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ADSL Speed and Performance Part 1 : Physical Limits


Let's get physical Right at the outset there are physical factors that can affect the performance of your ADSL service.

Let's get physical


Right at the outset there are physical factors that can affect the performance of your ADSL service.

If you read through How ADSL Works, you'll know that ADSL works on copper cables that run from your house back to the Telkom Exchange. It is actually a bit more complicated than that : the cable from your house to the nearest junction box meets up with the cables from your neighbour's houses at the nearest junction box and then travels from there back to the exchange. I want you to remember this part, because we'll revisit it later on when we talk about how you and your neighbours can impact on each other's ADSL experience.

Firstly though want to tell you about exchange distance and attenuation:

It's actually a very simple concept. The further the signal has to travel from your house to the exchange along the copper lines the more it attenuates (gets weaker). The amount of signal which is lost is measured in decibels, but at the end of the day it translates back into speed reduction. The weaker your signal the less likely you will be able to maintain a reliable high speed connection.

Rather than waffle on about this too much we put together this graphic that shows you the expected attenuation and maximum possible ADSL speed based on exchange distances. You do need to remember a few very important points though before you look at it:

Attenuation can be affected by other factors besides cable length (interference, poor connections).

The maximum speed is determined by exchange equipment available as well as cable length.

Cable distance is not measured in straight lines.

Your phone line may not actually run to the exchange closest to you. It doesn't sound right, but if for example your phoneline was in place before a newer exchange was built your signal may be travelling further than you think.

Also please note that the speeds indicated are the theoretical possible maximums under ideal conditions.  In real world terms we know from experience that obtaining a 2mb connection at 5km range, with 69dbs of attenuation is practically impossible.  For more information about poor attenuation values you can view this guide on dropped connections.



Here are some other physical factors that can affect the quality of your service:

  • Interference
    High powered electrical equipment near routers and phone lines can interfere with the ADSL signal. Typical culprits are pool pumps and electric fences which are becoming increasingly common place in South Africa. Electrical fences being turned on at certain times during the day are often linked to dropped ADSL signals and deterioration of signal quality.
  • Poor local cables
    The cable in your home can also affect signal quality. Make sure the phone cable running to your router is not too long and is a good quality cable. Check for loose connections at wall boxes and splitters and try to keep your cables out of the way and risk free from pulls and crimps.
  • ADSL Saturation
    This is slightly theoretical, but remember that ADSL services are a hybrid system running on top of existing phone lines which were not designed to carry heavy duty data signals. What we do know is that when a lot of cables run alongside each other the signals can interfere with each other and this can certainly become more of a factor if more of those cables are carrying high speed data signals. We've definitely noticed over the last few years of Telkom upgrades that there is a relationship between signal to noise ratios deteriorating as more users in a particular area sign on for high speed data services.



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