t high frequencies and improving voice frequency response on longer telephone lines. Loaded pairs cannot pass frequencies above 3,500Hz. Because ADSL uses the higher frequencies, it does not work on lines that have load coils. Taps-Taps are open points on a cable bundle that are left so that techni- cians will be able to easily splice off a pair to bring additional service to a home or bring service to a new home. Taps generally cause too much dis- tortion to be used with DSL. Loop carriers-Loop carriers, or remote concentrators, are not compatible with most of the DSL family. xDSL, therefore, must work around the exist- ing loop carrier systems, or the network operators must replace older- generation loop carriers with next-generation loop carriers designed to work with DSL modems. Currently only HDSL works with older digital loop carri- ers. Approximately 30% to 40% of the U.S. population is served by such digital loop carriers, and around the world, remote and rural locations are generally served with digital loop carriers. As a result, certain market territo- ries do not have quick and easy access to DSL. DSL access in those areas depends on the operator's ability and desire to upgrade the plant. Other external impairments-Leakage, impulse noise, narrowband inter- ference, and the general quality of the copper pair can all affect the quality of DSL service. Another characteristic of DSL is that it is a point-to-point connection that is always on. So when you have access to your ISP through a DSL line and your com- puter is powered up, the connection is on throughout the day. This has security implications: It is very important that you incorporate some form of firewall and security software to prevent the potential activities of a curious or malicious attacker. DSL provides high-bandwidth transmission over copper twisted-pair. It uses efficient modulation, or line-coding, techniques that enable it to carry more bits in a single cycle (i.e., Hertz) than older twisted-pair. It uses echo cancellation, which allo