Integrated Services Digital Network

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Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network system, designed to allow digital 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 user. The English term is a "backronym", thought better for English-language advertisements than the original, "Integriertes Sprach- und Datennetz" (German for "integrated voice and data net").

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

1 Configurations
2 Types of communications handled
3 A sample ISDN call
4 See also
- 4.1 Protocols
- 4.2 Other
5 External links

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Configurations

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 signalling and control (but can also be used for data).

There are two kinds of access to ISDN. Basic rate interface (BRI) — also Basic rate access (BRA) — 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 Primary rate access (PRA) — 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 and Australia it is 30B+1D, with an aggregate bit rate of 2.048 Mbit/s (E1).

Using bipolar with eight-zero substitution encoding technique, 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, this is specified in X.31. In practice, X.31 was only commercially implemented in France and Japan.

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Types of communications handled

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 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 phone (Plain Old Telephone Service) with few or no features. However 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 rather unappealing to the private customer.

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

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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                                          ....

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