The format of the frame is that it begins and ends with the bit pattern 01111110 or hex 7E called the frame delimiter.
Then there is an address and a control field followed by a data field that may be 0 to 5000 octets long. Then a Frame Sequence Check (FSC) is added and then the frame delimiter is added to the end. An important point is that the frame delimiter continues to be sent until another frame is sent. This generates a continuous bit pattern that looks like:
0111110011111100111111001111110 _____ ______ ______ ______ _ __ __ __ _This is used by the modems to train and synchronize their clocks via phase locked loops.