There are radio bands using VHF and FM frequencies, satellite links, and the Mode S transponder. The key to successful datalink is the timeliness of the information relayed, the capacity of the system to handle the traffic, and the cost per message. The ultimate linking device could be any of the above or a combination. The technology is still shaking out, and each has pros and cons. Ground-based datalinks utilizing VHF or Mode S require that the aircraft be within line of sight of a ground station, so many ground stations are required to achieve good coverage.
Satellite links have wide coverage but at present are prohibitively expensive.
While we don't have many midair collisions, that's not very comforting if you're the one who failed to see and avoid. Last year there were nine collisions, involving 18 aircraft. The number of close calls isn't well documented but certainly ran much higher than the actual collisions. For this test program, MIT used a less sophisticated version of the airline Traffic Collision Avoidance System, but it works very well. The sensor gets its input from transponders that ATC sees on radar.