A Cellular Data Network (CDN) identifies, collects and utilizes otherwise unused or unusable air time, in an existing Advanced Mobile Telephone System (AMPS). A Digital Cellular Overlay Network (DCON) is provided for performing CDN utilizing an underlying AMPS, where the DCON is transparent, with respect to the AMPS, and takes full advantage of existing AMPS equipment to keep the cost of performing CDN to a minimum. The resulting digital network may be used for high speed non-interfering data communications, i.e., data communications that do not degrade the underlying system's capacity and capability (qualitatively) to handle voice traffic. Expanded use of well-known radio control signals, in particular "busy bits" and "dotting sequences," provides a DCON capable of performing CDN in both a reliable and flexible manner.