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Ethernet Frame

The data link layer Protocal Data units (PDU) is frame. A frame contains the bits that are transmitted over an Ethernet LAN. The frame have three parts, frame header, frame trailer and the actual data that is being transmitted. As showed the picture below, the frame structure defined in Ethernet II and 802.3 have some slight differences: In Ethernet II frame, the 2-byte field Type represnets the Type of data to immediately follow in the Data field. In 802.3 frame, the 2 byte Legnth field specifies the length of the 802.2 Header and Data field. The protocol information is contained in 802.2 Header and Data field, which are at the LLC layer.

Ethernet II Frame


7 bytes1 byte6 bytes6 bytes2 bytes46-1500 bytes4 bytes
PreambleSOFDest. AddressSource AddressTypeDataFCS



802.3 Frame


7 bytes1 byte6 bytes6 bytes2 bytes46-1500 bytes4 bytes
PreambleSOFDest. AddressSource AddressLengthLLC header and Data (802.2 Header and Data)FCS


  • Preamble— synchronize the signals of the communicating computer, but really not counted as part of the 14-byte frame header.

  • Start-of-frame (SOF) delimiter: This field contains bits that signal the receiving computer that the transmission of the actual frame is about to start and that any data following is part of the actual frame.

  • Destination address— the address of the NIC to which the frame is being sent. Can be Unicast, multicast, or broadcast.

  • Source address— the address of the NIC of the sending computer. Always unicast.

  • Length— In 802.3, this field specifies the length of the data field. Valid length field (<= 1500 bytes or 0x05dc).

  • Type — In Ethernet II, this field identifies the network layer protocol.

  • Data— IEEE 802.3 specification includes requirement to pad the data to the 46-byte requirement, but the Ethernet II specification really doesn't. 802.3 frame data field actually contains a 802.2 Header and Data, which is at the LLC sublayer.

  • Frame check sequence— A 4-byte error detection CRC created by the sender and recalculated by the receiver to check for transit damage.

The Destination Address, Source Address and Type/Length fields are collectively called MAC Header, in contract with the LLC header and data field in the 802.3 frame.

800px-Ethernet_Type_II_Frame_format_svg

ICND1 and ICND2 break down

3 comments:

  1. but can u please tell me why the minimum size is 46 bytes
    what is the reason for this requirement?

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  2. The smallest packet size is 64 bytes.
    64 bytes - 14 bytes (MAC Header) - 4 bytes (CRC) = 46 bytes (Data).

    The mallest packet size is defined in RFC document (RFC894), I don't know why IEEE like the number 64. As far as I concern, physics set the low bound for the packet size.

    Short explanation:
    Ethernet LANs use CSMA/CD, all nodes shared a single coaxial cable and only one speaker is allowed at a time. Packets had to be of a minimum size to guarantee that the leading edge of the propagating wave of the message got to all parts of the medium before the transmitter could stop transmitting, thus guaranteeing that collisions would be discovered.
    Long explanation:
    http://xyznetwork.blogspot.com/2010/02/why-the-minimum-thernet-packet-size-is-64-bytes.html

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