What is ISDN: Integrated Services Digital Network

ISDN, Integrated Services Digital Network, enabled digital services to be offered to many businesses before broadband DSL became available.

ISDN Includes:
ISDN basics    

ISDN or Integrated Services Digital Network is an international standard for end to end digital transmission of voice, data and signalling.

ISDN 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 is a circuit-switched telephone network that carries packets data over copper lines and enabled existing copper wire based landline technology to be used to carry digital services.

Although ISDN has been in use for many years, and it is being retired in some areas, it is still widely used and some legacy services still make considerable use of it.

ISDN development

The concept for ISDN was developed when the analogue POTS, plain old telephone systems were the only real telecommunications systems available.

With computer technology developing fast and the Internet era about to dawn, companies needed the ability to communicate using data rather than analogue technology.

The first ideas for packet data systems had been developed int he 1960s, but with there being little need for data exchange between different companies and sites, it was not integrated into the customer facing side of the business. Telecommunications remained firmly analogue and circuit switched.

As technology developed, the International Telecommunications Union, ITU recommended the introduction ISDN, and only slowly did companies start to take up the new offering.

As digital technology, and also the Internet started to make its mark, more companies took up the idea of ISDN, with even some homes where home offices were required, looking at the use of ISDN.

ISDN performed very well when compared to the dial up modems that were used in the 1990s, but as DSL technology took hold, and speeds rose, ISDN became less attractive. Nevertheless, many legacy systems were used, and in some countries it offered the best performance and remained in use for many years. Accordingly many customers who used ISDN for their business telephone systems are now migrating to VoIP as this offers a high degree of capability when compared to analogue systems and even ISDN.

ISDN advantages

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.

With most companies now opting for DSL, or fibre connections, ISDN is used in some countries where the system have not migrated over fully yet. It can also be used as a backup in case the DSL or other digital systems fail.

ISDN network architecture

Concept behind the ISDN network and what it contained
ISDN network concept

ISDN configurations

Although the ISDN operation is relatively straightforward, it utilises a number of channels and interfaces.

There are two types of channel that are found within ISDN:

  • B or Bearer channels:   The bearer channels are used to carry the payload data which may be voice and / or data
  • D or Delta channels:   The D channels are 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, each 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 channels 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 connect 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.

ISDN operation

Call data is transmitted over the data (B) channels, with the signalling (D) channels used for call setup and management. Once a call is set up, there is a simple 64 kbit/s synchronous bidirectional data channel between the end parties, lasting until the call is terminated.

There can be as many calls as there are data channels, to the same or different end-points. Bearer channels may also be multiplexed into what may be considered single, higher-bandwidth channels via a process called B channel bonding.

The D channel can also be used for sending and receiving X.25 data packets, and connection to X.25 packet network. In practice, this was never widely implemented.

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. When it is being phased out VoIP phone systems are often taking over as they offer the advantages of a digitally based business phone system.

Wireless & Wired Connectivity Topics:
Mobile Communications basics     2G GSM     3G UMTS     4G LTE     5G     Wi-Fi     Bluetooth     IEEE 802.15.4     DECT cordless phones     Networking fundamentals     What is the Cloud     Ethernet     Serial data     USB     LoRa     VoIP     SDN     NFV     SD-WAN
    Return to Wireless & Wired Connectivity