A Model for the Organic Sychronization of Communications Systems

The timing of the switching actions at each switching center of a pulse code modulated (PCM) communications system is governed by a device called the "local clock." It may consist of a cyclic counter driven by an oscillator. Each cycle of the counter is then one clock cycle. In a geographically widespread PCM system, the local clocks may be either autonomous or synchronized. This choice should be made with the best possible knowledge of the available technology, as well as consideration of its functional and economic consequences. The choice is clearly a rather basic one, and it may have long term effects upon the evolution of the system. The time-multiplexed PCM signals arriving at any locality may have arbitrary, and usually scattered, points of origin. Some of them require 1705 1716 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , D E C E M B E R 196f> decoding into a common analog form. In particular, they may be voice signals. A homogeneous, time-multiplexed set of such signals is easily decoded by a common digital-to-analog converter, provided that the transmitted samples have been generated synchronously. A nonsynchronous alternative is to insert extra digits into the signals in order to permit multiplex transmission. Additional equipment is needed to remove these digits and smooth the timing of the demultiplexed samples before or after decoding them. This paper is only one of a number of studies of system synchronization, and it does not provide a complete solution to the problems touched upon.