A preamble is a 7-byte Ethernet synchronization sequence sent before every frame. Its alternating 1s and 0s help the receiver synchronize its clock and determine the precise length of each incoming bit. section 6.2.1
Key points:
- Uses 56 bits with the repeating pattern 10101010. section 6.2.1
- Precedes the 1-byte Start Frame Delimiter (SFD), whose 10101011 pattern marks the beginning of the frame. section 6.2.1
- Is transmitted with each Ethernet frame but is not part of the Layer 2 frame because it serves only a Layer 1 function. section 6.2.1 section 6.6
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 locate the preamble and SFD relative to the header and trailer. section 6.2
