I hope to get measurements from their neighbours in due course, then it'll be easier to see if it's something peculiar to them.

Are the speed-versus-linelength figures in your faq here still correct?

I have one person who is 3.5 Km from the exchange who measured download speed at 12.8 Mbps. My current view is that that is impossible (with ADSL 1) and there must be something wrong with the measurement (e.g. a faulty realtime clock on his pc).

Well as ADSL1 tops out at 8Meg, then yes impossible.
With ADSL2+ given the errors involved in estimating distance it is possible.

Why do you think it is the real time clock in the PC? More likely to be a user error, or a false reading from a speedtest, some do over estimate or interactions with AV suites cause issues.

As for the FAQ, yes they still stand, but would recommend abandoning distance use, and relying on attenuation where it is available.

In reply to a post by pvfpvf:

I have one person who is 3.5 Km from the exchange who measured download speed at 12.8 Mbps. My current view is that that is impossible (with ADSL 1) and there must be something wrong with the measurement (e.g. a faulty realtime clock on his pc).

Inaccurate measurements of download speeds can sometimes be due to anti-virus programs - e.g. Kaspersky IS 2011 could inflate download speeds by a very wide margin.

Thanks. But how does an antivirus program inflate the download speed? It cannot actually increase the rate at which data is received, so I imagine the only other thing it could do is give a false reading of elapsed time, which sounds implausible to me.

All the speed test readings I'm soliciting are from broadbandspeedchecker.co.uk - sorry about that, but I just thought the way to run it and its presentation was simpler for the lay person than the alternatives, including tbb's estimable one. I think it unlikely he misread the displayed result, so was trying to think of any technical effect that could give a totally wrong reading like 12.8 Mbps (in my view more than twice what he can attain).

There is no ADSL2 on my exchange.

A possibility that's just occurred to me is one of those software accelerators. I presume those work by redirecting the browser to a server, which gets the wanted data quickly, compresses it, sends it to the PC in compressed form, and then expands it in the PC, before giving it to the browser. That would give falsely fast readings, unless the file that is downloaded by the speed test cannot be compressed. That suggests the speed testers should use test data that cannot be compressed: e.g. a JPG photo that has been Winzipped.

Can't get attenuation data from lay users: too complicated.

Speed tests done with some AV products will show wildly different and erroneous results due to the scanning of network traffic and because of buffering of traffic. This only affects the numbers shown and is not related to the actual download speed

e.g. http://forum.kaspersky.com/index.php?showtopic=227894

Edited by 4M2 (Thu 30-Aug-12 13:02:03)

I can confirm that Kaspersky used to cause some speed tests to give absurd results, such as 100Mbps on an ADSL line.

This only applied to some testers. My memory is that speedtest.net and dslzone suffered, but that thinkbroadband and some others did not. Someone said it was flash based testers that failed with Kerspersky; I can't verify that.

The bt speed tester was usually OK - can not use it now because I'm LLU'ed however I think the thinkbroadband tester and iPlayer diagnostic results are about right when using KIS IS...

As others have suggested then sounds like that tester may have been affected by something like the AV software on their machine.

What happens is that the AV software delivers data in a bursty fashion, and unless the speedtest coder is aware of this, then the wrong results can be displayed.

Our Flash based tester is as simple as that one to use, and can post result to twitter for people.
http://www.thinkbroadband.com/speedtest/broadband.html
Broadband Speed Test Result - 6.27 Mbps (down); 0.84 Mbps (up) (via @thinkbroadband)

Thanks. Do you think your flash speed test would avoid being fooled by anti-virus burst delivery? If so, I'll ask him to try it.