I/O Interfaces & Buses
What does the PCI Express data link layer do about bit errors during transmission?
- AIt does nothing and passes the data on until software notices
- BIt powers off the device when an error is found
- CIt adds a CRC to detect errors and, on error, signals that the packet was not received so it is resent
Show answer and explanation
Answer: C. It adds a CRC to detect errors and, on error, signals that the packet was not received so it is resent
The data link layer sends each packet with a sequence number and a CRC (cyclic redundancy check) code. The receiver checks the CRC and returns ACK if it is good or NAK if it is not; the sender keeps each packet until it is acknowledged and resends it on NAK. Thanks to this, even when the physical layer occasionally flips a bit, the upper layers and software receive correct data.
- A. The data link layer detects and recovers errors.
- B. It does not cut power.
- C. Correct. It is retransmission based on ACK and NAK.