**Check Digit:**

A check digit is a mathematically calculated value that is added to data to ensure that the original data have not been altered. This helps in avoiding transposition and transcription errors.

Example:

Check digit is used by bank to ensure the correctness of bank account numbers assigned to customers, thereby helping to avoid transposition and transcription errors.

For instance, a bank account number is “630000241457”. The last digit is the check digit “7”, and if the other numbers are correct then the check digit calculation must produce 7.

Add the odd number digits: 6+0+0+2+1+5 = 14.

Multiply the result by 3: 14 × 3 = 42.

Add the even number digits: 3+0+0+4+4 = 11.

Add the two results together: 42 + 11 = 53.

To calculate the check digit, take the remainder of (53 / 10), and if not 0, subtract from 10. Therefore, the check digit value is 7.

**Parity Bits:**

The parity method of error detection is quite simple and simply requires adding an extra bit on the data. This extra bit is called a parity bit. This bit simply says whether the number of 1 bits is odd or even. Generally the parity bit is 1 if the number of 1 bits is odd and 0 if the sum of the 1 bits is even.

This parity is verified by receiving computer to ensure data completeness and data integrity during transmission.

Parity bits are used to check for completeness of data transmissions. A parity check is a hardware control that detects data errors when data are read from one computer to another, from memory or during transmission.

**Checksum:**

Checksums are exactly same as parity but able to identify complex errors also by increasing the complexity of the arithmetic.

**Cyclic Redundancy Checksums (CRC)/Redundancy Checksums:**

More advanced version of checksums by increasing the complexity of the arithmetic*.*

**Forward Error Control:**

Works on same principle as CRC. However FEC also corrects the error. FEC provides the receiver with the ability to correct errors.

**Atomicity:**

Atomicity is a feature of databases systems where a transaction must be all-or-nothing. That is, the transaction must either fully happen, or not happens at all. The principle of atomicity requires that a transaction be completed in its entirety or not at all. If an error or interruption occurs, all changes made upto that points are backed out.

**Point to remember for CISA exam:**

(1)When objective is to identify transcription and transposition error, answer should be check digit.

(2)When objective is to ensure accuracy, answer should be check digit.

(3)When objective is to identify transmission error, answer should be parity bits or checksum (higher version of parity bit) or CRC (higher version of checksum).

(4)When objective is to ensure completeness, answer should be parity bits or checksum (higher version of parity bit) or CRC (higher version of checksum).

(5)When objective is to ensure integrity, answer should be parity bits or checksum (higher version of parity bit) or CRC (higher version of checksum).

(6)For point no. (3), (4) and (5), first preferences to be given as follow:

(i)First preference to CRC

(ii)If CRC is not there as option then preference to be given to Checksum

(iii)If CRC and Checksum both are not there in option then preference to be given to Parity Bits.

(7)When objective is to correct (detect & correct) transmission error, answer should be Forward Error Control (FEC).

(8)When objective is to ensure that a transaction must either fully happen, or not happens at all, answer should be atomicity.