The designator AX.25 is associated with which amateur radio mode?

AX.25 (Amateur X.25) is a data link layer protocol derived from the ITU-T X.25 protocol suite, specifically adapted for amateur radio use. It serves as the foundational protocol for amateur packet radio networks worldwide. The protocol defines how data is framed, addressed, and transmitted over a radio link, establishing the rules for both connected and connectionless communication. It is most famously used in standard packet radio BBS systems and forms the underlying transport layer for APRS (Automatic Packet Reporting System). Other modes like RTTY, ASCII (when sent directly as raw serial data), or spread spectrum voice do not rely on the AX.25 framing and packet structure. Whenever you see AX.25 in amateur radio, it is completely synonymous with packet radio.