Integrated Services Digital Network

From Academic Kids

ISDN is also short for isosorbide dinitrate
Missing image
An ISDN Telephone

Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network system, designed to allow digital (as opposed to analog) transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds, than available with analog systems. More broadly, ISDN is a set of protocols for establishing and breaking circuit switched connections, and for advanced call features for the end user.


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 talk about 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discussion is from this point of view, but it should be noted that as a data connection service, ISDN has been mostly superseded by DSL.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 however a second viewpoint: that of the telephone industry, where ISDN is not a dead issue. 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. On a normal T1, the signalling is done with A&B bits to indicate on or off hook conditions and MF and DTMF tones to encode the destination number. ISDN is much better than this as 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 which is greatly desired by carriers who have to pay for line time and also by callers who become impatient while their call hops from switch to switch.

It is also used as a smart network technology intended to add new services to the public switched telephone network (the PSTN) by giving users direct access to end-to-end circuit-switched digital services.

ISDN has never gained popularity as a telephone network in the United States and today remains a niche product. 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. ISDN is used because of its "guaranteed" real-time, not-over-the-Internet service, and its superior audio fidelity as compared to POTS service.

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.

In the UK, British Telecom (BT) provides Home Highway and Business Highway, which are BRI ISDN services which offer connection from analog devices (such as normal phones) as well as ISDN devices (such as PCs equipped with terminal adapters). Home Highway has been bought by many home users, usually for Internet connection. Although not as fast as ADSL, it was available before ADSL, and in places where ADSL does not reach. BT also offers PRI ISDN.

In France, France Télécom offers ISDN services under their product name Numeris (2 B+D) of which a profesional 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.

In Germany, ISDN is very popular with an installed base of 25 mio. 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.


In ISDN, there are two types of channels, B and D:

  • B channels are used for data--this refers both to voice and data information, and
  • D channels are intended for signalling and control (but can also be used for data).

B stands for Bearer and D stands for Delta.

There are two kinds of access to ISDN:

  • Basic rate interface (BRI) - consisting 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, and,
  • Primary rate interface (PRI) - containing a greater number of B channels and a D channel with a bandwidth of 64 kbit/s, The number of B channels varies based on the country:
    • North America and Japan: 23B+1D, aggregate bit rate of 1.544 Mbit/s (T1)
    • Europe, Australia: 30B+D, aggregate bit rate of 2.048 Mbit/s (E1)

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.

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
  • U - defines the point between the NT-1 and the telco switch2

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 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.

Types of communications handled

Amongst 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 nowadays mostly used as an alternative to analog connection, most commonly for Internet access. Some of the services envisaged 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 (Plain Old Telephone Service) with little to no features. In North America, POTS phones have all of the features available to them marketed (Three Way Call, Call Forwarding, Caller ID, etc), but in Germany, the ISDN lines were marketed as being the phones for people who wanted advanced services.

Where an analog connection would require a modem an ISDN connection requires a terminal adapter (TA).

A sample ISDN 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                                          ....

Related topics


  • DSS1 (ETSI "Euro-ISDN", also used in many non-European countries)
  • NI-1 (US National ISDN Phase 1)
  • NI-2 (US National ISDN Phase 2)

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

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

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


External links

de:Integrated Services Digital Network es:RDSI fr:Réseau numérique à intégration de services gl:RDSI ko:ISDN it:Integrated Services Digital Network nl:Integrated Services Digital Network ja:ISDN pl:ISDN ru:ISDN fi:ISDN zh-cn:综合业务数字网


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