Tuning PPP over 2g gprs: MRU and MTU

From: http://www.net42.co.uk/ OS /linux/ppp_gprs_tuning.html

 

MTU & MRU seem to have a drastic effect on usability, but I 've not seen this discussed for GPRS.

This is just a quick note to report a drastic improvement in the usability
Of GPRS on Linux in 2G service areas (GSM not UMTS) by setting small values
For PPP's MTU and MRU parameters.

 

I have made a quick post to the PPP BOARD ON THE pharscape.org forums,
To see whether anyone knows a bit more ....

Update Feb 01. 2007: Tests Using Linux kppp with a Vodafone SIM card (APN: 'internet'), both on a 2g gsm phone
And in an option fusion Gt + datacard, suggest that Vodafone 2g gprs doesn' t need you
To tweak the MTU + MRU. So either Vodafone is doing this automatically, or there's something
About their UK network that makes this unnecessary.

 

By contrast, tmuk's 2g gprs network (APN: 'General. t-mobile.uk ')
Doesn't work properly unless you adjust MTU + MRU.

 

I 've tried un-commenting 'debug' (and setting 'kdebug' to 1) in
/Etc/PPP/options in order to see PPP option negotiations in
/Var/log/debug,
But although this shows a lot of stuff, it doesn' t show MTU + MRU being
Negotiated.
If I get time I'll try setting 'kdebug' to 5, but I think that will
Just show data packets as well.

  
   
 Posted: Mon Jan 29,200 7 am post subject: tuning kppp for 2G GPRS

Are there any rules of thumb for this?

Some quick tests of my own suggest that MRU/MTU really benefit from tweaking!
This is with T-Mobile UK using a Novatel wireless eu740 card in a non-3G service area.

With default settings in kppp: (only parameter set = "novj ")
========================================================
-Pings work OK
-But Web access is almost unusable
-Logs show few requests reach the Web Server
-Wireshark shows lots of repeated http get requests every few secs

With mildly optimised settings in KPP:
======================================

-Settings: "novj", "MRU 296", "MTU 296"
(Nothing magic about these numbers, but "Man pppd" suggests them
For very slow links)

-Ping times much the same

-Now the Web is much more usable! Not quick, but fairly predictable.
I'm posting this over 2G GPRS. Page fetch requests don't time out, they just take a while.

Presumably this setting will reduce the speed in 3G service areas, but I guess
It's probably worth it to make this thing usable.

I 'd be very grateful for your thoughts! And for any more tweaks,

Cheers

_________________
-- Martin.
(Novatel wireless eu740 on T-Mobile UK,
Kubuntu edgy EFT 2.6.17-10-generic)