The key to the successful emergence of the information society [1] is an access system that permits new and diverse communication services to flourish. A fundamental component of such a network is a digital end-to-end communication capability. This paper presents an approach to the design of a digital access system for application in both residential and business environments. Switch, customer premises, and distribution design concepts are presented, and the results of field and laboratory tests of a multichannel digital distribution system are described.

The advent of digital telephone switches, interconnected by digital carrier systems, makes it possible to route and transmit telephone and data signals virtually without deterioration over many links and through many switching nodes. No bits of the digitized speech or data, however, should be lost en route; an economical method to ensure this is to operate the entire digital telephone system (of a country) synchronously. A highly stable master frequency reference is established from which all digital switching centers derive their clocking, either directly by national distribution of the reference frequency, or through the means of the (pulse-code modulation) PCM transmission systems which interconnect the switches. Operating considerations require that the synchronized clock systems of every switch can survive outages of equipment or synchronizing links without major impact on telephone service. The present paper is concerned with the design and performance of the synchronized clock system as an integral part of a large digital telephone switching office. Clocking and synchronization hardware and software algorithms have been developed to allow a switching office to be synchronized by external reference standards or over digital carrier links. The components of the synchronization system of an office are distributed in peripheral interface hardware, digitally controlled clock hardware, and a feedback algorithm implemented in the software of the switching computer. Steady-state and dynamic behavior are analyzed for a single switch and a timing network. Mathematical analysis is supported by simulation results. Special attention is given to quantization effects and maintenance considerations.