What is an Integrated Services Digital Network?

ISDN allows transmission of data, voice, video, and graphics at high speeds over standard communication lines carried by bearer channels (B channels) occupying a bandwidth of 64 kbits per second. A data channel (D channel) handles signaling at 16 kb/s or 64 kb/s depending on the service type.

ISDN is not limited to public telephone networks alone, and can be transmitted via networks like telex, packet switched networks, CATV networks, etc. ISDN provides three logical, digital communication channels, which perform the following functions:

B-Channel: it carries user service information including digital data, video, and voice.
D-Channel: it carries signals and data packets between the user and the network.
H-Channel: it performs the same function as B-Channels, but operates at rates exceeding 64 Kbps.

There are two types of services corresponding with ISDN, Basic Rate Interface Service, and Primary Rate Interface Service.

This service provides two B channels and one D channel. The B-channel service, working at 64 kbps, is used to carry data. The D-channel service, working at 16 kbps, is used to carry control and signaling information, although it also supports data transmission under certain circumstances.

The D-channel signaling protocol contains Layers 1 to 3 of the OSI reference model. The International Telecommunication Union-Telecommunications Standards Section (ITU-T) defines the BRI physical layer specification.

This service provides 23 B channels and 1 D channel yielding a total bit rate of 1.544 Mbps. In Europe, Australia, and other parts of the world the service provides 30 B channels plus one 64-kbps D channel and a total interface rate up to 2.048 Mbps. The ITU-T I.431 defines PRI physical layer specification.

ISDN physical layer (Layer 1) frame formats depends upon whether the frame is outbound (from terminal to network) or inbound (from network to terminal). The frames are 48 bits long, from which data is represented by 36 bits. The bits of an ISDN frame are used as follows:
1) F: it provides synchronization.
2) L: it adjusts the average bit value

3) E: it ensures contention resolution when several terminals on a passive bus contend for a channel
4) A: it activates devices.
5) S: it is unassigned bit.
6) B1, B2 and D: handles user data.

Multiple ISDN devices can be attached physically to one circuit. In this configuration, collisions can occur when two terminals transmit simultaneously, to avoid which link contention is used.

Terminals cannot transmit data to the D channel unless they first detect a specific number of ones corresponding to pre-established priority. If TE detects a bit in the echo channel that is different from D bits, then it stop transmitting immediately. This technique guarantees that only one terminal can transmit its D-message at a time.

After successful D-message transmission, the terminal reduces its priority. Terminals cannot increase their priority until all other devices on the same line have had an opportunity to send a D-message. Telephone connections have the highest priority.

ISDN Layer 2 is signaling protocol called Link Access Procedure D Channel. LAPD is just like High-Level Data Link Control (HDLC) and Link Access Procedure Balanced (LAPB).

As the development of the LAPD acronym indicates, this layer is used across the D channel to check control and signaling information flows. The LAPD frame format is similar to that of HDLC. Like HDLC, LAPD makes use of information, supervisory, and unnumbered frames.

ISDN Layer 3 is used for ISDN signaling: ITU-T I.450 and ITU-T I.451. Both these protocols carry end-to-end, circuit-switched and packet-switched connections.

A variety of call-establishment, call-termination, information, and miscellaneous messages are described by using SETUP, CONNECT, RELEASE, USER INFORMATION, CANCEL, STATUS, and DISCONNECT. These messages provide the same functionality as that of the X.25 protocol.