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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.


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. image of waveform HDLC transmits with LSB first.


I’ll skip this part since APRS transmits synchronously.


  1. High-Level Data Link Control - Wikipedia