2.2 Methods of error detection

## Methods of Error Detection

### **Parity Checks**
- It uses the number of 1-bits in a byte
- Type Types -
  - Even - Even number of 1-bits
  - Odd - Odd numbers of 1-bits
- Example (Even Parity) -

| 0    | **1** | **0** | **1** | **1** | **0** |
| ---- | ----- | ----- | ----- | ----- | ----- |

- The LSB (Left-Most Bit) is the parity bit. As the number of 1s is even, the parity bit would be set to even.
  **Limitations with Parity Checks**
- Two bits may change during transmission; therefore error is not found
- Even though the parity checks would reveal the errors, the bit(s) changed wouldn’t be identified
### **Parity Blocks**

- To overcome the limitations of parity bits, Parity blocks would be used.

![image-20231217103611212](/Users/theo/Library/Application Support/typora-user-images/image-20231217103611212.png)

- Any changes in bits would be identified through the rows and columns

### **Checksum**

- Calculates a value from the bits/data (to be transferred) // by *example/description*
- Value is appended to the bits/data
* Value is transferred with the bits/data
* Receiver recalculates the checksum
* If both values are different error is detected // if both values are the same
* the transmission is successful

### **Echo Check**

- Once the data has been sent, The receiver will send the data back to the sender for verification.
- The sender would compare the received and original data for errors.
- The only downside is that we wouldn’t know if the error occurred when sending the data or sending the data back for verification.

### **Check Digits**

- Check digits are calculated from all the other digits in the data (ex-codes). The check digit would be the last digit of the code.
- These are used to identify mistyping errors such as -
  - 6372 typed as 6379
  - 8432 typed as 842

### **Automatic Repeat Requests (ARQs)**

- Uses acknowledgements and timeouts to make sure the user received the data
- The receiver would check the data for any errors; if none are found, a positive acknowledgement is sent to the sender. However, if errors are found, a negative acknowledgement will be sent, and the data will be sent again.
- The sender uses timeouts to wait for a pre-determined amount for the acknowledgement.
- If no acknowledgements are received after the timeout, the data will be sent again to the receiver.