Dwight Watt - Newspaper Article #166 9/12/2012

Question: What is a parity bit?


You may see messages appear on your network/Internet connection saying there were parity errors and usually you realize your network connection is slower, or that the web pages are not appearing as well. Parity errors are when information that is traveling across the network has been damaged in transit.

Usually it is because you have a bad connection. Disconnecting and reconnecting the network connection (right click on the network icon(picture) in the bottom right of your screen) will usually solve these. This could be that the connection to your ISP(Internet Service provider like AT&T, Charter, Comcast, etc.) has hit problems along the way or settings in your PC got confused. Less often it is an indication that equipment is failing which could be the network card in your PC, your Cable/DSL router or the equipment at your ISP. If it is equipment failing, then resetting the connection will not help.

Parity is a how networks determine if information being sent is received in-tact. When information is sent across the network it is sent as a bunch of bits or ones and zeros. These are grouped in bytes which basically represent one letter or number as we are used to. Most of the time it takes eight bits to represent one character we are used to. (Some newer machines are using 16 bits to represent a character).

When the computer sends a byte (8 bits or binary digits) it looks at how many ones are in the byte. Then it adds a parity bit to the byte. Assuming we are using even parity, if the number of ones in the byte (character) is an odd number it puts a one in the parity bit or if they are an even number it puts a zero. This now means all the bytes (characters) being sent have an even number of ones in them.

When the computer receiving the data looks, it first checks if the number of ones in the byte (character) is an even number. If it is, then we assume the byte(character ) was received as it was sent. If it is an odd number then we know that the byte was damaged in transit and that is when you will see messages about parity errors. The receiving computer requests the information be resent and you notice a slowdown as it has to be sent multiple times for one good receiving.

All of this about adding and checking a parity bit occurs behind the scene inside your computer so you do not know it is happening except when a large number of parity errors occur and then you see messages about network problems.