Re: [Bloat] DSLReports Speed Test has latency measurement built-in

[Neil Davies <neil.davies@pnsol.com>]

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On 7 May 2015, at 14:14, jb <justin@dslr.net> 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 <swmike@swm.pp.se> 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: swmike@swm.pp.se
> 
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