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The Digital Future Of The Telephone Network
A Study of Evolving Technology
By Lee Goeller
Originally published by Probe Research Inc. 1979. Reprinted by permission

Overview

Over the past hundred years, the telephone industry has grown from modest beginnings to one of the most complex technological achievements mankind has ever produced. However, because of its size and complexity, transmission, signaling and switching have developed more or less independently of each other, and each has gone out of its way to make the others think it is still the way it used to be when earlier generations of hardware froze interface designs. The coming of digital electronics, developed largely by the computer industry, has today made almost all of the traditional approaches to communications obsolete.

Analog transmission principles, developed to a very high degree, require elaborate means to adjust levels of signals at different points in switching and transmission systems. Complex administration and maintenance procedures have developed to implement the analog transmission plan and, as a result, many improvements and simplifications which digital techniques could make possible will be difficult to realize.

Although analog transmission advances came principally on long-distance trunks between central offices, switching developed primarily to serve local customers. Because of the simplifications required on a per-customer-line basis between the telephone set and the central office, local switching and transmission systems evolved as “2-wire" facilities; that is, they used one transmission path for signals going in both directions. This has led to systems using copper wire and certain kinds of switching equipment on a scale so vast that replacement with any other approach, no matter how desirable, will be both difficult and costly.

The evolution of switching and signaling shows how these systems developed, and how they have dictated the nature of future system development. However, the continuing growth of long-distance traffic and the driving impact of computer electronics are forcing a change in spite of the huge inertia that is obviously present.

An idealized system, taking advantage of modern techniques from within and without the telephone industry, is suggested in this study to show how both voice and non-voice traffic might be served in the future. The network proposed by AT&T is then analyzed to show what it can and cannot do. Finally, the transmission requirements imposed on digital central office switches of non-Bell manufacture are discussed to show how they may be a significant factor in holding overall digital development to that which can be accomplished through Bell non-digital local central office switches.

The continued growth of data and other non-voice communications may well increase the fragmentation of the overall telephone network. A possible solution, taking advantage of the concentration of non-voice communication needs at present in large business customer systems, is outlined in this study: PBXs, at least in the larger sizes, should be homed directly, via digital trunks, on No. 4 ESS, the Bell System's new digital toll switching system. This can facilitate the implementation of digital communications where they are needed, and permit the non-digital central offices presently used throughout the Bell companies to serve out their useful lives handling the predominantly analog traffic of residential customers.

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