III. Diversity of Assignment
There 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.
Simulation
In 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 Systems
Present 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.
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