Error propagation through plesiochronous demultiplexers

Activity time, the total disturbance time experienced by a channel, is used to illustrate the nonlinear way errors can propagate through a plesiochronous hierarchy. Frame structures at each level, originally specified to limit the propagation of error bursts by ensuring that loss and recovery times of the higher-level demultiplexers were generally shorter than the loss time of following level, are discussed. It is shown that the occurrence of uncontrolled slips effectively defeats the original design concepts and appears to be the critical mechanism by which errors propagate through the demultiplexer stacks. This is reflected in the sensitivity to both the input burst length and the propagation of a loss of alignment condition. A comparison of model results with hardware measurements shows good agreement down to 2 Mb/s