Fluctuating snr. What causes it?

Earlier this year, my line used to run happily at 6db and I never had any problems. However a fault appeared this year which BT could not/ would not resolve. This resulted in my once stable 6db starting to fluctuate and become unstable, giving me a fixed 2meg connection instead of the MAX 2.5meg previously. I have now moved isp's to a LLU connection. I believe the ADSL2+ has now been reduced to 2 in an attempt to give the line a bit more stability. It isn't working that way at the moment!!

Looking at my router stats gives me a range of -1.5db to 20db, although generally it stays around 9- 10db before steadily falling and dropping the connection. It then takes some time again for the router to re- sync at 9db.

I currently have nothing phone wise plugged into any sockets, have a filtered faceplate and the router plugged in via a cat5 ethernet cable. I am just wondering if someone can explain to me in "Noddy" terms why the fluctuation is so severe and what could be causing it? It is happening throughout the day, so isn't a streetlight related issue and it's far too early for Xmas lights

Thanks

Mark

Anything that can generate noise in the ADSL frequency range can cause it

So a treadmill, some low energy bulds, florescent lights, electric lawn mower, you name it, if it is electrical it is a possible source of noise.

Of course it may simply be a bad joint somewhere in the phone line.

The biggest source of noise on ADSL comes from MW radio stations, and if you turn on a MW radio at night, you'll hear a lot more stations. That's because there is a layer in the ionosphere which reflects radio waves back to Earth, and this layer is slowly destroyed by the action of energy from the Sun, so during the day noise is much less of a problem. In the summer, we only get about 5 hours of total darkness, so this layer never really builds up in any big way, so we get minimal interference on ADSL. Now that winter is here, the nights are longer than the days, so we see more noise on ADSL. A line which manages to hold up with a noise margin of 6dB in the summer with a night duration of only 5 hours, may not be able to do so when the nights are 12 to 14 hours long.

Edited by deleted (Tue 03-Nov-09 08:28:59)

Wow that's excellent. Thanks for the info

Mark

Mine tends to fluctuate all the time between about 12db and 15db. I had RouterStats running a few days ago, and it was shuffling between 14 & 15 db... then suddenly fell (at about midday) to 12db, and has been alternating between 11 and 13 since then. Nothing's changed on the line (to my knowledge).

It did this once before, and then there was a powercut and it went back up to 14 / 15 before falling slightly to about 12 and staying there. If you can explain that behaviour, then you're doing better than me!

Also, I don't find that it reduces at night particularly. If anything, mine slightly increases at night-time! I do have a Netgear DG834Gv3 though, and I've read the v3 is a bit wobbly when it comes to reporting the SNR margin... the v4 is a lot better so I may find different readings if I used a different router.

HI

The biggest source of noise on ADSL comes from MW radio stations, and if you turn on a MW radio at night, you'll hear a lot more stations. That's because there is a layer in the ionosphere which reflects radio waves back to Earth, and this layer is slowly destroyed by the action of energy from the Sun, so during the day noise is much less of a problem. In the summer, we only get about 5 hours of total darkness, so this layer never really builds up in any big way, so we get minimal interference on ADSL. Now that winter is here, the nights are longer than the days, so we see more noise on ADSL. A line which manages to hold up with a noise margin of 6dB in the summer with a night duration of only 5 hours, may not be able to do so when the nights are 12 to 14 hours long.

I think this is partly a myth. Yes more radio stations can be heard at night for all the reasons given, however those signals are very very weak and are easily rejected by ADSL modems. Tune a radio to a weak station you are getting due to it being night time then put that radio by your modem and most likely it will be drowned out by the ADSL signal.

A little while ago I was able to see something interesting that shows distant radio stations "arriving" at night doesn't cause anywhere near as much interference as we think. During the day my SNR margin is normally around 7 or 8db dropping to 5.5db at night. One night we had a powercut that wiped out a big area around me. Being on a UPS I still had broadband access and a working computer and checked my SNR, it had returned to better than daytime levels! This tells me that noise is coming from other electrical equipment being turned on at night, such as street lamps and normal household lighting, plus as winter approaches central heating pumps come on.

Regards

Phil

In reply to a post by PhilipD:

HI

The biggest source of noise on ADSL comes from MW radio stations, and if you turn on a MW radio at night, you'll hear a lot more stations. That's because there is a layer in the ionosphere which reflects radio waves back to Earth, and this layer is slowly destroyed by the action of energy from the Sun, so during the day noise is much less of a problem. In the summer, we only get about 5 hours of total darkness, so this layer never really builds up in any big way, so we get minimal interference on ADSL. Now that winter is here, the nights are longer than the days, so we see more noise on ADSL. A line which manages to hold up with a noise margin of 6dB in the summer with a night duration of only 5 hours, may not be able to do so when the nights are 12 to 14 hours long.

I think this is partly a myth. Yes more radio stations can be heard at night for all the reasons given, however those signals are very very weak and are easily rejected by ADSL modems. Tune a radio to a weak station you are getting due to it being night time then put that radio by your modem and most likely it will be drowned out by the ADSL signal.

A little while ago I was able to see something interesting that shows distant radio stations "arriving" at night doesn't cause anywhere near as much interference as we think. During the day my SNR margin is normally around 7 or 8db dropping to 5.5db at night. One night we had a powercut that wiped out a big area around me. Being on a UPS I still had broadband access and a working computer and checked my SNR, it had returned to better than daytime levels! This tells me that noise is coming from other electrical equipment being turned on at night, such as street lamps and normal household lighting, plus as winter approaches central heating pumps come on.

Regards

Phil

Well, if you can design an ADSL modem that can reject AM radio interference and still use the frequencies occupied by that station, then you ought to patent it, because you would make an absolute fortune !

You can't base an opinion on your own experiences alone. Of course there are lots of locally based systems that can interfere with ADSL. That's the definition of noise - something other than the wanted signal. When I first had ADSL in the days when it was fixed rate, I was amazed when I turned my wired house alarm off to change the battery, and found that the SNR on my ADSL went up by a good 8dB ! This alarm radiated even when it was disarmed, because it was sending anti-vandal signals around the wiring. Needless to say, this has now been replaced.

I think you would be quite surprised to see just how much power 20 feet of copper wire will pick up from AM radio stations - enough to directly power a pair of headphones with no amplification at all (i.e. a "crystal set").

Lastly, I would not worry unduly about central heating pumps - they use synchronous motors (like the ones used in mechanical electric clocks and timers), and the risk of interference from them is minimal, especially since they are all housed in earthed steel casings.

I still stand by my opinion that the biggest problem to ADSL is AM radio broadcasting, but what would I know ? I've only been a licensed radio amateur for the last 27 years !

Weak distant stations no real issue, nearby powerful ones, it may be just about OK during the day.

People can tell the frequency of stations from the bin plots, which effectively end up in a notched arrangement of bins. Early ANFP docs were concerned about AM radio being affected by ADSL.

Heating pumps not seen those as an issue, but spark igniters and timer relays have been an issue, i.e. people getting a new boiler and not finding out the problem until central heating is on.

When it comes to central heating the worst offenders are hot water tank thermostats, the type which attach to the outside of the tank and held in place with a spring wire. They are slow in operation and set up a lot of noise as they start to open, often causing a loss of sync.

My router used to be plugged into the same 4-way extension socket as my desktop PC. A while ago, when I turned my PC on, the whole router reset (as if it had been powered off/on at the mains). I thought it was a one-off bizarre event, but it happened again when I shut the PC down at night, and again the next day.

After that, I plugged the router into a different power socket (which isn't ideal as I have to have a long extension cable for it). I checked RouterStats after this, and normally when my desktop PC is shut down, there's a slight (brief) drop in SNR margin.

Would I be right in thinking that sometimes electrical noise can be fed via the mains, directly into the router, and that would cause SNR issues or affect the router in some other way? From my experience (re-told above), it certainly seems to be the case. Also, would a long power extension cable (as I now have for my router) act like an aerial to pick up any interference? I read a while ago that the Netgear power adapters can be rather susceptible to mains interference, and that can affect the router's behaviour.

And while we're at it, do ADSL splitters filter out external noise / noise on the phone line, or do they literally just split the ADSL / voice signals, and is it true that after time they can start to malfunction and not split things properly, leading to problems?

Thanks for any responses!

Edited by deleted (Sun 08-Nov-09 12:32:45)

Your scenario is one I've seen a number of times, so experimenting with different power plug arrangements can make a difference.

Hi

Well, if you can design an ADSL modem that can reject AM radio interference and still use the frequencies occupied by that station, then you ought to patent it, because you would make an absolute fortune !

Already done, it's called common mode noise rejection, which is why we use twisted pairs and is how the modem can reject the noise of distance MW radio stations.

I think you would be quite surprised to see just how much power 20 feet of copper wire will pick up from AM radio stations - enough to directly power a pair of headphones with no amplification at all (i.e. a "crystal set").

Which is why telephone cable is twisted pair and common mode noise rejection is used to cancel out the picked up noise. Yes I know all about crystal radio sets, a design to pick up radio waves, ADSL modems are designed to reject them.

I still stand by my opinion that the biggest problem to ADSL is AM radio broadcasting, but what would I know ? I've only been a licensed radio amateur for the last 27 years !

AM radio is a problem, I never said it wasn't. What I was saying is that noise at night isn't due to distant MW radio stations and their signals bouncing hundreds or thousands of miles around the globe. Those signals are very weak and very easily rejected by modern methods used in modems.

Regards

Phil

Edited by deleted (Sun 08-Nov-09 11:24:33)

The causes of this sort of problem are many and various, it can be caused by mains borne interferance, from the phone line itself, a faulty appliance, just about anything. So when dealing it with take a logical approach.

(1) if you can, put the router close to the master socket and use a filter faceplate. this often moves your router away from the rest of your kit and the filter faceplat will help to reduce anything picked up by extension wiring in your home Do try to do this, if only for a few days to see what happens.

(2) Check appliances you use to see if they can provoke the problem.

Interesting; I thought it may just have been some random thing I'd come across.

I don't know if it's an issue with Netgear routers, but they seem very sensitive to very slight power fluctuations. Our lights dimmed briefly the other week (I think the Americans would call that a power brownout!). Nothing was affected, except the router which was bizarrely sort of half-reset as it lost synch but the wireless stayed on.

Hi

Weak distant stations no real issue, nearby powerful ones, it may be just about OK during the day.

People can tell the frequency of stations from the bin plots, which effectively end up in a notched arrangement of bins. Early ANFP docs were concerned about AM radio being affected by ADSL.

I agree and see the same thing, my point was that distant weak radio stations that appear at night are not the main reason for increased noise levels and by design can and are rejected.

Heating pumps not seen those as an issue, but spark igniters and timer relays have been an issue, i.e. people getting a new boiler and not finding out the problem until central heating is on.

Not a good example really I know, I meant that during the night more electrical items are in use especially during the winter. Anything that is electrical is creating some form of noise and the background noise level goes up and we see reduced SNR margin and/or reduced sync speeds. Distant MW radio stations aren't to blame, it's us using all our electrical stuff/

I do wonder what effect the move to energy saving lights is/will have? The humble incandescent bulb doesn't create much in the way of electrical noise I would have thought, but switch all these to high frequency switching compact fluorescent tubes?

Regards

Phil

CFL's have a varying effect, i.e. some cause issues, some dont

In reply to a post by PhilipD:

Already done, it's called common mode noise rejection, which is why we use twisted pairs and is how the modem can reject the noise of distance MW radio stations.

Common mode rejection can stop interference from reaching the modem, but once it's there, the only way that a modem can reject the noise is by not using those frequencies. This will invariably cause a drop in sync speed, because of the reduced number of bins that can be used. Also, the number of twists in the pair are quite low, because a greater number would result in a longer wire length, which would have affects on the attenuation and the capactitance of the cable.

Hi

Common mode rejection can stop interference from reaching the modem, but once it's there, the only way that a modem can reject the noise is by not using those frequencies. This will invariably cause a drop in sync speed, because of the reduced number of bins that can be used. Also, the number of twists in the pair are quite low, because a greater number would result in a longer wire length, which would have affects on the attenuation and the capactitance of the cable.

Wow, for someone who said sarcastically I should patent the idea of common mode noise rejection what a turnaround

The modem will not pick up directly distant radio stations bounding in at night (which is the context of my original post) as it doesn't have an antenna tuned for receiving such signals..

I standby what I said, night time ADSL noise problems being attributed to distant MW radio stations bouncing off the ionosphere is more a myth than the reality.

Regards

Phil

In reply to a post by PhilipD:

Hi
Wow, for someone who said sarcastically I should patent the idea of common mode noise rejection what a turnaround

Oh dear, so you can't read either ?

What I actually said was "if you can design an ADSL modem that can reject AM radio interference and still use the frequencies occupied by that station"

I think maybe you ought to get a printout of all the tones that your router can't use, and match them up with MW radio stations. You may see a link. If a router cannot use a bunch of frequencies, this will reduce the efficiency of the router, especially when the range of frequencies that it is able to use is reduced by the effects of the length of the line. Sure, common mode rejection happening in the twisted pair cable will reduce the amount of interference picked up by the cable, but it won't completely stop it, and at night MW radio signals bouncing off the F layer can be extremely strong.

In reply to a post by PhilipD:

The modem will not pick up directly distant radio stations bounding in at night (which is the context of my original post) as it doesn't have an antenna tuned for receiving such signals..

LOL ! So you don't think a couple of hundred yards of copper cable would make a good antenna ?

Twisted-pair cables were first introduced when newly-installed electric trams were radiating impulse noise into the telephone cables in the 1880's, and twisted pair does also reduce the pickup of wireless signals, but it is nowhere near 100% effective. Sure, a lot of this wiring is underground, but in most installations you will have about 100 metres of overground cable. And guess what 1/4 wavelength of Radio 1 (909kHz) is ?

80 metres. 80 metres of copper wire will give you an excellent radio aerial for Radio 1 AM.

In reply to a post by PhilipD:

I standby what I said, night time ADSL noise problems being attributed to distant MW radio stations bouncing off the ionosphere is more a myth than the reality.

You can stand by what you say for as long as you like, chum, but it doesn't make it correct.

hi,

Have been drawn to post on this thread as all this talk from a poster about "ADSL modems not being tuned to radio stations" is utter rubbish..

Common mode rejection helps.. but it is not infinite.. overhead lines and drop lines are ready antennas for medium wave transmitters.. At night as mentioned these signals are propagated over considerable distances and as the transmitters that send them are usually transmitting megawatts then there is a good chance the signal hitting your line will reach considerable levels , fluctuating of course as various multi path signals from the source arrive in/out of phase etc..

An ADSL modem has no tuning, it is a wideband receiver, it lets in everything in its passband, it receives ALL ! , repeat they have no tuning!.

The ADSL modem on a long line is only receiving signals of -60dBm or less, this is way less than the field strength of a MW station being stormed in via reflection from the atmosphere. Even on short lines the upper frequencies are -60dBm or lower.. this means bins get wiped out and overall SNR takes a dive.

do hope that clarifies. it is no myth.

I had this problem last winter and bought a new router, never solved the problem, and now this winter it's back. Tiscali say it's my fault, but with a new router plugged into the BT test socket, I still get the line drop. The best SNR I get is 6db, but the new netgear a few weeks ago will hold the line down to SNR of 4 but after that it drops. The problem is worse at night, but the rain seems to be the main cause, getting anyone to listen though is a real problem.

With current rules they can simply say

a) Put up with it
or
b) We will mark the line as not suitable for ADSL

There are no requirements to fix or make good ANY faults that affect ADSL, even under the proposed USC they are likely to be get out clauses, as it won't be a legal right, and BT will not be the sole USC provider.

wow, thanks. basically it was put up, but now emailed them a 3- speed test results taken using this site, and the speed varies from 195Kbps to just over 1100Kbps on a good dry day, same time, just change in weather. Summer is fine, and so is the dry winter, so the line will clearly support an ADSL signal, at a reasonable speed, even though i'm 3.75Km from the exchange

In reply to a post by MrSaffron:

With current rules they can simply say

a) Put up with it
or
b) We will mark the line as not suitable for ADSL

There are no requirements to fix or make good ANY faults that affect ADSL, even under the proposed USC they are likely to be get out clauses, as it won't be a legal right, and BT will not be the sole USC provider.

That's not strictly true. Earlier this year the residents of this road were so fed up with the appalling service that we got with ADSL, which was degrading all the time, that we lobbied the CEO of BT and threatened a lot of bad publicity, and eventually they replaced over 500 metres of faulty cabling in this road. Although this resulted in dramatic speed increases for all the residents, we still get noise fluctuations which pull the speeds down. However, the difference is that I was getting 2M which often dropped down to 1M, and now I'm getting 6.5M which drops down to 5M.

Note the use of can, not will.

In reply to a post by MrSaffron:

Note the use of can, not will.

I agree, but you appeared to give the impression that there was nothing that the OP could do about their problems. What BT want to do, what BT are obliged to do, and what BT will do if threatened with bad publicity are three completely different things.

This week on the Gadget Show (where TBB got a mention incidentally), they had a feature on how to complain, and this included emailing the CEOs of companies, and in my experiences (not just with BT) this appears to work.

Got to agree with Zebedee and Kijoma here, PhilipD is living in cloud cuckoo land if he thinks RF doesn't interfere with ADSL.... We've had about 18 BT engineers and a REINS engineer trying to diagnose our fault for the past 18 months, exactly the same problem it drops out at night, but through the day it's up past 3.3Mbits....

The BT engineers always comment on how loud the MW stations are blasting through on their BAT phones... it's true.... twisted pair does cancel out interference but that is for a perfectly balanced pair... in other words if an spike/amplitude arrives on one leg the opposite amplitude/spike will arrive on the other thereby cancelling it out .... but it's rare for a telephone wire to have "exactly" equal lengths and of course any residual will appear across the terminals for the modem to deal with along with all the pops, cracks and whistles that a phone line receives....

As most of us know ADSL uses frequencies from approximately 26 to 138 kHz for the upstream and 138 kHz to 1104 kHz for the downstream connection although this range is extended for ADSL2 connections, and some VERY powerful transmitters are in that range, plus they are amplitude modulated which would spread them very wide off the centre carrier frequency in comparison to an SSB transmission or CW (morse) transmission...

This is all tied in with what the sun is doing, high levels of solar activity increases reflection, refraction, diffraction, absorption, polarization and scattering, which is why when we did long haul communications on HF (High Frequency) in the Royal Signals, we had to predict what the conditions would be, because even in the 1980's the Army was sending digital signals over both HF and VHF way before digital took off in civvy street ... propagation is highly variable, some days you'll get very little propagation other days you'll hear stuff from the other side of the world.... There isn't a modem on the market that can filter out that kind of variability of interference/signal.

Tune your AM radio to around 612Khz (about mid band for ADSL1 ) and put it next to your modem, you should hear the rhythmical beating of the modem with very little background noise, then try it again when your modem is having difficulty keeping its speed..... you'll be surprised at how different the modem sounds during the night when it's working harder

Btw I'm ex Royal Corps of Signals long haul communications specialist, and 23 years in Analogue/Digital component repair with computer and communications equipment .....what I don't know about interference over communications lines/airwaves you could write on the back of a postage stamp with a bingo marker pen ;¬)