Synchronizing and multiplexing in a digital communications network

Abstract

A concept of a totally digital communications network is described in which all signals are converted into digital form and remain so as they are multiplexed, switched, and transmitted. To permit time-division techniques for such network processing, and thus realize significant economic advantages, the digital signals must be either generated in synchronism or brought into synchronism. While the switching portion of this network is not yet in service, some of the transmission network already exists and the evolution toward a digital transmission hierarchy is well underway. The hierarchy consists of 1) terminals which convert analog Signals into digital form suitable for transmission, 2) transmission facilities which are available at various capacities, and 3) multiplexers which can derive several lower capacity digital facilities from a single high-capacity system. Such a network, when complete, can also disseminate time and frequency for other uses with the same accuracy as required by the network. The network requires that the relative phase difference between any two signals must be bounded, which means exact matching of the long-term average frequency throughout the network. Signal-processing time and variations in propagation velocity in various media-cable, radio, waveguide-control the short-term accuracy. This network can thus provide time-and-frequency information proportional to the observation time. Available techniques that can achieve such accuracy for the network are discussed.