Editing ISDN (section)

=== Reference points ===

A set of ''reference points'' are defined in the ISDN [[International standard|standard]] to refer to certain points between the [[Telephone company|telco]] and the end-user equipment.

*[[R interface|R]]&nbsp;– defines the point between non-ISDN terminal equipment, ''terminal equipment 2'' (''TE2''), and a ''[[terminal adapter]]'' (TA) which provides translation to and from such a device
*[[S interface|S]]&nbsp;– defines the point between ISDN terminal equipment, ''terminal equipment 1'' (''TE1''), or a TA and a ''Network Termination Type 2'' ([[Network termination 2|NT2]]) device
*[[T-interface|T]]&nbsp;– defines the point between the NT2 and [[network termination 1]] (NT1) devices.

Most NT-1 devices can perform the functions of the NT2 as well, and so the S and T reference points are generally collapsed into the S/T reference point.

In North America, the NT1 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 NT1 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.