III. Diversity of AssignmentThere is another and more common technique for using redundancy than in the method described above in which each station is assumed to have perfect switching ability. This alternative approach is called "diversity of assignment." In diversity of assignment, switching is not required. Instead, a number of independent paths are selected between each pair of stations in a network which requires reliable communications. But, there are marked differences in performance between distributed switching and redundancy of assignment as revealed by the following Monte Carlo simulation.
SimulationIn the matrix of N separate stations, each ith station is connected to every jth station by three shortest but totally separate independent paths (i=l, 2, 3,...,N; j=l,2,3,... ,N; i j). A raid is laid against the network. Each of the pre-assigned separate paths from the ith station to the jth station is examined. If one or more of the pre-assigned paths survive, communication is said to exist between the ith and the jth station. The criterion of survivability used is the mean number of stations connected to each station, averaged over all stations.
Figure 8 shows, unlike the distributed perfect switching case, that there is a marked loss in communications capability with even slightly unreliable nodes or links. The difference can be visualized by remembering that fully flexible switching permits the communicator the privilege of ex post facto decision of paths. Figure 8 emphasizes a key difference between some present day networks and the fully flexible distributed network we are discussing.
Comparison with Present SystemsPresent conventional switching systems try only a small subset of the potential paths that can be drawn on a gridded network. The greater the percentage of potential paths tested, the closer one approaches the performance of perfect switching. Thus, perfect switching provides an upper bound of expected system performance for a gridded network; the diversity of assignment case, a lower bound. Between these two limits lie systems composed of a mixture of switched routes and diversity of assignment.
Diversity of assignment is useful for short paths, eliminating the need for switching, but requires survivability and reliability for each tandem element in long haul circuits passing through many nodes. As every component in at least one out of a small number of possible paths must be simultaneously operative, high reliability margins and full standby equipment are usual.