No. 4 ESS: Digital Interface

The Digital Interface (DIF) is a newly introduced No. 4 ESS peripheral frame whose functions combine the operations performed by the Digroup Terminal (DT) and the Signal Processor 2 (SP2), with the exclusion of the supplementary matrix frame which is an SP2 optional adjunct.1,2 The main functions of a DIF are to terminate DS-1 level signals and to multiplex them to a form suitable for the No. 4 ESS digital switch; to perform the necessary signaling interchange between the transmission and switching facilities; and to provide adequate fault detection and reconfiguration capability. These functions are identical to those performed by the DT/SP2 complex but the DIF achieves them in a more compact and modern fashion. The DIF occupies less than one-half the 1131 space and uses two-thirds less power than the DT. This reduction was achieved in a number of ways: (i) by combining the equivalent of four DTS and an SP into a single structure thereby eliminating a number of duplicated functions and a complex interface, (ii) by using custom and catalog large-scale integrated circuits instead of the 1A Technology previously used, and (iii) by utilizing both metal oxide semiconductors and bit-sliced microcontrollers to replace the hardwired logic of the DT and SP controllers. As will be discussed in more detail later, these changes resulted in a frame with an entirely different internal philosophy which was more flexible and more consistent with the architecture of the other peripheral frames in the No.