HDLC
High-Level Data Link Control,
the framing AX.25 borrows:
a frame starts and ends with the 01111110 flag,
a 0 is stuffed after five 1s in a row so data never looks like a flag,
and a 16-bit FCS (a CRC) at the end catches corruption.
On air the bits are NRZI-coded,
a 0 as a change of tone or level and a 1 as no change.