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.
     
        FAQ
        (ftp://src.doc.ic.ac.uk/usenet/news-info/comp.dcom.isdn/).
     
        Usenet newsgroup: news:comp.dcom.isdn.
     
        (1998-03-29)