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 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.
FAQ (ftp://src.doc.ic.ac.uk/usenet/news-info/comp.dcom.isdn/).
Usenet newsgroup: news:comp.dcom.isdn.
