Here's a mystery I can't quite figure out so I thought I'd write about it to see what you think: There are a number of VoIP services I use frequently over a DSL link and they all work well. Over a 3G wireless connection, however, SIP via Sipgate.de isn't working as it should. While there is almost no noticeable delay in the speech path in downlink direction, the uplink speech path has a delay of almost 1000 ms!
One might argue, that's a 3G issue, but it definitely is not, as the delay in Skype in both directions is just fine. O.k. so maybe it's a combination of the mobile device I use for my SIP calls, a Nokia N95, and the 3G router that's causing this. No, I can rule that out, too, since I've tried it with a Huawei D100 3G router as well as with my Fritzbox DSL/3G router from AVM. I also use the Fritzbox over DSL and the N95 works well over that backhaul link, so that can't be the issue either.
My final idea: Maybe something is done in the network!? Possible but unlikely as the behavior is the same in Vodafone's network in Germany, the Mobilkom A1 network in Austria and Wind's network in Italy.
Other ideas, anyone?
Maybe operator are applying lower Qos strategy in order to dissuade SIP users ...
As Skype is using its own protocol, it pass through
the DSL operator Numéricable in france is doing this for torrent packets (even if it's encrypted, cisco routers are able to reconise unusual traffic)
Posted by: François | October 08, 2009 at 11:44 AM
Are you certain it is a constant delay?
Could this just be how the chosen codec reacts to a wide distribution of jitter
Perhaps this wireshark analysis would be insightful:
http://wiki.wireshark.org/RTP_statistics
Posted by: Robert Stonehouse | October 08, 2009 at 09:07 PM
Hi Robert,
interesting suggestion, I'll have a look!
Cheers,
Martin
Posted by: Martin | October 11, 2009 at 12:11 PM