Shared backup path protection (SBPP)
Fig. 1(a) Shared backup path protection (SBPP)
Shared backup path protection is also called SBPP for short. This is a type of path-based protection technique. Protection is carried out at the two end nodes of a working path. Specifically, a single end-to-end protection path and pre-planned protection capacity is reserved for each working path. Working path and protection path must be failure-disjoint. Upon a network failure such as link cut or intermediate node failure, switching-over actions are taken at the two end nodes of the working path to transfer the affected working traffic onto the protection path. SBPP is a kind of failure independent survivability technique. To recover a failure, SBPP does not need to know where the failure is. Once the two end nodes of the working path detect a network failure, they just perform the switching-over actions. Fig. 1(a) shows an example for the shared backup path protection (SBPP) technique, where a pair of link-disjoint working and protection paths is established and upon a failure the protection path recovers the failure for the working path.
Fig. 1(b) Spare capacity sharing of SBPP
Shared backup path protection is very efficient in spare capacity sharing. SBPP allows protection paths to share spare capacity on their common spans as long as their corresponding working paths do not share any common failure, i.e., common links or nodes. Fig. 1(b) shows an example of spare capacity sharing under the SBPP scheme, in which two protection paths are sharing protection capacity on their overlaying links. Rather than 3+4=7 protection capacity units, only a max{3, 4}=4 units of protection capacity units are required on these common fiber links due to efficient spare capacity sharing. The condition of spare capacity sharing is that each time there is only a single network failure. As shown in Fig. 1(b), SBPP will not be able to handle the situation of two-link failure that interrupts both working paths simultaneously. If that occurs, only one of the working paths is recovered, leaving all the traffic on the other working path totally lost.
Compared to the span restorable networks, an SBPP network often achieves much better spare capacity sharing efficiency due to its end-to-end protection and a wider range of spare capacity sharing. Typically, for a mesh network, a span restorable network can have a spare capacity efficient around 50-70%, while an SBPP network can achieve spare capacity efficiency around 30-40%. As a tradeoff, SBPP generally has a slower restoration speed than that of the span-restorable technique as it takes a longer time for failure detection and signaling in failure recovery.
