Digital telephone and switching system employing time division multiplex

Abstract

A digital telephone system is disclosed in which local groups of telephone lines are connected to respective concentrators which are themselves connected to switching centers by pulse code modulation highways. Data is transmitted on the highways as a succession of superframes where one superframe equals four multiframes, one multiframe equals 16 frames, one frame equals 30 channels and one channel equals eight digits. Channels 0 and 16 are used for signalling within the system and the remaining channels are used for data transmission between the telephone lines. The interpretation of channels 0 and 16 is determined by the particular frame and multiframe in which they occur. Each telephone line is identified by a combination of three channels representing a particular section of a concentrator, a particular block in the section, and a particular address in the block. Each concentrator has four sections and four highways. On each highway, the sections are scanned in turn, each for a period of one superframe. When all four sections have been scanned there is a one superframe period with no scan before the sections are scanned again on that highway. The superframe start times are mutually staggered by 16 frames on the four highways. Each highway transmits the relative superframe timings of the other three highways of the associated concentrator and this information is processed at the switching centers and used to control the superframe start time of the highway transmitting the information. The complement of the correct degree of stagger is added to the actual degree of stagger and the result compared with other totals similarly derived.