The preamble is bytes of alternating s and s (10101010...). It synchronizes the receiver's clock with the sender's. The SFD (Start Frame Delimiter) is byte: 10101011. That final signals "the real data starts now." Together, the preamble and SFD are bytes. They're not counted in the frame size because they're stripped before processing.
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$ curl repovive.com/roadmaps/network-design/physical-and-data-link-fundamentals/preamble-and-sfd
░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░████████████████████████████████████████████████████████████████████████████████████████████████