What field is included in the HDLC trailer?

The HDLC (High-level Data Link Control) trailer includes the Frame Check Sequence (FCS) field, which serves a critical role in ensuring data integrity during transmission. The FCS is a block of error detection code that is appended to the end of the frame. It allows the receiving device to verify whether the frame has been damaged or altered during transit over the network.

When a device receives an HDLC frame, it calculates the FCS based on the content of the frame and compares it to the received FCS value. If they match, the frame is considered valid and error-free; if there is a discrepancy, the frame is flagged as erroneous, and typically, the device will discard it or request retransmission.

This mechanism is essential for reliable communication in networking, as it helps to detect corruption of data that could occur due to noise or interference in the communication medium. Including the FCS in the trailer is a standard practice in various data link layer protocols, not just HDLC, as it helps maintain the integrity of the data being transmitted across the network.