D2 Channel Bank: Per-Channel Equipment

In PCM terminals, much of the signal processing takes place in common equipment, where the cost is shared by many or all channels in the system. Although this equipment is more costly than per channel equipment because of higher operating speeds and increased complexity, a net reduction in the per-channel cost is achieved because of timesharing. However, some functions cannot be performed economically in this way, and per-channel equipment is needed for this purpose. For the case of the D2 Channel Bank, this equipment includes (i) the lowpass filters and gates, and (ii) the interface equipment between the switching system trunk circuit (or other assigned circuit) and the D2 Channel Bank. The latter circuits are concentrated in 1659 1660 T H E BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1972 the channel units which contain the signaling scanning and converting circuits in addition to the voice frequency interface circuits. Looking at a cross section of the telephone plant will reveal a wide variety of switching and transmission equipment in current use. The continued effort to improve the quality of service, the rapid growth of the telephone plant, and the relatively long life of a switching machine have resulted in many switching machines, differing in functions and vintage, existing side by side. In addition, communication between these different machines has also evolved into many schemes. Any new equipment being designed to fit into the existing plant is faced with complex compatibility problems.