[techtalk] PPP/ISDN internal modem/Buffer overruns???

Kristina Pfaff-Harris kristina at dataengines.com
Mon Mar 20 13:09:00 EST 2000

Hi all.  I've about exhausted most possibilities on a problem I'm having,
so I thought I'd try the list.  I have a Zoom ISDN TA 2000 internal ISDN
modem.  It has it's own built-in 16550(A) UART, so pppd can use it just
like any other modem. (The modem itself takes care of multilink/channel

Here's the problem: even though I can get a good 128K multilink session
going, uploads (with ftp or scp) kill the network after 400K - 1meg of
transfer.  This does not happen in single-channel mode.  Ifconfig shows no
errors.  PPP shows no errors -- it doesn't even seem to realize the
connection has gone away.  Netstat shows the connection as "ESTABLISHED"
but with 50 - 60K in the Send Q. Then after killing pppd, is shows the
connection as being in "FIN-WAIT."

I've tried hardware *and* software flow control, irqtune, hdparm,
setserial to increase the baud base, asyncmap 0, setting the mru and mtu
to varying values from 296 - 1500, setting txqueuelen to anything from 1 -
10 under ifconfig for ppp0, patches for the kernel up through 2.2.15pre14
(currently using 2.2.14), and pppd from 2.3.5 through 2.3.11.  I can't use
isdn4linux as there's no support for AT&T 5ESS custom or DMS-100 switches,
spids, and so forth.  I've also seen that people have gotten this modem
working with kernel 2.0.38 and pppd 2.2, but unfortunately downgrading is
not an option.

Any ideas out there?  I swear I have read darn near every message on the
kernel development and ppp lists, in addition to many other web pages
including the Serial HOWTO, Serial Programming HOWTO, PPP HOWTO, Modem
HOWTO, etc, etc, etc.  At this point, I feel like there's just something
really simple I must be missing...any help hugely appreciate. :)


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