Using the Integrated Services Digital Network (ISDN) Basic Rate Interchange (Basic Rate Interface in ISDN) service, an NT1 (network terminating unit 1) is a device that accepts a two-wire signal from the phone company and converts it to a four-wire signal that sends and receives to and from devices within the home or business. In the U.K. and some other countries, the NT1 is located at the telephone company's central office. In the U.S., the NT1 is a separate box at the home or business or it can be integrated into one device. If it is a separate box, up to eight devices, such as telephones and computers, can be attached to it. If the NT1 is built into one device, then only that one device can be served by the line coming in from the phone company. Additional devices would require one or more additional lines.
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The ISDN Basic Rate Interface (Basic Rate Interface in ISDN) is the most common service offered by ISDN providers. BRI supports two separate 64 Kbps B channels and one 16 Kbps D channel. The phone company replaces your conventional analog circuit with a BRI circuit. The NT1 provides the entry termination at your home or business that is required by the phone company. You must have one NT1 for each ISDN line. Some ISDN equipment such as an ISDN terminal adapter (the ISDN equivalent of a modem) may already have a built-in NT1. To find out if your ISDN device has a built-in NT1, check to see if it is designed to connect directly to the public ISDN network. If it is, it does not require a separate NT1. Most U.S. ISDN devices have the NT1 built-in. If you wish to have more than one ISDN device per line, consider purchasing a separate NT1 box with multiple jacks. Some NT1 boxes have built-in analog conversion so you can use both ISDN and analog equipment on the same line. Your ISDN provider may occasionally "talk" to your NT1 box to perform routine testing and maintenance.