Re: Irqtune: some stats (was Fix for your serial/PPP problems)

[Gerry Jensen <gerry@optimed.com>]

> Just at a guess (and I don't know ppp internals that well), it
> might be PPP escaping the zero as a control character.  If the
> software had to handle the escape sequence for "N thousand"
> nulls, it might bog it down a bit, and those escape sequences
> might themselves take up more space, as they often do.  Really,
> though, I can't see that accounting for the difference on a
> modem that does compression.  The compression should be more of
> a factor than escaping every character even, assuming it IS a
> compressed link.
> 
> I don't know if PPP even escapes nulls.

You set up what characters you want escaped (in the receive direction) in
the /etc/ppp/options file (or ~/.ppprc) with the "asyncmap <map>" option. 
<map> is a 32 bit hex number with each bit representing a character to
escape.  "asyncmap 0x00000000" will escape no characters which is what one
would want in the general case (8-bit clean line, hardware flow control). 
If you omit the asyncmap option, *all* control characters (0-31 decimal)
are escaped.  So forgetting to put in the asyncmap option will slow down
your PPP connection by about 12.5% (if my math is correct) for random
data.  Transfering a file of all zeros should make your connection twice
as slow if your modem is not doing hardware compression.

Gerry