What is contained in the trailer of a data-link frame?

• A. logical address
• B. data
• C. error detection
• D. physical address

Answer: (C)

Error detection is what the trailer provides. In a data-link frame, the trailer holds the Frame Check Sequence, usually a CRC value computed over the frame’s contents. The sender calculates this CRC and appends it to the end of the frame; the receiver recalculates the CRC from what it received and compares it to the value in the trailer. If they match, the frame is considered intact; if not, the frame is discarded or flagged for error handling. This mechanism protects the data on the link from corruption as it travels between devices.

Addresses (logical/physical) and the actual data sit in the header and payload, not in the trailer, so the trailer’s sole purpose is verifying integrity through error detection.