RFC 1618 (rfc1618) - Page 2 of 6

PPP over ISDN

RFC 1618                     PPP over ISDN                      May 1994


   Transmission Rate

      PPP does not impose any restrictions regarding transmission rate,
      other than that of the particular ISDN channel interface.

   Control Signals

      PPP does not require the use of control signals.  When available,
      using such signals can allow greater functionality and
      performance.  Implications are discussed in [2].

      Control signals MAY be required by some of the framing techniques
      described, and is outside the scope of this specification.

   Encoding

      The definition of various encodings and scrambling is the
      responsibility of the DTE/DCE equipment in use, and is outside the
      scope of this specification.

      While PPP will operate without regard to the underlying
      representation of the bit stream, lack of standards for
      transmission will hinder interoperability as surely as lack of
      data link standards.  The D-channel LAPD interface requires NRZ
      encoding at the T reference point.  Therefore, as a default, it is
      recommended that NRZ be used over the B-channel interface at the T
      reference point.  This will allow frames to be easily exchanged
      between the B and D channels.

      When configuration of the encoding is allowed, NRZI is recommended
      as an alternative in order to ensure a minimum ones density where
      required over the clear B-channel, with caveats regarding FCS [2].

      Historically, some implementations have used Inverted NRZ (merely
      switching the sense of mark and space), in order to ensure a
      minimum ones density with bit-synchronous HDLC.  The use of
      Inverted NRZ is deprecated.

      Automatic Detection

         Implementations which desire to interoperate with multiple
         encodings MAY choose to detect those encodings automatically.
         Automatic encoding detection is particularly important for
         Primary Rate Interfaces, to avoid extensive pre-configuration.
         Only simple encodings are currently distinguished.

         The only reliable method of detection available is to switch
         modes between the supported encodings.  Transmission of the LCP



Simpson