No. 4 ESS: The Switched Digital Network Plan

The selection of a digital format for the No. 4 ESS switch represents a significant step in the continuing evolution of the intertoll network in the United States. This selection was dictated by the economics of advancing technology and by the requirements for a large toll switch which could economically satisfy current and projected volumes of intertoll service. Economic considerations, however, must be bounded by the requirements of a viable technical plan for the installation and operation of a digital toll switch in an evolving intertoll network which is now analog and which will remain substantially analog for many years in the future. This paper outlines the basic characteristics of the network plan for introducing digital switching into the intertoll plant. The plan proposes direct interconnection of digital transmission facilities, digital switches, and analog-to-digital converters to form a Switched Digital Network ( S D N ) . 1297 Facilities in the SDN will be the No. 4 ESS switch; a set of digital transmission facilities which carry digital bit streams on paired wire, coaxial, radio, and millimeter waveguide at rates from 1.544 megabits per second to 274 megabits per second; a set of analog-to-digital converters which include D-type channel banks using the n = 255 D2 coding law and the Voiceband Interface Frame (VIF); and a digital interface, the Digroup Terminal (DT), which permits direct digital interconnection between the No. 4 ESS switch and digital transmission facilities.