Latency
Latency
† This was at http://www.packeteer.com/solutions/voip/sld006.htm
For example:
|
Round-trip times from my office computer (as of 2015.08.05) for 10 pings (with DNS to IP cached) |
min (ms) |
avg (ms) |
max (ms) |
hops |
|---|---|---|---|---|
|
Local LANs (www.wireless.kth.se) |
0.450 |
0.507 |
0.542 |
4 |
|
to northern Sweden (ww Links to an external site.w Links to an external site..lt Links to an external site.u Links to an external site..s Links to an external site.e Links to an external site.) |
13.298 |
14.752 |
17.025 |
11 |
|
To US east coast (www.nyu.edu Links to an external site.) |
98.682 |
98.759 |
98.916 |
17 |
|
To US west coast (ww Links to an external site.w Links to an external site.. Links to an external site.stan Links to an external site.f Links to an external site.ord.edu Links to an external site.) |
177.430 |
178.960 |
182.306 |
21 |
|
To Australia (ww Links to an external site.w Links to an external site..u Links to an external site.o Links to an external site.w Links to an external site..edu.au Links to an external site.) {via the US west coast} |
333.704 |
333.900 |
334.297 |
24 |
Transcript
[slide33] What's the good news? Well, the good news is, if we look at quality versus delay, there's a curve, and we'll come to the details of it later, that basically says we have high usability, high quality with low delays, but as the delays increase, the perception is the quality is worse and worse. And a long time ago, internet telephony was out there in the 600 plus millisecond range. One of the first companies to introduce these systems in it was a company in Israel, and the quality was horrible. But the advantage was much cheaper than long distance calls. So people started using it, even though it was horrible. And then we had CB radio systems, satellite systems, but today, if we look at the delays on the internet, here's a sample of things from 2005, of the round-trip delays, locally to northern Sweden, to the east and west coast of the US, all the way to Australia and back. The one-way delays we see there, 180 milliseconds, no problem, good quality. It's perceived as very good quality. What happens when we get up to delays of about 250 milliseconds, which you get if you have satellite telephony service, with geostationary satellites? It becomes simplex. Because 250 milliseconds is long enough that both of you can hear nothing, so one of you starts to speak, but meanwhile, yes, the other person started to speak, 250 milliseconds later, yes, each of you hear the other speaking. So what do you do? You both stop. 250 milliseconds later, you realize both of you have just stopped. Of course, you were paying for that time, right? So it ends up being simplex. So you talk, and then you say, over, and then the other person talks, and they hand it over to you. But once we get below that barrier, we start to have conversational flows of the audio communications.