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Rules have been prescribed for the design of telephone subscriber loops. Adherence to the rules provides satisfactory transmission, while transmission is usually degraded if the rules are violated. The transmission of loops adhering to and violating the design rules is described. It is shown that insertion loss can be used as a criterion for detecting loops which have violated the rules. The insertion loss measurement can be made a Â¿pass-failÂ¿ type of test by using a sweep frequency generator with a shaped output versus frequency and by accounting for the loop resistance and length. Such a Â¿pass-failÂ¿ technique can be used for both loaded and nonloaded loops.