[nsp] atm encapsulation overhead
vandry at TZoNE.ORG
Thu Nov 20 17:49:12 EST 2003
On Tue, Nov 18, 2003 at 09:41:20AM +0100, Iva Cabric wrote:
> You can find information about overheads in different protocols
> (Ethernet, ATM, POS, ...) here:
I may be wrong, but isn't the description of HDLC byte stuffing incorrect
in this document?
It says: The flag is 0x7e, and bytes with values of 0x7e or 0x7d in the
payload are escaped with 0x7d. This is the way it works with asynchronous
PPP (like on a modem or RS232 line). In this case POS is being described,
so I believe the bit oriented (not byte oriented) HDLC algorithm should
be the one given:
Six consecutive 1 bits mark the end of a frame. A zero bit after six or
more consecutive 1 bits marks the start of the next frame (Note that
0x7e meets this). In the payload, a zero is inserted after every group
of 5 consecutive 1 bits in order to be certain that 6 consecutive ones
never appear in the payload.
The pathological case for async PPP (data all 0x7e's) is not nearly so bad
with sync (HDLC) PPP because of this.
More information about the cisco-nsp