Abstract:
A network ( 10 ) having an enhanced restoration architecture includes at least two Digital Access Cross-Connect Systems (DACS)  20  having the ability to uniquely identify a signal. When a restoration route is established in the network following a failure, the DACS situated at an origin and destination nodes of restoration route will launch and recover, the particularly identified signal provided continuity exists on the route. In this way, the DACS can automatically determine the continuity of the restoration route without the need to separately perform time-consuming continuity tests of the individual paths comprising the restoration route.

Description:
TECHNICAL FIELD 
     This invention relates to a technique for providing an alternate signal path in a communications network upon failure of a network link. 
     BACKGROUND ART 
     Present day telecommunications carriers, such AT&amp;T, carry large volumes of telecommunications traffic across their networks. While most carriers strive for high reliability, disruptions in their networks can and do occur. Such disruptions are often attributable to a failure in a link between two network nodes, each node typically comprising a telephone central office or network control center. The links, which may each take the form of a copper cable, optical fiber or a radio channel, may fail because of an act of nature, such as a flood, ice storm, hurricane or other weather-related occurrence. Link failures are sometimes man-made. Not infrequently, a contractor may accidentally sever a cable or fiber link during excavation along the link right-of way. 
     Regardless of the manner in which a communication link is severed, a link failure invariably disrupts communications services. For example, the loss of a single fiber in an optical cable usually results in many blocked calls. Each blocked call represents a loss of revenue for the carrier carrying that call. Thus, rapid restoration of traffic is critical. Typically, telecommunications carriers achieve traffic restoration by routing traffic on alternate routes. Since spare capacity often exists on many routes, traffic restoration is a matter of determining where such spare capacity exists and then establishing a path utilizing such spare capacity to bypass the severed link. 
     U.S. Pat. No. 5,182,744, “Telecommunications Network Restoration Architecture,” issued on Jan. 26, 1993, in the name of James Askew et al. and assigned to AT&amp;T (incorporated by reference herein) discloses a restoration technique for routing telecommunications traffic on an alternate route in the event of a severed communications link. The Askew et al. technique utilizes communications monitors at the nodes to detect the disruption of traffic across the communications links. Should a disruption occur because of a failed link, the monitor at one or both of the affected nodes notifies a central facility that determines an alternate route over which the traffic can bypass the failed link. After finding a restoration route, the central facility directs the nodes to conduct a continuity test of the restoration route. Upon successful completion of the continuity test, the disrupted traffic passes over the restoration route. 
     While the Askew et al. restoration technique represents a significant advance over past approaches, the continuity test performed prior to restoration is not instantaneous. At present, restoration of one hundred DS3 signals takes about five minutes. Even though such an interval may seem insignificant, as many as 50,000 calls may be blocked during this time. Thus, there is a need for a technique achieves network restoration quickly, to reduce the incidence of blocked calls. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly, in accordance with a preferred embodiment of the invention, a technique is provided for restoring traffic within a telecommunications network upon the failure of a link coupling a pair of nodes. In accordance with the restoration technique of the invention, the links are monitored to determine a possible link failure. The failure of any link is reported to a central facility, typically, although not necessarily, by wireless communication. Upon receipt of a report of a failed link, the central facility determines a restoration route. In practice, the restoration route is determined in accordance with the location of the spare capacity within the network and on the traffic priority. The central facility the directs nodes, via an appropriate command, to establish the restoration route. At each node, each piece of traffic to be routed on the restoration route is uniquely identified. The unique identity of the each piece of traffic is utilized to determine the passage thereof on the restoration route to establish the continuity of the route without the need for any separate, time consuming continuity tests. 
    
    
     BRIEF SUMMARY OF THE DRAWINGS 
     FIG. 1 is a block schematic diagram of a telecommunications network architecture in accordance with the present invention that provides for rapid restoration; and 
     FIG. 2 is flow chart diagram of the steps undertaken in connection with restoration of the network of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a communication network  10  that achieves rapid restoration of traffic in accordance with the invention. The network  10  comprises a plurality of nodes, represented by nodes  12   1 - 12   6 . Each pair of nodes is linked by one of links  14   1 - 14   6 . In the illustrated embodiment, the node pairs are linked as indicated in Table I 
     
       
         
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                   
                 Connecting 
               
               
                   
                 Node Pair 
                 Link 
               
               
                   
                   
               
             
             
               
                   
                 12 1 , 12 4   
                 14 1   
               
               
                   
                 12 2 , 12 4   
                 14 2   
               
               
                   
                 12 2 , 12 3   
                 14 3   
               
               
                   
                 12 4 , 12 5   
                 14 4   
               
               
                   
                 12 3 , 12 5   
                 14 5   
               
               
                   
                 12 5 , 12 6   
                 14 6   
               
               
                   
                   
               
             
          
         
       
     
     The number of nodes and connecting links is exemplary. As may be appreciated, the network  10  could include a larger or smaller number of nodes and links as desired. 
     In practice, each of links  14   1 - 14   6  comprises a multi-channel communications transmission medium that may take the form of a multiplexed radio channel, a multiple wire-pair cable, or one or more optical fibers that each carry a multiplexed, optically formatted signal. Each of the nodes  12   1 - 12   6  represents a communications facility such as telephone central office or network control hub. Each node includes at least one Line terminating Equipment (LTE)  16  for terminating a link coupled to the node. Thus, for example, the node  12   1  includes a single LTE  16  since a single link  14   1  terminates at that node, whereas the nodes  12   2  and  12   3  each include two LTEs  16 — 16  for terminating the two links, respectively, connected to these nodes. Each of the nodes  12   4  and  12   5  include three LTEs  16 — 16 — 16  for terminating the three links, respectively, connected to each of these nodes. 
     The nature of each LTE  16  depends on the nature of the link that it terminates. When the link comprises an optical fiber or set of fibers, the LTE  16  terminating such a link comprises an optical fiber interface that demultiplexes and converts the optically-formatted multiplexed signal into discrete electrical signals. Different types of LTEs terminate radio signals and the signals on a multi-pair wire cable, respectively. 
     Each node also includes a first Digital Access Cross-Connect System (DACS)  18  coupled to each LTE  16  associated with that node. Typically, each DACS  18  comprises a DACS III formerly manufactured by AT&amp;T that has the ability to Cross-connect DS 3  signals received at one LTE  16  to another LTE  16 . For example, the DACS  18  within the node  12   4  can Cross-connect a DS 3  signal received at the LTE  16  terminating the link  14   1  to either of the LTEs  16 — 16  terminating the links  14   2  and  14   4 , respectively. 
     In addition, each node also includes a second DACS  20  having the capability of terminating DS 3  signals and Cross-connecting and terminating T 1  signals. (For ease of illustration, only the DACS  20  in each of the nodes  12   1  and  12   6  is illustrated, the DACS  20  of each of the other nodes being omitted for purposes of clarity.) Each DACS  20  also has the ability to receive and report an alarm signal generated upon the failure of signals received on a corresponding link, as detected via its associated LTE. 
     In practice, each DACS  20  comprises a model 3/1 DACS manufactured by Alcatel SA of France. In addition of providing the cross-connect and termination functions discussed above, this particular model DACS has the ability to uniquely identify each piece of traffic (i.e., each DS 3  signal) that terminates at the DACS  20 . As will be discussed below, this identification ability, referred to as “path identification,” is utilized, in accordance with the invention, to assure path integrity without the need to perform a separate continuity test as required by the prior art. 
     The ability of the DACS  18  and  20  within each node to cross-connect signals between associated LTEs  16  facilitates the ability to re-route traffic in the event that a link fails. For example, assume that link  14   4  connecting the nodes  12   5  and  12   4  has failed. Under such conditions, the DACS  18  and  20  within the node  12   4  could route to node  12   2 , via link  14   2 , signals ordinarily destined for node  12   5 , via link  14   4 , but for the failure of that link. (In order to route such traffic on the link  14   2 , additional capacity must exist on that link.) Thereafter, the DACS  18  and  20  associated with the node  12   2  would route the signals via link  14   3  (assuming spare capacity on that link) to the node  12   3 . The DACS  18  and  20  within node  12   3  would then route the signals via the link  14   5  (assuming spare capacity) to the node  12   5 . In this way, the signals destined from node  12   4  to node  12   5  will bypass the failed link  14   4 . 
     Each of the nodes  12   1 - 12   6  includes a Restoration Node Controller (RNC)  22  which gathers alarm signals from the DACS  18  and  20  as well from each LTE  16  associated with a given node. For purposes of illustration, only the RNC  22  of each of nodes  12   1  and  12   6  is shown, the RNCs of the other nodes being omitted for clarity. Alarm signals received by the RNC  22  associated with each node are communicated to a Restoration And Provisioning Integrated Design (RAPID) central processor  24  through a packet communication network  26  that may comprise AT&amp;T&#39;s SKYNET wireless communications network, or a PINET, a derivative of AT&amp;T&#39;s ACCUNET® Packet communications service. In practice, the link between the RNC  22  and the central processor  24  provided by the network  26  comprises a satellite link. 
     The RAPID central processor  24  is also linked to a Transport Maintenance Administration System (TMAS)  28 , comprising part of a Failure Mode Analysis Center (FMAC)  30 . The TMAS  28  controls those LTEs  16 — 16  that terminate the links comprised of one or more optical fibers. In particular, the TMAS  28  has the capability to establish protection switch lockouts for LTEs that terminate optical fibers. To achieve path restoration, the RAPID central processor  24  needs the ability to control the protection switch lock-out capability, and hence the link between the RAPID processor and the TMAS  28 . 
     The process by which the RAPID central processor  24  restores the network  10  in the event of a failure is depicted in flow chart form in FIG.  2 . The restoration process commences upon receipt by an RNC  22  at one of the nodes  12   1 - 12   6  (step  100 ) of a failure message indicating failure of a link, LTE or a DACS. The RNC  22  reports the failure to the RAPID central processor  24  (step  102 ) via the network  26 . Following receipt of the failure message from the reporting RNC  22 , the RAPID central processor  24  then calculates the restoration path(s) (step  104 ). In calculating each restoration path, the RAPID central processor  24  takes into account the spare capacity, if any, on the non-failed links, as well as the traffic priority. In establishing each restoration route, the RAPID central processor  24  attempts to route high priority traffic before the low priority traffic. 
     Once the RAPID central processor  24  calculates the restoration path(s), the processor sends Cross-connect commands to each appropriate RNC  22  (step  106 ) via the network  26 . In response to the cross-connect command from the RAPID central processor  24 , each RNC  22  supplies a command to the appropriate one of the DACS  18  and  20  to effect the requested Cross-connections to establish each restoration path. To the extent that a restoration path includes a link comprising an optical fiber, the RAPID central processor  24  may need to send a protection lock-out request to the TMAS  28  (step  108 ). After initiating the cross-connect commands and the requested protection lock-outs (if necessary), the RAPID central processor  24  then requests alarm clearance of each RNC  22  that reported an alarm (step  110 ). 
     Following step  110 , the RAPID central processor  24  receives a report from the RNC  22  associated with the downstream-most node of the restoration route regarding its continuity (step  112 ). As discussed previously, the DACS  20  within each node advantageously has the capability of uniquely identifying each DS 3  signal. In this way, the DACS  20  associated with the downstream-most node can determine if a particular DS 3  signal was received. From a knowledge of the identity of a particular DS 3  signal launched at the origin of the restoration route, the DACS  20  associated with the downstream-most node can determine whether that particular signal traversed the restoration route. If the same signal launched at the origin of the restoration route is received at the end of the route, then the route is continuous. The failure to receive the same signal indicates a failure of continuity. 
     From a continuity standpoint, it is only necessary to detect the presence of a particular launched signal at the end of the restoration route. Any break or open along the restoration route will prevent the launched signal from traversing the path regardless of the location of such a break or open. However, it may be desirable to receive continuity reports from one or more intermediate RNCs  22 — 22  (i.e., those RNCs situated between the origin and destination nodes of the restoration route) for the purpose of determining the location of the break or open in the route in order to facilitate determination of an alternate restoration route. 
     Establishing continuity of the restoration path by confirming the identity of a particular DS 3  signal at the downstream-most node affords a savings in time, as compared to establishing continuity by a separate continuity test, as taught in the prior art Askew et al. patent (incorporated by reference herein). In practice, the overall time required to restore 100 DS 3  signals using the prior art technique embodied in the Askew et al. patent takes approximately five minutes because of the needed continuity tests. In contrast, the ability of the DACS  20  at least the downstream-most mode to establish continuity based on path identification allows 100 DS 3  signals to be restored in approximately one minute, a five fold reduction over the prior art. 
     Assuming that path continuity is found during step  112 , then the RAPID central processor  24  completes restoration by reporting the establishment of the restoration path to a Network Control Center (step  114 ). Note that reporting of the completion of restoration is not critical and potentially could be omitted if necessary. 
     The foregoing discloses a technique for restoring a communication network whereby continuity of a restoration path is established in accordance with the unique identity of signals traversing the restoration path. It is to be understood that the above-described embodiments are merely illustrative of the principles of the invention. Various modifications and changes may be made thereto by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.