Skip to main content

How Data Looks

Data is put into a set structure, Protocol Data Unit (PDU), called a Frame.

You need an addressing scheme, most commonly Media Access Control (MAC) Address:

  • Source address: from which the stream is sent from
  • Destionation address: adress which the stream is sent to

MAC: Media Access Control​

  • 48 bits in length.
  • Written in hexadecimal.
  • Universal Local (UL) Bit are the 7 most significant bits of the address (leftmost)
    • If those are turned off (MAC started with 00), it means MAC address should be universally unique.
    • If MAC address is locally assigned, those bits must be turned on.

PDU: Protocol Data Unit​

The PDU at Data Link Layer is Frame.

  • Destination: MAC
  • Source: MAC
  • Data
  • Frame Check Sequence (FCS): houses Cyclic Redundancy Check (CRC)
    • CRC: Before device transmitting a frame, it performs a calculation and stores the resulting number inside CRC. The receiving device performs the same calculation and compares results. This is used to check for data corrumption.