RFC 935 (rfc935) - Page 2 of 13
Reliable link layer protocols
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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