In North America (and some other countries), where the NT1 is not provided by the ISDN service provider, the Reference Point U is used to define the end of the wires provided to the user by the service provider. There is no international standard that defines Reference Point U.

It is therefore necessary for the user's own NT1 to be connected to reference point U. This will be necessary if the user wishes to attach more than one device. He would have to use equipment designed to operate on the S or T Reference Points.