Can it happen in the HDLC protocol that the content of the control field is the same as a FLAG byte (01111110)? If it can happen, how is the protocol protected it incorrectly as a frame boundary? from accepting a. Yes, it can happen, but knowing the position of the control field, the protocol also knows that it cannot be the end of the frame, so it does not accept it as frame boundary. b. No, it can never happen. Only the payload can contain an "accidental" FLAG or ESC byte (for example, in binary data), the header and tralier of the frame are designed so that this cannot occur. That is why the stuffing is only needed in the payload. c. It could only happen if the frame is an information frame and the contents of the SEQ, P/F and NEXT fields are 111, 1, 110, respectively. Athough then the control field would be identical to a FLAG byte, the protocol can still recognize the problem, since the NEXT field has smaller value than the SEQ field, which is an error. d. Yes, it can happen, but the byte stuffing algorithm will stuff an ESC byte before the FLAG, so this resolves the problem.