Wireless Waffle - A whole spectrum of radio related rubbish
ADSL: A Daily Service Losssignal strength
Monday 19 November, 2007, 11:53 - Much Ado About Nothing
For the past few months, every evening around sunset, my (ADSL) broadband connection at home has been 'drying up'. By this, I mean that the throughput has got smaller and smaller until eventually there has been no incoming or outcoming bandwidth available at all. If left alone, after an hour or two, the situation tends to correct itself but with sunset currently occuring during the working day, it's annoying to lose internet connectivity at these times.

At first I thought the problem might be to do with my ISP, Sky Broadband, but a quick scan of the rather useful, but unofficial, Sky User Forums didn't seem to indicate that others were suffering the same problems as me (though there were one or two veiled comments about how the connection sometimes dropped at night).

bt adsl filterMy next thought turned to my home installation. I'm quite a way from the local exhange and as such only get a connection speed of approximately 4 Mbps downstream and more like 0.4 Mbps upstream. According to my router, the available signal to noise ratio on the downstream link is only 8dB, that is to say that the ADSL signal on my phone line is only approximately 2.5 times higher than the noise on the line which isn't a great deal. But fiddling around with filters and connections makes next to no difference and the connection remains resolutely poor. During the 'dry periods' the signal to noise falls to 3dB or less (hence the loss of the connection), but re-booting the modem at these times oddly yields a return to 8dB.

Being a radio engineer type, and knowing that ADSL uses radio frequencies, I began to wonder whether the problem might be to do with increased radio interference on the line around sunset. ADSL uses frequencies from approximately 26 to 138 kHz for the upstream connection (i.e. from the home to the exchange) and in the range 138 kHz to 1104 kHz for the downstream connection (from the exchange to the home), though this frequency range is extended to just over 2200 kHz for the faster ADSL2+. Given my distange from the exchange, my connection is resolutely ADSL only (and not ADSL2+) and therefore if there was an increase in interference it would need to be in the frequency range 138 to 1104 kHz. However, as most ADSL modems are capable of running ADSL2+ even if the line is not capable of supporting it, it is likely that the receivers in them are not filtering out unwanted or unused frequencies making them susceptible to interference on frequencies up to 2200 kHz and quite possibly even higher even if those frequencies are not in use.

pole dancerNow as it happens, this frequency range is home, in Europe, to both long wave and medium wave radio transmitters (in the frequency range 148.5 to 285.5 kHz and 526.5 to 1606.5 kHz respectively) and there are many high powered radio transmitters in this frequency range. Could it be that the propagation that exists around sunset causes such a significant rise in the level of signals in this frequency range that it was knocking out my ADSL connection? The telephone connections in my area are via flown cables between telegraph poles (as they are still called despite telegraph having died out eons ago) and as such probably make rather good aerials, so it is quite possible that my modem is susceptible to incoming interference from high power long and medium wave broadcast transmitters.

When first booted up, the ADSL modems at both the exchange and the home end 'train', that is to say that they check all the available frequencies to see which have the best signal to noise and then use just these frequencies for the connection. This would perhaps explain why re-booting the modem during a dry period restores the signal to noise: clearly the frequency on which the interference occurs is one which is clean in the mornings when the modem is first switched on.

microfilter circuit diagramAs yet I have not been able to confirm my theory that interference from high power broadcast transmitters is knocking out my broadband connection, but it does seem eminently reasonable. Next steps are to do some tests (maybe connect a receiver to the phone line - bearing in mind it has 50 Volts on it!) Another option might be to add some filtering to the incoming line: surprisingly ADSL microfilters do not touch the ADSL frequency range they just stop the ADSL signal getting into/out of any phones connected (see the example circuit diagram on the right), which means, incidentally, that if you have nothing other than an ADSL modem connected to any phone socket in your house, you don't need a microfilter on it: you only need microfilters on the sockets where phones are connected. Perhaps, therefore, there's room for an improved microfilter that cleans up the incoming ADSL connection as well as keeping it out of the phones. I'll keep you informed of any progress - and if you experience these problems yourself leave a comment and I'll update you personally if I make any breakthroughs!

Update: 18 Dec 2007 Forcing the modem to use ADSL instead of ADSL2+ whilst reducing the connection speed by less than about 5% has almost eradicated the problem. It seems that purposefully avoiding the use of frequencies above 1.1 MHz does make a worthwhile difference.

Update: 11 Feb 2012 After much messing about Wireless Waffle has designed a filter which cures the ADSL drop-out problem that is described above. It costs next to nothing and can be made in minutes!

MDW 
Wednesday 9 January, 2008, 17:22
Very interesting article thanks very much.

David Anderson GM4JJJ 
Saturday 3 May, 2008, 09:27
Yes I concur, this is a definite problem with long line ADSL. Even though my telephone line is buried underground, I suffer from the nightime blocking of the ADSL signal by medium wave stations. I have been able to correlate the drop in noise margin of the ADSL with the appearance of MW AM radio stations at night. It causes the noise margin on my DG834 to drop from 15dB to below 0dB. I doubt much can be done to filter out the radio signals without also taking out the wanted ADSL signal.

I used a program called RouteStats to graph the Noise margin and the effect is very marked.

Much rubbish is written about the nightime problem being associated with streetlights!

I just wish that BT could drag itself away from the use of 19th century copper telegraph wires and switch to optical fibre.

Nigel Jones 
Saturday 10 May, 2008, 01:14
Interesting... I have similar feelings... I'm trying to track down what I think is noise in the 300-550 kHz range .. that's outside traditional LW/MW. Heck today I nearly bought a "scanner" to check it out.....

I'm about 5km away & get synced at 3600-4700 normally. This gets worse towards evening with probably a drop in SNR (at fixed speed) of ~4dB tops

But sometimes I get blasted such that my modem will resync to 2800, 2400 or today as low as 1700. This blast lasts anything from a couple of minutes to say 2 hours, although typically is 20-30 mins.

Once this stops I keep the same rate but my SNR shoots up to anywhere in the 15-21dB region.

So annoying -- I know do a nightly dsl reset, but I also noticed the "bin allocation" graph rather than gradually tailing off at higher frequencies I get a very significant (some bins missing) depression at around 300-550 khz which goes away when the interference stops.

I'm already filtering at the master socket, and have eliminated most electrical equip in my house. I'm clutching at straws now and thinking, could it be mains-bourne interference, would some mains filtering help? Should I plug the power adapter in somewhere else... but it's the timing and freq hit that's wierd. So there's a main transformer a few doors down, streetlights outside (but they're on far longer)... it does seem to happen more at weekends and after school. Radio ham perhaps? but presumably their transmissions are well "focussed" RF wise?

Ideas very very welcome!



Nigel Jones 
Saturday 10 May, 2008, 01:23
This got me thinking I should write it up on my blog - so I have on my blog together with some DMT snapshots showing the frequencing depression,.

Colin 
Thursday 22 May, 2008, 21:42
I can confirm that MW radio is definitely a problem affecting ADSL2+, although for you to lose service completely you'd have to be very close to a TX.

I suggest you Google for a freeware program called 'DMT Tool' which, if compatible with your ADSL2+ modem, will show you exactly the gaps in your ADSL2+ response caused by interference.

I used it for the first time yesterday and found large gaps corresponding exactly with frequencies broadcast from Brookmans Park AM transmitter which is 13 miles away.

QED. Clearly I'll have to live with it as there is no way to remove the issue short of AM broadcasts ceasing at Brookmans Part.

John 
Thursday 9 October, 2008, 16:30
This is a most informative article. Yesterday, I ran a simple experiment. Using RouterStats to monitor Rx Noise on my DG834G v3, I was able to show an obvious correlation between daytime and nighttime Rx Noise. During the day, I see typically 12dB and it's pretty constant varying by perhaps +/-1dB. However, at night, the Rx Noise wavers continuously dipping down to as little as 2dB. Amazingly, the DG834G very rarely loses sync but I'm wondering if I might get a faster connection if I could reduce the noise on the line at night. My line attenuation is 58dB so I'm quite a distance from the exchange.

Degradation in Rx Noise starts within 30 minutes of sunset and improves rapidly around sunrise. The cycle is repeated daily. Clearly, the change in ionospheric propagation of AM signals at night is increasing the interference on my ADSL signal. Referring to the statement that "Forcing the modem to use ADSL instead of ADSL2+......has almost eradicated the problem", how were you able to do this?

Thanks.

Mark Powell 
Tuesday 3 March, 2009, 22:19
In Manchester, on an ADSL2+ (BT21CN) service I have observed severe dips in the number of bits-per-tone around all the corresponding AM radio frequencies. The heaviest loss is around tones 337-340 with 338 as a "dead" 0 bit tone.

Tone 338 corresponds to 1458 kHz which is the Capital Gold service from Ashton Moss about 5 miles away from my location.

All the other tones which correspond to stations in my area also suffer from degradation in the order of about 6 or 7 bits compared to their neighbours.

Paul 
Sunday 27 September, 2009, 20:01
It is not only long lines that cause problems. I am just 3.5km from my exchange and all of my cabling is UNDERGROUND.

I use the DG834GT on TWO ISPs to my home and both have the exact same problem (the onlt difference is the Lins loss is slightly more (by 1dB) on one line. (I get around 11 mbs on one line and 9 mbs on the other during the day)

The modems have the DGTeam firmware (which allows ME to set the line levels, S/N ration etc. - highly recommended)

On one line the S/N falls from 9.5db to around 2.5dB and the other from 8.0 to 0.5 dB. ie around 8dB. If left to their own devices the lies will resync at around 5mb and 4mb and annoyingly never recover.

Using the DMT software it can be seen that the upper part of the spectrum from 1Mhz upward is trashed plus I have a local transmitter on 1.45MHz about 5km away (there are a lot of TXs on this frequency) which wipes out about 50 carriers (so much for being 4kHz wide)

As my cables are all underground I have grave concerns as to what exactly BT does with their lines to allow such strong signals to appear on "balanced lines" ie Twisted pairs unless of course they have crossed over pairs and messed up the signal balance.

Interestingly I spoke to BT Broadband and they said that they have the same problems and to get your ISP to contact BT Installations (separate company to BT BB) who MUST investigate the problem and fix it.

SO tommorrow I shall be contacting my ISPs and requesting they intialise an investigation .... Last time the ISPs said BT Install would not touch the problem. Well now I am armed with this information it is time to "get angry"

Maybe you should try the same.

I'll update in due course



John Tindle 
Sunday 20 December, 2009, 11:32
I too can confirm the problem. Looking at the detailed router stats, it is possible to see the signal to noise performance of each of the 256 4kHz channels. I see full blank outs on three channels and depressions on another 10. The noise levels are worse at night. The blank outs and depressions correspond perfectly with the highest received power AM transmitters.

A couple of points that have not been explored here:-
I am 100kM from the nearest transmitter, so when do we just grin and bear such noise and when do we complain (my line is not well balanced methinks!)
The sensitivity of my line is highly dependant on direction of signal. It is pretty much 2000m of overhead wire in a straight line and my biggest problems come from stations perpendicular to the axis.

Steve, GW0EZB 
Tuesday 27 July, 2010, 17:22
I've suffered some local RF interference at LF and MF. I'm going to try a Ferrite ring in several places on my DSL line and see if that helps

Dave 
Friday 4 March, 2011, 18:19
Hi,
I'd be really interested to know how you created a filter to block HF on your line and yet allow through everything below about 1.1Mhz.
The cable route here from the exchange is exactly 3.5km and results in a (slightly high) 53db attenuation but according to the engineers, the line is well balanced - by which I think they actually mean the loop resistance and capacitance of each leg.
I've got a nice three span antenna provided by BT and have spent several months trying to improve the situation as BT either aren't interested or the people they send have no understanding of RF. I've tried as far as I can to eliminate noise from the incoming main (rf filter) and even went so far as running a 12volt router off a battery and using a battery powered laptop to monitor the bins from the NTE5 test socket with main disconnected at the consumer unit.
I've tried around ten (yes 10 :) ) different routers and eventually left a 2wire 2700HGV on as it seems to be the best behaved of the bunch on this line.
Wouldn't a ferrite block on the adsl cable reduce the adsl signal as well as other RF?
Anyway, be interested in others ideas on this

tiffanyaltz 
Wednesday 15 June, 2011, 10:47
I was just surfing around the web looking for ideas to write some articles about and found your website, good stuff you have here thanks for the info.

Mark Payne 
Saturday 13 April, 2013, 19:17
I think its a term called REIN which you are experiencing, http://www.kitz.co.uk/adsl/rein.htm

Mark

shahid khan 
Tuesday 5 November, 2013, 05:46
the problem of radio interference due do the operation of a MW transmitter has disturbed almost 10000 subscribers in my locality. not only the adsl but also the basic telephone signal has been effected to a greater extent. the telephone sets are acting like radio receivers, although the use of a capacitive filter has showed some positive signs but still far away from being satiscatory. any helpful idea in this regard, particularly improving the quality of basic telephone, will be highly appreciated

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