A Digital Concentrator for the SLC-96 System

As the state of the art in electronics has advanced, so have the inroads of electronics into the loop plant as a supplement to or replacement for cable.1 The application of pair-gain systems to loops has been of particular importance in recent years. The SLCTM-96 subscriber loop carrier systems is the latest in a line of digital loop carrier systems to provide improved features and reduced cost.2 The SLC-96 system serves up to 96 single-party subscribers between a Central Office Terminal (COT) and Remote Terminal (RT) using standard pulse-code modulation (PCM) coding over facilities such as T l . System features include a variety of channel units, channel and drop testing provisions, and spare digital line switching. System versatility is further enhanced by three modes of operation (each mode uses one additional Tl line for protection): 121 Carrier only; 96 full-time channels on four working Tl lines. Mode II: Carrier concentrator; 96 channels on two working Tl lines. Mode III: Special services only; 48 special service channels on two working Tl lines. This article discusses the Time Assignment Unit (TAU) which is the concentrator employed in Mode II operation. In a fully equipped Mode II system there are actually two identical, but independent, concentrators, each concentrating 48 lines onto 24 time-slots of a Tl line, and thereby eliminating two main Tl lines (see Fig. 1). In many cases the saved Tl lines provide more advantage than is apparent. The benefits arise in applications limited by apparatus case size for holding repeaters, by small cables where the purpose of pair-gain systems is to avoid adding new cable, and, similarly, in situations that use several systems in parallel along the same route where the number of saved pairs can be very significant.