Explain what a FLAG is, and how bit stuffing handles the case when a FLAG sequence appears in the middle of data.
The correct answer and explanation is:
A FLAG is a specific bit pattern used in data communication to signal the beginning or end of a frame or packet of data. In many communication protocols, such as High-Level Data Link Control (HDLC) or other framed protocols, the FLAG is typically a unique sequence of bits that cannot appear anywhere else within the data frame itself. This ensures that the start and end of the frame can be reliably identified by both the sender and receiver.
In HDLC, the FLAG sequence is usually represented as 01111110, which consists of 8 bits. This sequence marks the boundaries of a data frame. When a FLAG sequence appears within the actual data, it can cause confusion because it might be mistakenly interpreted as the end of the frame, even though it is part of the payload. To avoid this issue, a technique called bit stuffing is used.
Bit stuffing is a process where extra bits are inserted into the data stream to prevent the FLAG sequence from appearing in the data. Specifically, if a sequence of five consecutive 1s is encountered during data transmission, a 0 is automatically inserted after these five 1s. This process ensures that the FLAG sequence (which consists of six consecutive 1s) will not appear in the data, thus avoiding confusion with the actual frame delimiter.
For example, if the data being transmitted contains the bit pattern 11111, the sender will insert an additional 0 after this sequence, making it 111110. This transformation prevents the FLAG sequence from appearing in the data payload. On the receiving end, the receiver checks the data and removes the inserted bits (i.e., removes the extra 0 after five consecutive 1s), thus restoring the original data without affecting the integrity of the transmission.
Bit stuffing helps maintain the clarity of frame boundaries and ensures that the data is transmitted accurately without misinterpretation.