Question

Consider the 802.3 MAC frame fields from left to right :
A. Preamble

B. Source address

C. SFD

D. Destination address

Choose the correct answer from the options given below :

• A. A, C, D, B
• B. A, C, B, D
• C. D, B, C, A
• D. B, D, C, A

Hint:

Ethernet is "polite" in a networking sense: it first announces it's talking (Preamble/SFD), then says WHO it wants to talk to (Destination), and only then identifies WHO is talking (Source).

Answer 1

The Correct Option is A

Concept:
• The IEEE 802.3 standard defines the MAC (Media Access Control) frame format for Ethernet networks.
• A frame is the unit of data transmitted at the Data Link layer.
• The fields must appear in a specific sequence to allow synchronization, addressing, and error checking between the sender and receiver.

Step 1: Identify the synchronization components (A and C)
The frame begins with a 7-byte Preamble (A) consisting of alternating 1s and 0s to synchronize the receiver's clock. This is immediately followed by the 1-byte SFD (C - Start Frame Delimiter), which signifies the beginning of the actual frame data.

Step 2: Identify the addressing components (D and B)
The network needs to know where the packet is going first so that non-target devices can stop processing the frame early. Therefore, the Destination Address (D) comes before the Source Address (B). Both are 6-byte MAC addresses.

Step 3: Arrange the sequence from left to right
Preamble (A) \(\rightarrow\) SFD (C) \(\rightarrow\) Destination address (D) \(\rightarrow\) Source address (B).
This matches the sequence in Option (A).