What digital protocol is used by APRS?

The Automatic Packet Reporting System (APRS) is built on top of the AX.25 packet radio protocol. AX.25 is the standard amateur radio adaptation of the X.25 protocol suite, specifically designed to handle data linking over RF. Since APRS transmits discrete packets of telemetry, location, and message data over radio, AX.25 serves as the perfect underlying data link layer to structure these transmissions. It formats the data into frames that include source, destination, and digipeater routing information. Distractors like PACTOR and AMTOR are HF modes typically used for continuous text or message forwarding, not the connectionless broadcasting used by standard APRS networks. QAM is a modulation technique, not a networking protocol.