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T interface

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Source updated May 12, 2026

A T-interface or T reference point is used for basic rate access in an Integrated Services Digital Network (ISDN) environment. It is a User-network interface reference point that is characterized by a four-wire, 144 kbit/s (2B+D) user rate.

Other characteristics of a T-interface are:

it accommodates the link access and transport layer function in the ISDN architecture
it is located at the user premises
it is distance sensitive to the servicing Network termination 1
it functions in a manner similar to that of the Channel service units (CSUs) and the Data service units (DSUs).

The T interface is electrically equivalent to the S interface, and the two are jointly referred to as the S/T interface.

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Related topics

R interface

R interface or R reference point defines the point between a non-ISDN device and a terminal adapter which provides translation to and from such a device.

S interface

The S interface or S reference point, also known as S0, is a user-network interface reference point in an ISDN BRI environment, characterized by a four-wire circuit using 144 kbit/s user rate. The S interface is the connection between ISDN terminal equipment (TE) or terminal adapters (TAs) and an NT1 (network terminator, type 1.) Not all TE or TAs connect externally to an S interface, but instead integrate an NT1 so they can connect directly to a U interface (local loop from central office.) Contrast to the T interface, which connects between an NT2 (PBX or other local switching device) and NT1.

U interface

The U interface or U reference point is a Basic Rate Interface in the local loop of an Integrated Services Digital Network (ISDN), connecting the network terminator (NT1/2) on the customer's premises to the line termination (LT) in the carrier's local exchange, in other words providing the connection from subscriber to central office. Unlike the ISDN S/T interfaces, the U interface was not originally electrically defined by the ITU ISDN specifications, but left up to network operators to implement, although the ITU has issued recommendations G.960 and G.961 to formalize the standards adopted in the US and EU. In the US, the U interface is originally defined by the ANSI T1.601 specification as a 2-wire connection using 2B1Q line coding.