Nortel Networks OPTera Metro 3500 - Manual (Page 63)

Operation, administration, and maintenance (OAM) features 2-25
Planning and Ordering Guide—Part 1 of 2 NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004
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 2-27 and Figure 2-13 on page 2-28 illustrate 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 2-29 shows
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 2-26 specifies the squelch map for Node D.