Fault-tolerant switches for photonic networks

In order to obtain an N X N Multistage Interconnection Network capable of passing all the N supplementary stages must be added to the standard ones. In this paper we prove that the completeness property of such networks is preserved even if some switching element suffers from a stuck-at fault. The critical stage is the central one which must be set in a fixed configuration, depending on the desired permutation. This problem is easily overcome if one additional stage is included. Moreover, the static full access capability is preserved in a complete network because of the redundancy of source to destination paths.