Abstract:
Recovery from link failure in a WDM ring network is implemented by forming an active ring and a protection ring through the same nodes. Failure in any wavelength channel of a link causes a node adjacent to the link to reroute any subsequent incoming signal from the source node and on the active ring to the protection ring and in an opposite direction. The oppositely directed signal is rerouted again at the other adjacent node back to the active ring to arrive at the destination node. The physical ring serving as the active ring to odd wavelength channels serves as the protection ring to even wavelength channels, and vice versa.

Description:
FIELD OF THE INVENTION 
     The present invention relates to optical ring networks, and more particularly, to fault recovery systems in optical wavelength division multiplexing ring networks. 
     BACKGROUND OF THE INVENTION 
     A typical ring network includes nodes, each node having a unique address. A demand will request that specific information be transmitted from a sending node to a receiving node. Traffic between different sending node/receiving node pairs is assigned to different wavelength channels, each of which may be considered either even or odd. 
     Adjacent nodes are interconnected by at least two fiber links, one being a clockwise-directed fiber for transmittal of even channels, and a second being a counterclockwise-directed fiber for transmittal of odd channels. The clockwise-directed fibers and the nodes between them constitute a clockwise-directed ring, and the counterclockwise-directed fibers and the nodes between them constitute a counterclockwise-directed ring. 
     For wavelength division multiplexing ring networks, each node has the apparatus for being a sending node and for being a receiving node. For this purpose, each node typically includes a frequency multiplexer and demultiplexer. The multiplexer is responsive to the demand so that if that node is the sending node specified in the demand, the information will be encoded, wavelength multiplexed to the appropriate channel, and added to the data stream arriving at that node. The demultiplexer is responsive to the data stream arriving at the node, so that it decodes each signal, determines if that node is the destination node and if so, drops the signal. 
     Occasionally, a link fails for one or more wavelength channels. In conventional systems for network recovery, the link failure is broadcast to each node. A switch on each node then reassigns signals from inoperative to operative wavelength channels. Such a system is expensive and cumbersome because of the broadcasting of link failure to each node and because each node must include a switch for each channel. 
     SUMMARY OF THE INVENTION 
     A network according to the principles of the invention includes a link failure detector and a failure correction apparatus. The link failure detector is in at least one of the links for detecting whether transmission of a signal intended for transmission between the pair of adjacent nodes which the link is between was successful or unsuccessful. The link failure detector also produces a link transmission signal indicative of success or lack of success. The link transmission signal is intended for transmission to the pair of nodes adjacent to the link. 
     The failure correction apparatus is in each node adjacent to a link having the link failure detector. The failure correction apparatus is responsive to the link transmission signal produced by the failure correction apparatus, so that in response to a link transmission signal indicative of unsuccessful transmission, any signal in either network arriving subsequently at the node is diverted to the other network, and so that in response to a link transmission signal indicative of successful transmission, any signal in either network arriving subsequently at the node is permitted to continue in the same network in which it arrived. Any signal subsequently produced at the node for transmission into either network is treated by the fault correction apparatus as a signal in the same network arriving subsequently at the node. 
     Such a system successfully diverts signals around any break in the network, and avoids the broadcasting of link failure to each node since only adjacent nodes are affected. Thus, it is simpler and less costly. Furthermore, each node need not include a switch for each channel. A fault correction apparatus on the node switches all nodes from one network to the other, regardless of which channels failed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The exemplary embodiments will be described with reference to the drawings, in which like elements have been denoted throughout by like reference numerals, and wherein: 
     FIG. 1 illustrates a unidirectional ring network. 
     FIG. 2 illustrates a mesh network. 
     FIG. 3 illustrates a bi-directional ring network. 
     FIG. 4 shows a bi-directional link realized by an optical fiber having two links. 
     FIG. 5 illustrates a node connected to two bi-directional links. 
     FIG. 6 illustrates fault correction apparatus. 
     FIG. 7 shows the operation of fault correction apparatus after successful transmission. 
     FIG. 8 shows the transmission signals in a network after fault correction apparatus has detected successful transmission of a previous signal. 
     FIG. 9 shows the operation of fault correction apparatus after unsuccessful transmission. 
     FIG. 10 shows the transmission signals in a network after fault correction apparatus has detected unsuccessful transmission of a previous signal. 
     FIG. 11 shows the transmission signals in a network after fault correction apparatus has detected unsuccessful transmission of a previous signal. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, FIG. 1 shows a unidirectional ring network  100 . The network  100  includes a plurality of links  105  for transmission of signals S L  and a plurality of nodes  110  for sending signals S S  through the links  105  and for receiving signals S R  from the links  105 . For purposes of illustration, the ring network  100  is shown as having 5 nodes  110  and 5 links  105 , but a ring network  100  can have as few as 3 nodes and as many as practical. 
     Each node  110  has a unique address, for example A, B, C, D, or E. Each node  110  also has exactly two adjacent nodes  110 . For example, nodes  110  with addresses A and C are adjacent to node  110  with address B. Each link  105  is disposed between adjacent nodes  110  and is for transmission of signals between those adjacent nodes  110  in a particular direction. For example, the link  105  disposed between nodes  110  with addresses A and B is for transmission of signals S L  from node  110  with address A to node  110  with address B. Transmission of signals through all nodes is in the same direction, in this case, counterclockwise. Thus the network  100  is unidirectional. 
     The links  105  can be for transmission using any feasible medium of transmission. An exemplary link  105  is a fiber optic cable for transmission of optical signals S L . 
     The nodes  110  and links  105  have the topology of a circle. Expressed in another manner, a signal S L  transmitted from node  110  with address A to adjacent node  110  with address B can then be transmitted unambiguously to the other node  110  adjacent to node  110  with address B, in this case, node  110  with address C. 
     To continue with this example, the signal S L  transmitted from node  110  with address B to node  110  with address C can then be transmitted unambiguously to the other node  110  adjacent to node  110  with address C, that is, node  110  with address D. The signal S L  transmitted from node  110  with address C to node  110  with address D can then be transmitted unambiguously to the other node  110  adjacent to node  110  with address D, that is, node  110  with address E. The signal S L  transmitted from node  110  with address D to node  110  with address E can then be transmitted to the other node  110  adjacent to node  110  with address E, that is, node  110  with address A. In other words, the signal S L  transmitted from node  110  with address A returns to the same node  110  with address A. 
     Referring now to FIG. 2, a mesh network  120  has  10  links  105  and  9  nodes  110 , each node  110  having a unique address, such as A, B, C, D, E, F, G, H and I. This mesh network  120  differs from the ring network  100  of FIG. 1 in that nodes  110  with addresses D and F each have  3 , rather than  2 , adjacent nodes  110 . The mesh network  120  is thus not a ring network. 
     Mesh network  120  can be considered as incorporating counterclockwise ring network  100 ′ and clockwise ring network  100 ″ with appropriate apparatus  130  at nodes  110  with addresses D and F for switching signals S L  and S L ′ between ring networks  100 ′ and  100 ″. Thus, the present invention applies to mesh networks  120  and other types of networks incorporating ring networks  100  as well as to ring networks  100 . 
     There are only two possible directions for the signal to travel in. The two directions will be referred to herein as a first direction and a second direction or as first parity and second parity. 
     Referring back to FIG. 1, a sending node  100 , for example node  100  with address A, will send a signal S S  to a receiving node  100 , for example node  100  with address B, in response to a demand S D . The source  140  of the demand S D  is not relevant to this invention, but examples of such sources are computers, telecommunications equipment, and sensor apparatus. The most basic information that must be contained in the demand S D  is the address of the sending node  110 , the address of the receiving node  110 , and the information to be passed from the sending node  110  to the receiving node  110 . The addresses and information must be included in the transmitted signal S L . The transmitted signal S L  complies with a specified protocol (for example, specification of the size of a header, data section and trailer). 
     Each node  110  has the physical capability of being a sending node  110 , and includes demand identification apparatus  142  (see, FIG. 5) capable of responding to a demand S D  when acting as a sending node. A sending node  110  produces, in response to a demand S D , a sending signal S S  which includes the addresses and information in conformity with the specified protocol. 
     Each node  110  also has the physical capability of being a receiving node  110  and so, has receiving apparatus  144  (see, FIG. 5) responsive to each transmitted signal S L  received at the node  110 . The receiving apparatus  144  determines if the signal S L  specifies that node  110  and, if so, receives the signal S R  and removes the signal S L  from the link  105 . 
     In order for the network  100  to process more than one demand S D  and transmit more than one signal S L , it includes a mechanism for producing and distinguishing different signals. Examples of such mechanisms are time modulation, wavelength modulation, and frequency modulation. In an example of time modulation, each signal S L  is of fixed duration and is known as a packet. Different demands S D  result in nonoverlapping signals S L  at different times so that different signals S L  do not overlap or collide. In an example of wavelength modulation, signals are wavelength modulated on carriers, such as light, which have differing wavelengths. 
     In an exemplary embodiment, the links  105  are optical fibers, and each node  110  includes demand identification apparatus  142  (FIG. 5) for identifying the pair sending node  110  address and receiving node  110  address with a unique wavelength. The signal S S  the sending node sends into the link  105  is modulated on a carrier of that wavelength. If the ring network  100  includes n nodes  110 , then the number of possible ordered pairs of sending nodes and receiving nodes is n★(n−1). For example, the network  100  shown in FIG. 1 has 5 nodes and so it would have 20 different ordered pairs of sending nodes  110  and receiving nodes  110 , and thus 20 different carrier wavelengths. These differing carrier wavelengths are considered differing channels. Thus, this exemplary network  100  must be capable of transmitting on any of 20 different wavelength channels. The actual channels might vary over time but, at any particular time, 20 carrier wavelengths are available for the demand identification apparatus  142  to transmit on. 
     To avoid misdirection of a signal, the nodes  110  of the network  100  use the same identification between ordered pairs of sending nodes  110  and receiving nodes  110  and channels. This information, referred to as channel identification apparatus  146  (see, FIG.  5 ), can be hardwired or broadcast to each node by a channel reference table signal S CRT . The latter approach would provide more flexibility in adding and removing nodes  110 . 
     As an example, on Table 1 each of the twenty pairs of sending nodes receiving nodes is identified with a channel. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Address of sending 
                 Address of receiving 
                   
               
               
                 node 110 
                 node 110 
                 Channel 
               
               
                   
               
             
             
               
                 A 
                 B 
                 01 
               
               
                 A 
                 C 
                 02 
               
               
                 A 
                 D 
                 03 
               
               
                 A 
                 E 
                 04 
               
               
                 B 
                 A 
                 05 
               
               
                 B 
                 C 
                 06 
               
               
                 B 
                 D 
                 07 
               
               
                 B 
                 E 
                 08 
               
               
                 C 
                 A 
                 09 
               
               
                 C 
                 B 
                 10 
               
               
                 C 
                 D 
                 11 
               
               
                 C 
                 E 
                 12 
               
               
                 D 
                 A 
                 13 
               
               
                 D 
                 B 
                 14 
               
               
                 D 
                 C 
                 15 
               
               
                 D 
                 E 
                 16 
               
               
                 E 
                 A 
                 17 
               
               
                 E 
                 B 
                 18 
               
               
                 E 
                 C 
                 19 
               
               
                 E 
                 D 
                 20 
               
               
                   
               
             
          
         
       
     
     Referring now to FIG. 3, a bi-directional ring network  150  has two links  160  and  170 , of first and second parity, respectively, between each pair of adjacent nodes  110 . First parity links are for transmitting signals S L1  in the direction as shown, and Second parity links are for transmitting signals S L2  in the opposite direction as shown. Ignoring, for now, the second parity links  170 , if all the first parity links  160  were operative, then the ring network  150  would have the same architecture as ring network  100 . If such an apparatus had the same functional apparatus as ring network  100 , then it would function as a ring network, and will be referred to herein as a first network  180 . Similarly, ignoring, for now, the first parity links  160 , if all the second parity links  170  were operative, then the ring network  150  would have the same architecture as ring network  100 . If such an apparatus had the same functional apparatus as ring network  100 , then it would function as a ring network, and will be referred to herein as a second network  190 . 
     Links  105  are functionally defined as being for transmitting signals S L  in a particular direction. Referring now to FIG. 4, a single structure, such as an optical fiber  200 , could transmit signals S L1  and S L2 , one in each direction, and thus include links  160  and  170  of opposite parity. Such a structure  200  is bi-directional. 
     Referring now to FIG. 5, a node  110  of a ring network  150  using wavelength modulation has as receiving apparatus  144  responsive to each transmitted signal S L  received at the node  110 , a wavelength demultiplexer in communication with a channel identification apparatus  146 . Since ring network  150  has first direction links  160  and second direction links  170  for transmitting in opposite directions, node  110  has two such receiving apparatuses  144 , one for each direction. 
     The demand identification apparatus  142  is downstream from the receiving apparatus  144 . The stream of signals S ND  which are not dropped by the receiving apparatus  144  reach the demand identification apparatus  142  and if a signal S S  is added to the stream, the demand identification apparatus  142  does so. In response to a demand S D , if node  110  is the sending node  110 , the wavelength-multiplexer modulates the information S I  contained in the demand S D  with the appropriate wavelength carrier S λ , in communication with the channel identification apparatus  146  to add a signal to the signal S ND  so as to produce a signal S L  leaving the node  110 . 
     An exemplary optical node  110  includes a pair of optical add-drop mechanisms (hereinafter “OADM”)  210  and  220 , each of which includes a wavelength-demultiplexer and a wavelength-multiplexer. The first optical add-drop mechanism  210  is for dropping a signal S R  from the first network  180  or adding a signal S S  to the second network  190 . The second optical add-drop mechanism  220  is for dropping a signal S R  from the second network  190  or adding a signal S S  to the first network  180 . 
     Each channel is associated with either the first or second parity network  180  and  190 . Such association is performed by the channel identification apparatus  146 . As an example of such association, Table 2 defines an association in which odd channels are associated with the first parity network  180  and even channels are associated with the second parity network  190 . 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Address of 
                 Address of 
                   
                 Parity of 
               
               
                   
                 sending node 
                 receiving node 
                   
                 Associated 
               
               
                   
                 110 
                 110 
                 Channel 
                 Network 
               
               
                   
                   
               
             
             
               
                   
                 A 
                 B 
                 01 
                 first 
               
               
                   
                 A 
                 C 
                 02 
                 second 
               
               
                   
                 A 
                 D 
                 03 
                 first 
               
               
                   
                 A 
                 E 
                 04 
                 second 
               
               
                   
                 B 
                 A 
                 05 
                 first 
               
               
                   
                 B 
                 C 
                 06 
                 second 
               
               
                   
                 B 
                 D 
                 07 
                 first 
               
               
                   
                 B 
                 E 
                 08 
                 second 
               
               
                   
                 C 
                 A 
                 09 
                 first 
               
               
                   
                 C 
                 B 
                 10 
                 second 
               
               
                   
                 C 
                 D 
                 11 
                 first 
               
               
                   
                 C 
                 E 
                 12 
                 second 
               
               
                   
                 D 
                 A 
                 13 
                 first 
               
               
                   
                 D 
                 B 
                 14 
                 second 
               
               
                   
                 D 
                 C 
                 15 
                 first 
               
               
                   
                 D 
                 E 
                 16 
                 second 
               
               
                   
                 E 
                 A 
                 17 
                 first 
               
               
                   
                 E 
                 B 
                 18 
                 second 
               
               
                   
                 E 
                 C 
                 19 
                 first 
               
               
                   
                 E 
                 D 
                 20 
                 second 
               
               
                   
                   
               
             
          
         
       
     
     As another example of such association, Table 3 defines an association in which all channels are associated with the first parity network  180  and no channels are associated with the second parity network  190 . 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Address of 
                 Address of 
                   
                 Parity of 
               
               
                   
                 sending node 
                 receiving node 
                   
                 Associated 
               
               
                   
                 110 
                 110 
                 Channel 
                 Network 
               
               
                   
                   
               
             
             
               
                   
                 A 
                 B 
                 01 
                 first 
               
               
                   
                 A 
                 C 
                 02 
                 first 
               
               
                   
                 A 
                 D 
                 03 
                 first 
               
               
                   
                 A 
                 E 
                 04 
                 first 
               
               
                   
                 B 
                 A 
                 05 
                 first 
               
               
                   
                 B 
                 C 
                 06 
                 first 
               
               
                   
                 B 
                 D 
                 07 
                 first 
               
               
                   
                 B 
                 E 
                 08 
                 first 
               
               
                   
                 C 
                 A 
                 09 
                 first 
               
               
                   
                 C 
                 B 
                 10 
                 first 
               
               
                   
                 C 
                 D 
                 11 
                 first 
               
               
                   
                 C 
                 E 
                 12 
                 first 
               
               
                   
                 D 
                 A 
                 13 
                 first 
               
               
                   
                 D 
                 B 
                 14 
                 first 
               
               
                   
                 D 
                 C 
                 15 
                 first 
               
               
                   
                 D 
                 E 
                 16 
                 first 
               
               
                   
                 E 
                 A 
                 17 
                 first 
               
               
                   
                 E 
                 B 
                 18 
                 first 
               
               
                   
                 E 
                 C 
                 19 
                 first 
               
               
                   
                 E 
                 D 
                 20 
                 first 
               
               
                   
                   
               
             
          
         
       
     
     A channel is referred to herein as having the same parity as the network with which it is associated. Thus, for the association defined in Table 2, odd channels are of first parity and even channels are of second parity. For the association defined in Table 3, all channels are of first parity and no channels are of second parity. 
     All such associations have in common the properties that the set of channels includes the two mutually exclusive sets of first parity channels and second parity channels and every channel is of either first or second parity. 
     The network, be it a first parity network  180  or a second parity network  190 , with which a channel is associated, is referred to herein as the operative network. The other parity network is referred to herein as the backup network. For a channel of first parity, the operative network is the first parity network  180 . The signal S S  is added to the first parity network  180  at sending node  110 , transmitted on first parity links  160  on the first parity network  180 , and signal S R  is received at the receiving node  110  from the first parity network  180 . The second parity network  190 , the backup network for this channel, is essentially inactive as far as this first parity channel is concerned. Similarly, for a channel of second parity, the operative network is the second parity network  190 . The signal S S  is added to the second parity network  190  at sending node  110 , transmitted on second parity links  170  on the second parity network  190 , and signal S R  is received at the receiving node  110  from the second parity network  190 . The first parity network  180 , the backup network for this channel, is essentially inactive as far as this second parity channel is concerned. 
     Systems according to the principles of the invention are not limited to networks  150  with just two links between each pair of adjacent nodes  110 . The same principles of this invention would apply equally well for networks with three, four, or more links between each pair of adjacent nodes  110 . 
     Referring now to FIG. 6, fault recovery apparatus in the network  150  includes a link failure detector  230  in at least one link  160  or  170 . In an embodiment of the present invention, every link  105  in the network  150  includes a link failure detector  230 . The link failure detector  230  produces a link failure signal S F  responsive to each signal S L  that enters the link  105 . For purposes of this discussion, only a link failure detector  230  in a first parity link  160  will be considered. It will be clear to a person of ordinary skill in the art how to extend the ideas of this invention to fault detection and recovery in a second parity link  170 . 
     Each node  110  adjacent to the link  105  containing a link failure detector  230  has one failure correction apparatus  240  for each link failure detector  230 . Thus, for the link failure detector  230  in first parity link  160 , each adjacent node  110  has one failure correction apparatus  240 . If the second parity link  170  also has a link failure detector  230 , then each adjacent node  110  has one more failure correction apparatus  240 ′. 
     The link failure signal S F  is produced by the link failure detector responsive to every signal S L  that enters the link  105 . The link failure signal S F  detector is indicative of whether transmission of the signal S L  through the link  105  is successful or not. The signal S F  can be at its most basic a binary signal, that is, “yes” or “no”. This signal S F  is intended for transmittal to and use only by the fault correction apparatus  240  in the adjacent nodes  110 . It is not intended that this signal S F  be broadcast to any other node  110 . 
     An exemplary fault correction apparatus  240  includes an optical crossbar  250 , an optical coupler  260 , and an interlink  270  for transmission from the optical crossbar  250  to the optical coupler  260 . 
     Each fault correction apparatus  240  is logically disposed between the demand identification apparatus  142 /receiving apparatus  144  pair and an adjacent link. In particular, the fault correction apparatus  240  associated with a link failure detector  230  on a first parity link  160  is between that first parity link  160  and the demand identification apparatus  142 /receiving apparatus  144  pair. The fault correction apparatus  240  associated with a link failure detector  230  on a second parity link  170  is between an other second parity link  170  adjacent to the node  110  and the demand identification apparatus  142 /receiving apparatus  144  pair. The fault correction apparatus  240  does not act on signals S ND , but rather on signals S L  before or after they have been processed by the demand identification apparatus  142 /receiving apparatus  144  pair. 
     The principles of operation of the fault recovery apparatus  230 - 240  will be discussed first for an operational link  160 . Referring now to FIG. 7, the initial state for the crossbar  250  is transmission through. A signal on S L1  channel x on the first parity network  180  which enters link  160  is successfully transmitted therethrough. In response to this successful transmission, the link failure detector  230  generates a signal S F  indicative of success and, upon receipt of this signal S F , the crossbar  250  in the failure correction apparatus  240  remains in the transmission through state. Assuming that the link  160  is operational on channel y of the first parity, subsequent signals S L1  on channel y on the first parity network  180  from sending node  110  to receiving node  110  with addresses A and B are transmitted as shown by thick lines in FIGS. 8 and 9, and subsequent signals S L2  on the second parity network  190  from sending node  110  to receiving node  110  with addresses D and C, respectively, are transmitted as shown by thick lines in FIGS. 8 and 9. 
     If, on the other hand, link  160  ceases to be operational for channel y of first parity, then signal S L1  on the first parity network  180 , which enters link  160 , is not successfully transmitted therethrough. Referring now to FIG. 9, in response to this failure, the link failure detector  230  generates a signal S F  indicative of failure and, upon receipt of this signal S F , crossbar  250  in failure correction apparatus  240  switches to the cross-state. The crossbar  250  remains in this state until it is reset to the transmission through state. It is to be noted that the signal S L1  which was not successfully transmitted through link  160  does not reach its intended receiving node  110 . 
     After such a failure, link  160  which failed in its transmission of signal S L1  is referred to as a “break”  280 . As above, it is appropriate to consider a subsequent signal S L1  on channel z on the first parity network  180  from sending node  110  to receiving node  110  with addresses A and B. If the path on the first parity network  180  from nodes  110  with addresses A and B does not cross the break  280 , for example, if the break  280  is on the link  160  between nodes  110  with addresses B and A, then the signal S L1  is transmitted as shown in FIG.  8 . It is also appropriate to consider a subsequent signal S L2  on the second parity network  190  from sending node  110  to receiving node  110  with addresses D and C, respectively. If the path on the second parity network  190  from nodes  110  with addresses D to C does not cross the break  280 , for example, if the break  280  is on the link  170  between nodes  110  with addresses D and C, then the signal S L2  is transmitted as shown in FIG.  8 . 
     Referring now to FIGS. 9 and 10, the state for the crossbar  250  is crossover. If the path on the first parity network  180  from nodes  110  with addresses A and B does cross the break  280 , then subsequent signals S L1  on channel z on the first parity network  180  from sending node  110  to receiving node  110  with addresses A and B are transmitted as shown by thick lines in FIGS. 9 and 10. Upon reaching the optical crossbar  250  in the failure correction apparatus  240  of the node  110  just before the break  280 , the signal S L1  is crossed over and directed to the interlink  270  for transmission from the optical crossbar  250  to the optical coupler  260 . Upon reaching the optical coupler  260 , the signal S L   1  is brought into the stream of signals in the second network  190 , the protection network for signal S L1  of channel z. The signal is then transmitted around the second network  190  until it reaches the optical crossbar  250  in the failure correction apparatus  240  of the node  110  just before the break  280 , the signal S L1  is crossed over and directed to the interlink  270  for transmission from the optical crossbar  250  to the optical coupler  260 . Upon reaching the optical crossbar  250  in the failure correction apparatus  240  of the node  110  just before the break  280 , the signal S L1  is crossed over and directed to the interlink  270  for transmission from the optical crossbar  250  to the optical coupler  260 . Upon reaching the optical coupler  260 , the signal S L1  is brought into the stream of signals in the first network  180 , the working network for signal S L1  of channel z. The signal S L1  then continues on the first network  180  until it reaches the receiving node  110  with address B, at which point it is dropped. 
     If the path on the second parity network  190  from nodes  110  with addresses D to C does cross the break  280 , then the signal S L2  from nodes  110  with addresses D to C is transmitted as shown in FIG.  11 . The signal is looped back in the opposite direction on the other node twice, just as the signal S L1  is. In fact, the two signals S L1  and S L2  might overlap over some links  105 . 
     Referring back to FIG. 6, the effect of a failure detector  230  in second parity link  170  and failure correction apparatus  240 ′ in adjacent nodes  110  is very similar to the effect of a failure detector  230  in first parity link  160  and failure correction apparatus  240  in adjacent nodes  110 , and will not be described in further detail. 
     The effect of the failure detector  230  and failure correction apparatus  240  in responding to a break is to permit all signals to travel over both first and second networks  180  and  190  and convert the double loop architecture of network  150  into a single loop architecture. 
     The foregoing descriptions of the exemplary embodiments are intended to be illustrative and not limiting. It will be appreciated that numerous modifications and variations can be made without departing from the spirit or scope of the present invention.