Error detection (parity, CRC, checksum) in Computer Networks - Step-by-Step Execution

Concept Flow - Error detection (parity, CRC, checksum)

Data to send

↓

Add error detection code

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Send data + code

↓

Receive data + code

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Check error detection code

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Accept

Data is sent with extra bits for error checking. Receiver uses these bits to detect errors and decide if data is correct or needs resending.

Execution Sample

Computer Networks

Data bits: 1011001
Parity bit: 1 (even parity)
Send: 10110011
Receiver checks parity
If parity matches, accept data

This example shows adding a parity bit to data and checking it at the receiver to detect errors.

Analysis Table

Step	Data bits	Parity bit	Parity Check Result	Action
1	1011001	Calculate parity bit (even parity)	Parity bit = 1	Append parity bit to data
2	10110011	Received parity bit = 1	Check parity of received bits	Parity matches, no error detected
3	10110001 (error introduced)	Received parity bit = 1	Check parity of received bits	Parity mismatch, error detected
4	-	-	-	Request resend or error correction

💡 Error detected when parity check fails, triggering resend or correction

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Data bits	1011001	1011001	10110011	10110001	10110001 (with error)
Parity bit	N/A	1	1	1	1
Parity check result	N/A	N/A	Match	Mismatch	Mismatch

Key Insights - 3 Insights

Why does the parity bit help detect errors?

The parity bit makes the total number of 1s even (or odd). If a single bit flips during transmission, the parity will not match, signaling an error (see execution_table step 3).

Can parity detect all errors?

No, parity can detect only an odd number of bit errors. If two bits flip, parity may still match, missing the error.

What happens if an error is detected?

The receiver requests the sender to resend the data or uses error correction methods (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the parity check result?

A Parity matches, no error detected

B Parity mismatch, error detected

C Parity bit not calculated

D Data bits are incorrect

💡 Hint

Check the 'Parity Check Result' column at step 2 in execution_table

At which step does the error get detected according to the execution_table?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for 'Parity mismatch' in the 'Parity Check Result' column

If the parity bit was not added, how would the error detection change?

A Errors would still be detected reliably

B Errors might not be detected at all

C More errors would be introduced

D Data size would increase

💡 Hint

Refer to the role of parity bit in variable_tracker and execution_table

Concept Snapshot

Error detection adds extra bits (parity, CRC, checksum) to data.
Parity bit makes total 1s even or odd to detect single-bit errors.
CRC uses polynomial division for stronger error detection.
Checksum sums data parts to detect errors.
Receiver checks these codes to accept or request resend.

Full Transcript

Error detection methods add extra bits to data before sending. Parity bit ensures the total number of 1s is even or odd, helping detect single-bit errors. CRC uses a mathematical division method to detect more complex errors. Checksum adds parts of data to detect errors. When data arrives, the receiver checks these codes. If they match, data is accepted. If not, the receiver asks for resend or corrects errors. This process helps keep data accurate during transmission.