Cambridge O Level Computer Science · Syllabus 2210 · Data Transmission
Parity Check
What is Parity Check?
A parity check is an error-detection method in which an extra parity bit is added to a group of bits so that the total number of 1 bits is even (even parity) or odd (odd parity); the receiver recounts the 1 bits and a mismatch shows that an error has occurred.
This definition is part of the Data Transmission chapter in Cambridge O Level Computer Science.
Parity Check in context
A parity check works by adding one extra bit — the parity bit — to each group of bits before transmission. Under even parity, the parity bit is set so that the total number of 1 bits in the group is even. Under odd parity, it is set so that the total number of 1 bits is odd. Sender and receiver must have agreed in advance which is being used. After transmission the receiver counts the 1 bits in each group: if the count no longer matches the agreed parity, an error has been detected.
Automatic repeat query (ARQ) is a method of establishing that data has been received without error. It uses an error-detection method — such as a checksum or a parity check — on the received data, and then acts on the result. If the data passes the check, the receiver returns a positive acknowledgement. If it fails, the receiver returns a negative acknowledgement, and the sender retransmits. If no acknowledgement arrives at all before the sender's timeout expires, the sender assumes the data did not get through and retransmits anyway. The process repeats until the data is acknowledged or the system stops after the number of attempts it permits.
Questions students ask about Parity Check
What is the difference between a parity check and a checksum?
A parity check adds one extra parity bit to a group of bits so the total number of 1 bits is even or odd, as agreed in advance; the receiver recounts the 1 bits, and a mismatch shows an error. A checksum is a value calculated from a whole block of data using an agreed calculation; the sender transmits it with the data, the receiver repeats the calculation, and a difference between the two values shows the data changed during transmission. Neither method proves the data is completely correct if the values still match.
How does automatic repeat query (ARQ) get corrupted data resent?
ARQ uses an error-detection method, such as a parity check or checksum, on the data the receiver gets. If the data passes the check, the receiver sends back a positive acknowledgement. If it fails, the receiver sends a negative acknowledgement and the sender retransmits the data. If no acknowledgement arrives at all before the sender's timeout expires, the sender assumes the data did not arrive and retransmits anyway. This repeats until the data is acknowledged or the permitted number of attempts is used up.