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Integrated services digital network

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Integrated Services Digital Network (ISDN) is a type of telephone network system. More specifically, ISDN is a set of protocols for establishing calls and then tearing them down.

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 connection into the telephone/data network at home, something a little better than a modem connection. Most of the links that would be on the Internet are related to this point of view and talk about the merits of various ISDN modems, carrier’s offerings and tarriffing (features, pricing). Much of the following discussion is from this point of view and correctly points out that from this viewpoint, ISDN is mostly superseded by DSL as a viable connection to the Internet.

There is however a second viewpoint, that of the telephone industry, where ISDN is not a dead issue. The telephone network in some ways is 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 hate to wait 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.

National Services

ISDN has never gained popularity as a telephone network in the United States and today remains a niche product. 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.

In Japan, NTT, a dominant telephone company, provides an ISDN service with the names INS64 and INS1500, which are much less recognized than ISDN.

In the UK, British Telecom 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 ISDN, there are two types of channels, B and D:

  • B channels are used for data, 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 64 kbit/s digital data channels and a 16 kbit/s digital signalling channel, designated as 2B+D, and,

  • Primary Rate Interface (PRI) – containing a greater number of channels, 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)

Calls are made 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 users, lasting until the call is terminated. There can be as many calls as there are data channels, to the same or different end-points. Data channels may also be multiplexed into what may be considered single, higher-bandwidth channels.

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 communcations 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.

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.

Where an analog connection would require a modem an ISDN connection requires a Terminal Adapter, or TA.

A Sample ISDN Call

The following is an example of a Primary Rate (PRI) ISDN call showing the Q.920/LAPD and the Q.930/Network message intermixed (i.e. exactly what was exchanged on the D-channel. 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 ….

The specifications defining the physical layer and part of the data link layers of ISDN are:

  • ISDN BRI: ITU-T I.430.

  • ISDN PRI: ITU-T I.431.

Image http://commons.wikimedia.org/wiki/File:T-Concept-ISDN.jpg

Licensed under the GNU Free Documentation License. It uses materials from the Wikipedia.

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