Overview

This chapter presents fundamentals of data communications in preparation for a discussion of data services in forthcoming chapters. It relies on the introductory material presented earlier for terms of reference and background. In particular, it builds on the definitions and descriptions of digital electrical signals, binary bits, error detection and correction, data terminal equipment (DTE), digital carrier, time division multiplexing (TDM), and digital circuit switching presented in Chapters 1 through 6.

A significant difference between voice and data service is the extent to which human intervention is required to ensure end-to-end communications integrity, including diagnosis and recovery under failed or inadequate service conditions. For example, if an American places a telephone call to Japan that is answered by someone who cannot speak English, human intelligence is relied upon to seek an interpreter or to take alternative action. Similarly, if a call cannot be completed due to a network failure, a human determines the problem and takes corrective steps.

By contrast, data services are provided with minimal human intervention. As a consequence, more elaborate mechanisms are required to ensure that transmitting and receiving DTEs "speak the same language," and that service restoration actions are promptly taken under network failure conditions. This generally requires higher levels of hardware and software compatibility among DTEs and intervening data network elements than is required in voice networks.

For private data networks, it might be feasible to specify hardware and software from a single source, achieving compatibility through proprietary design. For public networks relying on...