HDLC Frame Flag Explained | Generated by AI
Question: In the HDLC frame format, what is the flag field F?
Answer:
HDLC Flag Field
The correct answer is D: 01111110
Explanation
HDLC (High-Level Data Link Control) uses a special bit pattern as a flag to mark the boundaries of a frame.
The flag field value is:
01111110
This pattern serves dual purposes:
- It marks the end of the previous frame
- It marks the beginning of the next frame
Why 01111110?
The flag 01111110 contains six consecutive 1s surrounded by 0s. To prevent this pattern from appearing accidentally within the data payload, HDLC uses a technique called bit stuffing:
- Sender side: After every five consecutive 1s in the data, a
0is automatically inserted - Receiver side: After receiving five consecutive 1s, if the next bit is
0, it is automatically removed (destuffed)
This ensures the 01111110 pattern can only appear as an actual frame delimiter, never as part of the data content.
Why the other options are wrong?
| Option | Value | Reason |
|---|---|---|
| A | 11111111 | All ones — not the HDLC standard flag |
| B | 10000001 | Not the defined flag pattern |
| C | 01111101 | Close but incorrect — only five 1s, missing one |
| D | 01111110 | ✅ Correct — standard HDLC flag |
Option C (01111101) is a common trap because it looks similar, but it has the wrong bit arrangement and does not match the HDLC specification.