T interface

A T-interface or T reference point is used for basic rate access in an Integrated Services Digital Network environment. It is a User–network interface reference point that is characterized by a four-wire, 144 kbit/s 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 and the Data service units.

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