o another pair and causes interference (noise) there. Crosstalk exists in several flavors. Near-end crosstalk (NEXT) occurs when the transmitter at one end of the link interferes with the signal received by the receiver at the same end of the link, while far-end crosstalk (FEXT) occurs when the transmitter at one end of the circuit causes problems for the signal received by a receiver at the far end of the circuit. Similarly, problems can occur when multiple DSL services of the same type exist in the same cable and interfere with one another. This is referred to as self-NEXT or self-FEXT. When different flavors of DSL interfere with one another, the phenomenon is called foreign-NEXT or foreign-FEXT Other problems that can cause errors in DSL, and therefore a limitation in the maximum achievable bandwidth of the system, are simple radio frequency interfer- ence (RFI) that can find its way into the system; and impulse, Gaussian, and random noise that exist in the background but can affect signal qual- ity even at extremely low levels. Physical Impairments The physical impairments that can have an impact on the performance of a newly deployed DSL circuit tend to be characteristics of the voice Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Access Technologies Access Technologies network that typically have minimal effect on simple voice and low- speed data services. These include load coils, bridged taps, splices, mixed gauge loops, and weather conditions. Load Coils and Bridged Taps We have already mentioned the problems that load coils can cause for digital transmission; because they limit the frequency range that is allowable across a local loop they can seriously impair transmission. Bridged taps are equally problematic. When a multipair telephone cable is installed in a newly built-up area, it is generally some time before the assignment 