Multiplex communications – Fault recovery – Bypass an inoperative switch or inoperative element of a...
Reexamination Certificate
1998-02-04
2004-04-27
Jung, Min (Department: 2663)
Multiplex communications
Fault recovery
Bypass an inoperative switch or inoperative element of a...
C370S400000, C370S254000, C345S440000
Reexamination Certificate
active
06728205
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to communication or power networks and more particularly to a method and apparatus for planning and implementing automatic protection switching in networks.
BACKGROUND OF THE INVENTION
As is known in the art, a network includes a plurality of processing sites generally referred to as stations or nodes connected by one or more physical and/or logical connections. When the connections establish transmission of a signal in at least one direction between the nodes, the connections are generally referred to as links. Each node typically performs a switching function and one or more additional functions.
The nodes may be coupled together in a variety of different network structures typically referred to as network topologies. For example, network nodes may be coupled in a circular structure, generally referred to as a ring topology. Other topologies such as star topologies and tree topologies are also known.
The transmission of a signal from a first or source node to a second or destination node may involve the transmission of the signal through a plurality of intermediate links and nodes coupled between the source node and the destination node. Such a succession of links and nodes between a source node and a destination node is referred to as a path.
When a link or node in a path fails, communication between a source node and a destination node in that path is disrupted. Thus, to continue communications between the source and destination nodes, an alternate path must be found and the signal being transmitted from the source node to the destination is routed through the alternate path.
A self-healing network refers to a network which automatically restores connections among nodes in the event of a link or node failure in a path from a source node to a destination node. There is a growing trend and reliance on such networks owing to increasing reliance on and use of high-speed communication networks and the requirement that these communication networks be robust in the case of certain failures. Self-healing networks are central not only to applications where failures occur very often, such as military networks under attack, but also in public networks where failures, albeit rare, can be extremely disruptive. Self-healing networks can also be used in power transmission grids to aid in the distribution of power signals in a power network. Thus self-healing networks have use in a wide variety of applications including but not limited to communications networks and power systems networks.
Self-healing networks typically detect and report a failure, establish and connect a restoration path and then return the network to normal communications, not necessarily in that order. Such self-healing characteristics are incorporated, for example, in the Synchronous Optical Network (SONET) protocols.
A network may be represented as a graph which includes nodes representing the network nodes and edges representing bidirectional connections between the nodes. For a network to be capable of having a self-healing feature which leaves all nodes mutually connected even after the failure of a node and/or an edge, a graph representing the network must be either node and/or edge redundant. A node redundant graph is a graph in which the nodes remain mutually connected even after the elimination of any node in the network. Similarly, an edge redundant graph is a graph in which the nodes remain mutually connected even after elimination of any edge in the network. Node or edge redundancy ensures that all nodes remain connected even after the failure of a node or edge, respectively. One problem, however, is that not all self-healing techniques are guaranteed to work over an arbitrary redundant network (i.e. a network having some portions which are node redundant and other portions which are edge redundant). For instance, a technique which performs self-healing locally may not be able to use the redundancy afforded by a more distant part of the network. Node or edge redundancy is thus the minimum topological requirement for a network to perform self-healing which allows all remaining nodes to remain mutually connected after failure of a node or edge.
Self-healing networks can be classified according to the following three criteria: (1) the use of line (or link) rerouting versus path (or end-to-end) rerouting, (2) the use of centralized versus distributed schemes and (3) the use of precomputed versus dynamically computed routes. The criterion of link rerouting versus path rerouting maps to the criterion, common in optimization, of local versus global. The different criteria are not in practice, selected independently of each other for self-healing networks. Because path rerouting is less local in nature than link rerouting, path rerouting schemes are not usually distributed.
Furthermore, since path rerouting tends to require a relatively large amount of computation and covers a relatively large span of a network compared with link rerouting, path rerouting is typically implemented using precomputed routes. Link rerouting, which usually considers only a few hops away from the rerouted link in the network, is better suited to dynamically computed routes and thus may be done in a distributed fashion.
For example, in a bidirectional self-healing ring (SHR), after a link or node failure, traffic that was previously carried along one link is carried around the rest of the ring using a technique generally referred to as loopback.
One problem with this approach, however, is that a limitation may exist with respect to the number of nodes which may be traversed to replace the failed link. If there is a hop limit, for example, link rerouting may not be possible in a network having a ring topology.
One of the most common ways in SONET to restore rapidly network functionality is to combine SHRs and diversity protection (DP), using add-drop multiplexers (ADMs), for automatic protection switching (APS). Systems using One-to-n (1:n) DP have one back up link for n working links. SHRs perform loopback, which may be regarded as a special case of APS. SHR architectures may be classified into unidirectional rings, in which the duplex channel travels over a different path than the forward channel, and bi-directional rings where the forward channel and the duplex channel may travel the same path. Bi-directional rings typically include two or four fibers. Using mechanical ADMs, the restoration time is typically about 50 milliseconds (ms) while path switching typically requires less than 20 ms and loopback switching typically require under 80 ms. DP or SHR architectures typically require about 10 ms to detect and 50 ms to complete the switch.
One problem with the DP and SHRs approaches is that they require built-in excess capacity to handle failures. Moreover such systems may be difficult to upgrade as more nodes are added to the network. A system utilizing one-to-one (1:1) DP requires the most spare capacity, since the spare capacity is the same as the capacity used during normal operation. SHRs may require as much spare capacity as DP, depending upon network traffic patterns.
Furthermore, placing constraints on possible network topologies may result in increased network cost. For example, a ring topology may be used in a particular application to implement APS. The ring topology, however, may not be the most cost effective topology for the application. This results in a relatively expensive network.
It would, therefore, be desirable to provide a system which allows APS over any arbitrary network topology. Such a system can be used with any existing topology, allows relatively inexpensive expansion of existing networks regardless of topology and allows construction of new and relatively inexpensive networks.
One approach to self-healing uses optical switches such as acousto-optical switches. Optical switches allow switching to a backup fiber, in a time delay which is in the order of micro-seconds (&mgr;s). For four-fiber bi-directional rings, optical switches and amplifiers have bee
Barry Richard A.
Finn Steven G.
Medard Muriel
Daly, Crowley & Mofford LLP
Jung Min
Lee Andy
Massachusetts Institute of Technology
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