SIP return voice is more delayed than outgoing?


We have a dedicated Asterisk 1.6 server running as a front end SIP server for authenticated and internal users only, using SDSL 2MB/2MB, in the UK. The provider of out internet also is the provider of our outgoing SIP, so there are a few hops with a total of 30ms to the sip server.

  • Everything works fine in the office. No delays.
  • Everything works fine using extension dialling from two outside lines. Ie bot extensions are at home using home br= broadband of 512mb… there is roughly 100-200ms delay on both channels- which is fine.

The problem occurs when any extension, office, or home user dials out using a Trunk. Out Main one or even voipcheap.

The outgoing audio get to the “mobile” phone for test sake in not more than 250ms- that’s perfect. But when the person on the “mobile” replies, the audio returns after 500-750ms.

I have not timed anything exactly but that’s what it feels like. I tried running channel stats in CLI - jitter is 0, lost is 0

I mainly use aLaw codec, but i started forcing other like g729, g726 - Jitter is off, but even when its on it makes no difference (to my ear) the returning delay is still there.All these codecs are supported by my SIP servers, and all SIP clients connect using aLaw codec, and the aLaw has the lest translation time… but I am not sure if it actually translating it somewhere along the line, at should just pass through.

I do not know how to debug/trace the returning channel issue. Where is the delay, what can i do to try and resolve this.

The problem is that people start talking over each other because the delay out and in is different and confuses people, it would be better if the delay was the same. but i think 500ms is too much for the time and effort we spent setting up dedicated internet and server. :frowning:

Does anybody have some advice?

Google “buffer bloat”. Note that 250ms outbound is uncomfortably long.

Thanks - got to the bufferblaot page. I know all that. And i know 250ms is a bit laggy. But why is the same return route twice as bad…
if i do traceroutes and ping to my voip server over internet its 35ms. Both my modem and the providers modems are VoiP QoS optimised and do not traffic anything else except VoiP packets. There are no proxies inbetween us, no firewalls, my jitter buffer is off.

I cannot understand why the outgoing channel is faster than the returning one… is there a way to trace the return route of voice packets. Obviosly they are being buffered somewhere on the way back, or going via a proxy… But RTP is set to symmetrical so it should not.