ITPEC FE Morning April 2021 Question 6

Topic: Basic Theory

Which of the following is an appropriate description of parity check (vertical parity) that handles transmission errors on a communication line?

a)A parity check can correct 1-bit and 2-bit errors.b)A parity check can detect a 1-bit error.c)An odd parity check can detect a 1-bit error, but an even parity check cannot even detect a 1-bit error.d)An odd parity check can detect any odd number of bit errors, and an even parity check can detect any even number of bit errors.

Explanation

It detects a 1-bit error — flipping one bit changes the count of 1s from even to odd or back, so the parity no longer matches.

A parity bit is added to each character so that the number of 1s is even (even parity) or odd (odd parity). The receiver counts the 1s again: a mismatch means an error. It works the same way for both conventions.

•One wrong bit, or any odd number of wrong bits, changes the parity and is detected
•Two wrong bits, or any even number, cancel out and pass undetected
•The check shows that a character is wrong but not which bit, so it cannot correct anything

Answer (b)

Why not others:

•(a) — parity only detects; correcting an error needs to locate the bit, as a Hamming code or horizontal plus vertical parity does
•(c) — even parity detects a 1-bit error exactly as odd parity does; the two differ only in which count is expected
•(d) — neither convention detects an even number of bit errors; both detect only an odd number

Key rule: A single parity bit detects any odd number of bit errors, misses any even number, and corrects nothing.

Basic Theory · AI-written, may contain errors

Previous Next

Practice this exam

This question comes from an official ITPEC past paper. ITPEC Practice is an independent study tool and is not affiliated with ITPEC. See the official FE past-paper collection or Report an issue.