All about cable types & their suitability for ADSL.

A question often asked is how long can I run an extension for ADSL and what cable should be used ?

The answer is not as simple as it may seem as it depends on a number of variables such as how far you are from the exchange and how much interference the line is likely to be subjected to. Suggested lengths are suggested ideals for best performance based on a typical installation. Some people with very poor lines may not achieve this and others with excellent line quality will no doubt be saying their cable is 4x the length mentioned here.

Proper telephone cable is twisted pair (the official standard is BT reference number CW1308). It is typically manufactured from 0.50mm solid tinned copper and each of the pairs of wires is colour coded and loosely twisted together. This is the cable that a BT engineer will use if you have them install an extension. The same spec cable is used in the exchanges and in the street, only the cable used on the BT network typically contains 200 pairs in each cable and it is weather proof.

This cable is used for and works well for voice, ISDN and now ADSL as its twisted pair construction provides some resistance to interference. A good long run of CW1308 cable can be used for ADSL, 50m can typically be easily achieved without any degradation.

Most telephone cables that you can buy in a DIY store are not twisted pair. They are cheaper, general purpose low voltage cable as used for house alarms. You often see it labelled as telephone/alarm cable. This cable has straight through connections that have no rejection to interference. This cable is fine for voice but can only be used in short runs in a typical home/office environment where there are lots of sources of electrical interference. Your mileage may vary but it's not a good idea to run more than 20m of this type of cable and if you are near the limits for receiving ADSL it should be avoided altogether.

Another common cable is the flat type often used in RJ11 cables and on ready made flexible telephone extensions. This cable is also not twisted pair and so is not suited to long runs. We only supply a maximum length of 10m for our RJ11 cables to avoid problems but you can probably safely use 20m in most installations.