F. ISDN — Integrated Services Digital Network

ISDN (Integrated Services Digital Network), also known as RNIS (Réseau Numérique à Intégration de Services), is an all-digital broadband network designed to carry voice, data and video, which until then had been transmitted over more or less specialised networks. In fact, the only way to switch these three types of data simultaneously is through digital switching combined with a multiplexing protocol.

ISDN technology also enables the use of new applications based on high-speed transmission: internet access, teleworking, videoconferencing, distance learning, broadcasting and remote audio transmission. It offers a variety of communication interfaces based on circuit switching.

1. Principle

We want to digitise the signal from start to finish. To digitise the voice (which ranges from 300 to 3,400 Hz) using traditional techniques, 64 kbps is required. The control room receives the call and sends it over the bus, with the data travelling via a B-channel. The user can receive a fax at the same time as a call. It is also possible to transmit at 2 × 64 kbps by using the two B channels between two computers.

The channels defined by ISDN are:

Channel B (Bearer): 64 kbps data rate — data or voice channel.

Channel D16: 16 kbps data rate — basic access signalling channel.

Channel D64: 64 kbps data rate — primary access signalling channel.

H0 channel: bitrate of 384 kbps.

Channel H11: bitrate of 1,536 kbps (24 B-channels, North American standard).

Channel H12: bitrate of 1,920 kbps (30 B channels, European standard).

2. Basic access (BRI — Basic Rate Interface)

Basic BRI access allows for the connection of five terminals and the establishment of two simultaneous calls. On the public network side, this access is provided via a T0 interface; on the subscriber side, via an S0 interface. On ISDN digital buses, time-division multiplexing divides the time slots into three channels:

· 2 B channels (Bearer Channel) at 64 kbps: for the transmission of data, voice and video in circuit-switched mode.

· 1 D-channel at 16 kbps (D-channel protocol): used for call setup and signalling in packet-switched mode.

The total bandwidth of the BRI connection is 64 + 64 + 16 = 144 kbps. This synchronous 144 kbps full-duplex transmission takes place over two wires.

3. Primary Rate Interface (PRI)

Where users have significant communication requirements, the CCITT has defined a higher-speed access option. This is known as primary access, intended primarily for PABXs. Its characteristics are:

· European version (E1): 30 B channels at 64 kbps + 1 D channel at 64 kbps, totalling 2,048 kbps (T2/S2).

· North American and Japanese versions (T1): 23 B channels at 64 kbps + 1 D channel at 64 kbps, totalling 1,544 kbps (23B+D).

4. ISDN components

· R interface (Rated): provides a non-ISDN interface between non-ISDN-compatible user equipment and a terminal adapter (TA).

· S Interface (Subscriber): separates the user part of the terminal from its network functions.

· T-interface (Terminal): separates the network provider’s equipment from the user’s equipment.

· U interface (User): the interface between the network equipment and the transmission line to the operator’s switch.