XVI. DSL Diagnostics

What do the ADSL line signal figures mean ?

Products:
Vigor 2820
Keywords:
Loop Attenuation
SNR Margin
Sync Speed

The router's online status screen (as well as the telnet logs) will show two figures which relate to ADSL line signal level/quality. A good ADSL signal is more reliable and produces less errors. When you order your ADSL line, the telephone company will test your line, and if the attenuation is too high (because you are too far from the exchange) you cannot have ADSL. The two measures indicated by the router of the line quality are Signal to Noise Ratio (SNR) and Loop Attenuation.

SNR Margin is, as its name suggestions the ratio of signal (the bit we want) with noise (the bits we don't want) and the difference between the SNR required for the obtained synch speed and the actual observed SNR. Noise can be introduced by poor quality lines, bad wiring (including your own extension wiring) etc. The ratio is expressed as in dB and the higher the number, the more stable the line is likely to be (i.e. if the number is higher, there is much more signal than noise). The target SNR Margin is controlled by the ISP, and the figure is best understood when used in conjunction with the synch speed. For example an SNR margin of 6 with a downspeed of 14MB would indicate a better line than a SNR margin of 12 with a synch speed of 1MB

A lower target SNR will provide a faster synch speed buta similar line that's on a higher target SNR may be more stable.

Loop Attenuation is a measure of the quality of the line from the point of view of how much the signal is impeded. The longer a line, the higher the attenuation will be (hence the 'quieter' the signal). If the Attenuation is over 60 then the distance from the exchange may be great and there could be a limit on the line speed that the ISP is willling to offer. Typical attenuation values from a good working ADSL line are between 30 and 40, but if yours is higher, this isn't anything to worry about if it works fine.

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