The New L Multiplex - System Description and Design Objectives

The coaxial cable and radio relay facilities of the Bell System employ the same form of frequency division multiplex. This paper describes the terminal multiplex arrangements in broad terms and discusses the design objectives for a radically new family of multiplex terminals having message channel capacities ranging from 60 to 1800. Dependent upon the A-type channel bank 1 for the first step of modulation from voice frequencies to group carrier spectrum and the reverse function of demodulation, the new multiplex accepts such groups and assembles them into a single broadband signal for transmission via the various high- 208 T H E BELL SYSTEM TECHNICAL J O U R N A L , MARCH 19(33 frequency media. It is an order of magnitude smaller than its predecessor and offers operating advantages not formerly available. The first commercial installations were placed in service in mid-1962. Circuit and equipment design features of the new multiplex are covered in detail in companion papers. 2 3 ' 4 2.1 General Realization of the potential of a wire or radio system to transmit many messages simultaneously requires multiplexing equipment at the terminals of the transmission medium. Multiplexing techniques may be divided into two categories, frequency-division and time-division. Broadband facilities capable of handling hundreds of channels have used the former exclusively, and this paper is concerned with modern versions of such arrangements. Essentially the transmitting circuits of a multiplex convert signals from a number of independent voice-frequency channels into a single broadband signal suitable for transmission over a high-frequency medium.