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Integrated Services Digital Network
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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.