RE: USB - ignorant questions

From: Dunlap, Randy (
Date: Tue Aug 01 2000 - 18:45:17 EST

> Hello all,
and to you,

> I have some very ignorant questions about USB support in
> Linux. It seems
> to be available and relatively stable for 2.2.

There's a backport patch and it's in a few distributions.
I think that when 2.4.0-final is released and it has lots
of "testing," we'll hear about some problems that we
haven't heard yet.

> Does it work well with USB
> modems, and can you connect a USB cable between two PC's and
> use PPP across it?

It works with USB modems that fit the ACM (Abstract Control
Model) subclass of the USB Communications Device Class (CDC) spec.
Actually it may not work with all of the ACM modems.
The CDC spec allows modems to use bulk or isochronous transfers
for data but the linux-usb acm driver only supports modems that
use bulk transfers (or pipes or endpoints).
The best thing to do before buying a USB modem is to ask if
it is known to be working (supported) on one of the linux-usb
mailing lists (see
or to look at the Working Devices list (at, but it's not really updated enough.

For connecting 2 systems with a USB cable, it can't be done
with a "normal" USB cable. They are strictly for host-to-device
connections (master-slave relationship). You can get
a "smart" USB cable that makes each host think that it's
talking to a device (since there is a device in the cable).
And we have a driver for this smart cable if you get the
Prolific Technology device (Acer also sells the Prolific

> Second, and more important, is there any possible way to use the USB
> subsystem support in any older kernel, such as 2.0? I know
> this sounds
> rediculous, but I'm wondering how hard it would be to
> back-port (if it's not already available).

I don't know of a 2.0 backport. Go for it and report back
on it in a few months. It would be a fairly large task.


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