NT1s—Connecting Devices to an ISDN Line

Data circuit-terminating equipment (DCE) devices are needed for all analog and digital data communicating devices. Modems are the DCE devices for analog, POTS lines. However, digital transmission techniques such as ISDN and T-1 services also require DCE gear. As reviewed in Chapter 6, ISDN enables voice, data and video to share one telephone circuit. On ISDN, the signaling is out-of-band. Dialing, ringing, busy signals, reorder tones, and dialed and calling telephone numbers are all carried in an out-of band channel.

Devices such as video teleconference units, PCs, PBXs, key systems and multiplexers that are connected to ISDN lines need an NT1 as the DCE interface to the ISDN line. NT1 stands for network termination type 1. The NT1 corrects the voltage on the signals and performs the Layer 1 functions as defined in the ITU standards. These are the electrical and physical terminations of the 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.