IEEE 802.3 SFD Sequence Explained | Generated by AI
Question: In the IEEE 802.3 standard, what is the Start Frame Delimiter (SFD) field sequence in the MAC frame?
A. 10101011 (left=LSB, right=MSB)
B. 11010101 (left=LSB, right=MSB)
C. 01111110 (left=LSB, right=MSB)
D. 01111111 (left=LSB, right=MSB)
Answer:
The correct answer is A. 10101011 (read from left to right as LSB to MSB).
Detailed explanation:
In IEEE 802.3 (Ethernet), the frame structure includes:
- Preamble: 7 bytes of 10101010 … 10101010
- Start Frame Delimiter (SFD): 1 byte = 10101011
This makes the complete 8-byte synchronization sequence:
10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011
The purpose of changing the last byte from 10101010 to 10101011 is to provide a clear, unambiguous indication that the frame data is about to start (breaking the repeating 1010 pattern).
Key points:
- IEEE 802.3 uses 10101011 as SFD.
- This differs from Ethernet V2 (DIX Ethernet), which also uses 10101011 but has an 8-byte preamble of all 10101010.
- The choice of 10101011 in IEEE 802.3 was also made to improve compatibility/distinguishability with IEEE 802.4 (Token Bus) and IEEE 802.5 (Token Ring) frame formats.
Your choice was B (11010101), which is incorrect.
11010101 is not used as the SFD in IEEE 802.3 Ethernet.
Correct answer: A
References:
- IEEE 802.3 standard (Clause 4 - MAC frame format)
- https://standards.ieee.org/ieee/802.3/2022/ (official IEEE 802.3 standard documentation)