The ADTRAN Type 400 NT1 provides network termination for the 2B1Q basic rate interface between the customer terminal equipment and the ISDN network. Basic Rate ISDN consists of two 64 kbps B channels and a single 16 kbps D channel. The U-interface is used for connection to the ISDN network, while the S/T-interface is used for the subscriber terminal equipment. The U- and S/T-interfaces conform to ANSI and CCITT standards and perform all layer 1 functions. The NT1 contains lightning protection circuitry to protect from lightning and line transients on the network interface. Three LED status indicators on the front panel aid in fault isolation.The NT1 operates as a standalone unit or in a multiple Type 400 mounting such as the ADTRAN Type 400 Shelf, part number 1150028L1 or 1150024L1. The NT1 monitors the S/T-interface for an overcurrent fault condition. In the event of such a condition, the NT1 automatically limits the amount of current being delivered until the fault condition is removed. The S/T-interface may have as many as eight passive bus devices connected to it as either multiple terminals within 50 meters of each other over the full 1, 500 meter range of the interface, or multiple terminals of arbitrary spacing within 200 meters. Termination impedance options include the US standard of 100 ohm or the European unterminated configuration.