Digital Transmission Systems

Abstract

British Telecom (BT) is engaged in a program of installing digital transmission systems with the intention of providing for the growth in circuit demand and replacing the existing analog network. These digital systems conform to the standard CCITT hierarchy with bit rates from 2. 048 to 139. 264 Mbit/s. At the primary multiplexing level 30 analog telephony channels are converted to digital form and combined together to produce a bit stream at 2. 048 Mbit/s. Optical fiber systems will, in the future, also be used at all the bit rates. To make the necessary measurements during the commissioning of new systems and for their subsequent maintenance a variety of transmission test equipment is required. This paper describes the major items of such equipment that are used within BT indicating how they are used and the measurements made.