Multiplex communications – Fault recovery – Bypass an inoperative station
Reexamination Certificate
1998-04-22
2001-08-21
Ngo, Ricky (Department: 2664)
Multiplex communications
Fault recovery
Bypass an inoperative station
C370S228000, C370S242000, C359S199200, C359S199200
Reexamination Certificate
active
06278689
ABSTRACT:
TECHNICAL FIELD
This invention relates to a technique for restoring optically formatted traffic in a ring subnetwork in case of a double ring failure.
BACKGROUND ART
Many network providers of telecommunication services, such as AT&T, now utilize Synchronous Optical NETwork (SONET) rings interconnected in a network for transporting optically formatted traffic. A typical SONET ring subnetwork comprises a plurality of nodes, each linked to its two neighbors by an optical fiber service path and protection path in a ring-like arrangement. At each individual node, an Add-Drop Multiplexer (ADM) typically routes traffic onto, and off the ring subnetwork to permit traffic to pass on the ring between nodes. In practice, inter-ring traffic received at a node on one ring is coupled by a Digital Cross-Connect System (DCS) for routing on a second ring subnetwork to its ultimate destination.
During normal operation, optically formatted traffic within a ring passes from one node to another via the optical fiber service path. Should the optical fiber service path connecting a pair of nodes become unavailable to carry traffic, due to a fiber cut, for example, the traffic that would otherwise pass on the service path passes on the protection path. U.S. Pat. No. 5,182,744, issued on Jun. 26, 1993, in the name of James Askew et al., and assigned to AT&T Bell Laboratories (incorporated by reference herein) describes a technique for restoring optically formatted traffic in a mesh network in case of a failure of the optical service path between nodes. The Askew et al. technique can be utilized in a ring subnetwork to restore traffic in case of a “single” ring failure, i.e., the failure of a optical service path or a node linked to such a path.
In addition to suffering single failures as discussed above, a SONET ring subnetwork can also suffer, for example, a “double” ring failure. A double ring failure occurs when optical service and protection paths on a given span in a SONET ring fail, due to a cable break for example, and a “non-associated” protection path (i.e., a protection path on a span different from the span with the cable break) also fails. A double ring failure is generally not restorable via the ring itself, giving rise to a service outage which is costly from both a revenue and customer satisfaction perspective.
Thus, there is a need for a technique for restoring service in an optical subnetwork, such as a SONET ring subnetwork, in case of a double failure.
BRIEF SUMMARY OF THE INVENTION
Briefly, in accordance with a preferred embodiment, the present invention provides a technique for restoring service in an optical sub-subnetwork, such as a SONET ring, comprised of nodes connected by optical paths in a deliberate manner with predetermined protection paths for single failures. In accordance with the invention, the service optical paths in the subnetwork are connected through an optical cross-connect device (OXCD) at each end of the optical path. These OXCDs connect to each other, and other OXCDs associated with other subnetworks, in a matrix to form an optical cross-connect mesh network that provides a plurality of optical paths between each pair of OXCDs.
Also in accordance with the invention, a centralized monitoring system monitors traffic in each subnetwork to detect the failure of a path, service-bearing equipment within a node, a node itself, or any combination of such failures. Once the monitoring system detects a failure, the system determines the particular nature of the failure. In other words, the monitoring system determines whether the failure is a “single failure” (i.e. a failure for which a predetermined protection path is available) or a “double failure”, (i.e. a path failure combined with another failure for which the predetermined protection path cannot restore traffic). Upon detection of a double failure, the monitoring system determines an alternate optical path through the OXC mesh between the OXCDs on either end of the failed optical service path in the failed subnetwork. The alternate optical path can be found from unused capacity (spare or not-in-use protection paths for other subnetworks) in the optical mesh. Further, the alternate optical path can be established without regard for optical impairments if the OXC mesh is an “opaque” optical network, as taught in Afferton U.S. patent application Ser. No. 2,612,812, filed on Dec. 30, 1997 in the names of) Thomas Afferton et al. and assigned to AT&T (hereinafter incorporated by reference). The alternate path through the OXC mesh network restores the failed optical path in the failed subnetwork, reducing the failure to a “single failure”, thus allowing the subnetwork to protect itself and restore the traffic.
The technique of the present invention affords the advantage that the subnetwork itself can accomplish restoration in case of a single failure. However, in case of a double failure, where ring protection is not possible by the subnetwork itself, the monitoring system restores the failed optical path via an optical cross-connect mesh, thereby reducing the failure to a single failure. The subnetwork can then restore itself and minimize traffic downtime.
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U.S. Patent Application Serial No. (Afferton 2-6-1-2-8-1-2), filed on Dec. 30, 1997 in the names of Thomas Afferton et al. and assigned to AT&T.
Afferton Thomas S.
Strand John Lester
AT&T Corp.
Levy Robert B.
Ngo Ricky
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