ISDN Tutorial

- summary, overview or tutorial about ISDN, Integrated Services Digital Network, and the services that ISDN can offer in the telcommunications arena.

ISDN or Integrated Services Digital Network is an international standard for end to end digital transmission of voice, data and signaling. It can operate over copper based systems and allows the transmission of digital data over the telecommunications networks, typically ordinary copper based systems and providing higher data speeds and better quality than analogue transmission. The ISDN specifications provide a set of protocols that enable the set up, maintenance and completion of calls.

ISDN, Integrated Services Digital Network, provides a number of significant advantages over analogue systems.

In is basic form it enables two simultaneous telephone calls to be made over the same line simultaneously.

Faster call connection. It typically takes a second to make connections rather than the much longer delays experienced using purely analogue based systems.

Data can be sent more reliably and faster than with the analogue systems.

Noise, distortion, echoes and crosstalk are virtually eliminated.

The digital stream can carry any form of data from voice to faxes and internet web pages to data files - this gives the name 'integrated services'

ISDN Usage

ISDN is in use around the world, but with the introduction of ADSL it is facing strong competition. The technology never gained much market share in the USA, although it used in other countries. In Japan it became reasonably popular in the late 1990s although it is now in decline with the advent of ADSL. The system was also introduced in Europe where providers such as BT, France Telecom and Deutsche Telekom introduced services.

ISDN Configurations

There are two types of channel that are found within ISDN. These are the 'B' and 'D' channels. The B or 'bearer' channels are used to carry the payload data which may be voice and / or data, and the d or 'Delta' channel is intended for signalling and control, although it may also be used for data under some circumstances.

Additionally there are two levels of ISDN access that may be provided. These are known as BRI and PRI.

BRI (Basic Rate Interface) - This consists of two B channels, eac of which provides a bandwidth of 64 kbps under most circumstances. One D channel with a bandwidth of 16 kbps is also provided. Together this configuration is often referred to as 2B+D.

The basic rate lines connect to the network using a standard twisted pair of copper wires. The data can then be transmitted simultaneously in both directions to provide full duplex operation. The data stream is carried as two B channesl as mentioned above, each of which carry 64 kbps (8 k bytes per second). This data is interleaved with the D channel data and this is used for call management: setting up, clearing down of calls, and some additional data to maintain synchronisation and monitoring of the line.

The network end of the line is referred to as the 'Line Termination' (LT) while the user end acts as a termination for the network and is referred to as the 'Network Termination' (NT). Within Europe and Australia, the NT physically exists as a small connection box usually attached to a wall etc, and it converts the two wire line (U interface) coming in from the network to four wires (S/T interface or S bus). The S/T interface allows up to eight items or 'terminal equipments' to be connected, although only two may be used at any time. The terminal equipments may be telephones, computers, etc, and they are connected in what is termed a point to point configuration. In Europe the ISDN line provides up to about 1 watt of power that enables the NT to be run, and also enables a basic ISDN phone to be used for emergency calls. In North America a slightly different approach may be adopted in that the terminal equipment may be directly connected to the network in a point to point configuration as this saves the cost of a network termination unit, but it restricts the flexibility. Additionally power is not normally provided.

PRI (Primary Rate Interface) - This configuration carries a greater number of channels than the Basic Rate Interface and has a D channel with a bandwidth of 64 kbps. The number of B channels varies according to the location. Within Europe and Australia a configuration of 30B+D has been adopted providing an aggregate data rate of 2.048 Mbps (E1). For North America and Japan, a configuration of 23B+1D has been adopted. This provides an aggregate data rate of 1.544 Mbps (T1).

The primary rate connections utilise four wires - a pair for each direction. They are normally 120 ohm balanced lines using twisted pair cable. Primary rate connections always use a point to point configuration.

Primary rate lines are widely used to conenct to Private Branch eXchanges (PBX) in an office etc. Typically this may be used to provide a number of POTS (Plain Old Telephone System) or basic rate ISDN lines to the users.


Although ISDN is has been overtaken by technologies such as ADSL it is nevertheless still widely used in many areas, particularly where existing services need to be maintained, or where compatibility needs to be guaranteed. As such it is still an important technology that will be encountered for many years to come.

By Ian Poole

Read more telecommunications technology tutorials . . . . .

ATM E carrier & E1 Fibre optics ISDN
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