The nominal 500 kBit/s ADSL service can be supplied over circuits with up to 60dB loss at 300kHz. When deploying ADSL service it is useful to know if the circuit looking back from the test point meets this requirement.
By placing a device that provides a known attenuation at 300kHz between the test equipment and the test point the capability of the circuit can be verified.
Two such devices are currently available that provide a nominal 20dB and 40dB attenuation at 300kHz. By connecting the two in series a nominal 60dB attenuation is realised.