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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 Speech and Data Net").
In a videoconference, ISDN provides simultaneous voice, video, and text transmission between individual desktop videoconferencing systems and group (room) videoconferencing systems.
In the phrase "Integrated Services Digital Network",
- 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, without forcing the purchase of multiple analog phone lines at a much higher transmission rate.
- Digital refers to its purely digital transmission, as opposed to the analog transmission of plain old telephone service. If you're using an analog telephone modem for Internet access at this moment, your Internet service provider's modem has converted this site's digital content to analog signals before sending it to you, and your modem converts those signals back to digital when receiving (the same thing happens with every keystroke and mouse click you transmit). When you connect with ISDN, there is no analog conversion. ISDN transmits data digitally, resulting in a very clear transmission quality. There is none of the static and noise of analog transmissions that can slow transmission speed.
- 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 include all of the telecommunications and switching equipment in between.
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 three ISDN implementations. 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). 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.
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.
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
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 modems, making ISDN more pleasant 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.
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 ....
See also
Protocols
- DSS1 (ETSI "Euro-ISDN", also used in many non-European countries)
- DSS2 (Digital Subscriber Signalling System No. 2)
- INS-NET 64/1500 (Japanese national/NTT carrier-specific protocol)
- DACS used in the UK by British Telecom it uses non standard D channel signalling for Pair gain
- FTZ 1 TR 6 (obsolete German national protocol)
- TS.013/TS.014 (obsolete Australian national protocol)
- VN2/VN3/VN4 (obsolete French national protocols)
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)
Other
External links
- http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/isdn.htm
- http://hea-www.harvard.edu/~fine/ISDN/
- http://www.itu.org
- http://www.ralphb.net/ISDN/
- Linkbit Online ISDN Message Decoder
- http://www.whatlaptop.co.uk/YcPXCrBoJoyL1w.html
- http://www.publications.parliament.uk/pa/cm200203/cmhansrd/vo030714/debtext/30714-21.htm
ITU-T recommendations | Network access | telephony | Integrated Services Digital Network
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"Integrated Services Digital Network"