Abstract:
A restoration procedure for use in bidirectional multiplex section-switched ring transmission systems re-establishes a connection path by using a loopback connection at nodes bordering the failure and by then establishing a jumpered connection path, for each active tributary affected by the failure. The jumpered connection path at affected node eliminates any unnecessary loop formed in the re-established connection path caused by the loopback connection at the border nodes. There is stored in memory at each node (1) the provisioned connection path, (2) the jumpered path, and (3) a jumper flag is set indicating that a jumper connection exists. When the failure is cleared the (1) jumpered path is removed and its record erase from memory, (2) the provisioned connection path is restored, and (3) the jumper flag is reset. Advantageously, the this operation can be programmed into existing node controllers and is compatible with existing restoration procedures.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to bidirectional multiplex section-switched ring transmission systems and, more particularly, to a failure restoration method and apparatus for use in such systems. 
     BACKGROUND OF THE INVENTION 
     In prior known bidirectional multiplex section switched self-healing ring transmission systems (also referred to herein as bi-directional line-switched rings (BLSRs)), bridging and switching, in the presence of a fault, was restricted to switching at the BLSR nodes which border to the fault. A problem with such an arrangement, in long distance networks, is that the restoration path can be extremely long. In certain applications, for example, transoceanic BLSR transmission systems, the length of the restoration path would be looped and may cause signals to traverse the ocean three times for particular fault conditions making the path extremely long, causing long delays and degraded system performance. The long delays and degraded service is extremely undesirable. 
     The notion of eliminating BLSR restoration path delay is not a new one, prior U.S. Pat. No. 5,341,364 entitled “Distributed Switching in Bi-directional Multiplex Section-Switched Ring Transmission Systems” by Marra et al., issued on Aug. 23, 1994 provided one solution to the problem. However, the method described in this patent has the disadvantage that it may not be compatible with conventional BLSR nodes, since it explicitly relies on the absence of BLSR loopbacks at the nodes bordering the failure. BLSR compatibility is important for at least two reasons: the first is mixed-vendor operation, and the second is software upgrades. What is needed is a method and apparatus which eliminates BLSR node restoration path delay yet which is compatible with existing BLSR loopback operation. 
     SUMMARY OF THE INVENTION 
     The prior restoration problems resulting from a system transmission path degradation are overcome, in accordance with the principles of the invention, by the use of a jumper flag indicating an establishment of a jumpered connection path, for each active tributary affected by the path degradation. The jumpered connection path at an affected node eliminates any unnecessary loop formed in the re-established connection path caused by the loopback connection at that node. 
     In accordance with the invention, there is stored in memory at each node entries identifying (1) its provisioned (or normal) service connection path and (2) a jumper flag indicating if a jumpered connection path exists. 
     In response to a loopback setup complete message received at a node, from both the first and second directions, it is determined if a jumpered connection is needed at that node. If a jumpered connection is not needed, conventional BLSR processing is performed. If a jumpered connection is needed, the provisioned connection is taken down, the jumpered connection is put up, a jumper flag is set, and conventional BLSR processing is performed. 
     In response to a loopback takedown message received at a node, from both the first and second directions, conventional BLSR processing is performed. 
     In response to a signal fail/degrade or a signal fail/degrade clear message received at a node, from both the first and second directions, it is determined if a jumper flag is set at that node. If the jumper flag is set, the jumpered connection is taken down, the provisioned connection is restored, and the jumper flag is reset. If the jumper flag is not set, conventional BLSR processing is performed. 
     Advantageously, the operation of the present invention can be programmed into existing node controllers and is compatible with existing restoration procedures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawings 
     FIG. 1 shows, in simplified block diagram form, BLSR node transmission system  100  in which the present invention may be utilized, the system illustratively includes ring nodes  101  through  104 ; 
     FIG. 2 shows an illustrative block diagram of a ring node including an embodiment of the invention; 
     FIG. 3 shows an illustrative block diagram of a receiver used in the ring node of FIG. 2; 
     FIG. 4 shows an illustrative block diagram of a transmitter used in the ring node of FIG. 2; 
     FIG. 5 is an exemplary ring node ID map included in memory of the controller of FIG. 2; 
     FIG. 6 shows the format of the switch request message (K 1 ) and switch acknowledgment message (K 2 ); 
     FIG. 7 shows communications tributary traffic pattern table for ring node  104 , also included in memory of the controller of FIG. 2; 
     FIG. 8 shows communications tributary traffic pattern table for ring node  102 , also included in memory of the controller of FIG. 2; 
     FIG. 9 shows communications tributary traffic pattern table for ring node  101 , also included in memory of the controller of FIG. 2 
     FIG. 10 is a flow chart illustrating the bridge and switch operation of the controller of FIG. 2 in response to failure, clear, loopback completion, and takedown switch messages; 
     FIG. 11 shows the system after a complete fiber cut fault between ring nodes  101  and  104  and the effect on a signal “X” tributary traffic pattern after each fault bordering node has propagated a “failure” message through to other system body nodes; 
     FIG. 12 a  shows the signal “X” connection after each border node has received the “failure” message which originated from the other border node, but has not yet performed a loopback connection and any protection switching; 
     FIG. 12 b  shows the signal “X” connection after each border node has performed a loopback connection and any protection switching, and has propagated a “loopback complete” message to other system body nodes; 
     FIG. 13 a  shows the signal “X” connection after each body node has received the “loopback complete” message without requiring jumpering at border nodes; 
     FIG. 13 b  shows the signal “X” connection after each body node has received the “loopback complete” message requiring jumpering at border nodes; 
     FIG. 14 shows the signal “X” connection after the border nodes have each propagated a “clear” message through the body nodes; and 
     FIG. 15 shows the signal “X” connection after each border node has received the “clear” message which originated from the other border node, has taken down the loopback connection and restored any provisioned switching connection, and propagates a “takedown” message complete through to the other system body nodes. 
    
    
     DETAILED DESCRIPTION 
     In the following description, each item or block of each figure has a reference designation associated therewith, the first number of which refers to the figure in which that item is first located (e.g.,  104  is located in FIG.  1 ). 
     FIG. 1 shows, in simplified form, BLSR transmission system  100 , which for brevity and clarity of exposition is shown as including only ring nodes  101  through  104 , each incorporating an embodiment of the invention. It will be apparent that additional or fewer ring nodes and different orientation of ring nodes may be employed, as desired. Ring nodes  101  through  104  are interconnected by transmission path  110 , including service path  110 -S and protection path  110 -P, in a counter-clockwise direction, and by transmission path  120 , including service path  120 -S and protection path  120 -P, in a clockwise direction. In this example, transmission paths  110  and  120  are each comprised of two (2) optical fibers. It will be apparent, however, each of transmission paths  110  and  120  could be comprised of a single optical fiber. That is, bidirectional multiplex section-switched ring transmission system  100  could be either a two (2) optical fiber or a four (4) optical fiber system. In a two (2) optical fiber system, each of the fibers in transmission paths  110  and  120  includes service bandwidth and protection bandwidth. In the four (4) optical fiber system shown, each of transmission paths  110  and  120  includes an optical fiber for service bandwidth and a separate optical fiber for protection bandwidth. Such bidirectional multiplex section-switched ring transmission systems are known. In this example, transmission of digital signals in the CCITT Synchronous Digital Hierarchy (SDH) digital signal format is assumed. However, it will be apparent that the invention is equally applicable to other digital signal formats, for example, the ANSI SONET digital signal format. In this example, it is assumed that an optical STM-N SDH digital signal format is being utilized for transmission over transmission paths  110  and  120 . In one example, N=16. Details of the SDH digital signal formats are de-scribed in CCITT Recommendations G.707, G.708 and G.709 entitled “Synchronous Digital Hierarchy Bit Rates”, “Network Node Interface For The Synchronous Digital Hierarchy” and “Synchronous Multiplex Structure”, respectively. 
     It is noted that requests and acknowledgments for protection switch action are transmitted in an Automatic Protection Switch (APS) channel in the SDH multiplex section overhead accompanying the protection paths  110 -P and  120 -P on each of transmission paths  110  and  120 . The APS channel, in the SDH format, comprises the K 1  and K 2  bytes (shown in FIG. 6) in the SDH overhead of each of protection paths  110 -P and  120 -P. For purposes of this description, a “communications circuit” is considered to be a AU-4 SDH digital signal having its entry and exit points on the ring. 
     Each of ring nodes  101  through  104  comprises an add-drop multiplexer (ADM). Such add-drop multiplexer arrangements are known. For generic requirements of a SDH based ADM see CCITT Recommendation G.782. In this example, the ADM operates in a transmission sense to pass, i.e., express, signals through the ring node, to add signals at the ring node, to drop signals at the ring node, and to bridge and switch signals, in accordance with the principles of the invention, during a protection switch at the ring node. Note that in the event of a loop failure, normal “loopbacks” of the affected signals in ring nodes adjacent to (i.e., border) the failure occur in a well known manner in these bidirectional multiplex section-switched ring transmission systems. 
     FIG. 2 shows, in simplified block diagram form, details of ring nodes  101  through  104 . In this example, a clockwise digital signal transmission direction is assumed in the service path  110 -S and the protection path  110 -P on transmission path  110 . It will be apparent that operation of the ring node and the ADM therein would be similar for a counter-clockwise service path  120 -S and the protection path  120 -P on transmission path  120 . Specifically, shown are service path  110 -S and protection path  110 -P entering the ring node and supplying STM-N SDH optical signals to receiver  201 -S and receiver  201 -P, respectively, where N is, for example,  16 . Similarly, shown are service path  120 -S and protection path  120 -P entering the ring and supplying STM-N SDH optical signals to receiver  202 -S and receiver  202 -P, respectively, where N is, for example, 16. Details of receivers  201  and  202  are identical, and are shown in FIG. 3, to be described below. 
     The SDH STM-N optical signals exit the ring node on service path  110 -S as an output from transmitter  203 -S, on service path  120 -S as an output from transmitter  204 -S, on protection path  110 -P as an output from transmitter  203 -P and on protection path  120 -P as an output from transmitter  204 -P. Details of transmitters  203  and  204  are identical and are shown in FIG. 4, to be described below. 
     AU-4 SDH output signals from receiver  201 -S are routed under control of controller  210  either to transmitter  203 -S, i.e., expressed through to service path  110 -S, to interface  206 -S to be dropped, also to interface  206 -S for protection switching to interface  206 -P where it will be dropped or to transmitter  203 -P to be supplied to protection path  110 -P. In similar fashion, AU-4 SDH output signals from receiver  202 -S are routed under control of controller  210  either to transmitter  204 -S, i.e., expressed through to service path  120 -S, to interface  207 -S to be dropped, also to interface  206 -S for protection switching to interface  206 -P where it will be dropped or to transmitter  204 -P to be supplied to protection path  120 -P. (Note that this invention does allow looping back of the AU-4 SDH signals to either protection path  110 -P or protection path  120 -P, as per conventional BLSR operation.) The AU-4 signals from receiver  201 -P are supplied either to transmitter  203 -P, i.e., expressed through to protection path  110 -P, to interface  206 -S to be dropped or to transmitter  203 -S to be supplied to service path  110 -S. In similar fashion, AU-4 signals from receiver  202 -P are routed under control of controller  210  either to transmitter  204 -P, i.e., expressed through to protection path  120 -P, to interface  207 -S to be dropped or to transmitter  204 -S to be supplied to service path  120 -S. Note that if needed, looping back of the AU-4 SDH signals from service path  110 -S to the protection path  110 -P occurs using the interfaces  206 -S and  206 -P. Similarly, looping back of the AU-4 SDH signals from service path  120 -S to the protection path  120 -P occurs using the interfaces  207 -S and  207 -P. AU-4 SDH signals being added and dropped via interface  206 -S can be bridged to transmitter  203 -P and, hence, protection path  110 -P and can be switched from receiver  202 -P and, hence, from protection path  120 -P, all under control of controller  210 . Similarly, AU-4 SDH signals being added and dropped via interface  207 -S can be bridged to transmitter  204 -P and, hence, protection path  120 -P and can be switched from receiver  201 -P and, hence, from protection path  110 -P, all under control of controller  210 . 
     Interfaces  206 -S,  206 -P,  207 -S and  207 -P are employed tointerface to particular duplex links  216 -S,  216 -P,  217 -S and  217 , respectively, and could include any desired arrangement. For example, interfaces  206  and  207  could include a CEPT-4 digital signal interface to a DSX, a STM-1E (electrical) SDH digital signal interfacing to a DSX, an optical extension interface to an STM-1 SDH optical signal or the like. Such interface arrangements are known. 
     In accordance with the present invention, controller  210  uses the program shown in the flow charts of FIG. 10 (which are stored in memory  220 ) to control the adding, dropping, and bridging of the signals via interfaces  206  and  207 , as well as, the direct bridging and switching of the AU-4 tributaries being added and dropped to and from protection paths  110 -P and  120 -P. Controller  210  also monitors the status of interfaces  206  and  207  and the digital signals supplied thereto via the control bus arrangement. Specifically, controller  210  monitors interfaces  206  and  207  for a signal failure condition, i.e., loss-of-signal, loss-off-frame, coding violations and the like. The controller also monitors for loopback completion, takedown completion, clear, and other messages. 
     Controller  210  operates to effect the jumpering (signal path to protection path connection), bridging, and switching of communications tributaries at ring nodes, if necessary. Controller  210  communicates with receivers  201  and  202 , transmitters  203  and  204  and interfaces  206  and  207  via a control bus arrangement. Specifically, controller  210  monitors the incoming digital signals to determine loss-of-signal, SDH format K bytes (of FIG. 6) and the like. Additionally, controller  210  causes the insertion of appropriate K byte messages (of FIG. 6) for protection switching purposes, examples of which are described below. To realize the desired bridging and switching of the communications tributaries, controller  210  is advantageously provisioned via bus  212  with the identities (IDs) of all the communications tributaries passing through the ring node, as well as, those communications tributaries being added and/or dropped at the ring node (stored in tables of FIG.  7 - 9 ), the identity of all the ring nodes in system  100  and the positions of the ring nodes in system  100  (stored in FIG.  5 ). The bridging and switching of communications tributaries under control of controller  210  to effect the invention is described below. 
     FIG. 3 shows, in simplified form, details of receivers  201  and  202  of FIG.  2 . The receiver includes an optical/electrical (O/E) interface  301 , demultiplexer (DEMUX)  302  and driver and router  303 . An STM-N SDH optical signal is supplied to O/E  301  which converts it to an electrical STM-N signal. In turn, DEMUX  302  demultiplexes the STM-N signal, in known fashion, to obtain up to N AUG SDH signal, namely, AUG (1) through AUG (N). Again, in this example, N=16. The AUG (1) through AUG (N) signals are supplied to driver and router  303  where they are routed under control of controller  210  via the control bus as AU-4 (1) through AU-4 (M) SDH signals. As indicated above, each STM-N signal can include N AUG tributaries, in this example. The AU-4 (1) through AU-4 (M) signals are routed under control of controller  210 , as described above regarding FIG.  2 . DEMUX  302  also re-moves STM overhead (OH), and supplies the APS channel K bytes to controller  210  via the control bus. 
     FIG. 4 shows, in simplified form, details of transmitters  203  and  204  of FIG.  2 . The transmitter includes select unit  401 , multiplexer (MUX)  402  and electrical/optical interface (E/O)  403 . The AU-4 (1) through AU-4 (M) signals are supplied to select unit  401  where the particular tributaries AUG (1) through AUG (N) are selected under control of controller  210  to be supplied to MUX  402 . Again, in this example, N=16. The AUG tributaries are supplied to MUX  402  where overhead (OH) is added to yield an electrical STM-N SDH signal. In turn E/O interface  403  converts the STM-N into an optical STM-N for transmission on the corresponding fiber transmission path. MUX  402  also inserts appropriate K byte messages under control of controller  210  via the control bus. 
     FIG. 5 shows a ring node map table including the node identification (ID) of and relative location of each of ring nodes  101  through  104  of system  100 . The ring node map table is provisioned via  212  in memory of controller  210 . 
     With reference to FIG. 6 there is shown the format of the switch request message (K 1 ) and switch acknowledgment message (K 2 ). These K byte messages are both generated and monitored by controller  210 . The K 1  byte indicates a re-quest of a communications tributary for switch action. The first four (4) bits of the K 1  byte indicate the switch request priority and the last four (4) bits indicate the ring node identification (ID) of the destination ring node. The K 2  byte indicates an acknowledgment of the requested protection switch action. The first four (4) bits of the K 2  byte indicate the ring node ID of the source ring node and the last 4 bits indicate the action taken. The first four bits of the K 1  bytes are “priority” field which indicated the type of system message, e.g., idle, SF-loop, clear, loopback complete, takedown, etc. The fifth bit of K 2  bytes is a long/short bit which indicates the path length. The last three bits of the K 2  bytes are called “action taken” field, e.g., idle, FERF (far end remote failure), etc. 
     FIGS. 7-9 are illustrative node traffic tables for ring nodes  104 ,  101 , and  102 , respectively. These node traffic tables include the identification of the ring node communications traffic, i.e., the active communications tributaries, in both the clockwise (CW) direction and the counter-clockwise (CCW) direction of transmission. The active communications tributaries include those being added, dropped, bridged or expressed through the nodes  104 ,  101 , and  102 . Our illustrative tributary signal “X” connection, shown in FIG. 1, enters node  104  and is routed via node  101  and exits at node  102 . The tables of FIGS. 7-9 include the IDs of active communications tributaries in the clockwise (CW) direction (shown as  701 ,  801 , and  901 , respectively) and counter-CW (CCW) direction (shown as  710 ,  810 , and  910 , respectively). These tables identify the tributaries (using AU-4#s), the “provisioned” destination paths of those tributaries, and the jumper statuses. Shown in the node  104  table of FIG. 7 is the AU-4 tributary identification, i.e., X in our example. As previously noted, the number of AU-4 tributaries can be up to 16. As shown in FIG. 1, the X tributary enters node  104  in the CW direction and exits node  101 . The provisioned connection  711  is designated  102  (s 7 ) indicating that CW service channel  7  carries the X signal to node  102 . In node table  102  shown in FIG. 8, the provisioned connection  811  is designated  104  (s 7 ) indicating that CW service channel  7  carries the X signal from node  104 . In node table  101  shown in FIG. 9, the provisioned connection  911  is designated T (S 7 ) indicating that CW service channel  7  carries the X signal in an express manner, i.e., the X signal passes through rather than entering or exiting at this node. As will be discussed in a later paragraph, the X signal path after the transmission path break is shown as  721 ,  821 ,  921  in FIGS. 7,  8 , and  9 , respectively. 
     FIG. 10 is a flow chart illustrating the operation of controller  210 , in accordance with the invention, in controlling the operation of the ring nodes in order to effect the bridging and switching of tributary traffic paths in the presence of a ring impairment or removal of the impairment. A ring impairment is defined as a failure or degradation of the signals from any cause including failure of the transmission paths or of the equipment. It should be noted that all so-called part-time service which was being transported on the protection paths  110 -P and  120 -P is preempted upon detection of the failure. Thus, the part-time service is taken off of the protection paths  110 -P and  120 -P. 
     At each node, the controller runs the process shown in the flowchart. The controller  210  loops between steps  1003  and  1001  perpetually checking for a change in the content of K bytes (of FIG. 6) of an incoming STM-N signal. If a change in the content of the K bytes is detected, then the process continues on to  1005 ; if not, it returns to  1001 . 
     At  1005 , the controller will take one of three branches depending on whether the new K-byte content indicates one of the following: (1) signal failure/degrade or clear, (2) loopback setup complete, or (3) loopback takedown complete. All other K-byte content changes are handled according to the rules laid down in the BLSR processing document (ITU-T G.841) 
     (1) For a signal failure/degrade or clear message, control passes to  1007 , where a check is made to see if the jumper flag for that node is set indicating that a jumper already exists. If it does not, then control passes to  1011 . If it does, control passes to  1009 , where the jumpers are removed, the record of the jumpers is erased, the connections existing prior to the jumpers are reestablished, and the jumper flag is reset; control is then passed to  1011 . 
     When control is received at  1011 , conventional BLSR processing takes place, Such processing could take various forms depending on the nature of the failure message and the position of the node (i.e., whether it is a border node or a body node). For instance, if the node is a body node, then it will put up a (full or partial, depending on the type of message received) protection pass-through and propagate the message onward; if, on the other hand, it is a border node then it will perform loopback switching and generate a loopback setup complete message in the reverse direction. A key aspect and merit of the invention is that it is impervious to the details of this processing, since by virtue of the previous step, it has undone the jumpering and restored the node to a valid state germane to BLSR operation. 
     At the completion of the conventional BLSR processing, control is passed to  1013 , which returns the controller to the wait loop of  1001 . 
     (2) For a loopback setup completion message, control passes to  1015 , at which point the controller checks the node traffic pattern to determine if any of the circuits need jumpering. A jumper is needed when the node is an end-point (a node where traffic is added/dropped to/from the ring), unless the ring is segmented by multiple failures and the two end-points are not on the same segment. If the circuits do not need jumpering, then control is transferred to  1019 , where conventional BLSR processing takes place. If a circuit needs jumpering, then control is transferred to  1017 , where the controller removes the normal circuit connection and establishes the jumpered connection (this consists of disconnecting the dropped channel from the service line and reconnecting it to the same numbered channel on the protection channel coming in from the opposite direction.) When this is complete for all such circuits, a flag is set to indicate that this node is in a jumpered state, and a record of the jumpered connections is made; control is then transferred to  1019 , where conventional BLSR processing takes place. 
     In this case, the conventional processing consists of putting up protection channel pass-throughs for all non-jumpered circuits, and propagating the loopback completion message unless the node happens to be a border node, in which case no further propagation is done. 
     At the completion of the conventional BLSR processing, control is passed to  1013 , which returns the controller to the wait loop of  1001 . 
     (3) For a loopback takedown completion message, control passes to  1021 , where conventional BLSR processing takes place, which consists of propagating the loopback completion message unless the node happens to be a border node, in which case no further propagation is done. 
     At the completion of the conventional BLSR processing, control is passed to  1013 , which returns the controller to the wait loop of  1001 . 
     This last case is actually the same as case (1), since the step involving the jumpered circuits is a null step, for no jumpers can exist at this point. However, it has been explicitly culled out to make it symmetrical with step (2). 
     FIGS. 11 through 16 provide an illustration of the operation of the present invention for a single failure occurring within a normally operating ring, and will make reference to FIGS. 7 through 10. The following description assumes that an X signal tributary connection has been established, in a well known manner, to enter node  104  and traverse node  101  and exit at node  102 , as is shown in FIG.  11 . The node tables for nodes  104 ,  102 , and  101 , shown in FIGS. 7-9, respectively, have stored therein the “provisioned” connection information, in a well known manner, as depicted by  711 ,  811 , and  911 . 
     In FIG. 12 a,  a complete fiber cut fault has occurred between nodes  101  and  104 . As shown, such a cable cut interrupts the transmission of the X signal tributary (the two directions of the signal being indicated by bold lines). Nodes  101  and  104 , which border the cable cut, determine that a failure has occurred and propagate a failure message via the K-bytes in both CW and CCW directions. Thus at this time both border nodes  101  and  104  are concurrently sending messages to the next node of the system (i.e., node  101  is sending to node  102  and node  104  to node  103 ). Note the following description traces the function performed at the different nodes (1) as the messages that originated at node  101  are propagated through nodes  102 ,  103  and terminate at  104  and (2) as the messages that originated at node  104  are propagated through nodes  103 ,  102  and terminate at  101 . In the following description, all nodes other than the border nodes are referred to herein as body nodes. 
     When the failure message is received at node  102 , the controller, having detected a change in the K-bytes, ceases looping at  1003 , checks the message at  1005 , and proceeds to step  1007 , where it checks the jumper flag. In this example, we are assuming that the ring was unimpaired prior to the fiber cut fault, hence the jumper flag is not set, and therefore, control passes directly to step  1011 . Normal BLSR processing is performed, which consists of putting up a protection pass through  1201 , which allows onward propagation of the K-bytes to node  103 . 
     At node  103 , similar events take place as at node  102 , resulting in a protection pass through  1202 , and the K-bytes propagating on to node  104 . Note that this passthrough may already have been set up due to the receipt of K-bytes from node  104 , which is closer. Again, all this is as per normal BLSR processing. 
     Referring now to FIG. 12 b,  the receipt of the K-bytes at node  104  causes its controller to pass to step  1005 . There are no jumpers, so control passes on to  1011  and normal BLSR operation takes place—in this case, this comprises the setting up of signal loopback  1203 . 
     Meanwhile, the K-byte messages originating at node  104  and propagating through nodes  103  and  101  (in like fashion to the above) result in the setting up of signal loopback  1204  at node  101 . 
     Thus FIG. 12 b  shows the signal “X” connection  1205  after each border node  104  and  101  has received the “failure” message which originated from the other border node and has performed a loopback connection. 
     In accordance with normal BLSR processing in step  1011 , the controllers of border nodes  101  and  104  then propagated a “loopback complete” confirmation message to the other system nodes. Control at the border nodes then returns, via step  1013 , to looping at steps  1001 - 1003 . 
     At node  102 , the “loopback complete” message from node  101  is received before the one from node  104  arrives, step  1014  is performed, and control is transferred to normal BLSR processing in step  1019 , which does nothing further than pass on the “loopback complete” K-bytes. 
     When the “loopback complete” message from node  104  is received at node  102 , it performs step  1014  and proceeds on to step  1015 , since the other loopback complete message (i.e., the one from node  101 ) has already been received. In step  1015 , a jumper is needed, since the X signal tributary will need to be received from (and transmitted to) the right-hand side in order to eliminate the loop formed to the left. Step  1017  causes node  102  to remove provisioned connection  1205  and passthrough connection  1206  of FIG. 12 b,  and put in jumpers ( 1301  of FIGS. 13 a  and  13   b ), and set the jumper flag (column  804  of row  821  of FIG.  8 ). Note that the record of the originally provisioned connection is maintained in the node&#39;s memory (column  803  of FIG. 8) for subsequent restoral. 
     Node  102  then propagates the loopback completion message CCW to node  101 , where per normal BLSR operation, steady state is reached. Control at  102  also returns to steady state (looping at  1001 - 1003 ) via step  1013 . 
     Note that it is not necessary to put up jumpers at node  104  since it will have the same effect as the loopback at that node (there being no extended loop to truncate), and the present invention does not call for such jumpers to be put up. However, optionally, one may indeed put up such jumpers (FIG. 13 b ) if that is more desirable. 
     At this point in time, the system routing connection of the X signal, appears as shown in FIG.  13 . As shown in FIG. 13, the resulting restored path is shorter for signal X. 
     The controller at all of the nodes cycles through steps  1001  and  1003  until another message is received. We assume that the next message is a “clear” message indicating that the system cable cut has been repaired. Referring now to FIG. 14, the clear message is originated at both of the border nodes  101  and  104  and is propagated via node  102  and  103 , respectively. When node  102  receives the clear message from  101 , control is transferred to step  1007  where the jumper flag is checked; since the flag is set indicating that a jumper exists at node  102  (see column  804  and row  821  of FIG. 8) the controller performs step  1009 , and (1) the jumpered connection is taken down, (2) normal (i.e., provisioned) circuit connections restored, and (3) the jumper flag reset. Control then moves to step  1011 , whereupon normal BLSR processing is performed, which in this case means that the nodes propagates the clear message to the next node,  103 . Control at node  102  then passes to step  1013  and to step  1001 . 
     The clear message is similarly processed in node  103 , where it is merely passed through since there are no jumpers set at that node. 
     With regard to the clear message sent from  104  to  103 , since there are no jumpers at  103 , no special actions take place there other than normal BLSR processing (the message merely gets propagated), and likewise at  102 . At this time the routing of the X signal and other node connections appears as shown in FIG. 14, where the signals are flowing through the conventional BLSR loopback connections and not through the jumpered paths. 
     When the clear message originated at  101  reaches node  104 , since no jumper flag is set, control passes to step  1011  and it executes normal BLSR procedure, whereby the loopback connection  1401  is taken down. Per BLSR procedure, a takedown confirmation message is then sent from node  104 . Control then passes to step  1017  and then to step  1001 . 
     Likewise, the clear message sent from  104  results in loopback connection  1402  to be taken down and a takedown confirmation message to be sent from node  101 . 
     At this time the routing of the X signal and other node connections appears as shown in FIG. 15, which is the normal provisioned state. 
     The takedown confirmation messages from node  104  is propagated back through nodes  103  and  102  to  101 , and similarly the takedown confirmation messages from node  101  is propagated back through nodes  102  and  103  to  104 , with regular BLSR processing being performed at all nodes per step  1021  in FIG. 10, and stable operation is reached. 
     While we have described the operation of the present invention for a single signal tributary X, it should be noted that our invention can process multiple signal tributaries entering and exiting from different nodes at the same or different times. Thus, our single tributary operational description was merely illustrative of the operation of the present invention. Additionally, the above-described circuits and arrangements are, of course, merely illustrative of the application of the principles of the invention. Other arrangements may be devised by those skilled in the art without departing from the spirit or scope of the invention.