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Low SNR Margin & SNR Problems

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Not so long ago this was a fairly rare event. However, since the recent bulk speed upgrades, many users have found themselves on a faster connection, but they get frequent loss of sync to the exchange.

This FAQ should be read in conjunction with my page on Line Stats which explains attenuation and SNR.

~ Why does this happen?

The most common problem on longer lines is that your SNR is too low to maintain a steady connection.

Traditional Fixed 512/1Mb/2Mb
For each upgrade between the speeds you can expect to lose on average 6dB. Therefore if you upgraded from 512kbps to 2Mbps then your SNR Margin is likely to have dropped in the region of 12dB.
Once your SNR Margin starts to drop below 10dB on a traditional fixed rate line then you can experience problems.

DSL Max works slightly differently and is more tolerant at working at lower SNR Margins. In many cases it may depend on your modem/router, but generally below 6dB is when problems may start to occur. There are some routers that seem to cope better with low SNR Margins than others. For example there are many cases of the Netgear DG834(G) hanging on right down to 0db and below.

~ So what is happening?

SNR can vary on an hour to hour basis, not only is it affected by the speed of your connection but there are other things may reduce your SNR. Hot weather can cause a long line to expand. Wet weather on a connection somewhere could cause dampness and deteriorate the signal. Even street lighting and flashing xmas tree lights have been known to affect SNR.

Your router or modem can either hear the signal from the exchange or it cant. If it cant "hear", or to put it correctly can't synchronise with the exchange, then your adsl connection is lost and you are said to have "lost sync".

~ What are the symptoms?

Obviously the first thing you will likely notice is that you wont be able to surf etc. If you look at your router/modem the green adsl light will be flashing, rather than a steady green light you get when your modem syncs to the exchange.

Some routers provide logs that tell you when you have lost sync - my router would show as follows:-

Date/time : MAJOR ALARM : ATM Interface Down : Interface - atm-0
Date/time : MAJOR ALARM : DSL Interface Down

Other symptoms of low SNR are lots of HEC and CRC errors in your router log.

Disconnects can be very random, you may get several all at once, you may get them just in the evenings, you may go through a bout of disconnections and then go days/weeks before you get anymore. - It all depends upon the severity of your problem.

~ Are there any tests I can do?

You can check your line stats from your router/modem to find out your SNR Margin.
Details how to get your line stats for common routers/modems.
Interpreting your line stats.

Its also very worth while checking and comparing your line stats to an those taken from the test socket behind the master telephone socket.
Simply remove the 2 screws from the faceplate and you will see a test socket on the lower RH side.
If you get a better SNR Margin and or higher sync speed from the test socket then you should definitely think about purchasing a NTE5 adsl faceplate.

When checking from the test socket it is important that you only remove the faceplate and do not tamper with any of BT's wiring.

test socket
BT test socket.
photo adapted from

~ Can I do anything about low SNR?

Depending upon the severity there are different things you can try:-

  • Routers are known to provide a more stable connection than usb modems and the purchase of an adsl router has done the trick for many.
    Be aware that some routers do perform better than others on Low SNR Lines, and although there is no guarantee, it is something very well worth trying.
    The Speedtouch 585 + 576, Netgear DG834, Westell 6100, D-Link DSL-G604T are some routers that have specifically been known to maintain a more stable connection on a low SNR line.

    ~ The Netgear DG834 and DG834G have a very good reputation for being able to hold on to a connection at low SNR. I personally have seen one hang on to a line that frequently drops to 0dB (and below). Other users of the DG834G have reported similar. There's also a few instances where it worked when others wouldn't [link]
    ~ The D-Link DSL-G604T has also been reported* to successfully stabilise a low SNR line which was about to be declared as incapable of supporting adsl. [link].

    ~ Most Speedtouch routers seem to work well on long lines and have a good reputation.

  • Adsl Filters. Make sure your filters are fitted correctly. Fit good quality filters, some definitely are better than others - adslnation carries an article which shows the difference in between some filters. Or better still...

  • NTE5 Filtered Faceplate. Fit an NTE5 master socket adsl faceplate, which is proved to have the best success rate for many people.

~ What is an NTE5 adsl faceplate?

The NTE5 faceplate for the master socket is something that you can purchase and fit yourself.
It splits your voice and adsl data at the point of entry to your property and provides the most effective form of filtering available.
Fitting an adsl faceplate negates the need for separate filters in your home.

* Note although it is legal to fit a new NTE5 faceplate, you are not allowed to fit a complete NTE5 box, or tamper with the wiring on the BT side of things.

bt adsl faceplate

~ How do I install a NTE5 master socket faceplate?

See page Installing an adsl NTE5 faceplate.

If you have old style "star wiring" without an NTE5 master socket, then you can still legally fit an NTE5 line box by following the clarity guide here.

~ Where can I purchase a NTE5 faceplate?

~ Noise Spikes

A lot of low SNR problems may be related to noise spikes where "noise" comes from an outside source or electrical interference. You may be able to monitor your SNR Margin and identify a pattern which correlates to your low SNR. Common causes may be microwaves, central heating pumps, street lighting, xmas lights, treadmills, the list is practically endless.

Some users have reported success in tracking down the source of the noise using a portable AM radio. The radio should be tuned in between stations somewhere in the region of 612 Khz until you hear static noise. For more information on this see REIN faults.

~ How can I re-wire user side internal telephone cabling with Category5e/6 cable?

Re-wiring your internal telephone cabling with Cat 5-e cable may help reduce interference caused between the point of your master socket and extension. You may find this useful only if you get a good stable reading from the master socket but need to connect from an extension socket.
For more information see CAT5e Extension.

However there is something else you could try which has proved beneficial for many others.........

~ One last trick - remove the ringwire.

Disconnecting the ringwire may possibly help improve your SNR Margin. This trick is particularly useful if you have an older style star (or spur) wired telephone system and you cant fit a NTE5 faceplate.

The orange ring wire can act as an antennae and introducing additional noise on the line. Adsl filters have their own capacitor, therefore taking the ringwire out of the loop still means that your phones will ring. For information on how to do this see Removing the Ringwire.

NTE5 ringwire filter


With effect from late 2007, new installations should be fitted with a bell circuit filter, these NTE5's are identifiable from the front cover by being marked "Openreach".

Thanks to "Soms" for the image and info.

~ Unstable long line with slow speed?

Several ISPs of late seem to be proclaiming that BT no longer offer the Fixed Rate lines. This is not quite true and they are still available from BT wholesale -but they are more expensive for the ISP than the "Up to 8Mb" products. In this instance you should be able to request something called Max Capped Rate Profile.


*Thanks to J Housden for contacting me with info on the DSL-G604T, which successfully stabilised his 5dB SNR Margin line.
  No other supplied routers could even sync and the line was about to be declared as incapable of supporting adsl..

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