Solution
The correct answer is Ox7E
Key Points
- Bit stuffing is a technique used in communications protocols to ensure that a special sequence of bits (the flag sequence) does not appear within the data portion of a frame. This is important to differentiate the start and end of the frame from its content. The flag sequence, often used to signify the start and end of a frame, must be unique, meaning that it shouldn't be mistaken for or mixed up with the payload data.
- In particular protocols such as High-Level Data Link Control (HDLC) and the related Point-to-Point Protocol (PPP) in byte-oriented mode, each frame starts and ends with a specific bit pattern, 0x7E in hexadecimal or 01111110 in binary. This specific sequence of bits is recognized by the receiving device as the start or end marker of a data frame.
- To avoid confusion between the special flag sequences and the data being transmitted, a process called bit stuffing or byte stuffing (based on whether the operation takes place on an individual bit or a whole byte of data) is used. If the sender recognizes that the same sequence of bits (the flag byte) is to be sent within the data, it inserts (stuffs) an extra bit (usually 0) into the data to differentiate the flag byte from the corresponding sequence in the content of the message.
- When the receiver identifies an extra 0 bit following five consecutive 1 bits within the data portion of the frame, it removes (de-stuffs) the 0 bit to retrieve the original data. However, if it spots the flag sequence (01111110), it interprets it as the start or end of a frame.
Therefore, bit stuffing is essential in differentiating between the data payload and the flag byte to ensure reliable communication.
