Broadband SNR tweak
Don’t allow BT’s Digital Line Management (DLM) system in the exchange to make your broadband connection super slow. This page describes a brilliant tweak that could more than double your downlink connection speed on its own. When combined with some of our other recommendations, such as an ADSL faceplate and a more robust router/modem, you could get even greater broadband speed improvements.
What are ‘SNR’ and ‘target SNR margin’?
ADSL broadband services are surrounded by many confusing technical terms, which make it harder for users to understand how their broadband services work and how they can increase their broadband speeds. Two terms that are often bandied around are ‘SNR’ and ‘target SNR margin’. As we will show, the target SNR margin is particularly important in defining your downlink broadband speed.
Signal-to-Noise Ratio (SNR) is simply the ratio of the measured broadband signal (the wanted signal) to the noise on the line (which is unwanted and can cause errors). The downlink SNR is measured by your modem (and reported back to the BT exchange equipment) and the uplink SNR is measured by the BT exchange equipment. For an ADSL service to work reliably it is beneficial for the wanted signal to be significantly higher than the noise to minimise errors on the line.
When an ADSL broadband connection is first established, your modem and the BT exchange equipment measure the downlink and uplink SNRs and then try to set the connection speed such that there is a specific SNR margin, which is sufficiently high to guarantee a reliable connection that does not regularly drop and/or create an unacceptable number of errors. This specific margin is called the target SNR margin. The higher the target SNR margin, the lower your connection speed.
As we will discuss, since your modem measures the downlink SNR and reports this measurement back to the BT exchange equipment (which then sets the downlink connection speed), it is possible (with some modems) to adjust this measurement so that the BT exchange equipment believes that the SNR is higher than it really is so that it sets a higher connection speed than it would have done.
BT can increase your downlink target SNR margin to 18dB
When your broadband service is first switched on, the SNR target margin is set by the BT exchange equipment at 6dB. On a good line, BT considers that a 6dB target margin is appropriate to maintain a connectoin without a significant number of drops and errors.
As part of its Digital Line Management process, the BT exchange equipment continuously monitors your broadband connection, for example recording the number of times that it drops and the number of errors it suffers. If the line experiences any issues (such as bursts of errors at a particular time), the exchange equipment can increase the target SNR margin – usually in steps of 3dB. This action – aimed at stabilising the line – reduces the connection speed.
BT’s DLM system, or BT’s manual intervention, can increase the target SNR margin to a maximum value of 18dB.
Many things can cause the exchange equipment to increase the target SNR margin, including:
- not using a filtered faceplate (see our ADSL faceplate page for more information)
- a poor or faulty modem (see our broadband router/modem page for information on the best modems)
- temporary interference (e.g. from Christmas lights or faulty electrical equipment)
- regularly switching your modem on and off
- thunderstorms
- a faulty broadband microfilter, or forgetting to install a microfilter on all extension phone sockets.
Once the BT exchange equipment has increased the target SNR margin, it can take months for the target SNR margin to be reduced and, in some cases, the SNR margin is never reduced. In the worst case scenario, you may be saddled with very slow downlink speeds caused by an 18dB target SNR margin when your line could work perfectly with a 3dB (or less) target SNR margin. We will show you how to adjust your target SNR margin to any value that you desire. You will no longer be a slave to BT’s Digital Line Management system!
Relationship between downlink speed and target SNR margin
To demonstrate the impact of different target SNR margins on downlink connection speeds, here are some results from an ADSL connection based in rural Cambridgeshire. With the modem used (the Billion 7800N), it has been possible to demonstrate how the downlink connection speed is affected by the target SNR margin.
In this example, the downlink speed is only 3008kbps when the target SNR margin is set at 18dB, which is substantially lower than the 5568kbps achieved by the BT default target SNR margin of 6dB. With this particular connection, it was possible to reduce the target SNR margin to only 1dB, while maintaining the reliability of the connection, to achieve an amazing 6624kbps downlink connection speed. This connection speed is 120% faster than the connection speed with an 18dB SNR target margin.
| Downlink target SNR margin | Downlink connection speed |
| 18dB (maximum ‘worst case’ margin set by BT) | 3008kbps |
| 15dB | 3616kbps |
| 12dB | 4096kbps |
| 9dB | 4768kbps |
| 6dB (BT’s initial default) | 5568kbps |
| 3dB | 6240kbps |
| 1dB | 6624kbps |
Table: Effect of target SNR margin on downlink connection speed
Please bear in mind that lowering the downlink SNR target margin can increase the number of line drops or line errors, and not all lines with be stable at 3dB SNR or lower. However, you can try to see what target SNR margin works best for your line, to get the optimum balance between reliability and connection speed. Particularly where you have made significant improvements to your line (for example, purchasing a new modem or installing a faceplate broadband filter) your line may be able to tolerate substantial reductions in target SNR margin without adversely affecting reliability.
How can I tweak the target SNR margin?
The ability to tweak the downlink SNR target margin depends critically on the make and model of your modem. Depending on your particular model of broadband modem, there are a number of methods for tweaking the downlink SNR target margin, which include:
- using a modem that has an integrated SNR target margin adjustment setting (such as the Billion 7800N) – which is the easiest solution
- for Netgear DG834XX modem owners owners, installing the custom DG Team firmware, which incorporates SNR target margin adjustment
- RouterStats software
- connecting to the modem using telnet, and issuing the ‘adslctl configure –mod snr X’ or command.
You can easily tweak the target SNR margin with the Billion 7800N
Of all the methods, by far the easiest is to purchase the excellent Billion 7800N router/modem, which incorporate a very simple SNR margin adjustment page (and full instructions are provided on our dedicated Billion 7800N page. SNR adjustment with this modem is very simple, and the modem stores the SNR setting even when it has been switched off. If you’re considering replacing your existing modem, the Billion 7800N is the perfect modem for broadband users wanting to get the very highest speeds from their broadband connection and we strongly recommend it.
If you wish to purchase the Billion 7800N, you can buy the Billion 7800N through Amazon. This will generate a small commission for us, which will help keep this site going, while you will pay the same low price from Amazon. Thank you!
DGTeam has develop custom firmware with SNR margin adjustment
DGTeam provides unofficial custom firmware developed for selected Netgear modems. This firmware allows the downlink target SNR margin to be adjusted (which cannot be done using the standard Netgear firmware). The firmware is free of charge to download, and versions are available for the following models:
- Netgear DG834GT
- Netgear DG834PN
- Netgear DG834N-V1
- Netgear DGN2000
- Netgear DG834(G) V1 V2
- DG834(G) V3
- DG834(G) V4
- Netgear DG834GTB (Annex B)
- Netgear DG834PNB (Annex B)
We have regularly used the DGTeam firmware over a number years, and have found it very reliable and easy to use. We still, however, find that broadband connections made with our favourite modem – the Billion 7800N – are generally more robust than with Netgear modems – with or without the DGTeam firmware.
Please note that the target SNR margin adjustment using the DGTeam firmware is limited to about -5.5dB in the downward direction. This means that if BT has increased your target SNR margin to 15dB, for example, you will only be able to decrease this to about 9.5dB.
RouterStats allows you to monitor your connection stats and adjust the target SNR margin with a supported modem
RouterStats is powerful free software (which runs with Windows or Linux) that connects to your modem to provide a comprehensive range of statistics and graphs. For example, you can monitor (over time) the actual SNR margin and the number of errors experienced on your line, which can help to diagnose problems. In addition, RouterStats can be used to adjust the target SNR margin in some modems. RouterStats can be configured to work with a broad range of modems, including:
- Netgear DG834
- Netgeat DG834G
- Netgear DG834GT
- Netgear DG834N
- Netgear DG834PN
- Netgear DG2000
- Netgear DGN2000
- Thomson ST585 v6 and v7
- Draytek Vigor 2600G
- Linksys WAG54G
- Linksys HG200
- Linksys WAG160N
- Linksys WAG200G
- Linksys WAG354G
- Netgear DM111P
- Orange Livebox (Inventel)
- Speedtouch 510 v4 (Firmware 4.3.2.6)
- Speedtouch 585 v6 & v7
- Voyager 2100
- Voyager 220V
- Zoom X5v
On the RouterStats website, the developers warn that the RouterStats software is not ‘plug and play’. They point out that all routers are different and users may need to do some detective work to get RouterStats working with a particular modem. We find the RouterStats software is relatively complicated to use, but if you have the time to search the Internet for support information, then it may provide a cost-effective way to change the target SNR margin. If you don’t have the time, you’re probably best buying the Billion 7800N.
You can tweak some Broadcom-based routers using telnet
Some Broadcom-based modems allow the target SNR margin to be tweaked using the ‘adslctl configure’ command in telnet (or ‘adsl configure’ depending on the modem).
From your PC, configure a telnet connection to your modem.
So, if your modem is set up as http://192.168.1.254 then, from the Command Prompt, type:
telnet 192.168.1.254
and then enter the login ID and password.
Then issue the following command:
adslctl configure –snr N
or
adsl configure –snr N (depending on the modem)
where N is the number shown in the table below. The optimum value of N is determined by BT’s current SNR target margin and your desired SNR target margin.
| Your current target SNR margin |
Your desired SNR target margin |
Number you should enter |
| 6 dB | 4.5 dB | 75 |
| 6 dB | 3 dB | 50 |
| 9 dB | 6 dB | 50 |
| 9 dB | 4.5 dB | 25 |
| 9 dB | 3.5 dB | 1 |
| 12 dB | 9 dB | 50 |
| 12 dB | 6.5 dB | 1 |
| 12 dB | 6 dB | 65550 |
| 12 dB | 4.5 dB | 65525 |
| 12 dB | 3 dB | 65500 |
| 15 dB | 12 dB | 50 |
| 15 dB | 9 dB | 65550 |
| 15 dB | 6 dB | 65500 |
| 15 dB | 4.5 dB | 65475 |
| 15 dB | 3 dB | 65450 |
| 18 dB | 15 dB | 50 |
| 18 dB | 12 dB | 65550 |
| 18 dB | 9 dB | 65500 |
| 18 dB | 6 dB | 65450 |
| 18 dB | 4.5 dB | 65425 |
| 18 dB | 3 dB | 65400 |
Please note that this tweak will not work with some recent modems, because the ‘adslctl configure’ or ‘adsl configure’ commands have been deliberately disabled in the firmware of the modem by the manufacturers.
Also, note that the target SNR margin settings will be reset once the modem is rebooted, severely limiting its practical usefulness.
We would love to hear from you!
We hope that you have gathered from this page that the downlink target SNR margin is a very important factor in determining your downlink connection speed. You pay a high price in terms of broadband connection speed if the target SNR margin is increased by BT’s Digital Line Management from its default 6dB setting. We have documented a number of ways in which you may be able to reduce your downlink target SNR margin to substantially increase your speeds. If you have found this site useful, we would love to hear from you. Please email us at info@increasebroadbandspeed.co.uk.
