Frame Format

  • Each frame is added a header and trailer where header helps in differentiating one message from source to destination or other message to other destination.
  • The trailer contains error detection and correction bits. This is commonly known as cyclic redundancy check(CRC).
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Preamble( 7 bytes): This is kind of signal to the receiver about the frame arrival, used for synchronization with the incoming data stream.

Start Frame Delimiter(SFD – 1 bytes): This is to indicate the start of the frame and is of 1 byte that is set 10101011

Destination Address(6 bytes): This is the physical address(mac) of the destination nodes.

Source Address(6 bytes): This is the physical address(mac) of the sender. It is also of 6 bytes.

Length or type(2 bytes): This is for indicating the type of the layer-2 protocol. It can either be Ethernet or Token Ring or ATM.

Data Field: This is the data field from above layer.

Trailer(CRC-4 bytes):

The last field contains error detection information.


  • Thus total length of the header is 14 bytes ( DA(6)+ SA(6) + Type(2))
  • And trailer is 4 bytes, so overall 18 bytes
  • Remember preamble and SFD are not part pf frame, they are added by physical layer.

Categories: Networking

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