The ADTRAN DSU 56/64 and DSU 5600 provide cost-effective solutions for termination of 56 or 64 kbps DDS services. The units are a very economical solution for single port point-to-point, multi-point or frame relay applications. Extensive use of VLSI technology provides a highly reliable feature-rich DSU. The enclosure for both the DSU 56/64 and DSU 5600 is a small, attractive, slim-line package that is wall mountable, rack mountable or can sit on a desktop. The DSU 56/64 provides both a physical and electrical V.35 DTE interface, which is mounted on the rear panel. The DTE interface operates at the same rate as the network, either 56 or 64 kbps. Also located on the rear panel is a set of option DIP switches along with an RJ-48S network interface. The option switches allow the user to select the Network/DTE data rate, control lead options, remote digital loopback, and scrambler options. Both units respond to standard loopback commands from the service provider. In addition, the units have built-in test capabilities, which include V.54 remote loopback, DTE, DTE and loop, DTE with test pattern, and loop only. Included with the remote loopback is the capability to select a 2047 BERT test pattern, with results displayed on the front panel. The front panel of the DSU 56/64 and DSU 5600 provide a set of eleven LED indicators. Five of the indicators show the status of the V.35 DTE or EIA-232 interface while the remaining LEDs indicate the various test modes and loop status. Two front panel keys allow the user to select one of the several test modes the unit supports.