Ethernet Frame Format

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• transceiver implements a link at the physical layer
• Ethernet interfaces perform addressing and framing at the Data Link layer
• Ethernet encapsulates the payload from higher layer protocols within a protocol data unit (PDU)
  • called a frame
• Ethernet frame and header format:

Preamble

The preamble and start frame delimiter (SFD) are used for clock synchronization and as part of the CSMA/CD protocol to identify collisions.

• preamble consists of 8 bytes of alternating 1s and 0s
  • SFD is two consecutive 1s at the end
• not considered part of the frame
• indicates the beginning of the frame

EtherType

• EtherType field is usually used to indicate the type of protocol in the frame payload
  • 2-bytes
  • E.g.,
    • frame carrying IPv4 packet would have EtherType value of 0x0800
    • frame carrying IPv6 data would be 0x86DD

Info

• 2-byte field may be called EtherType/length field
• when Ethernet was being developed, there were several frame formats
  • one used the 2-byte field to indicate the frame length
• to maintain compatibility, EtherType values are 0x0600 (1536 in decimal) or greater
  • anything less is interpreted as the payload length

Error Checking

Error checking field contains a 32-bit (4 byte) checksum called a cyclic redundancy check (CRC) or frame check sequence (FCS).

• calculated based on the contents of the frame
• receiving node performs the same calculation
  • if matches, accepts the frame
• no mechanism for retransmission if damage is detected
  • CRC is not completely accurate at detecting damage
  • these are functions of protocols in higher layers

Cycle Redundancy Check (CRC)

Calculation of a checksum based on the contents of a frame used to detect errors.