Question E8D06:
What is the advantage of including parity bits in ASCII characters?  

Choices:  
A: Faster transmission rate  
B: Signal-to-noise ratio is improved  
C: A larger character set is available  
D: Some types of errors can be detected    

Correct Answer is  D: Some types of errors can be detected  

To include a parity bit with 7-bit ASCII, one extra bit is added to make the total number of 1s either even (even parity) or odd (odd parity). This provides simple error detection.

Why D is correct (Some types of errors can be detected):
- The parity bit lets the receiver check whether the total number of 1s in the received character is as expected.
- For even parity: choose parity bit $p$ so that
  $$b_1 \oplus b_2 \oplus \cdots \oplus b_7 \oplus p = 0$$
  Equivalently,
  $$p = b_1 \oplus b_2 \oplus \cdots \oplus b_7$$
- If a single bit flips in transmission, the parity check fails, and the receiver detects an error.

Why the other choices are incorrect:
- A: Faster transmission rate — Adding a parity bit adds overhead (an extra bit per character), slightly reducing net throughput.
- B: Signal-to-noise ratio is improved — SNR is a property of the channel, not the coding; parity does not change the physical SNR.
- C: A larger character set is available — Standard ASCII is 7-bit (128 codes). Adding a parity bit does not add data capacity; it’s used for error checking, not for more symbols.

Useful rules of thumb:
- Parity detects any odd number of bit errors in a character (1, 3, 5, ...).
- Parity cannot detect an even number of bit errors (2, 4, ...).
- Parity provides error detection only; it does not correct errors.