The ISDN user - to - network interface standard defines the hardware components needed for ISDN communication and these components' interfaces to other hardware components. ISDN provides standards for the following components and interfaces:
Terminal Equipment 1 (TE1). This equipment is workstation equipment, such as the ISDN telephone or ISDN-enabled computer.
Network Termination 1 (NT1). A device that connects the U interface (the telephone wall jack) to the interior phone circuit on the user premises. The NT1 device converts the two-wire circuit of the U interface to four-wire circuits that allow multiple terminal devices to access ISDN B channels. Typically, the NT1 device provides the Subscriber/Termination interface for ISDN-BRI service.
Network Termination 2 (NT2). An intelligent device such as a ISDN router that exists between the NT1 and TE1 devices. The NT2 device provides services that map to the Data Link and Network layers of the OSI mode.
Subscriber/Termination (S/T) Interface. The ISDN standard for the S/T Interface defines the conversion of two-pair to four-pair circuits and also defines distance limitations. The maximum allowable distance between the NT1 device on the user premises and a user computer is approximately 3,500 cable feet.
U Interface. The telephone wall jack on the user premises. The ISDN standard for the U interface defines the maximum allowable distance between the telephone company's central office equipment and the U interface, which is approximately 18,000 cable feet.
These hardware and interface components are described in the following sections.