A t.d.m.-exchange system is described in which the subscribers' line equipments have permanently allocated channels. Ifnchannel multiplexes are used, the subscribers' lines are effectively divided intonseparate space-division exchanges with common registers and junctions. Existing trunking theory still applies, but with the unusual aspects that the division bynresults in an aggregation ofnsmall, or even very small, exchanges, and that the relatively high cost of the switching elements justifies greater care in trunking design to minimise the number of switching elements.