⚠️ 1. Why Error Detection is Needed

📖 Definition:

Error detection is the process of identifying whether data has been corrupted during transmission or entry.

🔍 Why errors occur

When data is transmitted, it can be affected by interference (noise) in the communication channel.

🚨 Types of errors:

• Data loss → bits missing
• Data gain → extra bits added
• Data change → bits altered (e.g. 0 → 1)

📌 Causes:

• Electrical interference
• Weak signal
• Network congestion
• Hardware faults

📌 Example:

Original data: 1011001
Received data: 1010001 → ❌ bit changed

📝 Exam Tip:

Always mention:

• interference/noise
• data loss, gain, or change

🔢 2. Methods of Error Detection

✅ A. Parity Check

📖 Definition:

A method where an extra bit (parity bit) is added to ensure the total number of 1s is odd or even.

🔹 Types of Parity

1. Even Parity

• Total number of 1s (including parity bit) must be even

2. Odd Parity

• Total number of 1s must be odd

🔧 How it Works (Step-by-Step)

1. Count number of 1s in data
2. Add parity bit:
   • Even parity → make total even
   • Odd parity → make total odd
3. At receiver:
   • Count 1s again
   • If incorrect → error detected

📌 Example (Even Parity):

Data: 1011001 → 4 ones (already even)
Parity bit = 0

Sent: 10110010

If received as 10110011 → ❌ error detected

📦 Parity Byte

• Used when sending multiple bytes
• Each byte gets its own parity bit

📊 Parity Block Check (2D Parity)

• Data arranged in a table
• Parity added:
  • Horizontally (rows)
  • Vertically (columns)

✅ Advantage:

• More accurate than simple parity
• Can locate exact error position

✔️ Advantages:

• Simple
• Quick

❌ Disadvantages:

• Cannot detect all errors (e.g. two-bit errors)
• Cannot correct errors

📝 Exam Tip:

Mention:

• parity bit
• odd/even
• counting 1s

➕ B. Checksum

📖 Definition:

A value calculated from data and sent with it to detect errors.

🔧 Process:

Sender:

1. Data split into blocks
2. Blocks are added together
3. Result = checksum
4. Checksum sent with data

Receiver:

1. Add received data blocks
2. Add checksum
3. Compare with expected value

📌 Example:

Data blocks:

1010
1100

Sum = 10110 (example)

Checksum sent with data.

If result differs at receiver → ❌ error detected

✔️ Advantages:

• More reliable than parity
• Detects more types of errors

❌ Disadvantages:

• More complex
• Still cannot guarantee detection of all errors

📝 Exam Tip:

Always include:

• adding data
• sending checksum
• comparison at receiver

🔁 C. Echo Check

📖 Definition:

Data is sent back to the sender to verify accuracy.

🔧 Process:

1. Sender sends data
2. Receiver sends exact copy back
3. Sender compares:
   • If identical → ✔ correct
   • If different → ❌ error → resend

📌 Example:

Sent: 1101
Returned: 1101 → ✔ correct

Returned: 1001 → ❌ error

✔️ Advantages:

• Very accurate
• Simple to understand

❌ Disadvantages:

• Doubles transmission time
• Uses more bandwidth

📝 Exam Tip:

Key phrase:
“Data is sent back and compared with original”

🔢 3. Check Digit (Data Entry Errors)

📖 Definition:

A digit added to a number to detect errors during manual input.

🔧 How it Works:

• Calculated using a formula
• Added to end of number
• When entered:
  • Recalculated and checked

📌 Examples:

📚 ISBN (International Standard Book Number)

Used for books

🛒 Barcodes

Used in shops

📌 Example:

Number: 12345
Check digit calculated → 6
Final number: 123456

If entered incorrectly → system detects mismatch

✔️ Advantages:

• Detects human input errors
• Simple and effective

❌ Disadvantages:

• Cannot detect all errors
• Does not correct errors

📝 Exam Tip:

Mention:

• used in ISBN and barcodes
• detects input errors

🔁 4. Automatic Repeat Query (ARQ)

📖 Definition:

A method where the receiver requests retransmission if errors are detected.

🔧 How ARQ Works

Step-by-Step:

1. Sender sends data
2. Receiver checks for errors
3. Receiver sends:
   • ACK (Acknowledgement) → data correct
   • NACK (Negative Acknowledgement) → error found
4. If NACK → sender resends data

⏱️ Timeout

• If no response is received within a certain time:
  → sender resends data automatically

📌 Example:

• Data sent
• No reply → timeout → resend
• Error detected → NACK → resend

✔️ Advantages:

• Ensures data accuracy
• Reliable

❌ Disadvantages:

• Slower (due to retransmission)
• Uses more bandwidth

📝 Exam Tip:

Include:

• ACK / NACK
• retransmission
• timeout

📊 Summary Table

Method	How it Works	Pros	Cons
Parity Check	Counts 1s	Simple	Not very reliable
Checksum	Adds data blocks	More accurate	More complex
Echo Check	Sends data back	Very accurate	Slow
Check Digit	Verifies input	Good for typing errors	Limited
ARQ	Resends data	Reliable	Slower

🧠 Key Definitions (Memorise!)

• Error detection → Identifying corrupted data
• Parity bit → Extra bit for checking
• Checksum → Calculated value for verification
• Echo check → Data sent back for comparison
• Check digit → Digit to verify input
• ARQ → System that resends data if errors occur
• ACK/NACK → Signals for success/failure

🔥 Common Exam Questions

1. Describe parity check (4 marks)

✔ Include:

• parity bit
• odd/even
• counting 1s

2. Explain checksum (4–6 marks)

✔ Include:

• adding blocks
• sending checksum
• comparison

3. Describe ARQ (4 marks)

✔ Include:

• ACK / NACK
• retransmission
• timeout

4. Give examples of check digits

✔ Answer:

• ISBN
• Barcodes

🚀 Final Revision Tips

• Always state how the method works step-by-step
• Include advantages + disadvantages
• Use binary examples where possible
• Learn key terms exactly (examiners look for them!)