tel. and bb

Hello everyone

I was wondering if someone could explain please - whenever we use the landline, our SNR drops from 6db to 5.5db or lower. Now, we used to suffer from frequent disconnections - they're less frequent now (thanks to a Broadcom chipset based router), but they've not gone completely. Whenever our SNR drops to 4db or around, we get errors in the router stats. What would this point to? Is it normal for SNR to drop whenever the tel. is used? It's a Siemens c610a tel. and I'm using an ADSL nation filter.

Thanks for your time and help!

Edited by deleted (Sat 28-Jan-12 21:15:33)

There really should not be any noticeable drop in the SNRM when the telephone is used.

Please borrow and then test test with;

(1) a different filter
(2) a different telephone.

Thanks very much.

I've tried with another phone - an old one with a dial (love them!) and the same thing occurs - a drop by 0.4db in both down and upstream SNR. Maybe it's not a lot - I don't know, but earlier this afternoon it was down from 6db to 4.4db and it caused some errors, as shown in the router stats, unless it was something else. I'll try a different filter (and from the test socket) and report back.

Also, there was a noticeable hiss, louder on the phone with a dial than on the remote one, on both phones. I think I've asked about it before and I think it was down to the wireless signal from the router, but shouldn't be heard on the wired phone.

Anyway, thanks - you're all great, very helpful and knowledgeable!

Edited by deleted (Sat 28-Jan-12 23:52:06)

In reply to a post by edinburgh2011:

I'll try a different filter (and from the test socket) and report back.

That is a good idea and -- oops -- something I should have mentioned previously.

Also, there was a noticeable hiss, louder on the phone with a dial than on the remote one, on both phones. I think I've asked about it before and I think it was down to the wireless signal from the router, but shouldn't be heard on the wired phone.

The general rule of thumb is to keep the base station for a cordless phone and a modem/router as far apart as is practical.

It is possible to notice inductive coupling between a wired telephone and a modem/router (even one which has its WiFi disabled) if they are too close to each other.

A drop in SNRM of less than 0.5 dB (if possible to detect and display such fractional values) when a telephone "loops the pair" would be perfectly understandable and acceptable. A greater than 1 dB drop most definitely indicates that "something is not right".

I will disagree with that. There will be a change to either SNR, received power level, or attenuation. When the the phone is taken off hook a low impedance is placed across the filter outputs. All filters are single stage and changing the load impedance may change the inputs complex impedance and the frequency characteristics of the filter.

Predominately the impedance will change and even for the DSL frequencies it will be slightly lower and that will add additional load to the DSL signal and thus result in a slightly lower level being received by the modem. That will be reflected as a lower SNR.

Going on "gut feel" and remembering doing a few checks years back, I would think that up to 1dB difference would be normal and maybe on some line 2dB could be seen.

You will know that a decent modem can cope with an SNR down to about 3dB from a 6d sync and excellent modems such as the 2700 can cope with slightly negative SNRs.

FWIW
Whenever I've experienced lower SNRM due to a phone being used it's always turned out to be a line or equipment fault!

I shall respect your opinion on that point. My own personal observations are that the reduction in SNRM is very minimal -- to the point of being virtually unnoticeable. Whenever I have noticed a reduction in SNRM, it has been the result of a line fault -- more often that not a HR or semi-conducting joint.

Being an enthusiast for the 2Wire devices, your last point is well known.

ACK. That agrees with my observations.

I come from an electronics design background and having designed very some very complex telephony switches, line impedances and attenuation were something we had to pay a lot of detail. A lot of detailed analysis was done on differing lines, handsets at each end of tolerance, effects of filters and more. On some, the effect was minimal and almost immeasurable whilst on others it was significant.

I will have a look at one first thing tomorrow morning and see what happens. I know it is a good stable line ...

In reply to a post by MHC:

I come from an electronics design background and having designed very some very complex telephony switches

That much I have deduced from reading your postings in these fora.

line impedances and attenuation were something we had to pay a lot of detail. A lot of detailed analysis was done on differing lines, handsets at each end of tolerance, effects of filters and more. On some, the effect was minimal and almost immeasurable whilst on others it was significant.

Interesting background information.

I will have a look at one first thing tomorrow morning and see what happens. I know it is a good stable line ...

Thank you. Your experimental observations are duly awaited.