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What is FCS (frame check sequence)

arminderkaur192
  1. What is FCS?
    • The Frame Check Sequence (FCS) is an essential component in networking used to verify the accuracy of data transmission.
    • It is primarily employed in data transmission protocols to ensure that frames (data packets) have been transmitted without errors.
  2. Purpose of FCS:
    • FCS serves as an error detection mechanism.
    • It detects errors in frames transmitted across a network, including the address field, control field, and information field.
  3. How FCS Works:
    • When a message is divided into data frames for transmission, each frame has FCS bits appended to it.
    • These FCS bits are calculated based on the entire frame (including the payload) and serve as a checksum.
    • At the destination, the receiver computes the running sum of the entire frame (including the FCS bits).
    • The calculated FCS is then compared with the FCS bits in the received frame.
  4. Error Detection Process:
    • If the calculated FCS matches the received FCS, the transmission is considered successful.
    • If the FCS does not match, it indicates that the frame has been corrupted during transmission.
    • In such cases, the frame is discarded.
  5. FCS Size:
    • The size of the FCS field varies:
      • It can be either 2 bytes or 4 bytes.
      • The specific size depends on the communication protocol being used.
  6. Error Recovery:
    • FCS is responsible only for error detection, not error recovery.
    • The recovery process depends on the transmission protocol:
      • Ethernet: Frames are discarded upon error detection, and no retransmission occurs.
      • TCP: Corrupted frames are discarded, and TCP initiates error recovery by retransmitting the affected frames.
  7. Example Scenarios:
    • Ethernet:
      • If an Ethernet frame gets corrupted during transmission, its FCS bits will change.
      • At the destination, the calculated FCS is compared with the received FCS.
      • If they don’t match, the frame is discarded, resulting in data loss.
    • TCP:
      • TCP also discards corrupted frames but takes additional action:
        • It initiates error recovery by retransmitting the affected frames.
        • This ensures that data is not lost.

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