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Ethernet/802.3 Frames
Data on an Ethernet is transmitted in frames. This
term is used to describe, in general, any "link level" (ie,
within-network) PDU which encapsulates higher-layer data for local
delivery over a LAN. The Ethernet frame format is:
- Preamble
- 7 bytes of
0101010101... This is used to
synchronise the receiver's clock circuitry.
- Start Of Frame
- 1 byte, thus:
01010111 -- notice the two "1" bits
at the end.
- CRC
- A 4-byte checksum which allows the receiver to
be absolutely certain that the frame does not contain any physical
errors. If it does, then the frame is simply discarded at the
receiver.
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