Information Technology Reference
C. Recovery from Failure
If a link on the path between two nodes (where each node belongs to one of the two
classes) fails, destination node will not receive data on the primary path. However, it can
recover data by using results of the encoding operation.
3.10. Level of Service Algorithm (LSA) 
In priority-based routing scheme, traffic is divided into high-priority (HP) traffic request
and low-priority (LP) traffic request. The HP traffic accepts the shared backup path protection
and its backup path can share the resource with the risk-disjoint HP traffic path. Moreover,
the HP traffic's backup path can use the working resources of LP traffic. The LP traffic will
adopt the restoration scheme, and there is no backup resources allocated to this traffic. When
failure occurs, the LP traffic immediately starts searching for a recovery path.
The procedure steps of the heuristic algorithm, called Level of Service Algorithm (LSA),
are presented as follows: when an HP traffic request arrives, a primary path is computed
according to the standard shortest path algorithm. Then, a risk-disjoint backup path is
calculated through the standard shortest path algorithm, the routes are recorded and the
wavelength is allocated. Moreover, the network state is updated. If an LP traffic request
arrives, a primary path is computed according to the standard shortest path algorithm. Then,
the routes are recorded and the relevant wavelength is allocated. Moreover, the network state
is updated. If the algorithm cannot find any primary or backup paths, the connection request
will be blocked.
4. M ULTICAST T RAFFIC P ROTECTION S CHEMES
Multicast-based services are popular applications in WDM networks . Since a single
fiber cut may cause much more significant breakages to a light tree than an individual
lightpath, therefore, protection of a multicast session is very important.
Multicasting involves the propagation of information from one source to multiple
destinations at the same time . A point-to-multipoint connection from a source to multiple
destination nodes is defined as a light-tree to maintain a multicast session on WDM networks
and it is the generalized expansion of a lightpath . An efficient approach for multicasting
is to establish a multicast tree, which contains the source node as the root and all the
destination nodes as the leaves or intermediate nodes . Network nodes must be
multicasting-capable optical cross-connect switches (MC-OXCs), where these switches split
the received optical signal into multiple paths in order to support multicasting in a WDM
network. Note that each path of the optical signal is switched to a desired output port . In
the following, major techniques to protect multicast traffic are reviewed (see Figure12).