Use of COBS within RS-232 HCI Interface
Original Post: COBS (eGroups
Msg.) Date: 2001-01-03
In the core specification of HCI RS-232 interface, it
is said that in the event RTS/CTS are not available, then the COBS
(Consistent Overhead Byte Stuffing) protocol can be implemented as a means to
detect an error and to resynchronise.
The Consistent Overhead Byte Stuffing is a recent
proposal to PPP that yields less than 0.5% overhead, regardless of the data
pattern. It uses two steps to escape the delimiter, 0x7E. The first step is
eliminating zeros and then replacing all 0x7E with 0x00 between the beginning
and ending delimiters.
COBS Is something like AHDLC which is generally used to
TX frames in PPP
It uses a scheme like this
Code (n) Followed by: Meaning
-------- ------------ -------
00 Unused (framing character placeholder)
01-CF n-1 data bytes The data bytes, plus implicit trailing zero
D0 n-1 data bytes The data bytes, no implicit trailing zero
D1 Unused (resume pre-empted packet)
D2 Unused (reserved for future use)
D3-DF nothing a run of (n-D0) zeroes
E0-FE n-E0 data bytes The data bytes, plus two trailing zeroes
FF Unused (PPP error)
More detail can be found at
http://rescomp.stanford.edu/~cheshire/draft-ietf-pppext-cobs-00.txt
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