NT1s and TAs for ISDN

ISDN lines need NT1s and terminal adapters (TAs) to make them compatible with the public network and customer equipment. The network termination type 1 (NT1) provides the electrical and physical connections to the carrier's network. On BRI ISDN (Basic Rate Interface Integrated Digital Services Network) services, NT1 devices change the ISDN circuit from 2 wires that come into the building from the CO to the 4 wires needed by ISDN equipment. In addition, the NT1 provides a point for line monitoring and maintenance functions.

In the US, the FCC requires that the customer be responsible for supplying NT1. In the rest of the world, telephone carriers supply the NT1.

Terminal adapters (TAs) perform the multiplexing and signaling function on ISDN services. Multiplexing enables one line to be used simultaneously for multiple voice or data calls. NT1s and TAs are often built into videoconferencing equipment and routers.