As an ADSL n00b I wanted to understand how the technology works and how to get the most out of it. In essence if you want to improve speed and increase stability you need to familiarise yourself with SNR (signal to noise ratio) and attenuation. Many ADSL-routers will provide you with those stats.

SNR or Signal to Noise Ratio:
Describes the ratio of usable data-signals on your line. You can associate the “signal” with the data traveling across your ADSL-line and the “noise” as the unwanted interference affecting the signal. The higher the number the better for this measurement. In some instances interleaving can help raise the noise margin to an acceptable level.

6dB or below is bad and will experience no synch or intermittent synch problems
7dB-10dB is fair but does not leave much room for variances in conditions
11dB-20dB is good with little or no synch problems
20dB-28dB is excellent
29dB or above is outstanding

If your SNR is below 12dB you are pretty much screwed and will not get a consistent level of ADSL-service. You could try to convince Telkom to rewire the cabling – but this is unlikely to happen.

Attenuation:
Is the reduction in signal strength on your phone line. In ADSL this may be reported as “loop loss” and is the natural deterioration of the ADSL signal over distance from the exchange. Attenuation is normally directly linked to the length of your line. Copper is traditionally used in the local loop and the higher gauge of copper will give the best signal, however some lines may have some aluminium or aluminium joints on the line which will increase resistance… as will oxidisation of joints. The lower the dB the better for this measurement.

20dB and below is outstanding
20dB-30dB is excellent
30dB-40dB is very good
40dB-50dB is good
50dB-60dB is poor and may experience connectivity issues
60dB or above is bad and will experience connectivity issues

The standard signal attenuation spread for a given speed is somewhere in the region of 25-30dB for ADSL1 speeds (if we had ADSL2/2+ this would be).

The following guide (distance vs. attenuation vs speed) gives you an guestimate what you can achieve:
<1km should be 23-24Mbit (nice speed, but doesn’t it bug you that Telkom people walk through your bedroom?)
1.0km = 13.81dB = 23Mbit
1.5km = 20.7dB = 21Mbit
2.0km = 27.6dB = 18Mbit
2.5km = 34.5dB = 13Mbit
3.0km = 41.4dB = 8Mbit
3.5km = 48.3dB = 6Mbit
4.0km = 56dB = 4Mbit
4.5km = 62.1dB = 3Mbit
5.0km = 69dB = 2Mbit
>5.0km (you are pretty much poked — sorry for you)

You will not be able to fix attenuation (unless you relocate straight next to the exchange). You will however be able to influence Telkom or your ISPs – use tooling such as MRTG to measure your variances and log incidents against them. It is unlikely that they will move the exchange closer, but you could combat SNR (check your wiring, climb into the manhole and see if your underground wire is of the cheap kind etc).

Update – 2009/02/20: If you own a Netgear DG834XX ADSL router, you can try a custom firmware which allows you to lock in the SNR – check out the following post.