The Tele-Path Model 550B ISDN Portable Test Set is a compact, battery operated device used to conduct testing of ISDN circuits at the customer premise, the input and output of the ISDN repeater housing, or at the main distribution frame, The Model 550B is designed both for rugged durability and ease of use, with menu driven screens. The Model 550B’s operating modes allow it to function as an NT1 (Network Termination), as TE (Terminal Equipment), as both (NT1/TE), or as LT (Line Terination). It may be connected to either the DSL (Digital Subscriber Loop) U interface, or the SFT interface. It is compatible with AT&T AMI, and 2B1Q DSL interfaces, and EBS (P-Phone) and POTS Line Interfaces. The Model 550B allows placement of additional loss on the circuit, either at the "LINE" U Interface or the "S/T" interface for margin analysis testing. The Model 550B ISDN Portable Test Set allows you to select a loopback for either of the B channels, or for the full bandwidth (2B+D). The Model 550B also responds to network controlled loopbacks at the NT1. ISDN testing and measurements made by the Model 550B include: • FEBE (Far End Block Errors) and FEBE Errored Seconds in all ISDN "U" interface modes, • CRC Errors and CRC Errored Seconds with 2B1Q Interface selected, • AMI Frame Errors and Frame Errored Seconds with AT&T AMI interface selected, • S/T FRAME Errors and Errored Seconds, and Bipolar Violations (BPV’s) and Errored Seconds on the S/T Interface in NT1 and TE modes, • Bearer Service selections: - Circuit-Switched Voice/Data and - Packet Data, • Dual Call Testing, • Bit Error Testing, • D-Channel message analysis, • Cause Messages, • Layer 1, 2, 3 States, • Optional Service Testing: - EBS (P-Phone) and - POTS, • Pre-Qualification of Loop: - LT Emulation, generating Layer 1 2B1Q signal, and - Generation of 40KHz Test Tone