Needs to reliably detect the beginning of a packet in the presence of errors as high as 10% to provide margin for FEC.

8-bit HDLC "flag" unsuitable due to short length.

Use pseudorandom sequence with good autocorrelation properties, e.g., a PN sequence.

The longer the PN sequence, the greater the margin between missing frame sync and falsely detecting one in noise.  64 looks good, and is easy to implement in software on a 32-bit CPU.

Reliable sync enables use of negative acknowledgements (NAKs) to request additional information to aid decoding.