Tuning PPP over 2G GPRS: MRU and MTU
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From: http://www.net42.co.uk/os/linux/ppp_gprs_tuning.html
This is just a quick note to report a drastic improvement in the usabilityof GPRS on Linux in 2G service areas (GSM not UMTS) by setting small valuesfor 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 phoneand in an Option Fusion GT+ datacard, suggest that Vodafone 2G GPRS doesn't need youto tweak the MTU + MRU. So either Vodafone is doing this automatically, or there's somethingabout 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 beingnegotiated.If I get time I'll try setting 'kdebug' to 5, but I think that willjust show data packets as well.
Posted: Mon Jan 29, 2007 1:13 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)
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