Attenuation in DSL connections
What is attenuation?
Attenuation is the reduction in the amplitude of a signal in travelling from one point to another. In a DSL connection it normally refers to the proportion of the signal lost between the exchange and the user's premises. The attenuation is made up of resistive losses in the cable (a function of the length, diameter and material of the cable) and high frequency losses due to capacitive and inductive effects. Other factors such as the quality of the joints in a cable can also have an effect.
How does a modem/router measure attenuation?
A DSL signal is made up of many different frequency tones (or 'bins'), and high frequency losses in a cable increase as the frequency increases. So each tone has its own level of attenuation, made up of the resistive loss of the cable combined with the high frequency loss at that particular frequency. The modem/router averages all the different levels of attenuation for the tones used in the signal, and reports this average value as the attenuation for the connection. The averaging is probably weighted according to how many bits are allocated to each tone, so the calculation is fairly complex, and different modem/routers can use different algorithms for making the calculation.
Some modem/routers also report the attenuation at a specific frequency (300 kHz), but the average value is the one which is generally used for assessing a DSL connection.
Attenuation of an ADSL2+ connection
It's common experience when upgrading from ADSL(1) to ADSL2+ that the reported attenuation increases by about 3 dB. This doesn't imply that the line has somehow become more lossy, but is simply the result of that way attenuation is measured. ADSL(1) and ADSL2 use frequencies up to 1.1 MHz (tone 255); ADSL2+ uses frequencies up to 2.2 MHz (tone 511). Because attenuation increases with frequency, the use of these higher frequencies for ADSL2+ means that, when the modem/router averages the attenuation over the range of tones used, the calculated result is higher than it would be if only tones up to 255 were used.