HDLC frames can be transmitted over synchronous or asynchronous serial communication links, thus the beginning and end of each frame has to be identified by a flag: 01111110(0x7E). It must be ensured that the flag doesn’t appear as data inside the frame.
Synchronous
Seeing that the flag has six 1s in a row, the other data is encoded such that it never contains more than five 1s consecutively. This is done by Bit stuffing: any time 5 consecutive 1s appear in the transmitted data, the data is paused and a 0 is transmitted.
Knowing of this method, every time it receives 5 consecutive 1s, the receiving device would strip out the following 0. If the sixth bit is 1, it is
"a flag" if seventh_bit==0 else "an error"
When idle, a frame delimiter will be continuously transmitted. The HDLC spec allows the 0 bit at the end of the frame delimiter to be shared with the next one, thus 011111101111110…… . Note that some hardware doesn’t support this.
Asynchronous
I’ll skip this part since APRS transmits synchronously.