Trends in digital communication by wire

The introduction and rapid spread of pulse code modulation must be examined in the framework of overall capability, efficiency, and cost. However, apart from television, the only field of application that can presently justify a massive investment in a new transmission system is telephony, because it represents the dominant communication market. Other communication services-such as data and facsimile transmission, telegraphy, broadcast sound, and closed-circuit television-must in general travel as passengers on telephony networks if they are to be economically feasible. Hence, the most fruitful approach to an examination of digital communication is to begin by studying PCM telephony and then to see how other services can be coordinated with it.