Integrated Services Digital Network

communications (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 SwitchedTelephone 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 RateInterface (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.

FAQ.

Usenet newsgroup: news:comp.dcom.isdn.

(1998-03-29)