RFC 1598 (rfc1598) - Page 2 of 7

PPP in X

RFC 1598                      PPP in X.25                     March 1994


2.  Physical Layer Requirements

   PPP treats X.25 framing as a bit synchronous link.  The link MUST be
   full-duplex, but MAY be either dedicated (permanent) or switched.

   Interface Format

      PPP presents an octet interface to the physical layer.  There is
      no provision for sub-octets to be supplied or accepted.

   Transmission Rate

      PPP does not impose any restrictions regarding transmission rate,
      other than that of the particular X.25 interface.

   Control Signals

      Implementation of X.25 requires the provision of control signals,
      which indicate when the link has become connected or disconnected.
      These in turn provide the Up and Down events to the LCP state
      machine.

      Because PPP does not normally require the use of control signals,
      the failure of such signals MUST NOT affect correct operation of
      PPP.  Implications are discussed in [2].

   Encoding

      The definition of various encodings 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, X.25 requires NRZ encoding.



3.  The Data Link Layer

   This specification uses the principles, terminology, and frame
   structure described in "Multiprotocol Interconnect on X.25 and ISDN
   in the Packet Mode" [4].

   The purpose of this specification is not to document what is already
   standardized in [4].  Instead, this document attempts to give a
   concise summary and point out specific options and features used by
   PPP.




Simpson