No. 4 ESS: Transmission/Switching Interfaces and Toll Terminal Equipment

The nature of No. 4 ESS, together with evolving equipment concepts, has resulted in a streamlined switching/transmission interface with 1057 Fig. 1--Transmission/switching interface system. advantages in cost, space, and maintenance effort. The terminal equipment and arrangement that compose this interface are the subjects of this paper. The interface system is shown in Fig. 1. The streamlining of the terminal arrangement is clearest in the case of digital transmission facilities. The No. 4 ESS switching network is, of course, a digital time-division network. Serial links into and out of the network use the DS-120 format--an 8.192 Mb/s PCM stream that accommodates 120 voice-frequency channels. Terminals for digital transmission facilities terminate the facility and provide the multiplexing/demultiplexing to interface the transmission format with the DS-120 format. In particular, there is no need to derive each analog channel. Avoiding the conversion step eliminates unnecessary signal degradation as well as the need for a host of per-channel equipment: trunk circuits, distributing frame appearances, etc. The Digroup Terminal (DT) is the interface for digital facilities in No. 4 ESS. The DT consists of up to eight Digroup Terminal Units (DTUs). Each DTU terminates DS-1 level signals (1.544 Mb/s, 24 VF channels), providing multiplexing of five DS-1 signals for a DS-120 port of the time-division network. The DT also extracts/inserts signaling information from/to the DS-1 streams.