pppd and it's baud rate setting with HSDPA gsm modems

pppd and it's baud rate setting with HSDPA gsm modems

[Jar]

Hello

I am using usbserial+option kernel modules with my HSDPA gsm modem. The
dialer is wvdial.

wvdial.conf

[Dialer Defaults]
Modem = /dev/ttyUSB0
Baud = 3600000
...
...

When I use "Baud = 3600000" in wvdial's config file, the pppd complains
"pppd[12246]: speed 3600000 not supported". But still I get the 1Mbit/s
download speed. However if I set "Baud = 460800" the I get also 1Mbit/s
download speed. But 460800bit/s < 1Mbit/s ??

Is this kind of "USB serial link" some special case, where pppd doesn't
use the speed setting at all?


-- 
Best Regards, Jar

Re: pppd and it's baud rate setting with HSDPA gsm modems

[Paulius Zaleckas]

Jar wrote:
> Hello
> 
> I am using usbserial+option kernel modules with my HSDPA gsm modem. The
> dialer is wvdial.
> 
> wvdial.conf
> 
> [Dialer Defaults]
> Modem = /dev/ttyUSB0
> Baud = 3600000
> ...
> ...
> 
> When I use "Baud = 3600000" in wvdial's config file, the pppd complains
> "pppd[12246]: speed 3600000 not supported". But still I get the 1Mbit/s
> download speed. However if I set "Baud = 460800" the I get also 1Mbit/s
> download speed. But 460800bit/s < 1Mbit/s ??

Option driver doesn't care about baudrate. It can be 9600 and you would
still get 1Mbit/s. This is virtual serial port and it has little common
with real UART.

> Is this kind of "USB serial link" some special case, where pppd doesn't
> use the speed setting at all?

thats driver who is not using speed setting, not pppd.

Re: pppd and it's baud rate setting with HSDPA gsm modems

[James Cameron]

Setting the speed on a HSDPA or GPRS modem is done using AT commands,
and this is part of the negotiation between the modem firmware and the
service provider's radio network.  The service provider may constrain
the speed.  The received signal strength and bit error rate may
constrain the speed.  The base station's uplink may constrain the speed.

To fiddle with the Quality of Service settings to change the speed, look
for the modem documentation.  The exact command depends on the modem,
but I've been able to reduce the speed using one of these: +CGEQREQ
+CGEQMIN +CGQREQ +CGQMIN ... I've never got it to go faster without
fixing the root cause of why it is slow.

The speed between the modem and the host is constrained by the USB
drivers.  Ensure you are using a USB 2.0 port.  Ensure you are using a
USB driver that pre-queues URBs.  The current usbserial and option
module in kernel 2.6.27 do this fine.

-- 
James Cameron
http://ftp.hp.com.au/sigs/jc/