Integrated Services Digital Network
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integrated services digital network[′in·tə‚grād·əd ¦sər·vəs·əz ′dij·əd·əl ′net‚wərk]
Integrated services digital network (ISDN)
A generic term referring to the integration of communications services transported over digital facilities such as wire pairs, coaxial cables, optical fibers, microwave radio, and satellites. ISDN provides end-to-end digital connectivity between any two (or more) communications devices. Information enters, passes through, and exits the network in a completely digital fashion.
Since the introduction of pulse-code-modulation (PCM) transmission in 1962, the worldwide communications system has been evolving toward use of the most advanced digital technology for both voice and nonvoice applications. Pulsecode modulation is a sampling technique which transforms a voice signal with a bandwidth of 4 kHz into a digital bit stream, usually of 64 kilobits per second (kbps).
Many aspects of telecommunications are improved with digital technology. For example, digital technology lends itself to very large-scale integration (VLSI) technology and its associated benefits of miniaturization and cost reduction. In addition, computers operate digitally. Digital transport provides for human-to-human, computer-to-computer, and human-to-computer interactions. The ISDN is capable of transporting voice, data, graphics, text, and even video information over the same equipment. See Data communications, Digital computer, Integrated circuits
The customer has access to a wide spectrum of communications services by way of a single access link. This is in contrast to existing methods of service access, which segregate services into specialized lines.
Associated with integrated access and ISDN is the concept of a standard interface. The objective of a standard interface is to allow any ISDN terminal to be plugged into any ISDN interface, resulting in terminal portability, flexibility, and ease in operation. See Electrical communications
Integrated Services Digital Network(communications)
ISDN was first published as one of the 1984 ITU-T Red Book recommendations. The 1988 Blue Book recommendations added many new features. ISDN uses mostly existing Public Switched Telephone Network (PSTN) switches and wiring, upgraded so that the basic "call" is a 64 kilobits per second, all-digital end-to-end channel. Packet and frame modes are also provided in some places.
There are different kinds of ISDN connection of varying bandwidth (see DS level):
DS0 = 1 channel PCM at 64 kbps T1 or DS1 = 24 channels PCM at 1.54 Mbps T1C or DS1C = 48 channels PCM at 3.15 Mbps T2 or DS2 = 96 channels PCM at 6.31 Mbps T3 or DS3 = 672 channels PCM at 44.736 Mbps T4 or DS4 = 4032 channels PCM at 274.1 Mbps
Each channel here is equivalent to one voice channel. DS0 is the lowest level of the circuit. T1C, T2 and T4 are rarely used, except maybe for T2 over microwave links. For some reason 64 kbps is never called "T0".
A Basic Rate Interface (BRI) is two 64K "bearer" channels and a single "delta" channel ("2B+D"). A Primary Rate Interface (PRI) in North America and Japan consists of 24 channels, usually 23 B + 1 D channel with the same physical interface as T1. Elsewhere the PRI usually has 30 B + 1 D channel and an E1 interface.
A Terminal Adaptor (TA) can be used to connect ISDN channels to existing interfaces such as EIA-232 and V.35.
Different services may be requested by specifying different values in the "Bearer Capability" field in the call setup message. One ISDN service is "telephony" (i.e. voice), which can be provided using less than the full 64 kbps bandwidth (64 kbps would provide for 8192 eight-bit samples per second) but will require the same special processing or bit diddling as ordinary PSTN calls. Data calls have a Bearer Capability of "64 kbps unrestricted".
ISDN is offered by local telephone companies, but most readily in Australia, France, Japan and Singapore, with the UK somewhat behind and availability in the USA rather spotty.
(In March 1994) ISDN deployment in Germany is quite impressive, although (or perhaps, because) they use a specifically German signalling specification, called 1.TR.6. The French Numeris also uses a non-standard protocol (called VN4; the 4th version), but the popularity of ISDN in France is probably lower than in Germany, given the ludicrous pricing. There is also a specifically-Belgian V1 experimental system. The whole of Europe is now phasing in Euro-ISDN.
See also Frame Relay, Network Termination, SAPI.
Usenet newsgroup: news:comp.dcom.isdn.