ISDN requires the phone company to install services within their phone switches to support the connections. ISDN, while gaining in popularity, has been slowed by high installation costs and inconvenient setup procedures. To work, xDSL requires the phone company to install xDSL equipment at both ends and remove bandwidth limiting devices such as loading coils. ADSL transmissions typically have a greater download speed than upload speed, adjusting perfectly to typical Internet usage patterns. At the far end, there are fewer problems with crosstalk, so bandwidth is greater from the CO to the office.
Higher frequencies suffer a greater amount of attenuation.
The reason that the upstream speed in ADSL is generally much less than the downstream rate is due to this fact. In the downstream direction, the high frequencies still attenuate, but at the customer end, they have a better chance of avoiding crosstalk since most customers don't have large bundles of wires in their premises. Attenuation is signal loss due to the diminishing signal power as a signal traverses its path. This is a natural part of signal transmissions and signal repeaters may be used to boost signal energy along a path. If the signal becomes too weak, the far end may be unable to recover it.