ADSL FAQS
We have been asked alot recently how speeds are determined on broadband lines (adsl) so we have re-issued an old page we long ago retired but still as relevant now as it always was.
The full post bellow
Lately with broadband speeds increasing we have been receiving alot of enquiries from people rather confused with the array of choices regarding Broadband so we have decided to put this general Broadband FAQs together.
ADSL teminology
What is SNR / Noise Margin ?
What distance am I from my exchange? How long is my phone line?
What maximum line speed can I expect?
ADSL ( Broadband ) Terminology
DSL (Digital Subscriber Line) is a family of technologies that provide a digital internet connection over the copper wires of the local telephone network (PSTN).
Example DSL technologies (sometimes called xDSL) include:
* ADSL (Asymmetric Digital Subscriber Line)
* HDSL (High Bit Rate Digital Subscriber Line)
* RADSL (Rate Adaptive Digital Subscriber Line)
* SDSL (Symmetric Digital Subscriber Line)
ADSL is most common for residential connections, with RADSL on the rise.
ADSL:
Asymmetric means your download speed is higher than your upload speed. This technology always syncronises at the line speed you are subscibed to, i.e. if your ordered a 1Mbit/s line it will syncronise at 1Mbit/s or not at all.
RADSL:
Modified version of ADSL. Rate Adaptive means it will syncronise at the highest speed your line supports, up to the maximum you are subscribed to. So, if you are subscribed to a 4Mbit/s service but your line only supports 3.5Mbit/s (because your are too far away from the exchange or your line is of bad quality), then it will automatically syncronise at 3.5 Mbit/s.
SDSL:
Symmetric means your upload and download speeds are the same. This is mostly used in businesses.
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What is SNR / Noise Margin ?
SNR (Signal to Noise Ratio) or Noise Margin shows how easily the modem “understands” the ADSL signal. It is a measure of how much stronger the ADSL signal is compared to the noise (unwanted signals) on your line. Since the SNR is measured in decibel (db) an increase of 3db means doubling of the ADSL signal strength compared to the noise.
If the SNR margin drops to 6dB or lower then most modems will start to perform intermittently and you will experience disconnects. SNR is a direct measure of your line quality. The higher the value the better is your line and the more stable is your connection.
How far am I from my exchange? How long is my phone line?
You can check your straight line distance from the exchange on Sam Knows
Click on Availability checker, type your post code (you won’t see the distance if you check your phone number) and on the result page click on Locality and it will show your distance.
This is, however, only the straight line distance. The actual cable length is normally quite a bit longer because the telephone cable follows the road.
If you want to know the exact length, your line provider and/or BT should be able to tell you.
As a rule of thumb the BT cabling from the exchange to your place has an attenuation of about 14db/km. This means if you know the downstream line attenuation (there is now a category in the FAQ explaining how to find out), then you just divide the value by 14 and the result is your approximate cable length.
However, this is not very accurate as BT seems to use different quality cable and therefore the attenuation per length can vary.
What maximum line speed can I expect?
In order to tell what maximum speed you can expect you need to know your line length or downstream line attenuation.
Below is theoritacal Broadband speeds based on distance:
(we would like to thank intermode systems for usage of the graph)
Below is approximate speeds based on line attentuation and SNR
This is more accurate because the attenuation actually determines your max sync speed. Attenuation can be different for 2 lines of the same length depending on what quality cable BT used.
The speed in both charts is the maximum sync speed for a given line length or attenuation. Noise on the line will reduce the achiveable speed.