Channel zero switching arrangements for digital telecommunication exchanges

摘要

It is an emerging international telecommunications requirement that all 32 channels of the p.c.m. multiplex must be switchable. The switching of channel zero relates to the "spare bits" not defined for synchronization purposes and these bits may be used as a data-bearer for network administration or control purposes. Digital telecommunication switching network, therefore, must be capable of concentrating channel assemblies of these spare bits into one transmit multiplex which may be connected to a spare bit data processor remote from the switching network or co-located with it. The passing of channel zero into the switch block, however, is dangerous because if this data is passed across the switch block to another digital line termination unit DLT in a time slot other than channel zero, then that receiving DLT will be called upon to transmit data containing two valid sets of sync and non-sync patterns which could confuse the synchronization circuits associated with the digital transmission system connected to that DLT. Such an arrangement is prevented using equipment in the receive and transmit DLT's. In the receive DLT after the incoming synchronization channel has fulfilled its synchronization purpose it is deliberately modified such that when switched to a transmit multiplex it will not be erroneously recognized as a sync channel. The modification does not of course cause loss of information contained in the spare bits or alarm bit. At the transmit DLT a decision is made as to whether the international bit in the sync pattern and the international bit and spare bits in the non-sync pattern should be transmitted to line as data ones or the patterns received.