alright, in my effort to be nice.. ill write whats in my textbook about parity..and whats in the excel book about parity.. then u get 2 general ideas which SHOULD help you guys to understand.. if it doesnt, then shoot me.
TEXT BOOK:
Parity checking is a method of checking for errors in data transmission using an additional bit called a parity bit.
This bit is used only for the purpose of identifying whether the bits being moved have arrived successfully. When data is represented using an 8-bit ASCII, then a ninth bit is used that holds a 0 or a 1. When an
odd parity is chosen, the
number of 1s in the 9 bits must be odd. This means that
. The sender and receiver can also decide to send an even parity, in which case the number of 1s in the 9 bits must be even. If an error has occured in a single bit, then the parity will be different and an error in transmission has occured.
EXCEL BOOK:
Seven bits represent the normal ASCII character set when an 8-bit byte is used in a computer. The 8th bit can be used to check whether that byte has been received correctly by using the ideas of parity.
It works as follows:
- the originating computer counts the number of 1s and determines whether there is an odd or even number.
- the 8th bit in the byte is then used to create a byte with an even number of 1s if
even parity is set, or an off number of 1s if
odd parity is set in the protocol governing the particular communication session.
The transmission of the ASCII character 'R' demonstrated this process.
0 1 0 1 0 0 1
0 (Parity = bold)
Because the
number of 1s is three, which is odd, we add 0 as the parity bit to maintain
odd parity checking.
Problems with parity checking
- it can detect 1 error but is stumped if there are two.
- if two bits flip value, then the parity bit will not alert the receiver.
- Any byte with an even numbers of errors will slip through undetected.
make sense?
