[linux-elitists] USB-Serial and pppd

Greg KH greg@kroah.com
Thu Jun 12 10:43:53 PDT 2008

On Thu, Jun 12, 2008 at 01:27:41PM -0400, Matthew Galgoci wrote:
> > Yes, the "generic" usb-serial driver is slow, as designed.  The sierra
> > driver is not using the "generic" driver, but only the usb-serial
> > framework, and as such, should be much faster.
> >
> > What kernel versions are you using in your above configurations?
> Greg,
> I've got a Verizon USB720 branded Novatel Wireless CDMA device. It's
> currently using the "option" driver* by Matthias Urlichs in conjunction
> with usb-serial**.
> Kernel version is the current Fedora 9 errata kernel***.
> It seems fairly snappy but in light of your comments, I'm wondering if
> this is suboptimal and if there is a way to make it go faster. Any
> suggestions?

Yes, the option driver can (and should) be changed to go faster.  It's a
simple one line change.

In drivers/usb/serial/option.c, change the line:
	#define N_OUT_URB 1
to be:
	#define N_OUT_URB 4

Your upload throughput should increase a bit.

Another number to look at is the output buffer size, if you are doing
lots of uploads.  Change:
	#define OUT_BUFLEN 128
	#define OUT_BUFLEN 4096

The combination of the two changes should help out a lot.

The dowload speed should be pretty much as fast as the network allows,
but that all depends on your local connection.  If you want to test it
out, you can change the line in the driver:
	#define N_IN_URB 4
to be a larger number, like 8:
	#define N_IN_URB 8

It probably will not matter that much, but it might.

If it does, let me know and I'll increase it.  With 3G support for a lot
of these devices, I should probably increase the buffers anyway, as it
can't hurt.

I can't do this testing myself very easily, because while I have a lot
of these modems, my testing SIM card is tied to one specific device
(damm wireless phone companies) and my basement office doesn't provide a
good signal.

If anyone knows of a way to "unlock" these modems to work with any SIM
card (as long as the SIM card has a data plan), please let me know.


greg k-h

More information about the linux-elitists mailing list