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A network termination unit (NTU) is a device that links the customer-premises equipment (CPE) to the public switched telephone network (PSTN). The device serves as the last point of connection between the two. Typically, the service provider is the owner of the NTU, which usually supports various communication standards, including protocols and voltages, to allow different device types to communicate with the PSTN.
An NTU connects ISDN-based CPE to ISDN lines. It allows the CPE to be linked to the switching device at the central office (CO).
A digital ISDN CPE, like an ISDN phone or a router, normally includes an ISDN S/T interface, whereas the ISDN termination at the customer's end generally includes a U interface with an RJ-45 connector. The NTU transforms the ISDN line's U termination into one or more basic coding S/T interfaces. These interfaces are ideal for connecting routers, digital ISDN-ready phones as well as other types of equipment to an ISDN line.
An NTU works differently from an ISDN terminal adapter, which allows users to connect faxes, analog telephones and similar devices to an ISDN line. An NTU generally supports either a multipoint or a point-to-point connection across distances as long as 5.4 kilometers on 26-AWG twisted-pair copper cabling. Some modern ISDN-enabled devices have a built-in U interface, which makes an NTU unnecessary.