Start Frame Delimiter (SFD) - Ethernet Header

Definition

• The Start Frame Delimiter (SFD) is the 1-byte (8-bit) field that comes immediately after the Preamble. Its bit pattern is 10101011.
• Its role is to tell the receiver, "Synchronization is complete. The Ethernet frame starts with the very next byte."

What Problem It Solves

The Preamble only helps the receiver match the sender's timing. The SFD provides the exact point where the real Ethernet header begins, preventing the receiver from treating synchronization bits as actual data.

What Happens If Not Used

The receiver would know a frame is coming but might not know which bit is the first real bit of the frame, leading to incorrect interpretation of the header.

Easy Wording

The SFD says, "Ready? The real Ethernet frame starts now."

Layman Example

A race official says, "On your marks... Get set..." (Preamble), then fires the starting pistol (SFD). Only after the gunshot does the race officially begin.

Technical Example

• Your browser sends a request to a web server.
• The NIC creates an Ethernet frame.
• Preamble lets the switch synchronize its clock.
• SFD marks the exact beginning of the frame.
• The switch now starts reading the first actual field: the Destination MAC Address.

Limitation: The SFD only identifies where the frame begins. It doesn't indicate who should receive the frame. Destination MAC Address, solves that problem.