Digital Data System: Local Distribution System

An i m p o r t a n t design consideration in planning t h e Digital D a t a System was the method of providing local distribution, the interconnection of the customer's terminal equipment to the serving DDS office. Since this interconnection is supplied to each customer, factors such as economy, ease of installation, and reliability were considered. To minimize the investment required for installation of new telephone plant, a decision was made to use the existing type of subscriber loop plant for DDS local distribution. Thus, the copper pairs from the subscriber's location to the serving central office may be used for DDS as well as for telephone and other services. Since not all local subscriber offices are DDS offices, interoffice trunk cable will also be used as part of the DDS loop. The local transmission system consists of station equipment, either a data service unit (DSU) or a channel service unit (csu), to couple the data signals to the cable pairs; two cable pairs, one for each direction of transmission; and central office equipment, an office channel unit (ocu) to convert the digital signals present on the local loop into regenerated bits formed into 8-bit bytes for interconnection to the digital multiplexers. 1 In addition to transmitting the basic digital signals, means must be provided for transmitting control mode information over the local 919