The TDU 120e (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 120e 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. The TDU 120e also provides complete monitoring and control of the T1 network with embedded simple network management protocol provided through both a SLIP and a 10BaseT Ethernet interface. 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. major feature of the TDU 120e 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 a Nx56/64 (V.35), Dual Nx56/64, Router, NxIQ, DSX-1, OCU DP, 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.