On 7 May 2015, at 14:14, jb <[email protected]> wrote: > I thought would be more sane too. I see mentioned online that PDV is a > gaussian distribution (around mean) but it looks more like half a bell curve, > with most numbers near the the lowest latency seen, and getting progressively > worse with > less frequency.
That's someone describing the typical mathematical formulation (motivated by noise models in signal propagation) not the reality experienced over DSL links > At least for DSL connections on good ISPs that scenario seems more frequent. > You "usually" get the best latency and "sometimes" get spikes or fuzz on top > of it. "Good ISPs" (let's, for the moment define good this way) are ones in which the variability induced by transit accross them is small and bounded - BT Wholesale (access network) has - in our experience - delivers packets (after you've removed the effects of distance and packet size) from the customer to the retail ISP with <5ms delay variation (~0%loss) and from the retail ISP to the customer <15ms delay variation <0.1% loss. The delay appears to be uniformly distributed. The major (in such a scenario) cause of delay/loss is the instantaneous overdriving of the last mile capacity - that takes the typical pattern of rapid growth followed by slow decay that would expected for a queue fill/empty cycle at that point in the network (in that case the BRAS) An example (not quite what described above - but one that illustrates the isssues) can be found here; http://www.slideshare.net/mgeddes/advanced-network-performance-measurement Neil > > by the way after I posted I discovered Firefox has an issue with this test so > I had > to block it with a message, my apologies if anyone wasted time trying it with > FF. > Hopefully i can figure out why. > > > On Thu, May 7, 2015 at 9:44 PM, Mikael Abrahamsson <[email protected]> wrote: > On Thu, 7 May 2015, jb wrote: > > There is a web socket based jitter tester now. It is very early stage but > works ok. > > http://www.dslreports.com/speedtest?radar=1 > > So the latency displayed is the mean latency from a rolling 60 sample buffer, > Minimum latency is also displayed. and the +/- PDV value is the mean > difference between sequential pings in that same rolling buffer. It is quite > similar to the std.dev actually (not shown). > > So I think there are two schools here, either you take average and display + > / - from that, but I think I prefer to take the lowest of the last 100 > samples (or something), and then display PDV from that "floor" value, ie PDV > can't ever be negative, it can only be positive. > > Apart from that, the above multi-place RTT test is really really nice, thanks > for doing this! > > > -- > Mikael Abrahamsson email: [email protected] > > _______________________________________________ > Bloat mailing list > [email protected] > https://lists.bufferbloat.net/listinfo/bloat
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