Problems include limited availability, complicated provisioning challenges, and poor service and tech support. The first restriction refers to the length of the copper phone cable that provides phone service to the premises. If this line is longer than 18,500 feet, ADSL services are pretty much ruled out. Keep in mind that the straight-line distance from the premises to the CO doesn't account for all the copper line used in the connection, so even if you are within 18,500 feet, DSL service still may not be available. The second barrier is the presence of load coils and bridged taps.
Load coils are induction devices that shift the frequencies that typically carry voice upward.
This compensates for excessive wire capacitance, which can occur in copper loops, particularly those over 18,000 feet long. Unfortunately, voice traffic gets shifted into the frequencies normally used for DSL traffic, resulting in unacceptable interference. A short bridged tap or two may not prevent a line from being capable of carrying DSL, but the echo and additional noise that the taps add to the line can quickly make delivery of DSL untenable. The third restriction is fiber optic cable. Therefore, the signal cannot be transmitted over media using digital transmission, such as fiber optic cable.