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Re: [Linphone-users] Audio vs. video priority?

From: Simon MORLAT
Subject: Re: [Linphone-users] Audio vs. video priority?
Date: Tue, 16 Dec 2003 15:51:59 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.5) Gecko/20031105 Thunderbird/0.3

Hello Matt,

Is the problem is an overflow when receiving, I think there's no solution for the receiver. The awfull audio comes that audio rtp packets are received too late. For video it's different, because there are less video packets (20/s for audio), but they are bigger, and it does not disturb much our eyes if an image is seen 50ms too late. For audio it's different, you can't catchup time, if 20ms of audio arrive too late, the only the receiver can do is to play 20ms of silence, hoping that's it going be better later. The good solution would be that some router at your internet provider makes a kind of priority queue with packets it sends to you. In case it sees there's too much data, it sends audio rtp packets and keep video packets for later. This is possible to do this on linux, with iptable/netfilter (it's for advanced users but I'm not). However recognizing a video stream from an audio stream for netfilter is not easy. However if we arbitrary decide that the audio port will always be 8000 and video port 9000, then it's easy. Quality of Service is a complex problem. I'm sure it exists ietf protocols for telling routers to prioritize some packets, but I'm not sure it is implemented yet.
Anyway, changing MSSync to MSNoSync will not solve this problem.

Matt Lawson wrote:

I am using Linphone with Asterisk and it works pretty well, particularly when bandwidth is plentiful. However, we are also trying to connect some machines which are using residential DSL lines (maximum 256kbps upstream) and we always eventually run out of bandwidth (remember they may have other things happening than just Linphone).

What I notice is that the video stays together pretty well, but the audio breaks up horribly when bandwidth starts to run out.

My question is, is there any way to give the audio stream "priority" over the video? With a video call, the image is a 'bonus' but the users will expect the audio to be perfect, or at least understandable.

The way our Linphone is now (it is not exactly like the CVS version, because it is a few months old), it uses the normal audio timer sync (20ms or whatever it is) and uses "no sync" for the video. We discovered that this gave the best image quality. But I wonder, is that causing the audio to be lost in favor of video? Should I put the video on a timer also? Or should I change the audio to "no sync" as well? Is there any other way to tweak it so that audio will be sure to come through at the expense of the video?

Suggestions welcome.  Thanks.

- Matt

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