Integrated Services Digital Network



(ISDN) A set of communications standards allowing a single wire or optical fibre to carry voice, digital network services and video. ISDN is intended to eventually replace the plain old telephone system.

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.