📚 Knowledge Library — Topic 2.5B — Data Transmission

Checksum, Echo Check ARQ & Check Digit

Understand how different error detection methods check whether data has arrived correctly, and why some methods are used for transmission while check digit is used for input.

Start LearningWorked Example
1. Invitation

Some errors need more than a simple count.

Parity is useful, but it is not the only way to check data.

Checksum, echo check and ARQ all help systems detect problems and respond when data may be wrong.

💡 Remember: error detection is automatic. The computer system checks the data, not the user.
Figure 1.1
Automatic Checking
Data sent
↓
System checks
↓
OK or resend
2. Big Idea

Most methods compare before and after.

The sender creates or sends something that can be checked later.

The receiver or sender then compares values. If the values do not match, an error is detected.

💡 Same pattern: send data → check data → compare → detect error.
Figure 2.1
Compare Pattern
Before value
↔
After value

Match = OK
No match = error
3. FutureLogic Bridge

Think of a receipt total.

A receipt total helps you check whether the items in a bag are correct.

A checksum works in a similar way. A value is calculated from the data before sending, then recalculated after receiving.

💡 Bridge: checksum is like checking the total on a receipt.
Figure 3.1
Receipt Model
Items
+
Total
↓
Check again
4. Worked Example

Checksum checks a calculated value.

A checksum value is calculated from the data before transmission.

The receiver recalculates the checksum. If the two values are different, an error has been detected.

Checksum process

Sender
calculates
Receiver
recalculates
Compare
values
If the values do not match, request the data again.
Figure 4.1
Checksum
Data
↓
Checksum value
↓
Recalculate + compare
5. Echo Check

Echo check sends a copy back.

In an echo check, the receiver sends the received data back to the sender.

The sender compares the returned copy with the original data. If they do not match, the data is sent again.

🎯 Exam Tip: receiver sends the copy back; sender compares it with the original.
Figure 5.1
Echo Check
Sender
→
Receiver
←
Copy returned
Like reading information back over the phone.
6. ARQ

ARQ handles the resend.

Automatic Repeat reQuest uses acknowledgements and timeouts.

If data arrives correctly, the receiver sends an acknowledgement. If no acknowledgement arrives before the timer runs out, the sender resends the data.

ARQ cycle

send packet → start timer → wait for ACK → resend if needed
Figure 6.1
ARQ
Send
↓
Timer starts
↓
ACK? yes = next
ACK? no = resend
7. Check Digit

Check digit is different.

A check digit is used to check data when it is entered into a computer, such as a barcode or ISBN.

It is not used for checking data transmission between computers.

⚠️ Common Mistake: do not list check digit as a transmission error detection method.
Figure 7.1
Check Digit
Barcode / ISBN
↓
Input check

Not network transmission
8. Exam Tip

Know the different jobs.

Checksum detects whether data may be wrong. ARQ deals with what happens next.

These two methods are often used together because they do different jobs.

MethodMain job
ChecksumDetects mismatch
Echo checkSends copy back to compare
ARQRequests retransmission
Check digitChecks input data
🎯 Exam Tip: checksum detects; ARQ resends.
Figure 8.1
Different Roles
Detect
+
Confirm
+
Resend
9. Summary

Advanced error detection in one screen.

Checksum uses a calculated value. Echo check sends a copy back. ARQ uses acknowledgements and timeout to resend data.

Check digit is used for input checking, not network transmission.

💡 Key idea: error detection methods protect data accuracy during or before processing.
Figure 9.1
Summary
Checksum = value
Echo = copy back
ARQ = resend
Check digit = input