Broadband Terms

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Dave
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Broadband Terms

Post by Dave »

Below are some common broadband terms and their meanings.

ADSL

ADSL stands for A-symetric Digital Subscriber Line. ADSL uses frequencies that the normal copper phone line can pass through, but that are not used for Voice calls (0-4KHz).

Some space is not used in the frequency spectrum to avoid interference with voice calls, so ADSL uses 28KHz-1.1MHz, and ADSL2+ uses 28KHz-2.2MHz.

Attenuation and Signal Noise

The copper wiring that is in the Local Loop (between the exchange and the home) has it's own characteristics and is subject to noise and interference, which varies depending on factors that affect each individual line.

Attenuation

Attenuation is the phenomenon where the signal amplitude (strength) degrades with distance. A longer line in the local loop will have a greater attenuation, and a poor quality line similarly.

It is measured in dB. A standard policy is that 45dB attenuation generally is the cut off for ADSL2. This is the reason that ADSL2+ is not available beyond a certain distance from the exchange.

A high attenuation requires that the DSLAM and Router transmit at higher power levels, which in turn affects the speed of the signal.

Noise

Noise is electromagnetic interference on the line; things like crosstalk, electrical wires, high voltage devices, line joints, EMP..... all induce a current in the line that is not the signal. This obviously can interfere with the signal, causing errors, and loss of data - this all slows down the ADSL, as data has to be re-transmitted.

A key measure of line "health" is the Signal to Noise Ratio (or SNR). This is also measured in dB. This measures how "clear" the signal is compared to the noise on the line. Below a specified limit, then the ADSL line profile has to be changed to account for this. A high SNR indicates a very clear signal.

A low SNR will generally lead to a dropped connection. This leads to a tradeoff between line stability and speed.

Line Profile

The ISP's equipment takes account of both line attenuation and SNR for individual lines, combined with a more general knowledge of line characteristics in the local loop, to create "Line Profiles".

There are generally a range of these, to account for different customer situations and requirements. A lower speed profile, as a rule, can cope with a lower SNR, and higher attenuation. Those with "better" lines can be set to a more aggressive profile, which will give higher speed.
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