After the logic filter comes the data payload. This is the data to be transferred by the link. Some sort of error detection follows the data payload. There are several ways to implement error detection and they are generally chosen based on the available resources of the receiving processor.  Simple checksums add up all of the 1s in the packet. More complicated schemes use algorithms to XOR the packet with a number. The receiver performs the same calculation and compares its result with the value received in the packet. If they match, then the packet is good. Whatever the method used, the goal is to ensure that the received data is correct.