Date: Thu, 04 Sep 1997 23:56:28 +0100 From: Brian Somers <brian@awfulhak.org> To: Mike Smith <mike@smith.net.au> Cc: Greg Lehey <grog@lemis.com>, Nate Williams <nate@mt.sri.com>, Jaye Mathisen <mrcpu@cdsnet.net>, hackers@FreeBSD.ORG Subject: Re: Anyway to get connect speed with usermode ppp/tun0 device? Message-ID: <199709042256.XAA26318@awfulhak.demon.co.uk> In-Reply-To: Your message of "Thu, 04 Sep 1997 10:21:41 %2B0930." <199709040051.KAA00434@word.smith.net.au>
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[.....]
> > 1. The line is flaky, and it's causing errors.
> > 2. The modem isn't falling back.
> > 3. LQR is counting those errors and reporting them.
>
> This is all well and good until you add an error-correcting layer to
> the equation.
>
> If 1., then the receiving end will request a retransmission. If the
> errors are really bad, then one or both ends will attempt 2. At any
> rate, there should be no errors for 3. to detect; the error-correcting
> protocol is meant to guarantee error-free data transmission.
Ah, but if 1., where the ``line'' means the serial cable between the
UART & the modem ("It always seems to happen when I switch on the
radio", or "My ISP had a guy with an electric screwdriver playing
with their modem rack"), or if we've got sio overflows, or if the other
side is just sending us crap, *this* is where we see the FCS errors.
The error correction stuff, as you say, will hide this from the LQR
and application levels.
Interrestingly enough, I guess a modem that "falls forward"
(increases its data rate) may force sio overflows by essentially
catching up with the DTR speed (if DTR is set pretty low).
> > > and might be related to the other hanging things up.
> >
> > I don't understand what you're saying here.
>
> I'm not sure either; the most common cause of FCS errors on modern PPP
> links is flow control problems; it's possible that the terminal server
> you were connected to just dropped its guts, or the modem likewise.
Cosmic rays are common too :-)
> mike
>
>
--
Brian <brian@awfulhak.org>, <brian@freebsd.org>
<http://www.awfulhak.org>;
Don't _EVER_ lose your sense of humour....
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