Error detection versus error correction

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Quick Questions

Shade in the A or B circle for each question.

1. A receiver finds that a parity bit does not match the data. What has the parity check done?

A Repaired the data so it is correct

B Shown that an error has happened

2. After an error is detected in a received packet, what usually happens next?

A The receiver asks for the data to be sent again

B The check method fixes the bits automatically

3. Which statement about error detection is correct?

A It tells you something went wrong, not what the right data was

B It always gives back the original correct data

4. Where does a checksum travel when data is sent?

A On a separate trip after all the data has been checked

B With the data, usually in the packet trailer

5. A packet passes its parity check. Does this prove the data is error free?

A No, some errors can still be missed

B Yes, a passed check guarantees no errors

Match the Pairs

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

Left column		Right column
1parity bit = 1 -> received byte has an odd number of 1s		Apacket = header + data + trailer(checksum)
2IF error found THEN request resend		BIF parity OK THEN data may still contain errors
3The error check travels with the data, so the receiver can compare it with the data that arrived		CA matching checksum does not guarantee there are no errors, because some errors can still be missed.
4A method can miss some errors, so a passed check does not prove the data is perfect		DDetect then resend: the receiver asks the sender to send the data again
5parity error found -> the receiver fixes the byte automatically		EDetection only shows that an error happened. It does not repair the byte, so the data must be resent.
6checksum matches -> the data is 100% error free		FAn error is detected, but the receiver does not know which bit is wrong, so the data is not repaired

Definitions

error detection

Fill in the Blanks

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

IF checkReceived __ checkCalculated THEN OUTPUT "Error detected" ENDIF

IF errorFound = ____ THEN OUTPUT "Please send again" ENDIF

checkValue ← CalculateCheck(data) packet ← data + __________ SEND packet

bitsChanged ← 2 missed ← (bitsChanged ___ 2) = 0 OUTPUT missed

REPEAT RECEIVE packet _____ errorFound = FALSE

Spot the Error

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

Question 1

A RECEIVE data, check

B IF check fails THEN

C ACCEPT data anyway

D ELSE

E ACCEPT data

Question 2

A ones ← COUNT(1s), even parity

B IF ones is odd THEN

C PRINT 'No error detected'

Question 3

A sent ← [3, 5], checksum ← 8

B got ← [5, 3], sum ← 8

C PRINT 'Error detected'

Question 4

A REPEAT

B SEND packet, check

C RECEIVER repairs the data

D RECEIVER sends ACK or NAK

E UNTIL reply = ACK

Question 5

A RECEIVE packet

B SPLIT into data, trailer

C calc ← CHECKSUM(data)

D IF calc = trailer THEN NAK

E ELSE ACK

Answer Key

Error detection versus error correction

Section A: Quick Questions

B — Shown that an error has happened

A — The receiver asks for the data to be sent again

A — It tells you something went wrong, not what the right data was

B — With the data, usually in the packet trailer

A — No, some errors can still be missed

Section B: Match the Pairs

parity bit = 1 -> received byte has an odd number of 1s → An error is detected, but the receiver does not know which bit is wrong, so the data is not repaired

IF error found THEN request resend → Detect then resend: the receiver asks the sender to send the data again

The error check travels with the data, so the receiver can compare it with the data that arrived → packet = header + data + trailer(checksum)

A method can miss some errors, so a passed check does not prove the data is perfect → IF parity OK THEN data may still contain errors

parity error found -> the receiver fixes the byte automatically → Detection only shows that an error happened. It does not repair the byte, so the data must be resent.

checksum matches -> the data is 100% error free → A matching checksum does not guarantee there are no errors, because some errors can still be missed.

Section C: Definitions

error detection — Checking received data to show that an error has happened. It does not repair the data, so the receiver usually asks for it to be sent again. Some errors can still be missed.

Section D: Fill in the Blanks

IF checkReceived <> checkCalculated THEN
  OUTPUT "Error detected"
ENDIF

IF errorFound = TRUE THEN
  OUTPUT "Please send again"
ENDIF

checkValue ← CalculateCheck(data)
packet ← data + checkValue
SEND packet

bitsChanged ← 2
missed ← (bitsChanged MOD 2) = 0
OUTPUT missed

REPEAT
  RECEIVE packet
UNTIL errorFound = FALSE

Section E: Spot the Error

C — Error detection only shows that an error has happened. The receiver cannot repair the data, so when the check fails it should request a resend, not accept the data.

B — With even parity the number of 1s should be even. An even count means no error was detected. An odd count would signal an error, so the test should be 'is even'.

C — Swapping the two values still gives a sum of 8, so the checksum matches and this error is missed. The program should print that the checksum matches. A method can miss some errors.

C — The receiver only checks the data to detect errors. It does not repair it. A NAK makes the sender resend the packet, and the loop repeats until an ACK arrives.

D — A NAK should be sent when the calculated checksum is different from the trailer value (<>), because that shows an error was detected. A match means ACK.