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Integrated Services Digital Network

Integrated Services Digital Network

Integrated Services Digital Network or Isolated Subscriber Digital Network (ISDN), originally "Integriertes Sprach- und Datennetz" (German for "Integrated Speech and Data Net"), is a telephone system network. Prior to the ISDN, the phone system was viewed as a way to transport voice, with some special services available for data. The key feature of the ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. There are several kinds of access interfaces to the ISDN defined: Basic Rate Interface (BRI), Primary Rate Interface (PRI) and Broadband-ISDN (B-ISDN).

ISDN is a circuit-switched telephone network system, that also provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better voice quality than an analog phone. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kbit/s. Another major use case is Internet access, where ISDN typically provides a maximum of 128 kbit/s in both upstream and downstream directions (which can be considered to be broadband speed, since it exceeds the narrowband speeds of standard analog 56k telephone lines). ISDN channels may use bonding to achieve a greater data rate, typically 3 or 4 BRIs (6 to 8 64 kbit/s channels) are bonded.

ISDN should not be mistaken with any specific protocol, like Q.931. In a broad sense ISDN can be considered a digital communications medium existing on layers 1, 2, and 3 of the OSI model. ISDN is designed to provide access to voice and data services simultaneously.

However, common use has reduced ISDN to be limited to Q.931 and related protocols, which are a set of protocols for establishing and breaking circuit switched connections, and for advanced call features for the user. They were introduced in the late 1980s.

In a videoconference, ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group (room) videoconferencing systems.

ISDN elements

  • Integrated Services refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice, video, and fax, over a single line. Multiple devices can be attached to the line, and used as needed. That means an ISDN line can take care of most people's complete communications needs at a much higher transmission rate, without forcing the purchase of multiple analog phone lines.
  • Digital refers to its purely digital transmission, as opposed to the analog transmission of plain old telephone service (POTS). Use of an analog telephone modem for Internet access requires that the Internet service provider's (ISP) modem converts the digital content to analog signals before sending it and the user's modem then converts those signals back to digital when receiving. When connecting with ISDN there is no digital to analog conversion.
  • Network refers to the fact that ISDN is not simply a point-to-point solution like a leased line. ISDN networks extend from the local telephone exchange to the remote user and includes all of the telecommunications and switching equipment in between.

The purpose of the ISDN is to provide fully integrated digital services to the users. These services fall under three categories: bearer services, supplementary services and teleservices.

Basic Rate Interface

The entry level interface to ISDN is the Basic Rate Interface (BRI), a 144 kbit/s service delivered over a pair of standard telephone copper wires. The 144 kbit/s rate is broken down into two 64 kbit/s bearer channels ('B' channels) and one 16 kbit/s signaling channel ('D' channel or Delta channel).

BRI is sometimes referred to as 2B+D

The Interface specifies three different network interfaces:

  • The U interface is a two-wire interface between the exchange and the Network Terminating Unit which is usually the demarcation point in non-North American networks.
  • The T interface is a serial interface between a computing device and a Terminal Adapter, which is the digital equivalent of a modem.
  • The S interface is a four-wire bus that ISDN consumer devices plug into; the S & T reference points are commonly implemented as a single interface labeled 'S/T' on an NT1
  • The R interface defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device.

BRI-ISDN is very popular in Europe but is much less common in North America.

A graph includes "U,T,S,R" can be found at

Primary Rate Interface

The other ISDN service available is the Primary Rate Interface (PRI) which is carried over an E1 (2048 kbit/s) in most parts of the world. An E1 is 30 'B' channels of 64 kbit/s, one 'D' channel of 64 kbit/s and a timing and alarm channel of 64 kbit/s. In North America PRI service is delivered on one or more T1s (sometimes referred to as 23B+D) of 1544 kbit/s (24 channels). A T1 has 23 'B' channels and 1 'D' channel for signalling (Japan uses a circuit called a J1, which is similar to a T1).

In North America, NFAS allows two or more PRIs to be controlled by a single D channel, and is sometimes called "23B+D + n*24B". D-channel backup allows you to have a second D channel in case the primary fails. One popular use of NFAS is on a T3.

PRI-ISDN is popular throughout the world, especially for connection of PSTN circuits to PBXs.

Even though many network professionals use the term "ISDN" to refer to the lower-bandwidth BRI circuit, in North America by far the majority of ISDN services are in fact PRI circuits serving PBXs.

Data Channel

The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-Channels can also be used to carry data, since they are nothing more than digital channels.

Each one of these channels is known as a DS0 (dee-ess-zero).

Most B channels can carry a 64 kbit/s signal, but some were limited to 56K because they traveled over RBS lines. This was more of a problem in the past, and is not commonly encountered nowadays.

Signalling Channel

The signalling channel (D) uses Q.931 for signalling with the other side of the link.


X.25 can be carried over the B or D channels of a BRI line, and over the B channels of a PRI line. X.25 over the D channel is used at many point-of-sale (credit card) terminals because it eliminates the modem setup, and because it connects to the central system over a B channel, thereby eliminating the need for modems and making much better use of the central system's telephone lines.

X.25 was also part of an ISDN protocol called "Always On/Dynamic ISDN", or AO/DI. This allowed a user to have a constant multi-link PPP connection to the internet over X.25 on the D channel, and brought up one or two B channels as needed.

Frame Relay

In theory, Frame Relay can operate over the D channel of BRIs and PRIs, but it is seldom, if ever, used.

Consumer and industry perspectives

There are two points of view into the ISDN world. The most common viewpoint is that of the end user, who wants to get a digital connection into the telephone/data network from home, whose performance would be better than an ordinary analog modem connection. The typical end-user's connection to the Internet is related to this point of view, and discussion on the merits of various ISDN modems, carriers' offerings and tarriffing (features, pricing) are from this perspective. Much of the following discussion is from this point of view, but it should be noted that as a data connection service, ISDN has been mostly superseded by DSL.

There is a second viewpoint: that of the telephone industry, where ISDN is a core technology. A telephone network can be thought of as a collection of wires strung between switching systems. The common electrical specification for the signals on these wires is T1 or E1. Between telephone company switches, the signaling is performed via SS7. Normally, a corporate or other PBX is connected via a T1, and the signalling was done with A&B bits to indicate on-hook or off-hook conditions and MF and DTMF tones to encode the destination number. ISDN is much better because messages can be sent much more quickly than by trying to encode numbers as long (100 ms per digit) tone sequences. This translated to much faster call setup times. Also, a greater number of features are available and fraud is reduced.

ISDN is also used as a smart-network technology intended to add new services to the public switched telephone network (PSTN) by giving users direct access to end-to-end circuit-switched digital services and as a backup or failsafe circuit solution for critical use data circuits.

United States and Canada

ISDN-BRI (Basic Rate Interface) has never gained popularity as a telephone access technology in Canada and the US and today remains a niche product. The service was seen as a solution in search of a problem, and the extensive array of options and features were difficult for most customers to understand and utilize. ISDN has long been known by several derogatory acronyms highlighting these issues, such as It Still Does Nothing, Innovations Subscribers Don't Need, and I Still Don't kNow. One of the few places where BRI still exists is in videoconferencing. High-end videoconferencing made by companies such as Sony, Polycom and Tandberg bond up to 6 B-channels together (using a BRI circuit for every 2 channels) to provide digital, circuit-switched video connections to almost anywhere in the world. This is very expensive, and is being replaced by IP-based conferencing, but where cost is no object, and perfect quality required, BRI is the preferred choice. Another user of BRI lines is the US Federal government.

Part of the difficulty is that as BRI was coming into service, the concept of what the word broadband meant was being revised upward to mean at least 256 kbit/s incoming to the customer. As ADSL grew in popularity in the United States, the consumer market for BRI imploded. Its only remaining positive element is that while ADSL has a functional distance limitation, BRI has a greater limit. and can use repeaters. As such, BRI may be acceptable in situations where the customer is too remote for ADSL to work. This is further stymied by many North American CLECs having given up on BRI and will not install it as a new option for remote/rural exchanges.

However, most modern non-VoIP PBXs use ISDN-PRI (Primary Rate Interface) circuits. These are connected via T1 lines with the central office switch, replacing older analog two-way and Direct Inward Dialing (DID) trunks. PRI is capable of delivering Automatic Number Identification (ANI) in both directions so that the telephone number of an extension, rather than a company's main number, can be sent. It is still commonly used in recording studios, when a voice-over actor is in one studio, but the director and producer are in a studio at another location. The ISDN protocol delivers channelized, not-over-the-Internet service, powerful call setup and routing features, faster setup and tear down, superior audio fidelity (as compared to POTS service), lower delay and, at higher densities, lower cost.


In Japan, it became popular to some extent from around 1999 to 2001, but now that ADSL has been introduced, the number of subscribers is in decline. NTT, a dominant Japanese telephone company, provides an ISDN service with the names INS64 and INS1500, which are much less recognized than ISDN.

United Kingdom

In the UK, British Telecom (BT) provides ISDN2e (BRI) as well as ISDN30 (PRI). Until April 2006, they also offered Home Highway and Business Highway, which are BRI ISDN-based services that offer integrated analog connectivity as well as ISDN. Later versions of the Highway products also included built-in USB sockets for direct computer access. Home Highway has been bought by many home users, usually for Internet connection, although not as fast as ADSL, because it was available before ADSL and in places where ADSL does not reach. Virgin Media also use ISDN lines for customers of Virgin Broadband who live in cable TV areas.


France Telecom offers ISDN services under their product name Numeris (2 B+D), of which a professional Duo and home Itoo version is available. ISDN is generally known as RNIS in France and has widespread availability. The introduction of ADSL is reducing ISDN use for data transfer and Internet access, although it is still common in more rural and outlying areas, and for applications such as business voice and point-of-sale terminals.


In Germany, ISDN is very popular with an installed base of 25 million channels (29% of all subscriber lines in Germany as of 2003 and 20% of all ISDN channels worldwide). Due to the success of ISDN, the number of installed analog lines is decreasing. Deutsche Telekom (DTAG) offers both BRI and PRI. Competing phone companies often offer ISDN only and no analog lines. Because of the widespread availability of ADSL services, ISDN is today primarily used for voice and fax traffic, but is still very popular thanks to the pricing policy of German telecommunication providers. Today ISDN (BRI) and ADSL/VDSL are often bundled on the same line, mainly because the combination of ADSL with an analog line has no cost advantage over a combined ISDN-ADSL line.


In India, ISDN was very popular until the introduction of ADSL. Bharat Sanchar Nigam Limited, the largest communication service provider in India and a state owned company, is offering both ISDN BRI and PRI services across the country over its ISDN network. After the introduction of ADSL broadband technology with static IPs, the data transfer load is taken up by ADSL. But ISDN still plays a very big role as a backup network for point-to-point leased line customers and low cost reliable data network for organisations located all over India, such as banks, E-seva centres, Life Insurance Corporation of India, and so on.

International occurrence

A study of the German Department of Science shows the following spread of ISDN-channels per 1000 inhabitants in the year 2005:


In ISDN, there are two types of channels, B (for "Bearer") and D (for "Delta"). B channels are used for data (which may include voice), and D channels are intended for signaling and control (but can also be used for data).

There are two ISDN implementations. Basic Rate Interface (BRI), also called Basic Rate Access (BRA) in Europe — consists of two B channels, each with bandwidth of 64 kbit/s, and one D channel with a bandwidth of 16 kbit/s. Together these three channels can be designated as 2B+D. Primary Rate Interface (PRI), also called Primary Rate Access (PRA) in Europe — contains a greater number of B channels and a D channel with a bandwidth of 64 kbit/s. The number of B channels for PRI varies according to the nation: in North America and Japan it is 23B+1D, with an aggregate bit rate of 1.544 Mbit/s (T1); in Europe, India and Australia it is 30B+1D, with an aggregate bit rate of 2.048 Mbit/s (E1). Broadband Integrated Services Digital Network (BISDN) is another ISDN implementation and it is able to manage different types of services at the same time. It is primarily used within network backbones and employs ATM.

Another alternative ISDN configuration can be used in which the B channels of an ISDN basic rate interface are bonded to provide a total duplex bandwidth of 128 kbit/s. This precludes use of the line for voice calls while the internet connection is in use. The B channels of several BRIs can be BONDED, a typical use is a 384K videoconferencing channel.

Using bipolar with eight-zero substitution encoding technique, call data is transmitted over the data (B) channels, with the signaling (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 (actually implemented as two simplex channels, one in each direction) between the end parties, lasting until the call is terminated. There can be as many calls as there are bearer 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, or via use of Multi-Link PPP "bundling" or by using an H0, H11, or H12 channel on a PRI.

The D channel can also be used for sending and receiving X.25 data packets, and connection to X.25 packet network, this is specified in X.31. In practice, X.31 was only commercially implemented in UK, France and Japan.

Reference points

A set of reference points are defined in the ISDN standard to refer to certain points between the telco and the end user ISDN equipment.

  • R - defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device
  • S - defines the point between the ISDN equipment (or TA) and a Network Termination Type 2 (NT-2) device
  • T - defines the point between the NT-2 and NT-1 devices1

1 Most NT-1 devices can perform the functions of the NT-2 as well, and so the S and T reference points are generally collapsed into the S/T reference point.
² Inside North America, the NT-1 device is considered customer premises equipment(CPE) and must be maintained by the customer, thus, the U interface is provided to the customer. In other locations, the NT-1 device is maintained by the telco, and the S/T interface is provided to the customer. In India, service providers provide U interface and an NT1 may be supplied by Service provider as part of service offering

Types of communications

Among the kinds of data that can be moved over the 64 kbit/s channels are pulse-code modulated voice calls, providing access to the traditional voice PSTN. This information can be passed between the network and the user end-point at call set-up time. In North America, ISDN is now used mostly as an alternative to analog connections, most commonly for Internet access. Some of the services envisioned as being delivered over ISDN are now delivered over the Internet instead. In Europe, and in Germany in particular, ISDN has been successfully marketed as a phone with features, as opposed to a POTS phone (Plain Old Telephone Service) with few or no features. Meanwhile, features that were first available with ISDN (such as Three-Way Call, Call Forwarding, Caller ID, etc.) are now commonly available for ordinary analog phones as well, eliminating this advantage of ISDN. Another advantage of ISDN was the possibility of multiple simultaneous calls (one call per B channel), e.g. for big families, but with the increased popularity and reduced prices of mobile telephony this has become less interesting as well, making ISDN unappealing to the private customer. However, ISDN is typically more reliable than POTS, and has a significantly faster call setup time compared with POTS, and IP connections over ISDN typically have some 30–35ms round trip time, as opposed to 120–180ms (both measured with otherwise unused lines) over 56k or V.34/V.92 modems, making ISDN more reliable and more efficient for telecommuters.

Where an analog connection requires a modem, an ISDN connection requires a terminal adapter (TA). The function of an ISDN terminal adapter is often delivered in the form of a PC card with an S/T interface, and single-chip solutions seem to exist, considering the plethora of combined ISDN- and ADSL-routers.

ISDN is commonly used in radio broadcasting. Since ISDN provides a high quality connection this assists in delivering good quality audio for transmission in radio. Most radio studios are equipped with ISDN lines as their main form of communication with other studios or standard phone lines.

Sample call

The following is an example of a Primary Rate (PRI) ISDN call showing the Q.921/LAPD and the Q.931/Network message intermixed (i.e. exactly what was exchanged on the D-channel). The call is originating from the switch where the trace was taken and goes out to some other switch, possibly an end-office LEC, who terminates the call.

The first line format is <time> <D-channel> <Transmitted/Received> <LAPD/ISDN message ID>. If the message is an ISDN level message, then a decoding of the message is attempted showing the various Information Elements that make up the message. All ISDN messages are tagged with an ID number relative to the switch that started the call (local/remote). Following this optional decoding is a dump of the bytes of the message in <offset> <hex> ... <hex> <ascii> ... <ascii> format.

The RR messages at the beginning prior to the call are the keep alive messages. Then you will see a SETUP message that starts the call. Each message is acknowledged by the other side with a RR.

10:49:47.33  21/1/24  R  RR
0000  02 01 01 a5                                          ....

10:49:47.34 21/1/24 T RR 0000 02 01 01 b9 ....

10:50:17.57 21/1/24 R RR 0000 02 01 01 a5 ....

10:50:17.58 21/1/24 T RR 0000 02 01 01 b9 ....

10:50:24.37 21/1/24 T SETUP

   Call Reference       : 000062-local
   Bearer Capability    : CCITT, Speech, Circuit mode, 64 kbit/s
   Channel ID           : Implicit Interface ID implies current span, 21/1/5, Exclusive
   Calling Party Number : 8018023000 National number  User-provided, not screened  Presentation allowed
   Called Party Number  : 3739120 Type: SUBSCRB
0000 00 01 a4 b8 08 02 00 3e 05 04 03 80 90 a2 18 03 .......>........ 0010 a9 83 85 6c 0c 21 80 38 30 31 38 30 32 33 30 30 ...l.!.801802300 0020 30 70 08 c1 33 37 33 39 31 32 30 0p..3739120

10:50:24.37 21/1/24 R RR 0000 00 01 01 a6 ....

10:50:24.77 21/1/24 R CALL PROCEEDING

   Call Reference       : 000062-local
   Channel ID           : Implicit Interface ID implies current span, 21/1/5, Exclusive
0000 02 01 b8 a6 08 02 80 3e 02 18 03 a9 83 85 .......>......

10:50:24.77 21/1/24 T RR 0000 02 01 01 ba ....

10:50:25.02 21/1/24 R ALERTING

   Call Reference       : 000062-local
   Progress Indicator   : CCITT, Public network serving local user,
In-band information or an appropriate pattern is now available 0000 02 01 ba a6 08 02 80 3e 01 1e 02 82 88 .......>.....

10:50:25.02 21/1/24 T RR 0000 02 01 01 bc ....

10:50:28.43 21/1/24 R CONNECT

   Call Reference       : 000062-local
0000 02 01 bc a6 08 02 80 3e 07 .......>.

10:50:28.43 21/1/24 T RR 0000 02 01 01 be ....

10:50:28.43 21/1/24 T CONNECT_ACK

   Call Reference       : 000062-local
0000 00 01 a6 be 08 02 00 3e 0f .......>.

10:50:28.44 21/1/24 R RR 0000 00 01 01 a8 ....

10:50:35.69 21/1/24 T DISCONNECT

   Call Reference       : 000062-local
   Cause                : 16, Normal call clearing.
0000 00 01 a8 be 08 02 00 3e 45 08 02 8a 90 .......>E....

10:50:35.70 21/1/24 R RR 0000 00 01 01 aa ....

10:50:36.98 21/1/24 R RELEASE

   Call Reference       : 000062-local
0000 02 01 be aa 08 02 80 3e 4d .......>M

10:50:36.98 21/1/24 T RR 0000 02 01 01 c0 ....

10:50:36.99 21/1/24 T RELEASE COMPLETE

   Call Reference       : 000062-local
0000 00 01 aa c0 08 02 00 3e 5a .......>Z

10:50:36.00 21/1/24 R RR 0000 00 01 01 ac ....

10:51:06.10 21/1/24 R RR 0000 02 01 01 ad ....

10:51:06.10 21/1/24 T RR 0000 02 01 01 c1 ....

10:51:36.37 21/1/24 R RR 0000 02 01 01 ad ....

10:51:36.37 21/1/24 T RR 0000 02 01 01 c1 ....


See also


  • NI-1 (US National ISDN Phase 1)
  • NI-2 (US National ISDN Phase 2)
  • 4ESS (Lucent 4ESS specific protocol defined in AT&T TR 41459)

Specifications defining the physical layer and part of the data link layers of ISDN:

From the point of view of the OSI architecture, an ISDN line has a stack of three protocols

  • physical layer
  • data link layer
  • network layer (the ISDN protocol, properly)


External links

  • (list of published recommendations available in English, french and Spanish)

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