A Start Frame Delimiter (SFD) is a one-byte Ethernet pattern that marks the end of the preamble and the beginning of an Ethernet frame. Its bit pattern is 10101011. section 6.2.1 section 6.6
The SFD helps the receiving device synchronize its receiver clock and determine when incoming electrical signals represent the frame. Although sent with every frame, it is a Layer 1 field rather than part of the Layer 2 frame. section 6.2.1 section 6.2
Preamble followed by SFD
An Ethernet transmission begins with a seven-byte preamble of alternating 1s and 0s, such as 10101010 repeated, followed by the one-byte SFD 10101011. The SFD signals that the preamble is finished and the frame is starting. section 6.2.1
Figure 6.2 The contents of the Ethernet header and trailer. They are split into multiple fields, each serving a different purpose. The fields of the header are the Destination, Source, and Type/Length. The trailer consists of a single field: the Frame Check Sequence (FCS). Although not considered part of the Ethernet frame, the Preamble and SFD are sent with each frame.
Use the Ethernet frame diagram to distinguish the transmitted preamble and SFD from the Ethernet header and trailer. section 6.2
