CPSC 110-08: Computing on Mobile Phones Spring 2012 Homework: Parity Bit Error Detection Due: Friday 2/3 (before class)

CS Principles

• 5: The student can describe the combination of abstractions used to represent data.
• 6: The student can explain how binary sequences are used to represent digital data.

Introduction

This topic was covered in class on Monday.

Parity Bit Error Detection

Suppose you are sending a stream of data to a server. By adding a parity bit, you enable to the server to detect some basic transmission errors. For example, if the server expects that every byte will contain an even number of 1s and it detects a byte such as 0001 0101 with an odd number of 1s, it can tell that an error occured. Perhaps the user meant to send 0000 0101 but one of the bits was flipped from 0 to 1 during transmission.

A parity bit is a bit that is added as the leftmost bit of a bit string to ensure that the number of bits that are 1 in the bit string are even or odd.

To see how this works, suppose our data are stored in strings containing 7 bits.

In an even parity scheme the eighth bit, the parity bit, is set to 1 if the number of 1s in the 7 data bits is odd, thereby making the number of 1s in the 8-bit byte an even number. It is set to 0 if the number of 1s in the data is even.

In an odd parity scheme the eighth bit, the parity bit, is set to 1 if the number of 1s in the 7 data bits is even, thereby making the number of 1s in the 8-bit byte an odd number. It is set to 0 if the number of 1s in the data is odd.

The following table summarize this approach.

Data Bits (7)	Add a parity bit to get 8 bits
	Even Parity Total number 1s is even	Odd Parity Total number of 1s is odd
000 0000 (0 1s)	0000 0000	1000 0000
011 0010 (3 1s)	1011 0010	0011 0010
011 0011 (4 1s)	0011 0011	1011 0011
011 0111 (5 1s)	1011 0111	0011 0111

Quiz Yourself

• Parity Error Detection I
• Parity Error Detection II

Getting Help

In Class on Friday