Some results:

The line is fairly long and gives around 1.6 Mbps. It has not failed in 3 years and has been extremely consistent in performance with a 2wire 2700 business hub installed. The Netgear that was originally there have under 1Mb and would drop out most nights. Voice quality is clear/excellent - and the only interruption was a recent theft of underground cable. The drop wire terminates on the outside wall and comes straight through to a filtered face plate from which two extensions are taken.

I tried various combinations or 1, 2 or 3 phones off hook several times.

Taking a single phone off hook resulted mainly in differences of 0dB, -0.1 and -0.2 and one of -0.5dB.

With two off hook, again there were varying results, mainly in the -0.2 to -0.6dB range. Going to all three off the highest was -0.9dB.

Obviously, these are quick and dirty readings and changes in background SNR could be making the reading better or worse but it does show that you can get a minor change when a phone goes off hook.

I also looked at what happens when an incoming call occurs - 0dB each time.

In reply to a post by edinburgh2011:

I'm using an ADSL nation filter.

I'm going to be controversial here, and suggest that you replace this.

Reason?

I went through a couple of these (faceplate) filters in about 3 years. For some reason they didn't last particularly well on my line.

Can I suggest you try an Openreach-branded faceplate?

I'd also go with the suggestion of a 2wire 2700 as a cheap, effective bit of kit.

Thank you for performing the tests, although obviously not as rigorous as in a laboratory environment.

Purely for interest, how were you determining the SNRM values in that experiment?

Taken from the 2700 page ...

Right. So we would both say that a particular value was x.y +/- 0.1 dB.

Thanks guys for a very insightful discussion.

I've not managed to do my tests - I'll do them on Wed.

The router I'm using is a Broadcom based router - TP-Link 8960 (couldn't afford the Billion 7800).

I'm on a relatively short line, close to the exchange:

Downstream Upstream
Line Coding(Trellis): On On
SNR Margin (0.1 dB): 59 96
Attenuation (0.1 dB): 120 41
Output Power (0.1 dBm): 0 115
Attainable Rate (Kbps): 23964 888

the way I saw fluctuating was from the router's stats.

I'll report back as soon as I've done the tests and I'll post screen shots.

Thanks!

Lots of factors come into play, but it has to be assumed that the router is consistent in its measurement over the hour or so and thus +/- 0.05dB. So, what was registered as 0, could be -0.099 or +0.099dB and -0.9 could be anywhere from -0.801 to -0.999

It was getting light, so the overnight increase in atmospheric noise would be receding resulting in an increase in SNR, could there be additional domestic created noise possibly ... But to alleviate those, as much as possible, I used the SNR just before taking a receiver off hook, using an F5 page refresh for the new value.


It definitely showed that a handset can cause a drop in SNR. On a shorter line, it may be a little worse, a different handset could be better or worse - an older device maybe worse and a new DECT better ...

mine has an auto refresh function, so it was showing almost immediately.

there was a drop from 59 = (5.9db) to 57 = (5.7db) during the day.

when there was additional domestic noise (in the eve), the figures were much lower, approx. from 54 (5.4db) to 48 (4.8db) or slightly less.

it just happened i was looking at my router stats when my partner was on the phone!

I will disagree with that. There will be a change to either SNR, received power level, or attenuation.

Seconded.

The SNR figure is the easiest to use in most case. But it is actually the power seen by the modem input that is lower with noise staying constant and hence a lower SNR.

Glad to see that someone else has seen the same effect.

Edited by MHC (Mon 30-Jan-12 23:39:29)