Integrated Services Digital Network
IDSN
ISDN

    (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>.

   Usenet newsgroup: <news:comp.dcom.isdn>.

   (1998-03-29)