Frame Check Sequence (FCS) - Ethernet Trailer

Definition

• The Frame Check Sequence (FCS) is the last field of an Ethernet frame. It is 4 bytes (32 bits) long and stored in the Ethernet trailer, not the header.
• Its role is to help the receiving device detect whether the frame was corrupted while traveling across the network.
• It is calculated using a mathematical algorithm called Cyclic Redundancy Check (CRC-32).

What Problem It Solves

Electrical noise, interference, or damaged cables can change one or more bits during transmission. The FCS allows the receiver to verify that the frame arrived exactly as it was sent.

What Happens If Not Used

A corrupted frame could be accepted as valid, causing incorrect data to be processed, which might lead to broken web requests, corrupted files, or communication errors.

Easy Wording

The FCS is the package's quality check that tells the receiver whether the data arrived safely.

Layman Example

A factory weighs a sealed package before shipping it. The customer weighs it again after delivery. If the weight is different, something changed during transport, so the package is rejected.

Technical Example

• Your browser sends an HTTP request.
• The NIC creates the Ethernet frame and calculates the CRC-32 over the frame (excluding the Preamble, SFD, and FCS itself).
• The resulting value is placed in the FCS field.
• The switch forwards the frame to the router.
• The router performs the same CRC calculation on the received frame.
• If the calculated CRC matches the FCS, the frame is accepted.
• If it does not match, the frame is discarded, and higher-layer protocols (such as TCP, if used) handle retransmission if needed.

Limitation: The FCS can detect transmission errors, but it cannot identify which bits changed or repair the damaged frame. Error recovery is handled by higher-layer protocols (for example, TCP retransmitting lost or corrupted data).