Digital Data System: Digital Multiplexers

The Digital Data System employs all digital facilities for both shorthaul transmission within a digital serving area and long-haul transmission to interconnect digital serving areas. Except for the local loop which serves individual customers,1 the TI carrier system is used exclusively within a digital serving area, while several alternatives are available for the long-haul digital channel. To obtain efficient utilization of these facilities, a two-stage data-multiplexing hierarchy is employed. As illustrated in Fig. 1, each customer's loop is terminated in an office channel unit (ocu) that matches the loop's data rate, e.g., 56, 9.6, 4.8, and 2.4 kb/s. The output of a 56-kb/s ocu feeds directly into a port of the second-stage multiplexer. The other three, which are collectively termed subrate data rates, are gathered in groups of 5, 10, or 20 in the first-stage multiplexer. Each first-stage, or subrate, multiplexer, in turn, feeds a port of the second-stage multiplexer. Synchronization information is contained within the multiplexed output signal of a first-stage multiplexer. This permits it to operate independently of the second-stage multiplexer. The result is that any port of one second-stage multiplexer may be cross-connected to any spare port of another whether the signal source is a subrate multiplexer or an ocu. To further simplify the cross-connect process, all 893