), the ring network in row i of the old provisioning table is being deprovisioned. In this case, the corresponding TCC state machine for the deprovisioned ring network releases all memory used for data structures before being destroyed.   c) If row i of the new provisioning table is valid but row i of the old provisioning table is invalid, a new ring network is being provisioned. In this case, a new TCC state machine is created for the new ring.   d) If row i of the new provisioning table is valid and row i of the old provisioning table is also valid, the ring network identified in row i might have been modified. In this case, the contents of row i of the old and new provisioning tables are examined to determine what was modified. Then:
           i) If the link connecting an interface of NE  31  to the ring is being changed from a two-fiber to a four-fiber link or vice versa, the corresponding old TCC state machine is destroyed and replaced with a new TCC state machine.   ii) Any other changes are forwarded to the corresponding old TCC state machine for appropriate action. The incorporated and commonly-owned U.S. patent application Ser. No. 09/343,122, entitled “GENERATION OF DATA USED FOR NETWORK OPERATION” discusses some actions that are performed upon notification of modifications affecting the ring; also, see APPENDIX A in this disclosure.   
               

   In action  86  ( FIG. 4 ), a TCC provisioning message is sent from TCC card  42  to other cards in NE  31  including XCON card  44 . The TCC provisioning message includes the ring map, squelch table, and payload table generated by the TCC state machine of the newly provisioned ring network. Also in the TCC provisioning message are the ring network&#39;s Ring Index No., the Node ID of NE  31  in the ring network, and the interfaces of NE  31  (east interface and west interface) linked to the ring network. In XCON card  44 , the TCC provisioning message is received by an XCON BLSR task. One XCON BLSR task supports one ring network. 
   In action  87  ( FIG. 4 ), each XCON BLSR task waits for a switch request intended for the supported ring network. Processing of switch requests is later described below with reference to  FIGS. 5A and 5B . 
   Because the XCON BLSR tasks are isolated from one another in order to support multiple ring networks, the software variables used by the XCON BLSR tasks are uniquely identified by the Ring Index No. of their supported rings. For example, to access the ring map of each of the supported rings, an array of five (5) ring maps can be statically declared as
         tRingMap ringMap[5],
 
which could then be used as
   ringMap[ringIdx],
 
where ringIdx is the Ring Index No. of the supported ring network. The ring map of the ring network with a Ring Index No. of “0” can then be accessed using the variable ringMap[0], the ring map of the ring network with a Ring Index No. of “1” can be accessed using the variable ringMap[1], and so on.
       

   In one example, NE  31  uses a multi-tasking operating system such as the VxWorks Operating system from Wind River Systems, Inc. to allow software tasks in NE  31  (including the XCON BLSR tasks) to run concurrently. 
   In one example, each XCON BLSR task has three conventional software pipes (e.g., UNIX pipe) for communicating with other tasks: (i) a user command pipe, (ii) a pipe for receiving messages from an interrupt service routine, and (iii) a timer pipe. Each pipe, like the variables used by the XCON BLSR tasks, is also identified by the Ring Index No. of its supported ring network. 
   User commands, such as manual switch requests, are passed to an XCON BLSR task via the user command pipe. For example, a user command intended for the XCON BLSR task supporting RING B is passed to the user command pipe with a Ring Index No. of “1” (which is the Ring Index No. of RING B; see Table 1). 
   A software timer communicates with an XCON BLSR task using the timer pipe. For example, the software timer can inform the XCON BLSR task supporting RING A that a particular period of time has elapsed by passing a message to the timer pipe with a Ring Index No. of “0” (which is the Ring Index No. of RING A; see Table 1). 
   Once the XCON BLSR task of the newly provisioned ring network is initialized, the TCC BLSR task queries other network elements in the ring network to see if they are ready to send and receive SONET STSs. If so, the XCON BLSR task is enabled to recognize the new ring network. 
   As is well known, the Automatic Protection Switching (APS) protocol uses the so-called K-bytes of a SONET STS overhead to convey switching commands and error conditions. For example, a network element requesting a re-route of SONET STSs because of a locally detected link failure coordinates the switchover to a protection link using the K-bytes. In NE  31  ( FIG. 3A ), K-bytes are stripped by line interfaces  46 – 49  from the overhead section of received SONET STSs, and are placed in designated time slots of SCLs  41  for transmission to XCON card  44 . There, newly received K-bytes are compared against previously received K-bytes. An interrupt is generated when the new K-bytes are different from the old K-bytes. 
   An interrupt is also generated when line interfaces  46 – 49  locally detect link related problems such as signal degradation, signal failure, and loss of frame. Link related problems can be locally detected using hardware or software techniques that are well known to a person of ordinary skill in the art. The locally detected link conditions are placed by line interfaces  46 – 49  in designated high priority time slots of SCLs  41 , referred to as BSR (bi-switched ring) bytes, for transmission to XCON card  44 . An interrupt is generated when the new and old BSR bytes are different. 
     FIGS. 5A and 5B  illustrate an example process for handling switching requests in NE  31 . Of course, the just mentioned process can also be adapted to work in other types of network elements. In action  60 , line interface cards  46 – 49  strip the K-bytes of received SONET STSs for transmission to TCC card  42  via SCLs  41  (shown in  FIG. 3A ). Locally detected link conditions are also sent to TCC card  42  using the BSR bytes time slots of SCLs  41  (action  61 ). From TCC card  42 , the K-bytes and BSR bytes are forwarded to XCON card  44  via the SCL  41  linking the two cards (action  62 ). In XCON card  44 , the newly received K-bytes and BSR bytes are compared against those previously received (action  63 ). If either the K-bytes or the BSR bytes have changed, an interrupt service routine (ISR) is triggered (action  64 ). Otherwise, no action is required (action  76 ). The triggered ISR determines whether the BSR bytes have changed (action  65 ). If the BSR bytes have not changed, the interrupt must have been generated in response to a K-byte change. In that case, the ISR examines the K-bytes to determine if the change is directed to NE  31  (action  66 ). If not, the ISR ignores the K-bytes, which are then passed through NE  31  without being processed (action  77 ). If the K-bytes change are directed to NE  31  or if the BSR bytes have changed, the ISR determines which ring network is affected (action  67 ). 
   As previously discussed, each time slot of each SCL  41  is designated to carry a particular type of information. By storing the type of information carried by each time slot in a look-up table (e.g., map, memory, database), the ring network affected by the K-byte or BSR byte change can be readily determined by the ISR. For example:
         (i) if the byte of logical bus BUS 0  in time slot TS 5  of the SCL  41  between interface  46  and TCC card  42  is designated to carry a K-byte received by interface  46 ; and   (ii) if the byte of logical bus BUS 1  in time slot TS 7  of the SCL  41  between TCC card  42  and XCON card  44  is designated to carry the byte of logical bus BUS 0  in time slot TS 5  of the SCL  41  between interface  46  and TCC card  42 ; then   (iii) the byte of logical bus BUS 1  in time slot TS 7  of the SCL  41  between TCC card  42  and XCON card  44  affects RING A (because interface  46  is linked to RING A).
 
The design of a look-up table mapping the SCL  41  time slots, the K-bytes and BSR bytes, the interfaces, the ring networks coupled to the interfaces, and the Ring Index No. of each ring network is well within the capability of a person skilled in the art.
       

   Once the affected ring network is determined, the ISR checks the APS Lock flag of the XCON BLSR task supporting the affected ring network to determine if the XCON BLSR task is busy processing other switch requests (action  68 ,  FIG. 5B ). In this example, an APS Lock flag is used to prevent different switch requests from simultaneously changing the switching configuration of XCON card  44 . When the APS Lock flag is set, new switch requests are added to the processing queue (action  69 ) and wait until the previous requests are fully processed. Otherwise, the ISR passes the K-bytes and BSR-bytes to the XCON BLSR task supporting the affected ring network via the ring network&#39;s ISR pipe (action  70 ). The ISR then sets the APS Lock flag of the XCON BLSR task (action  71 ). 
   The XCON BLSR task processes the K-bytes and BSR bytes in accordance with the APS protocol (action  72 ) and, upon completion, clears the APS Lock flag (action  73 ). Actions  70 – 73  are repeated for each switch request pending in the processing queue (action  74 ). If there are no pending switch requests, the XCON BLSR task checks if there are user generated requests (action  75 ). User generated requests are administrative switch requests made, for example, to perform an equipment maintenance card swap or to change the switching configuration of XCON card  44  to add/remove customers. User generated requests are passed to the XCON BLSR task using the user command pipe identified by the Ring Index No. of the affected ring. User generated requests are conventionally processed (action  78 ) by reconfiguring the switch matrix of XCON card  44 . 
   While specific embodiments of this invention have been described, it is to be understood that these embodiments are illustrative and not limiting. For example, the present invention can be used in a variety of ring topology networks including Synchronous Digital Hierarchy (SDH) networks. Many additional embodiments that are within the broad principles of this invention will be apparent to persons skilled in the art.