What to Do When the Bits Get Bashed

What to Do When the Bits Get Bashed

Now imagine that the second packet shown in Figure 3-2 had an error. If both NICs think that +5 volts is a binary 0, and +10 volts is a 1, what happens if the voltage is 15 volts? Or 7.5 volts? Certainly, those voltages would confuse the NICs.

How do errors occur? Well, something as simple as someone running a vacuum cleaner near the cable could cause the voltage to change while the electricity is passing over the wire, a phenomenon called electrical interference. Formally, electrical interference is the effect of one electrical signal changing because of some other nearby electrical signal. You have all seen or heard electrical interference, such as when your television picture gets a little fuzzy, when you can't hear your AM radio station while you're driving under high-power electrical wires, or when you hear a buzzing noise from your stereo when you are running a blender in your kitchen.

Electrical interference happens. The people who create networking standards can do things to minimize the chance that interference causes transmission errors, but ultimately, electrical interference will cause at least some transmission errors. So, Fred might receive all three packets (refer to Figure 3-2), but one packet might have some bits that just could not be understood. Imagine the first packet had the first paragraph of the customer letter, the second packet had the second paragraph, and so on. Would Fred want to see the letter with the second paragraph missing? Of course not. So, there needs to be a standard for how Fred can realize the second packet had an error, and cause Wilma to send that packet again.

The solution to the transmission error problem typically happens inside software on the two computers with today's networking standards. To recover the errored packet, both Wilma and Fred's computer software must agree to use the same networking protocola protocol that provides a method to recognize and recover from errors. A protocol is like a standardin fact, in many ways, protocols and standards are the samebut the term protocol typically refers to a process. Figure 3-3 shows a simple protocol that might be used to recover the lost data.