 local loops connected at the cen- tral office. Since there are no repeaters and no loading coils in the signal path, the user can, in most cases, use the less-expensive lim- ited-distance modems. Limited distance modems may also be used on standard type 3002 unconditioned channels, depending on the length of the circuit and the data rate. Modem manufacturer can supply the specific details. Five types of C conditioning are available from Bell. C1 and C2 112 TELECOMMUNICATIONS conditioned lines may be ordered for point-to-point, multipoint, and switched configurations. C3 conditioining, which is similar to C2 conditioning, applies only to private switched networks with a max- imum of four trunks and two access lines in tandem. C4 conditioning can be ordered in two-, three-, and four-point circuits only. C5 conditioning can be ordered only in point-to-point circuits and is similar to C2 conditioning. C5 conditioning is pri- marily intended for overseas channels. The 3002-type channel with C conditioning has specific limits on attenuation distortion and on envelope delay distortion that exceed those for a 3002 unconditioned channel (see Bell's technical reference Publication 41004 for exact specifications). Since Bell charges a premium per conditioning type per termination point, the telephone company is required to keep line impairments from exceeding the limits set in the tariff. When transmitting digital data, it is necessary to transmit many state-changes per second. Attenuation distortion describes the extent to which a signal traveling through a channel is affected as the fre- quency of the signal changes. The higher the data rate, the more changes per second the channel must handle. The ability of a channel to handle these changes is called high-frequency response. Good high- frequency response is the same as low attenuation distortion. C con- ditioning improves the high-frequency response of the basic 3002 channel. Envelope delay distortion occurs because the amount of signal de- lay is not constant at a par