Re: [rtcweb] Consensus vs. Voting (was Re: Call for Consensus Regarding Selecting Recommended Audio Codecs)

[Roman Shpount <[e-mail]>]

On Thu, Jan 17, 2013 at 9:16 AM, Tim Panton <[e-mail]> wrote:

>
> Wait, you run the risk of double counting (at least to some extent). The
> overall jitter from Alice to Bob isn't changed by transcoding.
> I think you are implying that because we transcode in the middle, there
> will be 2 jitterbuffers and the sum of the depth of these jitterbuffers will
> exceed the depth of the single jitterbuffer that would be needed if there
> was no transcoding. I agree, but I dispute that the sum of these 2
> jitterbuffers would be more than 20ms greater than the individual
> jitterbuffer that Bob would run anyway.
>
> If you look at the typical deployment scenario for 722 - Bob is either a
> conference mixer engine or using a desktop phone on a wire to a reliable
> low loss company network. Most of the packet jitter (and loss) occurs in
> the first couple of hops from Alice's browser through the wifi and the
> cafe's DSL,
> so the jitterbuffer in the middle is probably the right place to put it.
>


Well not exactly double counting. If jitter on both calls legs are
independently normally distributed and if we have jitter of X from webrtc
client to transcoder, and jitter Y from transcoder to the end client, then
in case of jitter buffer in the middle the total delay is X+Y, in case of
end-to-end transmission it is sqrt(X^2 + Y^2). So in case of two 100 ms
jitters the difference is 200 ms of overall delay vs 141 ms of overall
delay.

Regarding the deployment scenarios, you are correct in some cases where
transcoder is sitting on the edge of well configured VoIP network. In such
cases your penalty is essentially equal to jitter within the VoIP network.
Unfortunately, there is still a use case when both end points are on
consumer IP connections, for instance, when you add webrtc client to a
hosted PBX service. Your legacy phones are located at homes or small
offices and your WebRTC clients are on similar connections. You would see
significant jitter on both.
_____________
Roman Shpount