This is how telemetry data is sent. The amount of phase shift used in modulating data onto the subcarrier is referred to as the modulation index, and is measured in degrees. The same kind of scheme is also used on the uplink. Demodulation is the process of detecting the subcarrier and processing it separately from the carrier, detecting the individual binary phase shifts, and decoding them into digital data for further processing. The same processes of modulation and demodulation are used commonly with Earth-based computer systems and fax machines transmitting data back and forth over a telephone line.
Modems use a familiar audio frequency carrier which the telephone system can readily handle.
More about radio waves and electromagnetic radiation.