NT Devices and S and T Interfaces

The interface between the ISDN network and a TE1 device or terminal adapter is called an S interface. The S interface connects to a network termination type 2 (NT2) device such as a PBX, or directly to the TE1 device or terminal adapter, as shown in Figure 7. The NT2 device is then connected to a network termination type 1 (NT1) device through a T interface. The S and T interfaces are electrically equivalent.

An NT1 device is a physical layer device that connects a home telephone network to a service provider carrier network. ISDN devices that connect to an NT1 device from the home network side use a 4-wire S/T interface. The NT1 device converts the 4-wire S/T interface into the 2-wire U interface that telephone carriers use as their plain old telephone service (POTS) lines.

In the United States, NT1 devices are user owned. In many other countries, NT1 devices are owned by the telephone service providers.

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