[wellylug] Modem upload problems

[Jethro Carr]

On Sat, 2005-06-04 at 15:22, Lyndsay Mountfort wrote:
> I recently gave up on the onboard modem on my laptop, and bought a
> PCMCIA modem from Dick Smith. Using Mepis 3.3 and kernel 2.4.29 it was
> detected and seemed to work ok. Turned out the standard 2.6.10 build for
> Mepis 3.3 was missing the relevant driver (serial_cs), but having
> compiled and installed the driver it appeared to work fine also.
> 
> Until I tried to send a reasonably large attachment to someone. It turns
> out that all uploads of significant size choke to a crawl, with long
> periods of inactivity showing in the KPPP statistics graph. A couple of
> hundred kb can take hours. Doesn't matter what software is uploading -
> email, browser (webmail), or scp from the Konsole. Downloads of any size
> work very well.
> 
> Looking at the PCMCIA HowTo, it suggested that slow performance might be
> related to an interrupt conflict, so I followed the instructions and
> used setserial to set it to polled mode (irq 0), and hey presto, uploads
> worked reasonably well (at least about 2kb/s, a considerable
> inprovement. The rating for the modem is 56k down, 33k up).  So from
> there I played around with various other interrupts, but none of them
> worked satisfactorily.
> 
> So I take it from that there is not necessarily a conflict, but it likes
> polled mode better, at least for uploads. Unfortunately, downloads are
> excellent (for a modem) using interrupts (>5kb/s), but using irq 0 they
> reduce to about 2k as for uploads.
> 
> Has anyone struck this before? Anyone got any ideas what I could try to
> get usable upload speeds, but keep good download performance?
> Initialisation strings or anything? It's all well outside my very
> limited hardware knowledge.

I've had simular problems (with a pcmcia modem & a serial modem). The
pcmcia modem was fixed when I recompiled pcmcia-cs to autodetect the
IRQ. The serial modem had a problem with flow control.

It sounds like you've tried all the irq's, so it could be a issue with
flow control.


I'm not very knowledgeable in this area, but I had a problem with a DSE
serial modem once like this which was caused by flow control.

Flow control is needed when uploading files, as the modem is slower than
the pc at transmitting data. If the pc sends data to it at full speed,
the modem gets 'overloaded' and drops all the data it can't fit into
it's small buffer.

Flow control tells the computer to slow down the sending of the data, to
match the upload speed of the modem.

I'mm not sure about kppp (I don't use it), but there might be some
settings in kppp, and see what the flow control is set to. There is
usually 3 options:
- none
- xon/xoff
- crt/crts

Try them and see if it works. Certain modems also need extra AT commands
passed to them to enable flow control. See your modem's manual.

hope this helps,

-- 
-- Jethro Carr

jethro.carr at jedolinux.com

http://jethrocarr.jedolinux.com
http://jethrocarr.jedolinux.com/index.php?page=cv/cv.php

http://www.jedolinux.com
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