The TDU 120 (T1 Distribution Unit) is an intelligent, full-featured, T1 DSU/CSU in a 1U high 19-inch rackmount enclosure. Designed to interface customers data terminal equipment (DTE) to T1 or Fractional T1 services, the TDU 120 is fully compatible with D4 or ESF framing formats with AMI or B8ZS coding. When in ESF framing mode, the unit satisfies ANSI T1.403 and AT&T 54016 modes for utilization of the facility data link (FDL). It uses option modules identical to those used with other ADTRAN TSU units. A base V.35 and DSX-1 (PBX) interface is provided on the rear of the unit. The V.35 interface can be configured to operate synchronously at Nx56 or Nx64 data rates where N=1 to 24. The channels used can be selected at random. The DSX-1 interface is a short haul T1 interface intended for connection to a PBX, cellular/PCS switch or similar device. The channels used by the DSX-1 port can also be selected at random.A major feature of the TDU 120 is an option slot on the rear of the unit that accepts a single expansion module. A number of application-specific modules are available to provide flexibility and room for growth. The optional plug-in modules currently available include Nx56/64 (V.35), Dual Nx56/64, DSX-1, OCU DP, DBU, FXS, FXO, 4-wire E&M, and Drop and Insert. Additionally, the plug-in modules are designed to accept a plug-on version of many of these modules. The TDU 120 is configured using a terminal or computer through a craft port located on the front of the unit or a rear-mounted daisy chain-able control port. The terminal displays an easy-to-follow menu structure allowing the user to configure, monitor, and test the TDU 120. An optional software package called T-Watch PRO is available to interface to the TDU 120. The T-Watch PRO software runs on a PC under Windows, and allows full access to monitor and test several local and remote TDU devices. Pop-up menus provide complete configuration and diagnostic control with a few clicks of the mouse. Management of remote units may be in-band or out-of-band. Optional SNMP management is available using the Agent Module.