Unit 2.2 - methods of error detection Flashcards
Explain what parity checks are and how they work
A parity check is one of the methods used to check for errors during data transmission. Since data is sent in binary, aka 0s and 1s, parity checks involve using a parity bit at the start of the binary number and sending it with the data.
There are 2 types of parity checks : even or odd. Even parity checks to see that the total number of 1s in the byte is even, and adds either 0 or 1 accordingly. Vice versa for add parity check.
What are some limitations of parity checks?
They cannot tell exactly which bit has been corrupted/changed during transmission. For example, if during an even a transposition error occurs (when a bit accidentally switches from 1 to 0 or vice versa) and the number of 1s is still even even though the position of bits has been messed up, error will not be found.
What is a parity block?
Parity blocks uses the same rules for error checking as parity checks, but instead of 1 byte, it takes a set of binary numbers and arranges them in a table, so that a parity but is used in each row and each column.Gives the ability to find exactly which bit was transmitted wrong.
What is cheksum and how is it used for error checking?
An error detection method in which data is sent in blocks with a calculated value known as the checksum. The receiving computer will perform the same calculations to regenerate the data and compare the two checksums.
If the sum of the bytes (1 byte sum is maximum 255) is less than or equal to 255, that is the checksum value. If the sum is begger than 255, divide it by 256 to get the quotient, round down, and then multiply by 256 to get the product. Minus the product from the sum and that is the checksum. In this method, the receiving computer that does the comparison.
What is an echo check?
A method of error detection where the receiving computer sends back an exact copy of the original data to the sending computer. The sending computer then compares the data it originally sent to the data it received back. If there is an error, the sending computer resents data and the process starts over. In this method, the sending computer does the comparison.
What is check digit?
An extra digit added to a number calculated from the existing digits in the number. Used to detect incorrect digits, transposition errors, extra digits and committed digits in a number. Used in barcodes and ISBNs. The check digit is added to the right of the numbers, and calculated by assigning 3s and 1s to every other number, starting from the right. The numbers are multiplied together (barcode digit and multiplier) and then added all together. The number is then rounded down to the nearest 10, and this is subtracted from the original number. The digit achieved from these series of operations is the check digit. Not used in detecting errors during data transmission.
Define algorithm
A sequence of steps taken when solving a problem which can show us how to solve a problem, or whether a problem has a solution or not.
What is ARQ?
Automatic repeat request/query is a set of established rules between sending and receiving computers to control errors.Regardless of error checking method, if computer receives data with error, it can request for the data to be resent. When data is received by the computer, it sends a short message called an acknowledgement, which indicates whether data transmission was successful or not.
Positive ack = transmission was completed without error
Negative ack - indicates errors occurred during transmission, receiving computer will request to resend data. This process is repeated until computer receives compltete data packet or time out is reached.
