DESC: The ADTRAN DSU III DBU, a high-performance rackmount data channel service unit (DSU/CSU), provides an interface between the digital data service (DDS) and the customers data terminal equipment (DTE) while providing automatic switched dial backup (DBU) of the dedicated circuit. This is the 1B+D ISDN DBU. When the DSU III DBU is configured for dedicated service with switched DBU, the user selects options to qualify a dedicated line failure. Upon line failure recognition, the DSU initiates connection to the remote unit over the switched network. Upon restoral of the DDS circuit, the unit automatically or manually reverts to the restored DDS circuit. ??Another feature is DTE-to-service rate matching, for dedicated and switched DBU modes. This feature allows slower rate DTE equipment to communicate over a higher speed digital line. In the DBU mode, dialing is accomplished automatically (with stored numbers) or manually. The unit responds to standard loopback commands from the telephone ...