Ethernet Frames

Data is transmitted through an Ethernet network in frames. The frames are of variable length, ranging from 64 octets to 1518 octets, including the header, payload, and cyclic redundancy check (CRC) value. Figure 11 shows the Ethernet frame format.

Figure 11: Ethernet Frame Format

Ethernet frames have the following fields:

• The preamble (PRE) in the frame is 7 octets of alternating 0s and 1s. The predictable format in the preamble allows receiving interfaces to synchronize themselves to the data being sent. The preamble is followed by a 1-octet start-of-frame delimiter (SFD).
• The destination address (DA) and source address (SA) fields contain the 6-octet (48-bit) MAC addresses for the destination and source ports on the network. These Layer 2 addresses uniquely identify the devices on the LAN.
• The length/type field is a 2-octet field that either indicates the length of the frame's data field or identifies the protocol stack associated with the frame. Following are some common frame types:
  • AppleTalk—0x809B
  • AppleTalk ARP—0x80F3
  • DECnet—0x6003
  • IP—0x0800
  • IPX—0x8137
  • Loopback—0x9000
  • XNS—0x0600
• The frame data is the packet payload.
• The frame check sequence (FCS) field is a 4-octet field that contains the calculated CRC value. This value is calculated by the originating host and appended to the frame. When it receives the frames, the receiving host calculates the CRC and checks it against this appended value to verify the integrity of the received frame.