Comparing the error detection methods

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Date:

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Quick Questions

Shade in the A or B circle for each question.

1. A librarian types an ISBN into a computer. Which method checks it for typing errors?

A Check digit

B Checksum

2. Which method sends a copy of the data back to the sender so the two can be compared?

A Parity bit

B Echo check

3. What extra data does a single parity check add to each byte?

A A whole extra byte

B One extra bit

4. Which method can find the exact bit that changed in a block of data?

A Parity block check

B Single parity bit check

5. Which method asks the sender to send the data again if there is a problem?

A ARQ

B Checksum

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1Parity bit		AOne extra bit added to each byte so the number of 1s is even or odd. It spots a single changed bit but cannot show which bit is wrong.
2Echo check		BThat describes a parity bit. A checksum is a value calculated from a whole block of data and sent with it.
3A method used at data entry rather than transmission: an extra digit is calculated from the other digits, so a mistyped number can be detected.		CThe receiver sends the data back to the sender, who compares it with the original. No extra check data is added, but a mismatch does not show whether the error happened on the way there or the way back.
4A method that does not add check data but asks for the data to be sent again when an error is found or no acknowledgement arrives.		DA check digit is used for data entry, such as typing in an ISBN. Transmission uses parity, parity blocks, checksums, echo checks or ARQ.
5A check digit is the best method for finding errors in data sent across a network.		ECheck digit
6A checksum is a single extra bit added to each byte so that the number of 1s is even.		FARQ

Definitions

error detection method

Fill in the Blanks

Complete each line of code by filling in the missing parts.

OUTPUT "Parity bit adds _ extra bit"

OUTPUT "Check digit is used at data _____"

OUTPUT "Echo check sends the data ____"

OUTPUT "ARQ asks the sender to ______ data"

IF where = "transmission" THEN OUTPUT "parity, checksum or echo check" _____

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A parity: used in data entry

B extra data: 1 bit per byte

C detects: a change in one bit

D misses: two bits changed

Question 2

A method: checksum

B extra data: 1 bit per byte

C receiver recalculates it

D used in: transmission

Question 3

A method: check digit

B extra data: 1 digit on number

C detects: typing mistakes

D used in: data transmission

Question 4

A method: echo check

B receiver sends data back

C receiver compares both copies

Question 5

A method: ARQ

B receiver sends ACK or NAK

C on NAK the data is corrected

D no reply: timeout, resend

Answer Key

Comparing the error detection methods

Section A: Quick Questions

A — Check digit

B — Echo check

B — One extra bit

A — Parity block check

A — ARQ

Section B: Match the Pairs

Parity bit → One extra bit added to each byte so the number of 1s is even or odd. It spots a single changed bit but cannot show which bit is wrong.

Echo check → The receiver sends the data back to the sender, who compares it with the original. No extra check data is added, but a mismatch does not show whether the error happened on the way there or the way back.

A method used at data entry rather than transmission: an extra digit is calculated from the other digits, so a mistyped number can be detected. → Check digit

A method that does not add check data but asks for the data to be sent again when an error is found or no acknowledgement arrives. → ARQ

A check digit is the best method for finding errors in data sent across a network. → A check digit is used for data entry, such as typing in an ISBN. Transmission uses parity, parity blocks, checksums, echo checks or ARQ.

A checksum is a single extra bit added to each byte so that the number of 1s is even. → That describes a parity bit. A checksum is a value calculated from a whole block of data and sent with it.

Section C: Definitions

error 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.

Section D: Fill in the Blanks

OUTPUT "Parity bit adds 1 extra bit"

OUTPUT "Check digit is used at data entry"

OUTPUT "Echo check sends the data back"

OUTPUT "ARQ asks the sender to resend data"

IF where = "transmission" THEN
  OUTPUT "parity, checksum or echo check"
ENDIF

Section E: Spot the Error

A — A parity bit checks data during transmission, not at data entry. Line 1 should read 'parity: used in transmission'.

B — A checksum sends a calculated value with the data block. It does not send one bit per byte (that is a parity bit). Line 2 should be 'extra data: calculated value'.

D — A check digit is used when data is entered, for example typing an ISBN or reading a bar code. It is not a transmission check. Line 4 should be 'used in: data entry'.

C — In an echo check the sender compares the original with the copy echoed back, because only the sender has the original. Line 3 should say the sender compares both copies.

C — ARQ does not correct data. A negative acknowledgement (NAK) makes the sender send the data again. Line 3 should be 'on NAK the data is resent'.