RFC 935 (rfc935) - Page 2 of 13

Reliable link layer protocols

RFC 935                                                     January 1985
Reliable Link Layer Protocols


   the character-oriented standards, in both synchronous and
   asynchronous environments, including all the major US mainframe
   manufacturers.

   The only other serial link layer protocol known to the author in as
   wide use as these is DEC's DDCMP (Digital Data Communications Message
   Protocol).  This protocol uses a character count instead of framing
   characters, but is in other respects a character-oriented protocol.

   The next sections of this note will compare the three protocols above
   on several bases, paying particular attention to the characteristics
   that make particular aspects of the protocol appropriate to the
   low-speed, asynchronous, serial environment.

Frame Structure

   All serial protocols divide the data to be transmitted into units
   known as frames.  A frame is typically one to several hundred
   characters in length.  The frame structure is the major difference
   used above to divide the protocols into three classes.

Character-Oriented Framing

   Character-oriented protocols use two techniques for defining a frame.
   First, it is necessary to determine where characters start and stop.
   The technique used for this purpose is to transmit a number of unique
   characters prior to the start of a frame.  The character generally
   used for this is the SYN character.

   Note that this is not required when using asynchronous transmission.
   Since each character is itself framed by start and stop bits, there
   is never a question of where characters begin and end.

   The main technique for structuring a frame is the use of special
   framing characters to delineate the start and end of a frame, and to
   delineate portions of the frame (such as header and text).  Some uses
   of character-oriented protocols require that these characters never
   appear in the header or text of the frame, while others allow
   "transparent" transmission.  Transparency is obtained by preceding
   each framing character by a unique control character, typically DLE.
   In this way, all characters may be sent as header or text, except for
   DLE.  In order to allow DLE to be sent in the header or text, the DLE
   is doubled.






Robinson