I Don't Understand a Thing You're Saying

I Don't Understand a Thing You're Saying

So, someone drives up to your house and parks in your driveway. (Why do we park in driveways, and drive on parkways, anyway?) The person comes up to the door, rings the bell, and you answer it. The stranger smiles and confidently says, "Jwl! Sfk oewp fkas lal. Wwjj. KK45i!!!" Of course, this makes no sense to you, so you ask the visitor to repeat himself, after which he says, "Hi, I'm Archie. If you're Bob, you just won our grand prize of $1 billion." "Oh my! Yes, I am Bob. Please come in."

With Ethernet, a similar process occurs for recognizing that a received frame makes no sense. When the receiving NIC interprets the incoming electrical signal as meaning some string of binary 0s and 1s, the receiver might misinterpret the meaning. That can happen because the electrical signal has changed as it passed over the wire, affected by resistance in the wire, EMI, collisions, and other factors. Simply put, the sender might send a 1 and the receiver might think it was a 0, or vice versa.

To recognize the error, Ethernet uses a field in the Ethernet trailer called the frame check sequence (FCS) field. The FCS field holds a 4-byte number that allows the receiver to notice that the frame that it received actually had bit errors in it.

To check for errors, the receiver must rely on the value of the FCS field as set by the sender. The sender runs a mathematical formula, with the input being the contents of the Ethernet frame to be sent, up to the Ethernet trailer. The sender places the results of the formula into the trailer FCS field. Upon receipt of the frame, the receiver applies the same formula to the same part of the Ethernet frame (everything up to the trailer). If the result of the formula as calculated by the receiving NIC is the same as the value in the transmitted FCS, then no errors occurred. If the values do not match, then an error has occurred. Figure 5-7 outlines the process.