Nortel Networks 3500, NTRN10AN manual Blsr nodal / multi-span failure scenario involves squelching

Operation, administration, and maintenance (OAM) features 2-25

BLSR nodal / multi-span failure scenario (involves squelching)

In the instance in which one or more nodes becomes disconnected because of multiple line failures and/or nodal failures, a BLSR network enters into a bidirectional protected state of operation. The protection operation is much the same as for the loss of a span, except that add-drop traffic at the affected node is lost.

Figure 2-12 on page 2-27and Figure 2-13 on page 2-28illustrate failure in Node C for OC-48 and OC-192 BLSR respectively. Traffic normally intended to pass through Node C is looped back to the appropriate protection timeslots at Nodes B and D. The traffic is then routed to the intended destinations as described for link failures. The nodes performing the protection switch are termed switch nodes.

During protection switching, traffic that normally exits the ring at the lost node has the potential to be misconnected to another path termination. To ensure that this does not happen, the nodes adjacent to the failed node (in the example, Nodes B and D) squelch the appropriate working and protection paths by inserting into them a path AIS (alarm indication signal) before completing the protection switch. These paths continue to be given path AIS until the ring returns to normal operation.

The squelching is performed by the switch nodes on the basis of a squelch map that is automatically derived from the node map and STS-1 cross-connection map when these maps are provisioned. The squelch map has an entry for each STS-1 cross-connection provisioned at the ADM node. Each entry contains the APS IDs of the nodes providing the service access point (SAP) and end node for that STS-1.

If a node loses communication with the SAP or end node for a particular STS-1 (for example, because of a failure of the SAP or end node or because of a ring segmentation isolating the SAP or end node), it can then squelch the path.

Pass-through connections at the failed node are not squelched, as these can be successfully rerouted over the protection path. Figure 2-14 on page 2-29shows an example of a four-node ring with four STS-1 paths (a, b, c, and d). The arrows indicate the direction of each path, from the originating node (SAP) to the end node.

Table 2-3 on page 2-26specifies the squelch map for Node D.

Planning and Ordering Guide—Part 1 of 2 NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004