Integrated Services Digital Network is set of communications standards, which allows a single wire or optical fiber to carry voice, video and digital network services. ISDN is proposed to replace the plain old telephone system.

Integrated Services Digital Network is a system with digitized phone connections, which has pure analogue connection. It is the first protocol which describes on a digital communications line. This allows transmission of data, voice, video and graphics at very 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. It is not limited to public telephone networks alone. It may be transmitted via networks like telex, packet switched networks, CATV networks, etc.

There are two types of services corresponding with ISDN:
1) Basic Rate Interface Service.
2) Primary Rate Interface Service.

ISDN Basic Rate Interface Service

The ISDN Basic Rate Interface (BRI) service provides two B channels and one D channel. BRI B-channel service works at 64 kbps is used to carry user data. BRI D-channel service works at 16 kbps is used to carry control and signaling information although it also supports user data transmission under certain circumstances. The D-channel signaling protocol contains Layers 1 to 3 of the OSI reference model. International Telecommunication Union-Telecommunications Standards Section (ITU-T) defines the BRI physical layer specification.

ISDN Primary Rate Interface Service

ISDN Primary Rate Interface (PRI) service provides 23 B channels and 1 D channel yielding a total bit rate of 1.544 Mbps. ISDN PRI in different country like Europe, Australia, and other parts of the world provides 30 B channels plus one 64-kbps D channel and a total interface rate upto 2.048 Mbps. ITU-T I.431 defines PRI physical layer specification.

Different services may be called by using different values in the "Bearer Capability" field in the call setup message. One ISDN service is telephony, which can be obtained using full 64 kbps bandwidth but will require the some special processing or bit diddling as ordinary Public service Telephone Network calls.
ISDN Specifications
This section describes the various ISDN specifications for Layer 1, Layer 2, and Layer 3.
Layer 1
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: it handles user data
Multiple ISDN devices can be attached physically to one circuit. In this configuration, collisions occur when two terminals transmit simultaneously. Therefore, ISDN avoids collision by using link contention.

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 without wasting time. This technique guarantees that at a time only one terminal can transmit its D message. 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 highest priority than all other devices.

Layer 2
ISDN Layer 2 is signaling protocol called as Link Access Procedure D channel (LAPD). 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 as of HDLC. Like HDLC, LAPD makes use of information, supervisory and unnumbered frames.

Layer 3
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 same functionality as that of X.25 protocol.

ISDN Channels
ISDN provide 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.