NT1S—Connecting Devices to an ISDN Line

Devices such as video teleconference units, PCs, PBXs, key systems and multiplexers that are connected to ISDN lines need an NT1 interface to the ISDN line. As reviewed in Chapter 6, ISDN enables voice, data and video to share one telephone circuit. The network termination type 1 (NT1) corrects the voltage on the signals. It provides the electrical and physical terminations to the carrier's network. In addition, the NT1 provides a point from which line monitoring and maintenance functions can take place. On BRI ISDN services, NT1 devices change the ISDN circuit from two wires that come into the building from the central office to the four wires that are needed by ISDN equipment. (BRI ISDN circuits carry two channels of voice, video or data.)

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

A terminal adapter (TA) that performs the multiplexing and signaling function on ISDN services is also required. Multiplexing enables one line to be used simultaneously for two voice or data calls. ISDN telephones have built-in terminal adapters. Figure 7.5 illustrates a videoconference system with an internal terminal adapter. Non-ISDN telephones, fax machines, video systems and computers can use ISDN if they are connected to an external terminal adapter or a terminal adapter included in their NT1 device.