Network failure restoration method with different recovery performances for different signal groups

Abstract: In a communications network, shared spare routes and dedicated spare routes are provided between nodes of adjacent pairs and a dedicated spare path are provided between terminal nodes. Each of the dedicated and shared spare routes comprises a link or a series of permanently connected links. A first signal requiring a quick fault restoration performance is transmitted over the first working path and a second signal not requiring the quick fault restoration performance is transmitted over the second working path. The working paths and all working links are monitored to detect a path or link failure. If the first working path is affected by a path failure, the first signal is instantly switched to the dedicated spare path, and if it is affected by a link failure, the first working path is reestablished using one of the dedicated spare routes. If the second working path is affected by a path failure, an alternate path is established between the terminal nodes using the shared spare routes and the second signal is switched to the alternate path and if it is affected by a link failure, an alternate route is established using one of the shared spare routes and the second working path is reestablished using the alternate route.

Description: The present invention relates generally to a communications network, and more specifically to the restoration of network resource from network failure. In a communications network, it is the usual practice to multiplex signals for a particular destination into a single multiplex signal and transmit it over a physical or virtual path chat is established between two terminal (i.e., end) nodes. The traffic carried by the path may include data signals for incer-bank transactions that require a high recovery speed with which they are protection-switched to a spare path, and others that do not require such high speed recovery performance and allow a delay of as long as a few seconds. Since the fault recovery speed of a network path is usually matched to the high speed restoration requirement of the signals it carries and high-speed restoration performance requires an extra cost or investment, it is not economically advantageous to multiplex low-speed requirement signals with high-speed requirement signals onto a network path to allow the former to share high cost restoration facilities with the latter. "Design and Control Issues of Integrated Self-healing Networks in SONET" are discussed by Y. Okanoue et al. in Proceedings of the Global Telecommunications Conference (GLOBECOM), US, New York, 2 December 1991, vol. 2 of 3, pages 730-735. It is therefore an object of the present invention to achieve economical provisioning of network resources by providing fault recovery performance at different levels corresponding to signals of different fault recovery speeds. This object is achieved by a method according to Claim 1 and a network according to Claim 3. The present invention provides, between the terminal nodes, a dedicated spare path, and provides, between nodes of adjacent pairs, a plurality of dedicated spare routes and a plurality of shared spare routes, each of the dedicated and shared spare routes comprising at least one spare link. A first signal requiring a quick fault restoration performance is transmitted over the first working path and a second signal not requiring the quick fault restoration performance is transmitted over the second working path. The working paths and links are monitored to detect a path failure and a link failure. If the first working path is affected by a path failure, the first signal is switched to the dedicated spare path, and if the first working path is affected by a link failure, the first working path is reestablished using one of the dedicated spare routes. If the second working path is affected by a path failure, an alternate path is established between the terminal nodes using the shared spare routes and the second signal is switched to the alternate path, and if the second working path is affected by a link failure, an alternate route is established between adjacent nodes using one of the shared spare routes and the second working path is reestablished using the alternate route. The present invention provides a communications network comprising a pair of terminal nodes and an intermediate node interconnected by links, first and second working paths established between the terminal nodes via the intermediate node, each of the working paths comprising a series of working links, the first working path transmitting a first signal which requires a quick restoration performance and the second working path transmitting a second signal which does not require the quick restoration performance. The network includes a dedicated spare path between the terminal nodes, a plurality of dedicated spare routes between the terminal nodes, and a plurality of shared spare routes between the terminal nodes. Each network node monitors the working paths to detect a path failure and a link failure. If the first working path is affected by a path failure, the first signal is switched to the dedicated spare path, and if the first working path is affected by a link failure, the first working path is reestablished using one of the dedicated spare routes. If the second working path is affected by a path failure, an alternate path is established between the terminal nodes using the shared spare routes and the second signal is switched to the alternate path and if the second working path is affected by a link failure, an alternate route is established between adjacent nodes using one of the shared spare routes and the second working path is reestablished using the alternate route. Each network node preferably comprises an optical cross-connect system for switching a wavelength-time-division multiplex signal at different levels of digital hierarchy. The present invention will be described in further detail with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of an exemplary communications network for describing the present invention; and Fig. 2 is a flowchart of the operation of each node of the network In Fig. 1, an exemplary communications network is shown as comprising nodes 10, 11 and 12 interconnected by optical fibers, or links. As illustrated, nodes 10 and 12 are terminal (end) nodes where logical communication channels, or "paths" are established over concatenated links connected by intermediate (transit) node 11. Each path supports a particular optical bandwidth of a wavelength-time-division multiplex (WTDM) signal. Each network node includes an optical cross-connect system for switching the WTDM signal at different levels of digital hierarchy. A suitable cross-connect system for implementing the present invention is disclosed in U. S. Patent 5,457,556 issued to Tatsuya Shiragaki. At each node, a WTDM signal may be demultiplexed into a plurality of optical TDM signals of particular wavelength or lower hierarchical level digital signals may be multiplexed into an optical TDM signal of particular wavelength. A path carrying such a single-wavelength signal between terminal nodes is a "wavelength path" and an intermediate node that supports a wavelength path provides no wavelength conversion and is termed a WP cross-connect node If wavelength conversion is provided in an intermediate node, the node is a VWP cross-connect node, and the path passing through this node carries signals of different wavelengths and is termed a "virtual wavelength path". Nodes 10 and 11 are interconnected by links 13 to 17 and nodes 11 and 12 are interconnected by links 21 to 25. Under normal conditions, a connection 26 is established in node 11 between links 13 and 21 to form a first working path 27 between terminal nodes 10 and 12. One end of the path 27 is connected to a local access line 28 via a connection 29 at node 10 and the other end of the path 27 is connected through a connection 30 to a local access line 31. In addition, a connection 32 is established in node 11 between links 17 and 25 to form a second working path 33 between terminal nodes 10 and 12. According to the present invention, traffic signals are classified into a high recovery-speed group and a low recovery-speed group according to their different tolerances to down time. Inter-bank data signals are one of the examples that are relatively intolerant of outage and require high speed restoration from failure. There are other signals that are relatively tolerant of down time and allow some delays. Such signals do not necessarily require high speed restoration performance. The signals that require quick recovery from failure are classified under the high recovery-speed group and carried by the working path 27, while the signals that do not necessarily require quick recovery are classified under the low recovery-speed group and carried by the working path 33. Spare facilities are provided in the network for protection-switching a failed working path or link. For this purpose, a connection 38 is permanently established in node 11 between links 14 and 22 to form a spare path 39 between terminal nodes 10 and 12. The spare path 39 is used as a dedicated spare path for the working path 27 if the latter should encounter a path failure. In addition, links 15 and 23 are reserved as dedicated spare links to be used for the working path 27 if the latter should encounter a link failure. On the other hand, links 16 and 24 are reserved as "shared" spare links for the working path 33 if the latter should encounter either a path failure or a link failure. The network may further include an intermediate node 50 between nodes 10 and 11 to establish a dedicated spare route 51 consisting of a series spare links 52 and 54 permanently connected by a connection 53 at node 50. In addition, a shared spare link 55 may be provided between nodes 10 and 50 and a shared spare link 56 between nodes 50 and 11. As described in the aforesaid Shiragaki U. S. patent, the cross-connect system at each network node is provided with fault detection and rerouting circuitry that monitors traffic signals, exchanges control signals with other network nodes when the monitored signals indicate the presence of either a path failure or a link failure, and automatically reestablishes switched connections to reroute the traffic signals to appropriate spare facilities. As illustrated in the flowchart of Fig. 2, the operation of the fault detection and rerouting circuitry of each network node starts with step 60 where all traffic signals are monitored for the presence of a path failure or a link failure. If a path failure is detected, flow proceeds from step 60 to step 61 to identify the recovery-speed group of the failed path. If the failed path is one that carries the signal of high recovery-speed group, flow proceeds from step 61 to step 62 to switch the affected signal from the failed path to the dedicated spare path that is associated with the failed working path. If the failed path is identified as one carrying the signal of low recovery-speed group, flow proceeds from step 61 to step 63 to exchange control signals with other nodes to find an alternate path. If such an alternate path is found (step 64), flow proceeds from step 64 to step 65 to switch the affected signal from the failed working path to the alternate path. If a failure is detected in a link, flow proceeds from step 60 to step 66 to identify the recovery-speed group of the failed link. If the failed link is one that carries the signal of high recovery-speed group, flow proceeds from step 66 to step 67 to switch the affected signal from the failed link to the dedicated spare link that is associated with the failed working link. If the failed link is identified as one carrying the signal of low recovery-speed group, flow proceeds from step 66 to step 68 to exchange control signals with other nodes to find an alternate link. If such an alternate link is found (step 69), flow proceeds from step 69 to step 70 to switch the affected signal from the failed working link to the alternate link. The operation of this invention will best be understood by the following description with reference to Fig. 1. If a path failure PF1 occurs in the working path 27, nodes 10 and 12 establish connections 40 and 41, respectively, in their cross-connect systems for connecting the dedicated spare path 39 between lines 28 and 31 and clear the connections 29 and 30. Since the dedicated spare path 39 is in readiness to carry signals therethrough by virtue of the preestablished connection 38, the path for down-time intolerant signals can be restored in a short period of time. If a link failure LF1 occurs in the link 13, nodes 10 and 11 establish connections 42 and 43, respectively, for connecting the dedicated spare link 15 between line 28 and link 21 and clear the connections 29 and 26, thus instantly switching the failed down-time intolerant signals to an alternate path. If a path failure PF2 occurs in the working path 33, nodes 10, 11 and 12 exchange control signals to find links 16 and 24 as appropriate path segments and establish connections 44, 45 and 46, respectively, to form an alternate path 47 between local access lines 35 and 37 and clear the connections 34, 32 and 36. Such an alternate path is a shared spare path which is created on an as-needed basis, rather than on a preassigned basis, and the links that comprise it are shared among a plurality of working paths that carry signals of the low recovery-speed group. Since the establishment of the shared spare path takes time, the as-needed approach for path recovery is economically advantageous for down-time tolerant signals. If a link failure LF2 occurs in the link 17, nodes 10 and 11 exchange control signals to find the link 16 as an alternate link and establish connections 44 and 48, respectively, for connecting the spare link 16 between access line 35 and link 25 and clear the connections 34 and 32. Such an alternate link is created on an as-needed basis and can be shared among working links that carry signals of the low recovery-speed group. Since the establishment of such a shared spare link takes time, the as-needed approach for link recovery is economically advantageous for down-time tolerant signals. It is seen that the dedicated spare route 51 can also be used instead of the dedicated spare link 15 in the event of link failure LF1, and shared spare links 55 and 56 can also be used instead of the shared spare link 16 in the event of link failure LF2. In the latter case, nodes 10, 50 and 11 exchange controls signals to establish a connection at node 50 between links 55 and 56.

Claims: A fault recovery method for a communication network wherein a plurality of nodes (10, 11, 12) are interconnected by communication links (13 ∼ 25) and terminal nodes (10, 12) are interconnected by a first working path (27) formed with a series of working links (13, 21) and a second working path (33) formed with a series of working links (17, 25), comprising: a) providing between said terminal nodes (10, 12) a dedicated spare path (39) comprising a permanently connected series of spare links (14, 22) and a shared spare path (47) comprising a permanently connected series of spare links (16, 24); b) transmitting first and second signals over said first and second working paths (27, 33) respectively, said first signal requiring quick fault restoration performance and said second signal not requiring fault quick restoration performance; c) monitoring said first and second working paths (27, 33) for detecting a path failure; and d) if the first working path (27) is affected by a path failure (PF1), reestablishing the first working path over the dedicated spare path (39), and if the second working path (33) is affected by said path failure (PF2), reestablishing the second working path over the shared spare path (47), characterized in that , the step (a) further comprises providing between adjacent nodes (10, 11) at least one dedicated spare route (15, 51) and at least one shared spare route (16, 55, 56), each of said dedicated spare route and said shared spare route comprising at least one of said communication links, in that the step (c) further comprises monitoring said first and second working paths (27, 33) for detecting a link failure, and in that the step (d) further comprises the steps of: d 1 ) if the first working path (27) is affected by a link failure (LF1), reestablishing the first working path (27) via said at least one dedicated spare route (15, 51); and d 2 ) if the second working path (33) is affected by said path failure (PF2) or affected by a link failure (LF2), reestablishing the second working path via said at least one shared spare route (16, 55, 56). A fault recovery method as claimed in claim 1, characterized in that said at least one dedicated spare route comprises a series of permanently connected spare communication links and said at least one shared spare route comprises a series of mutually disconnected spare communication links. A communications network comprising: a pair of terminal nodes (10, 12) interconnected via an intermediate node (11) by communication links (13 ∼ 25); a first working path (27) between said terminal nodes (10, 12) for transmitting a first signal which requires quick fault restoration performance, said first working path (27) comprising a series of working links (13, 21); a second working path (33) between said terminal nodes (10, 12) for transmitting a second signal which does not require quick restoration performance, said second working path (33) comprising a series of working links (17, 25); a dedicated spare path (39) and a shared spare path (47) between said terminal nodes (10, 12), said dedicated spare path and said shared spare path (47) comprising a permanently connected series of said communication links (14, 22); and each of said nodes being adapted to monitor said first and second working paths (27, 33) for detecting a path failure, each node being responsive to the first working path (27) being affected by a path failure (PF1) for reestablishing the first working path over the dedicated spare path (39), and responsive to the second working path (33) being affected by said path failure (PF2) for reestablishing the second working path over the shared spare path (47), characterized by : at least one dedicated spare route (15, 51) between adjacent nodes (10, 11), said at least one dedicated spare route comprising at least one of said communication links; and at least one shared spare route (16, 55, 56) between said adjacent nodes, said shared spare route comprising at least one of said communication links, in that each of said nodes is arranged to: monitor said first and second working paths (27, 33) for detecting a link failure, responsive to the first working path (27) being affected by a link failure (LF1) for reestablishing the first working path (27) via said at least one dedicated spare route (15, 51), and responsive to the second working path (33) being affected by said path failure (PF2) or affected by a link failure (LF2) for reestablishing the second working path via said at least one shared spare route (16, 55, 56). A communication network as claimed in claim 3, characterized in that said at least one dedicated spare route comprises a series of permanently connected spare communication links and said at least one shared spare route comprises a series of mutually disconnected spare communication links. A communications network as claimed in claim 3 or 4, characterized in that each of said nodes comprises an optical cross-connect system for switching a wavelength-time-division multiplex signal at different levels of a digital hierarchy depending on whether the multiplex signal is affected by a path failure or a link failure.

Inventors: SHIRAGAKI TATSUYA, SHIRAGAKI, TATSUYA
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1997
Main classification: H04L29
IPC codes: H04B10, H04L1, H04Q11, H04J3, H04J14, H04B1, H04L12, H04Q3