Abstract:
A method for reconfiguring a ring of nodes connected by a working fiber and a protection fiber upon disruption of the signal in either the working fiber or the protection fiber includes signaling the other nodes. Each node determines whether it is a node adjacent to the disruption. Those nodes that are not adjacent to the disruption undergo no change in their operation. A receiving node adjacent to the disruption forms a bridge isolating the disruption and sends an acknowledgement signal back to a signaling node. Upon receipt of the acknowledgement signal, the signaling node also forms a bridge. This results in the isolation of that portion of the ring having a disruption and the formation of a new ring.

Description:
This invention relates to communication networks, and in particular, to methods and systems for ensuring the integrity of data transmission in the event of an equipment failure within the network. 
     BACKGROUND 
     A communication network typically includes a large number of nodes connected by transmission lines. In a modem network, these transmission lines are often optical fibers. Such fibers are extremely thin and therefore susceptible to mechanical breakage. In addition, because fibers are so thin, the alignment between fibers at a junction must be extremely precise. These junctions are therefore easily disrupted by mechanical shock or vibration. Even slight kinks or bends in a fiber can cause internal reflections that lead to significant degradation in signal quality. 
     Although every attempt is made to isolate a fiber from mechanical disturbance, it is difficult to reliably do so. Buried fibers routinely fall prey to backhoes in construction accidents. Over the years, the accumulated effect of the vibration of passing subway trains can gradually degrade communication. Not all disruptions result from human activity, however. Even a minor earthquake can cause isolated disruptions in service. 
     A network can also fail as a result of disruption within a node. For example, the laser at the transmitting end of each fiber can gradually deteriorate. Since nodes can include complex electronic systems, they too are subject to failure from a variety of causes. 
     To avoid excessive service disruption in the event of network failure, it is desirable to provide the network with redundancy. One method of achieving this is to arrange the nodes of a communication network in a ring and to connect the nodes with both two independent fibers: a working fiber and a protection fiber. A ring connected in this way is referred to in the art as a UPSR (Unidirectional Path Switched Ring). 
     In a UPSR, a source node transmits two copies of a data frame to a destination node. A working copy of the data frame travels clockwise around the ring on the working fiber and a protection copy of the frame travels counter-clockwise around the ring on the protection fiber. If the destination node finds that the protection copy matches the working copy, it accepts the working copy. Otherwise, the destination node selects the better of the two copies. 
     As it makes its way to the destination node from the source node, a data frame can pass through many other nodes. In these intervening nodes, there may be data packets queued for transmission on the ring. In addition, there may be space within the data frame for accommodating some of these data packets. Because these empty spaces represent a waste of network resources, it would be useful to accommodate some of these queued data packets in those spaces. 
     Unfortunately, as soon as the data frame accepts a data packet from a node other than the source node, the working copy of the data frame will inevitably differ from the protection copy of the frame. Thus, upon comparing the working copy with the protection copy, the destination node will receive two different frames with no way to determine whether the difference is the result of additional data on the frame or a disruption in transmission. 
     SUMMARY 
     A communication network according to the invention circumvents the foregoing difficulties by providing nodes that do not rely on a comparison between two copies of a data frame in order to detect the existence of an error. Instead, each node adopts a signaling protocol that informs all the other nodes in the network of the condition of the signals arriving at that node from an adjacent node. In response to these signals, each node makes an independent decision as to whether to bypass its adjacent nodes on the network. 
     The communication network provides a method for reconfiguring a ring having a plurality of nodes connected by first and second channels. Examples of such rings include SONET (Synchronous Optical Network) rings and WDM (Wavelength Division Multiplexing) rings. 
     When a disruption occurs, there will be a first node and a second node adjacent to, and on either side of, the disruption. Upon the detection of the disruption, the first node signals each of the other nodes to cause that other node to determine if it is the second node, and, if so, to identify itself as such. If it is not, that node continues to operate in its normal mode. However, if that node determines that it is the second node, it sends an acknowledgement signal back toward the first node and forms a bridge between the first and second channels, thereby preventing data from proceeding further toward the disruption. Upon receipt of the acknowledgement, the first node likewise forms a bridge between the first and second channels, thereby preventing data from proceeding further toward the disruption. This results in the isolation of that disruption and the combination of the first and second channels to form a new ring that excludes the disruption. 
     In one aspect of the invention, the first node sends, by way of the first channel, a first fault signal indicative of a signal fault on the first channel. A second node monitors the second channel for information indicative of the signal fault. On the basis of this information and the first fault signal, the second node forms a first bridge and thereby disconnects a portion of the ring. In addition, the second node sends an acknowledgement signal, by way of the second channel, to the first node. 
     The information indicative of the signal fault can be a second fault signal. However, it can also be loss of signal on the second channel. This feature permits the data protection to function correctly when both the working channel and the protection channel are disrupted. 
     In response to the acknowledgement signal, the first node forms a second bridge, thereby disconnecting another portion of the ring. This results in a reconfigured ring in which no signal faults are present in either the first or the second channel. 
     In a typical communication network, there can be several intervening nodes on the first and second channels connecting the first node and the second node. The method of the invention can thus include routing the acknowledgement signal through a third node selected from the plurality of nodes forming the network. 
     Forming the first bridge can include directing data traffic arriving at the first node by way of the second channel out through the first channel. This is preferably accompanied by forming the second bridge by directing inbound traffic arriving at the second node outbound on the second fiber. 
     The method can also include detecting a signal fault on the first channel. The signal fault can be a loss of a signal on the first channel or a degradation of the signal on the first channel. The degradation of the signal can be manifested by an increase in the bit error rate of the signal on the first channel. 
     The fault signal is typically sent as part of the frame overhead for the protocol used on the ring. For example, in the case of a SONET ring, the fault signal is encoded on either the V4 byte or the Z4 byte. 
    
    
     These and other features of the invention will be apparent upon review of the following detailed description and the accompanying figures in which: 
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a network in which a working fiber and a protection fiber connect a plurality of nodes into a ring; 
     FIG. 2 is a typical node from the ring of FIG. 1 showing an internal architecture for protection of data following a signal fault on the working channel; 
     FIGS. 3A-3F show the state of a ring network at various times following a disruption in the network; 
     FIG. 4 shows the node of FIG. 2 redirecting traffic from an inbound protection channel to an outbound working channel; 
     FIG. 5 is a flow chart illustrating the logic followed by the node of FIG. 2 in implementing the data protection method of the invention; 
     FIG. 6 is the node of FIG. 2 showing the internal architecture for protection of data following a signal fault on the protection channel; and 
     FIG. 7 is a flow chart illustrating the logic followed by the node of FIG. 6 in implementing the data protection method of the invention. 
    
    
     DETAILED DESCRIPTION 
     A communication network  10  implementing the data protection method of the invention includes a plurality of nodes  12 ,  14 ,  16 ,  18  arranged in a ring  19 , as shown in FIG.  1 . Examples of such rings include SONET rings and WDM rings. 
     Within the ring  19 , a particular node  12  is connected to adjacent nodes  14 ,  18  by a working channel  20  and a protection channel  22 . For purposes of illustration, we adopt the convention that the working channel  20  carries a signal in a clockwise direction around the ring  19  and that the protection channel  22  carries the signal in a counterclockwise direction around the ring  19 . The channel can be a transmission line such as an optical fiber. However, the channel can also be one of the many channels carried on a single optical fiber. This feature allows the communication network to carry out the data protection method of the invention on a single channel carried by an optical fiber without affecting all the other channels on the optical fiber. 
     FIG. 2 shows a schematic illustration of a portion of the data protection system within a typical node. For the sake of simplicity in illustration and exposition, FIG. 2 shows only that portion of the data protection system associated with monitoring a signal on an inbound working channel Wi. In addition, because all the nodes  12 ,  14 ,  16 ,  18  have the same architecture, reference numerals for parts shown in FIG. 2 are used in connection with subsequent descriptions of the operation of each node. 
     The typical node shown in FIG. 2 includes a working channel input  24  that carries a signal normally routed to a working channel output  26  by the switch  28 , shown here in its normal configuration. The working channel input  24  and the working channel output  26  are connected to an inbound working channel Wi and an outbound working channel Wo respectively. The working channel input  24  is monitored by a first signal fault detector  30  and by a UFI (upstream fault indication) signal detector  32 . The working channel output  26  is in communication with a UFI generator  34  for generating a UFI signal to be detected by a UFI detector monitoring a working channel input  24  of an adjacent downstream node  14  on the working channel  20 . Also in communication with the working channel output  26  is a first LFI (Local Fault Indication) generator  21 . This first LFI generator  21  is used only in conjunction with the detection of a signal fault on the protection channel, as will be discussed below in connection with FIGS. 6 and 7. 
     The node also includes a protection channel input  36  that carries a signal normally routed to a protection channel output  38  by the switch  28 . The protection channel input  36  and the protection channel output  38  are connected to an inbound protection channel Pi and an outbound protection channel Po respectively. A DFI (Downstream Fault Indication) generator  39  and a second LFI generator  41  are both in communication with the protection channel output  38  for sending a signal to an adjacent downstream node  18 . A second signal fault detector  42  and a DFI (downstream fault indication) signal detector  44  monitor the protection channel input  36  for the presence of a signal fault or a DFI signal respectively. 
     The UFI detector  32 , the DFI detector  44 , and the first and second signal fault detectors  30 ,  42  cooperate to control the switch  28  through switch control elements  45 . These switch control elements  45  are represented in FIG. 2 by a first AND gate  45   a  that is armed by the UFI detector  32  and triggered by the second signal fault detector  42 ; a second AND gate  45   b  that is armed by the first signal fault detector  30  and triggered by the DFI detector  44 ; and an OR gate  45   c  that trips the switch  28  in response to the output of either the first AND gate  45   a  or the second AND gate  45   b.    
     The manner in which a ring of nodes having the architecture shown in FIG. 1 reconfigures the ring following a service disruption will be apparent from a detailed analysis of an example in which a disruption causes a signal fault on an inbound working channel leading to a node. The cause of the disruption is immaterial to the operation of the system. The disruption can arise from a fiber cut of one or both fibers that carry that channel, a degradation of a signal carried by one or more channels in one or both fibers, or from a disruption of an entire node. What is significant is that a signal fault in any fiber leading to any node in the ring initiates a sequence of events that inevitably results in the reconfiguration of the ring to avoid the disruption. 
     Referring now to FIG. 3A, a ring  46  includes a first node  48  in communication with an inbound working channel  48   a , an outbound working channel  48   b , an inbound protection channel  48   c , and an outbound protection channel  48   d . These channels are connected to the working channel input  24 , the working channel output  26 , the protection channel input  36 , and the protection channel output  38  of the first node  48  respectively. A disruption  50  in the inbound working channel  48   a  results in the detection of a signal fault by the first node  48 . 
     Referring back to FIG. 2, within the first node  48 , the first signal fault detector  30  monitors its working channel input  24  for a signal fault. A signal fault can be a total loss of a signal or merely a degradation of a signal. In either case, if the first signal fault detector  30  detects a signal fault at the working channel input  24 , it: instructs the UFI generator  34  to place a UFI signal on the working channel output  26 , and instructs the second LFI generator  41  to place an LFI signal at the protection channel output,  38 . 
     Referring back to FIG. 3A, because of the disruption  50  in the inbound working channel  48   a , the UFI generator  34  of the first node  48  operates in the manner described above to place a UFI signal on its outbound working channel  48   b  and an LFI signal on its outbound protection channel  48   d . This results in the UFI and LFI signals shown in FIG.  3 A. Note that the UFI signal is now present on the signal entering a second node  52 . The operation of this second node  52  is best understood with reference to FIG.  2 . 
     Referring again to FIG. 2, the working channel input  24  is also monitored by the UFI detector  32 . In response to the existence of a UFI signal on the working channel input  24 , the UFI detector  32  outputs a signal arming the first AND gate  45   a . In its armed state, the first AND gate  45   a  is prepared to place the switch  28  in its bridged state upon the occurrence of either a signal loss or an LFI signal on the protection channel input  36 . 
     The second node  52  passes the signal present at its working channel input  24  to its working channel output  26 . This places the ring  46  in the state shown in FIG. 3B, in which the UFI signal originally generated at the first node  48  is provided to the third node  53  by way of an outbound working channel  52   b.    
     The third node  53  is identical to the second node  52  and reacts to the UFI signal in exactly the same manner as already described above. The third node thus provides the UFI signal, originally generated by the first node  48 , to the working channel input of the fourth node  54 , as shown in FIG.  3 C. 
     The internal architecture of the fourth node  54  is identical to that of the second node  52 . Consequently, the operation of the fourth node  54  in response to the UFI signal present on its inbound working channel  54   a  is identical to that described above in connection with the second node  52 . The fourth node  54  therefore has within it a first AND gate  45   a  that has been armed by its UFI detector  32  in response to the UFI signal now present on the inbound working channel  54   a.    
     Consistent with the foregoing discussion of the operation of the first signal fault detector  30 , the first node  48 , in response to the existence of a signal fault at its working channel input  24 , instructed its second LFI generator  41  to place an LFI signal at its protection channel output  38 . This LFI signal is therefore present on the protection channel input  36  of the fourth node  54 . Because it has been armed by the UFI detector  32 , the first AND gate  45   a  generates a signal that passes through the OR gate  45   c  to place the switch  28  in a bridged state, as shown in FIG.  4 . In this state, the switch  28  redirects traffic on the inbound working channel  54   a  to the outbound protection channel  54   d . In addition, the signal from the first AND gate  45   a  causes the DFI generator  39  to place a DFI signal on the protection channel output. This places the ring  46  in the state shown in FIG.  3 C. 
     Depending on the location of the fault, the LFI signal may be detected by signal fault detector  30  or signal fault detector  42 . if both the protection channel and the working channel arc cut, or if a node fails altogether, it may be impossible for a node to detect an LFI signal. In order to extend the operation of the data protection system to such cases, it is preferable for the fourth node  54  to treat a loss signal in the same, manner as an LPI signal. 
     The DFI signal present on the outbound protection channel  54   d  associated with the fourth node  54  now propagates back through the third node  53 , as shown in FIG. 3D, and through the second node  52 , as shown in FIG.  3 E. Because neither the third node  53  nor the second node  52  ever transmitted an LFI signal out their respective protection channel outputs  38 , neither of those nodes ever armed their respective DFI detectors  44 . As a result, the DFI signal is passed unimpeded to.the protection channel input of the first node  48 . 
     Referring back to FIG. 2, the first node  48  did send an LFI signal on its protection channel output  38 . As a result, the first signal fault detector  30  of the first node  48  armed the second AND gate  45   b  of the first node  48 . This second AND gate is therefore ready to trigger the switch  28  upon receipt, by the first node  48 , of a DFI signal on the protection channel input  36 . This DFI signal is provided by the second node  52 , as shown in FIG.  3 E. 
     Upon receipt of this DFI signal, the second AND gate  45   b  of the first node  48  places the switch  28  in its bridged state, as shown in FIG.  4 . This places the ring  46  in the state shown in FIG. 3F, in which traffic entering the first node  48  on its inbound protection channel  48   c  is routed to its outbound working channel  48   b , thereby reconfiguring the ring  46  to avoid the disruption  50 . 
     It is apparent that since only one node in the ring  46  detects the fault on its inbound working channel and that only one node in the ring detects the LFI signal (or a loss of signal) on its inbound protection channel. As a result, only two nodes can be in a position to form a bridge. These two nodes are inevitably those nodes that are adjacent to the disruption  50 . 
     In the preferred embodiment, the LFI, DFI, and UFI signals are encoded in an overhead byte of the frame overhead associated with transmission of data. For example, in the case of a SONET ring, these signals can be sent over the Z4 or V4 bytes of the SONET path overhead. 
     FIG. 5 is a flow chart summarizing the operation of a typical node in the data protection method of the invention. As shown in FIG. 5, a node first checks to see if there exists a signal fault on its inbound working channel (step  56 ). If there is, the node transmits a UFI on the outbound working channel (step  58 ) and sends an LFI signal on its outbound protection channel (step  60 ). The node then monitors its inbound protection channel for the presence of a DFI signal (step  62 ). Upon receipt of a DFI signal, the node then forms a bridge, thereby routing traffic from its inbound protection channel to its outbound working channel (step  64 ). 
     If there is no fault present on its inbound working channel, the node checks to see if there is a UFI on its inbound working channel (step  66 ). If there is no UFI on its inbound working channel, then the ring is operating normally and no further action need be taken (step  68 ). However, if there is a UFI on its inbound working channel, the node must determine whether it is to form a bridge. 
     To determine whether it is to form a bridge, the node examines its inbound protection channel to determine whether there is either a loss of signal (step  70 ) or a signal fault (step  72 ). If neither of these are present on its inbound protection channel, the node recognizes that there is no need for it to form a bridge (step  68 ). If either a loss of signal or a signal fault is present on its inbound protection channel, the node sends a DFI signal on its outbound protection channel to signal whichever node initiated the data protection process that one bridge has been formed and that it too should form a bridge (step  74 ). At the same time, or shortly thereafter, the node forms a bridge, thereby routing traffic from its inbound working channel to its outbound protection channel (step  76 ). 
     The foregoing discussion describes the structure and operation of the system in connection with a disruption in the working channel. The operation of the system in connection with a disruption of a signal on the protection channel proceeds in an analogous manner, as indicated by the flow chart of FIG.  6 . 
     Referring to FIG. 6, when a first node detects a signal fault on its inbound protection channel (step  78 ), it sends a DFI signal on its outbound protection channel (step  80 ) and an LFI signal on its outbound working channel (step  82 ). The DFI signal propagates around the ring in the same manner that the UFI signal propagated around the ring when the signal fault was on the inbound working channel instead of the inbound protection channel. The first node then waits for a UFI signal on its working channel (step  84 ) and, upon receipt of such a signal, forms a bridge (step  86 ). 
     A second node that does not detect a signal fault on its inbound protection channel monitors its inbound protection channel for a DFI signal indicating a fault somewhere on the protection channel (step  88 ). If it detects no such DFI signal, the second node remains in its normal operating state (step  90 ). If it does detect such a signal, it must then determine whether it should form a bridge. To do so, the second node monitors the inbound working channel for either a loss of signal (step  92 ) or the presence of the LFH signal generated by the first node (step  94 ). If neither of these is present, the second node recognizes that it need not form a bridge, and it therefore remains in its normal operating mode (step  90 ). However, if the second node detects either a loss of signal or an LFI signal on the inbound working channel, it sends a UFI signal on its outbound working channel (step  96 ) and forms a bridge (step  98 ). It is this UFI signal that triggers the formation of a bridge by the first node (steps  84  and  86 ). 
     FIG. 7 shows the node in FIG. 2 but with only those interconnections between components that relate to the operation of the data protection system when a disruption is present on the protection channel instead of on the working channel. In practice, the interconnections shown in FIG.  7  and FIG. 2 are present on a typical node at the same time. 
     As shown in FIG. 7, the second signal fault detector  42  monitors the protection channel input  36  for the occurrence of a fault If the second signal fault detector  42  detects a fault on the inbound protection chancel it causes the first LFT generator  21  to send an ILO signal on the outbound working channel and the DFI generator  39  to send a DPI signal on the outbound protection channel. In addition, the second signal fault detector arms a third AND gate  45   e  to be ready to trip the switch  28  upon receipt of detection of a UPI signal on the inbound working channel by the UPI detector  32 . 
     The first signal fault detector  30  monitors the inbound working channel for the presence of a signal fault. If a signal fault exists on the inbound working channel, the first signal fault detector arms a fourth AND gate  45   d . Meanwhile, a DFI detector waits for a DFI signal on the inbound protection channel. As long as no DFI signal is present, the switch  28  is in its normal state. However, upon receipt of a DFI signal, the DFI detector signals the armed fourth AND gate  45   d . The fourth AND gate  45   c  then causes the UFI generator  34  to send a UFI signal on the outbound working channel and also causes the switch  28  to trip and form a bridge. 
     The data protection system of the invention thus includes a system for protection of data on the working channel operating in parallel with an analogous system for the protection of data on the protection channel. In addition, because the ring  46  has the same configuration as a UPSR, the conventional UPSR data protection system can operate in parallel with the data protection system of the invention.