5.1 ISDN Architecture
5.1.4 BRI functional groups and reference points

ISDN technology involves many functional devices, also known as functional groups. The ISDN reference points define the communication protocols of these devices. Each functional group has identical elements, but one group includes the Network Termination 2 customer premises equipment (NT2 CPE); the other does not. The following functional groups are illustrated in the Figure :

  • Terminal equipment 1 (TE1) designates a device that is compatible with the ISDN network. A TE1 connects to a network termination of either Type 1 or Type 2, such as a digital telephone, a router with ISDN interface, or digital facsimile equipment.

  • Terminal equipment 2 (TE2) designates a device that is not compatible with ISDN and requires a terminal adapter, such as terminals with X.21, EIA/TIA-232, or X.25 interfaces, or a router without ISDN interface (AGS+, and so on).

  • Terminal adapter (TA) converts standard electrical signals into the form used by ISDN, so that non-ISDN devices can connect to the ISDN network. An example would be converting V.35 or EIA/TIA-232 to ISDN or a router without ISDN interface (AGS+, and so on).

  • Network Termination type 1 (NT1) connects four-wire ISDN subscriber wiring to the conventional two-wire local-loop facility. The NT1 is part of the CPE in the United States and it is part of the local exchange in Europe.

  • Network Termination type 2 (NT2) directs traffic to and from different subscriber devices and the NT1. The NT2 is an intelligent device that performs switching and concentrating. Often, a private branch exchange (PBX) is the NT2 device.

  • Line termination (LT) (located at the exchange side) functions are identical to those of an NT1.

  • Exchange termination (ET) Subscriber line cards in the ISDN exchange LT and ET are sometimes referred to as the LE (local exchange), which is the ISDN switch for which we must configure.

Reference points are architectural definitions that may or may not have a physical realization as an interface. ISDN reference points are as follows:

  • U reference point (user reference point)-This reference point is located between the NT1 and LT (it corresponds with a subscriber line). There are no ITU-T standards for the U interface. This is the American National Standards Institute (ANSI) standard for the United States.

  • T reference point (terminal reference point)-This reference point is located between NT1 and NT2 (or between the NT1 and TE1 or TA, if there is no NT2 device). The T interface uses the same characteristics as the S interface.

  • S reference point (system reference point)-This reference point is located between NT2 and TE1 or TA. It connects the terminals to the ISDN network. This is the most important interface for the users. The S interface uses the same characteristics as the T interface.

  • R reference point (rate reference point)-This reference point is located between TA and TE2 (non-ISDN interface). The TE2 connects to the TA via a standard physical-layer interface. These standards include EIA/TIA-232-C (formerly RS-232-C), V.24, X.21, and V.35.

The S/T interface is governed by the ITU I.430 standard. The ANSI T1.601 or ITU I.431 standards govern the U interface, depending on the country. Some manufacturers define a V reference point in LEs between LT and ET. This reference point identifies the network node interface and is transparent to users. Given all the ISDN abbreviations such as T, S, U, S/T, and so on, what do all of these components and reference points look like in the real world?

As shown in Figure , a connection is made from the wall jack with a standard two-wire cable to the NT1, and then out of the NT1 with a four-wire connection to your ISDN phone, terminal adapter, Cisco ISDN router, or maybe to an ISDN fax. The S/T interface is implemented using an eight-wire connector to allow for powering the NT and TE capabilities.

Because all these connectors look similar (such as RJ-11, RJ-45s, and so on), you must be careful about what you plug in and where. The S/T reference point is a four-wire interface (TX and RX). It is point-to-point and multipoint (passive bus). It uses the ITU I.430 specification. The S/T interface defines the interface between a TE1 or TA and an NT.

The U interface defines the two-wire interface between the NT and the ISDN cloud. The R interface defines the interface between the TA and an attached non-ISDN device (TE2). An NT1 and NT2 combination device is sometimes referred to as an NTU.

ISDN Protocols
The ITU-T groups and organizes the ISDN protocols according to general topics:

  • E-series-Telephone network, for example E.164-International ISDN addressing

  • I-series-Concepts and interfaces, for example I.430-BRI interface

  • Q-series-Switching and signaling, for example q.921- Link Access Procedures on the D channel (LAPD)