Cisco Certified Network Associate (CCNA) Group Practice Exam

What is the purpose of the FCS field in a frame?

To Obtain the MAC Address of the Sending Node To Verify the Logical Address of the Sending Node To Compute the CRC Header for the Data Field To Determine If Errors Occurred in the Transmission and Reception

Correct Answer:

To Determine If Errors Occurred in the Transmission and Reception

Explanation:

The main concept is error detection using a Frame Check Sequence (CRC). The FCS is a CRC value appended to the end of the frame. The sender computes the CRC over the frame’s contents (header and data) and places the result in the FCS. When the frame is received, the NIC recalculates the CRC over what was received and compares it to the FCS. If they match, the frame is considered to have arrived without detectable errors; if they don’t, a transmission error occurred and the frame is discarded or flagged for error handling. This mechanism detects many bit errors, including burst errors, but it does not validate MAC addresses or logical addresses, and it applies to the whole frame rather than just the header.

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