ticular frequency. High-frequency signals are delayed differently from low-frequency signals. Since data sig- nals are transmitted by pulses, it is important that a channel preserve the pulse shape. In a channel with excessive delay distortion, various parts of the pulse arrive at different times and cause the reception of a distorted version of the original pulse. Since data transmission involves successive pulses spaced closely over time, the envelope delay distortion causes pulses to spill over into each other. This phenomenon is called intersymbol interference. C conditioning limits the permissable amount of envelope delay dis- tortion within specific frequency ranges. D conditioning, available in two types, specifically limits noise and harmonic distortion. D1 conditioning is offered for point-to-point SATELLITE AND CARRIER SERVICES 113 channels, and D2 for two-or three-point channels. Bell charges for D conditioning on a per-circuit-per-month basis rather than per ter- mination point per month. Minimizing noise and harmonic distor- tion is critical to data transmission because these impairments interfere with the accurate reproduction of the transmitted pulse shapes. Once these impairments change the original signal shape, a modem receiver is unable to recreate the signal shape regardless of its sophistication. To combat this problem, modem manufacturers maximize the differences between various pulse shapes. A signal that uses four voltage levels to define 2 bits per pulse is more resistant to noise and harmonic distortion than one that uses eight levels to de- fine 3 bits per pulse. High data rates such as 9.6 kbit/s on voice- grade circuits require the best protection against these impairments. Most 9.6 kbit/s modems operate satisfactorily with lines meeting only the basic 3002 specifications for noise and harmonic distortion. By minimizing these major impairments, modems can tolerate a greater number of other impairments, which makes D conditioning worthy. Unlike the analog circuits, the telephone compan