A set of reference points are defined in the ISDN standard to refer to certain points between the telco and the end user ISDN equipment.
- R - defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device
- S - defines the point between the ISDN equipment (or TA) and a Network Termination Type 2 (NT-2) device
- T - defines the point between the NT-2 and NT-1 devices1
1 Most NT-1 devices can perform the functions of the NT-2 as well, and so the S and T reference points are generally collapsed into the S/T reference point.
² Inside North America, the NT-1 device is considered customer premises equipment (CPE) and must be maintained by the customer, thus, the U interface is provided to the customer. In other locations, the NT-1 device is maintained by the telco, and the S/T interface is provided to the customer. In India, service providers provide U interface and an NT1 may be supplied by Service provider as part of service offering
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