Alternative routing methods for PNNI networks with partially disjoint paths

We consider the problem of offering a reliable service in a PNNI network, such that service is almost uninterrupted even if one of the links or nodes in its path goes down. This is done through the provision of alternative paths for each VC that requires this reliable service at the time of call setup. The same approach is also applicable to VP protection switching for establishing alternative soft VPs. We also assume that topological constraints on the network may make it impossible to setup completely disjoint alternative paths, so the alternative paths may be required to be partially disjoint from the primary path. If that is so, the network elements which are common to the primary and alternative paths must be extremely reliable. We formulate this problem as an integer programming problem and show that it can be cast as the well known transportation problem. We compare the performance of the solution of the transportation problem to that of a heuristic based on sequential determination of the alternative paths. The heuristic algorithm delivers good sub-optimal results with lower computational complexity than the optimal transportation algorithm, so it can be used in real-time scenario. For off-line calculations of partially disjoint VPs, the computational complexity of the transportation algorithm is suitable for PNNI networks.