No. 4 ESS: Peripheral System

Electronic switching systems, in modern terminology, are composed of two parts, the central processing unit and the peripheral system. In No. 4 ESS, the central processing function is performed by a new highspeed integrated circuit unit, the 1A Processor, described in a special issue of the B.S.T.J. 1 The essential interconnection, signaling and interface functions of the No. 4 ESS peripheral system are the subject of this paper. The switching network developed for No. 4 ESS features a new PCM time-division architecture that effectively integrates modern switching and transmission technologies. Interoffice trunks are handled via a multiplexed format, with no need to derive individual channels, and with no need for much of the per-channel equipment required in previous toll switching machines. Significant economies in administration and maintenance also result from this configuration. The multistage timespace-time topology chosen enables a large, high-traffic-occupancy 1029 switching network to be constructed, as required in large metropolitan area switching centers. The planning, design, and operation of this new switching network is detailed in Section II. Several special interoffice signaling equipment units have been developed for No. 4 ESS. These recognize and process information that identifies the address or destination of a call or any change of status of the call. A new Common Channel Interoffice Signaling ( C C I S ) unit is being introduced, permitting direct intercommunication via a data link between modern stored-program controlled switching offices.