IGCSE Computer Science 0478 — 2.2 Methods of Error Detection Hangman

The 39 key terms

1data transmission

The sending of data, such as text, images, audio or video, from a sender device to a receiver device.

2ASCII

A character set that gives each character a 7-bit code (128 characters), usually stored in 8 bits. Extended ASCII uses 8 bits for 256 characters.

3packet trailer

The last part of a packet. It marks where the packet ends and can hold error checking information, so the receiving device can check the data arrived correctly.

4error detection

Checking received data, because it can be damaged in transit and the receiver cannot see the original. Spotting the error is the first step to fixing it.

5interference

Unwanted disturbance to a signal, such as electrical noise or crosstalk, that damages data as it travels and can cause bits to be lost, gained or changed.

6data loss

When some bits or bytes that were sent never arrive, so the message that is received is shorter than the one that was sent.

7data gain

Data gain is when extra bits or bytes that were not sent are added to the data, so the message arrives longer than it was sent. It is a type of transmission error.

8data change

Bits arrive altered, such as a 0 flipping to a 1, so the same amount of data arrives but with wrong values. Comparing sent and received bits shows it.

9compare sent and received

Compare the data sent with the data received. Fewer bits or characters means loss, extra ones means gain, and the same length with different values means change.

11parity bit

One extra bit added to a group of bits (such as 7 data bits) so the total number of 1s follows a rule, even or odd, agreed by sender and receiver. It is used to detect errors.

12even parity

A parity bit is added so the total number of 1s in the byte, including the parity bit, is even. Odd count of 1s in the data gives parity bit 1; even count (even 0) gives 0.

13odd parity

A rule where the parity bit is set so the total number of 1s in the byte, including the parity bit, is odd. Even number of 1s in the data gives bit 1; odd number gives bit 0.

14count the 1s

Count the 1s in the 7 data bits, then set the leftmost parity bit to 0 or 1 so the total number of 1s in the whole 8-bit byte matches the agreed rule (even or odd).

15parity check

The receiver counts the 1s in the whole received byte, parity bit included. If the count breaks the agreed even or odd rule, an error has occurred and is detected.

16even number of bits change

A single parity check misses an error when an even number of bits change, because the count of 1s keeps the same parity. It also cannot show which bit is wrong.

17parity byte

An extra byte sent after a block of data bytes. Each of its bits is the parity bit for one column (bit position) taken down across all the data bytes in the block.

18parity block check

Checks a block of bytes in two directions: a parity bit on every byte (row) and a parity byte for the columns, so the wrong row and column pinpoint the error.

19parity block

Each byte gets a row parity bit, then a final parity byte holds the parity bit for each column. The corner bit is the parity bit of the parity bits, so it checks in both directions.

20failed row and column

In a parity block, the row and the column that both fail their parity check cross at the bit that changed. Flip that bit to correct a single-bit error.

21four-corner error

Four flipped bits at the corners of a rectangle keep every row and column parity correct, so a parity block check misses the error. Only one error can be located reliably.

23binary addition

Add two 8-bit binary numbers column by column from the right. Carry any 1 to the next column on the left. 1 + 1 = 10, so write 0 and carry 1. Check by adding the denary values.

24overflow

An error when an 8-bit addition gives a result over 255. It needs a 9th bit, so the carry out of the leftmost column is lost and the stored result is wrong.

25checksum

A value calculated from a block of data by a set rule and sent with it. The receiver repeats the calculation and compares the results. If they differ, an error occurred.

26checksum = sum(bytes) % 256

Add all the byte values in the block, then keep the remainder after dividing by 256 so the result fits in one byte. Example: 200+100+50=350, 350 mod 256 = 94.

27recalculate and compare

The receiver recalculates the checksum from the received data and compares it with the one sent. Match means no error detected; mismatch means error, so resend is requested.

29echo check

The receiver sends a copy of the received data back to the sender, who compares it with the original. A match means no error was detected; a mismatch means the data is sent again.

30echo check limit

An echo check cannot tell if the error happened on the way there or back, and it can miss errors if both trips are damaged the same way.

31validation vs verification

Validation checks entered data is reasonable and complete. Verification checks it matches the source data, for example by proofreading.

32check digit

An extra digit added to the end of a number, calculated from its other digits. It is recalculated on entry and compared, to detect data entry mistakes.

34weighted check digit

Multiply each digit by its weight, add them up, divide by 11 and use the remainder (often 11 minus it) as the check digit. A 10 is written X in ISBN-10.

35ISBN-13 check digit

The last digit of an ISBN-13, found by weighting the first 12 digits 1 and 3 alternately, adding them, and choosing the digit that makes the total a multiple of 10.

38check digit limitation

A wrong number can still pass the check digit test, and a valid number may belong to the wrong item. The check digit tests the digits only, not what they mean.

39ARQ

Automatic repeat query: the receiver checks data for errors and replies. If the data is bad or no reply arrives, the sender automatically resends it until it is received correctly.

40positive acknowledgement

A message sent by the receiver to tell the sender that data arrived without error. Often written ACK. It is sent only after the error check passes, so the sender can send the next data.

41negative acknowledgement

A message (NAK) the receiver sends to the sender when the error check fails, telling it the data arrived with an error so the sender must resend the same data.

42timeout

The set time a sender waits for an acknowledgement after sending data. If none arrives in time, it assumes the data was lost and resends it.

44ARQ limits

ARQ resends data when a separate check finds an error, so it misses errors that check misses, and a noisy link causes many resends that slow the transfer.

45error detection method

Parity, parity block, checksum and echo check find errors in transmission; a check digit finds them at data entry; ARQ asks for a resend. Each sends different extra data.

46Choosing a method

Decide if the error happens in transmission or data entry, pick the check that fits, then justify it by what it can and cannot detect.