Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of communicating the signals over long distances with very low loss.
To ensure high reliability and availability in communications networks, including optical communications networks, protection switching is often used. When implemented, protection switching typically provides a primary or “working” path for a network and a redundant or “protection” path for the network. Accordingly, each path may be monitored, and if a failure is detected on the working path, network traffic may be switched to the protection path. An example of protection switching may be Ethernet Linear Protection Switching (ELPS) as defined by the ITU G.8031 standard.
With protection switching in optical communication networks, an optical signal may be transmitted via two or more optical paths between the same source and destination node. A selector at the destination may include a photodetector for each path to monitor signals received from the two or more paths. Based on such received signals, the selector may select one of the signals to be forwarded to a transponder or receiver at the destination node. For example, the selector may determine, based on the photodetector monitoring, whether one of the paths has experienced a loss of signal or “loss of light.” If a particular path experiences a loss of light, then the selector may select another path to forward to the transponder or receiver. Such selection may be referred to as a “protection switch.”
The selector may operate in accordance with a protection switching protocol (e.g., ITU G.8031 or other standard). Each protection switching protocol may include a hierarchy for handling user-initiated and auto-failure initiated protection switching requests. Such hierarchy may be implemented via hardware, software, or a combination thereof.
In certain instances, a bi-directional working path of a protection switching group may be subject to an intermittent failure. Such failure may be of the nature that the working path may operate normally for periods of time, but intermittently fail due to a particular problem associated with a communication network. For example, among the reasons an intermittent failure may occur on a working path include, without limitation, a bent transmission medium (e.g., cable or fiber) within the working path; a cut transmission medium within the working path; incorrect or lose transmission media connections to network elements of the working path; a transmission medium short within the working path; electromagnetic interference in the working path; faulty connectors interfacing transmission media to network elements in the working path; loss of heartbeat messages (e.g., Continuity Check Messages) by network elements of the working path due to congestion, collision and/or other cause; and/or a degraded signal along the transmission path.
In traditional communication networks, such an intermittent bi-directional failure on a working channel may result in periodic traffic loss upon the occurrence of each intermittent failure, as traditional communication networks are typically configured such that the faulty working path may, in certain situations, redesignate a working path as the active path after a protection switch despite the occurrence of the intermittent failure on the working path. Thus, each time the working path is redesignated as the active path, the working path may again fail, leading to a protection switch and more traffic loss. This problem is described in greater detail below.