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)