How does the Error Detection field in the frame trailer let a receiver know a frame was corrupted?

The sender computes a Cyclic Redundancy Check (CRC) over the frame and stores it in the trailer; the receiver recomputes it and, if the values differ, discards the corrupted frame.

The Error Detection field (often called the Frame Check Sequence) supports the data link layer's job of rejecting corrupted frames:

* The receiver recomputes the CRC and compares it to the trailer's FCS — a mismatch means the frame is discarded. *

• The sender runs a mathematical calculation — a Cyclic Redundancy Check (CRC) — over the bits of the frame and places the result in the trailer.
• The receiver performs the same calculation on the received frame.
• If the receiver's result does not match the value in the trailer, the frame is assumed to be corrupted and is discarded.

Note: This is error detection, not correction — the data link layer simply drops the bad frame; recovery (retransmission) is left to higher layers.

Go deeper:

• Frame check sequence (Wikipedia) — exactly on-card: sender computes a CRC into the trailer, receiver recomputes, mismatch → discard; explicitly detection only, not correction.