Multiplexing and concentration

Obtaining a cost-effective teleprocessing network is postulated on efficient utilization of the communication links and processing equipment. A variety of line-sharing devices and procedures are commonly used for this purpose. The functional, economic, and application aspects of frequency-division multiplexing (FDM), synchronous time-division multiplexing (STDM), asynchronous time-division multiplexing (ATDM), message switching concentration (MSC), and line (or circuit) switching techniques are discussed. The motivations for line sharing stem from economies of scale in the cost of bandwidth and from the traffic smoothing effect which such devices produce when serving a large terminal population characterized by unscheduled requests for service. The discussion of these techniques includes a detailed contrast between multiplexing and concentration, two terms which are often (and unfortunately) used synonymously. It is shown that FDM and STDM are examples of conventional multiplexing, whereas message switching and line switching usually illustrate concentration. ATDM is shown to be a hybrid line-sharing scheme embodying certain features of both concepts. Thus it is often referred to as statistical multiplexing. The first part of the paper is devoted to a functional explanation of the various concepts referenced above. The balance of the paper is concerned with applications and systems-design considerations involving multiplexing and concentration techniques. The application section focuses on important economic factors pertaining to the selection and use of the various methods. The role of line-sharing devices in contemporary common-carrier and end-user networks is also considered. The economic and technical aspects of these contrasting application environments are emphasized and illustrate the multiplicity of uses for line-sharing devices. The concluding portion of the paper includes a discussion of system-design considerations. This section illustrates precisely how the decision to use multiplexers or concentrators in a typical computer-communication network is implemented. Various techniques for geographically positioning multiplexers and concentrators to minimize total costs are presented. The use of one of these procedures is demonstrated using a typical de- sign problem as a case study.