Loop Plant Electronics: Digital Loop Carrier Systems

The loop electronics overview to this issue1 examines the need for pair gain systems in the loop plant and traces the history of these systems. This article examines briefly the reasons for developing large pair-gain digital carrier systems developed for the Bell System, the SLMm Subscriber Loop Multiplexer System and the SLC'"-40 Subscriber Loop Carrier System; it then finally discusses, in detail, design breakthroughs which made the SLC-40 system possible. In the late 1960s, P. A. Gresh of Bell Laboratories and C. D. Howe of AT&T 2 studied the long subscriber route characteristics of the Bell System and concluded that the optimal size for a single carrier system would be between 75 and 100 lines. This size system could serve the 1129 largest amount of long route growth and save the most capital compared to a cable design. This analysis influenced the design of the SLM system, the first Bell Laboratories design of a large pair-gain digital system. A digital approach was chosen in 1967 when the SLM system design began, since a digital system offered the potential for high transmission quality and low cost with newly available integrated circuits. Further, digital systems offered the potential of lower cost by using time division concentration at the remote terminal, and could use the T1 trunk carrier hardware for the digital line. The SLM system, first introduced in the Bell System in 1971, is a combination carrier and concentrator system that has an 80-line capacity. The SLM system uses delta modulation encoding.