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Karl Auerbach comments on VOIP (yes he does have a life outside of ICANN. ;-) djf)
From: Dave Farber <dave () farber net>
Date: Sat, 15 Mar 2003 20:59:18 -0500
------ Forwarded Message From: Karl Auerbach <karl () cavebear com> Reply-To: Karl Auerbach <karl () cavebear com> Date: Sat, 15 Mar 2003 17:52:22 -0800 (PST) To: Dave Farber <dave () farber net> Subject: Re: [IP] some comments on Review of Vonage's VoIP broadband phone service Over the last few months I have been building an end-to-end test lab to evaluate the behaviour of voice-over-IP systems and phones under controlled degrees of network impairment. (Yes, I do have a life outside of ICANN. ;-) See the early report: http://www.iwl.com/Products/maxwell/VoIPReport.html and somewhat more product oriented: http://www.iwl.com/Products/maxwell/Maxwell_for_VoIP.html What we've done is to build a box (we call it "Maxwell" after James Clarke Maxwell's famous 2nd law of thermodynamics daemon). Max allows us to do stateful impairments of the traffic flows - losses, duplications, delays, reordering, packet rewritings, etc. Around this we've built various test rigs to let us inject analog voices and tones into phones and measure the analogue coming out of the other phones. This is somewhat different than other kinds of testing, mainly used with POTS phones, in which the phones themselves are not part of the test system. But with Voice-over-IP the quality of the RTP/RTCP implementation in the phones is one of the most critical aspects of voice quality. I spent several years at Precept Software/Cisco working with RTP/RTCP on IP/TV, a high quality audio/video application and I can attest first hand that even though the RTP/RTCP RFC's are not very thick, they are quite hard to implement well. Anyway, we've got a couple of dozen phones - soft and hard, SCCP and other call setup protocols, including the Cisco ATA 186 that Vonage uses. Although various setup protocols are used, all these phones ultimately use RTP/RTCP for the actual voice transport. We're still pretty early in our work on this, but the results are, to put it euphemistically, quite interesting. And its hard to nail things down into categories - there are a lot of adaptive algorithms that implementors use to handle jitter (variation in transit delay) and sharp changes in the perceived jitter (i.e. the variation in the variation of transit delay.) We've also noticed some more subtle effects - all voice-over-ip phones induce at least a slight delay (some home-brew softphones induce a couple of seconds of delay). At a certain point (the ITU says around 150milliseconds of delay) people start shifting to walkie-talkie mode of speach rather than natural conversation. We've noticed that before that happens, there is a potential for the parties to start to believe that the other person is responding slowly. This may lead to false conclusions that the slow person is under the influence. When evaluating voice quality one has to be careful to compare apples with apples and oranges with oranges - virtually every VOIP phone is software driven and the phone's behaviour will vary from software release to software release. In addition, the specific configuration settings - such as suppression of packets during periods of silence - can have an impact on quality. I'll be at the IETF in San Francisco this coming week if anybody wants to get together and talk about this stuff. --karl-- ------ End of Forwarded Message ------------------------------------- You are subscribed as interesting-people () lists elistx com To manage your subscription, go to http://v2.listbox.com/member/?listname=ip Archives at: http://www.interesting-people.org/archives/interesting-people/
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- Karl Auerbach comments on VOIP (yes he does have a life outside of ICANN. ;-) djf) Dave Farber (Mar 15)