The Discrete Address Beacon System (DABS) has been designed as an evolutionary replacement for the Air Traffic Control Radar Beacon System (ATCRBS). As with ATCRBS, DABS is a cooperative Air Traffic Surveillance System utilizing ground based sensors (interrogators) and airborne transponders. In addition to its surveillance function, DABS integrally accommodates ground-to-air and air-to-ground data link communication within the interrogations and replies. In DABS, each aircraft transponder may be individually interrogated, using its unique 24-bit address, giving the ground based interrogators freedom to schedule interrogations and replies to make efficient use of the channels essentially independent of the aircraft traffic distribution. The evolutionary constraint on DABS, requiring the capability for one-for-one replacement of ATCRBS ground sensors and transponders, dictated the need to maximize commonality between the two systems. Thus, the ATCRBS interrogation and reply frequencies (1030 and 1090 MHz) were prime candidates for DABS operating frequencies. This report presents the rationale for the selection of the DABS signalling waveforms and error control techniques. The main issues in arriving at the final link design were (1) affordable transponder cost, (2) electromagnetic compatibility with ATCRBS and TACAN, and (3) adequate performance in the channel environment, which includes interference from ATCRBS transmissions. The resulting DABS that of ATCRBS, and transmits ground-to-air data link messages with high reliability. This is accomplished without noticeably affecting ATCRBS performance, with less channel occupancy per target report than ATCRBS, and with transponders projected to cost approximately 160% of the cost of ATCRBS transponders.