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power cut can this effect the exchange

scootie
Grafter
Posts: 4,799
Registered: 03-11-2007

power cut can this effect the exchange

Last wensday we had a power cut lasting no more than five mins here.(chesterfield)
Ive had no problems with my sync speed/noise margin before this happend but now im geting a wide range of noise margin from 2db upto 20db had bein steady 12db near bouts since i moved to plusnet in novmeber.
and i never had a line drop till power cut im now getting 1 or 2 a day now plus my sync speed use to be 3800 give or take 100 now im geting as low 2900.
so what im getting at would a power cut affect the exchange as it seems here that it has?Huh
and  is a higher noise margin better than a low Huh
8 REPLIES
notheruser
Grafter
Posts: 139
Registered: 08-01-2008

Re: power cut can this effect the exchange

It is extremly unlikely that a power cut would affect an exchange. (Ever noticed that even when the mains fails, your phone still works? - unless it's a wireless one of course!)
High noise margins are best. The noise margin (also known as the signal to noise ratio) is the ratio between the signal you want, and the noise signal that you don't want. It doesn't really matter what the actual level of the signals are - it's the ratio between them that makes all the difference.
So for example, a 1 volt signal with 0.001 volt noise signal has a ratio of 1000:1 (which is 60dB). Equally, a signal of 5 volts with a noise figure of 0.005 volts also has a SNR of 1000:1.
In simple terms, its a measure of how easyily the receiving electronics can discriminate between the wanted signal, and the unavoidable "noise" - the higher the value, the easier it is to discriminate. The terminology comes from way back in the early telephony days, when "noise" really was heard as noise - i.e. audible hiss, hum, clicks, pops etc. from unwanted interference. The "signal" was the good audio which you wanted to hear.
VileReynard
Seasoned Pro
Posts: 10,912
Thanks: 256
Fixes: 10
Registered: 01-09-2007

Re: power cut can this effect the exchange

In simple terms, high noise margins are not best.
They cause a reduced sync rate, a lower BRAS rate and a low data transfer rate.

notheruser
Grafter
Posts: 139
Registered: 08-01-2008

Re: power cut can this effect the exchange

OK, so my original post was an over-simplified explanation - "Noise Margin" is not quite the same as "Signal to Noise Ratio", but the two are closely related.
"SNR" is the ratio of signal to noise. "Noise Margin" is the difference between the actual figure, and the SNR which the router considers necessary.
For an explanation of SNR and how it is calculated, see here:-
http://en.wikipedia.org/wiki/Signal-to-noise_ratio
for Noise Margin, see here,
http://en.wikipedia.org/wiki/Noise_margin
"In electrical engineering, noise margin is the amount by which a signal exceeds the minimum amount for proper operation"
Note - exceeds the minimum - and the more you exceed the minimum by, the better. Therefore having a high Noise Margin at any given speed is a good thing.
See this article:-
http://www.kitz.co.uk/adsl/linestats.htm
Note "The main reason why SNR margins fall as the line speed increases is that the required SNR increases. "
In other words, as your line runs faster (and therefore at higher sync rate etc.), the Noise Margin will be lower - but that does not make it a good thing!
Also "The SNR Margin is there to absorb fluctuations in interference on the line, and therefore to keep the service reliable. The figure of 6dB is normally accepted as enough headroom to make the link reliable, but some lines need more - 10dB is often accepted as a safer target. " Note that the higher value is regarded as safer.
If you're on ADSL Max, your router will adjust itself to run as fast as it can, while still maintaining a safe Noise Margin.
(For how this works, see here:-
http://www.thinkbroadband.com/files/broadband-max-myths-and-legends.pdf )
At any given speed, a high noise margin is best.
Note in BT's document "Broadband Max is a rate adaptive service and will always try to sync up to the highest line rate your line can support reliably." As it does this, the Noise Margin figure will reduce, and speed will increase - but this does not mean that having a low Noise Margin is a good thing - if there hadn't been a high Noise Margin to begin with, this couldn't have happened.
In an ideal world, you would be running at a line rate of 8128k kbps, have a BRAS profile of 7150 kbps, and have a huge Noise Margin. In reality, this never happens.
A low Noise Margin measurement indicates that your equipment is in all likelihood running at optimised speed - don't confuse this with meaning that high Noise Margin in itself is a problem. Just as in most other areas of comms, having a high Noise Margin (= Safety Margin) is a good thing - low margins are a compromise between "playing safe" and "high performance".
Again from BT's document "Modems can retrain for two main reasons, either due to the margin dropping below 0dB, or if large numbers of errors are detected." - i.e. if the noise margin goes too low, the modem has too retrain.
It sounds from scootie's description as though he may be retraining quite frequently. Although the power cut would not have affected the exchange equipment itself, the fact that his router switched on and off could have caused a problem. If the power failure was a "brown out" rather than a nice clean switch off / switch on, the router may have tried to reconnect a number of times, which definitely could cause problems.
I'd be inclined to report this as a fault - although it may stabilise again by itself.
scootie
Grafter
Posts: 4,799
Registered: 03-11-2007

Re: power cut can this effect the exchange

thank you for your knowledge of snr.
regards the power cut my router was switched of at the time as i turn it off over night (goverment and there green trip stuff) and the cut was at like 9am so it was still turned off. iam aware that the phone still works in a power cut.
its just funny that since ive had broadband at this address its bein very stable no line drops just sync profile goes from 2500 to 3000 every now and then and since power cut its droping and snr is not stable any more.
runhare
Aspiring Pro
Posts: 521
Thanks: 50
Fixes: 2
Registered: 09-10-2007

Re: power cut can this effect the exchange

What caused the original power failure? if it was lightening then the cable circuit you are connected to or one of its many components could have been damaged; carbonisation of contacts is common leading to poor contact between segments of line. (this is what I strongly suspect has happened to a friends ADSL connection that died after a similar short power outage near our homes three weeks ago)
scootie
Grafter
Posts: 4,799
Registered: 03-11-2007

Re: power cut can this effect the exchange

sounds a good cause that its a effected the line at some point.
but i dont think it was lighting as they was no storm at the time.
i thought it might be that exchange lost part of its power supply or all the customers routers in area effected due to power outage would have reconnected at same time and effected the hardware in the exchange.
its not a big problem that noise margin is up down to extremes and geting a couple of disconections aday.
just wanted sum views on why a power outage would effect adsl line sum days after outage
notheruser
Grafter
Posts: 139
Registered: 08-01-2008

Re: power cut can this effect the exchange

It's quite unlikely that the power failed at the exchange - BT take a "belt and braces" approach to power supplies. (Hence my point about the phone still working). Hundreds of routers all coming back on at the same time would certainly put the exchange equipment under pressure for a while, but it's not the sort of thing which would cause equipment damage. (There is no major power drain on the lines when the user equipment comes on). Bear in mind that exchange equipment is designed to cope with all sorts of faults - for example, a digger pulling up a cable and cutting/potentially shorting hundreds of circuits at once, or street cabinets flooding and shorting lines to earth.
i take it nothing else in your house has been giving problems since the outage? (Phones - not noisy? Anything else playing up?) There is an outside chance that your router was damaged by a surge when the power went off and on again.
I would be inclined to leave the router on for at least a few days - it can take a while to stabalise a screwed up connection, and if you're switching off every night, it may not settle properly.
VileReynard
Seasoned Pro
Posts: 10,912
Thanks: 256
Fixes: 10
Registered: 01-09-2007

Re: power cut can this effect the exchange

If you think you have audible noise present try dialling 17070 option 2. [quiet test].
This will also tell you your phone number.