Describe parity check (odd and even), checksum and echo check
Methods of Error Detection
Parity Check
📡 Parity check is like a quick “handshake” that tells you if the number of 1‑bits in a data word is even or odd. The sender adds one extra bit – the parity bit – so that the total number of 1‑bits becomes either even (even parity) or odd (odd parity). The receiver performs the same count; if the result doesn’t match the expected parity, an error is detected.
Odd parity – the total number of 1‑bits is odd. Even parity – the total number of 1‑bits is even.
| Data Bits | Parity Bit | Result |
|---|---|---|
| 1101010 | 1 | Odd parity ✔️ |
| 0110110 | 0 | Even parity ✔️ |
Tip: Parity can only detect an odd number of bit errors. If two bits flip, the parity may still be correct.
Checksum
🔢 A checksum is a simple sum of all the data bytes, usually taken modulo a fixed value (e.g., 256 for 8‑bit bytes). The sender appends this checksum to the message; the receiver recomputes the sum and compares it with the received checksum.
$$\text{checksum} = \left( \sum_{i=1}^{n} \text{byte}_i \right) \bmod 256$$
| Byte 1 | Byte 2 | Byte 3 | Sum | Checksum |
|---|---|---|---|---|
| 0x12 | 0x34 | 0x56 | 0x9A | 0x9A ✔️ |
Exam tip: Remember that the checksum is calculated before the checksum byte is added to the message.
Echo Check
📞 In an echo check, the receiver simply sends the received data back to the sender. The sender compares the echoed data with the original. If they match, the transmission is considered error‑free.
- Sender transmits data packet.
- Receiver receives the packet.
- Receiver sends the same packet back (echo).
- Sender compares echoed packet with original.
- If they match, no error; otherwise, request retransmission.
Analogy: It’s like shouting a secret word to a friend and asking them to repeat it back. If they say the same word, you know you spoke correctly.
Exam Tips for IGCSE 0478
- Know the difference between odd and even parity.
- Be able to calculate a checksum modulo 256 and explain its purpose.
- Understand the simple flow of an echo check and why it’s useful.
- Remember that parity can only detect an odd number of bit errors.
- Practice drawing small tables to show parity bits or checksum calculations.
Good luck, and keep practicing with real data examples! 🚀
Revision
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