Error Detection (2024)

Table of Contents

The ASCII (American Standard Code for Information Interchange) TABLE Parity Bits and Error Detection Example: Issues ____________________________________________________________________________________________________________________________ CheckSum Error Detection:

The ASCII (American Standard Code for Information Interchange) TABLE

3 most sb\4 least sb	0000	0001	0010	0011	0100	0101	0110	0111	1000	1001	1010	1011	1100	1101	1110	1111
000	NUL	SOH	STX	ETX	EOT	ENQ	ACK	BEL	BS	HT	LF	VT	FF	CR	SO	SI
001	DLE	DC1	DC2	DC3	DC4	NAK	SYN	ETB	CAN	EM	SUB	ESC	FS	GS	RS	US
010	SP	!	"	#	$	%	&	'	(	)	*	+	,	-	.	/
011	0	1	2	3	4	5	6	7	8	9	:	;	<	=	>	?
100	@	A	B	C	D	E	F	G	H	I	J	K	L	M	N	O
101	P	Q	R	S	T	U	V	W	X	Y	Z	[	\	]	^	_
110	'	a	b	c	d	e	f	g	h	i	j	k	l	m	n	o
111	p	q	r	s	t	u	v	w	x	y	z	{	\|	}	~	DEL

Parity Bits and Error Detection

Since ASCII consists of 7 bits we can add a "parity" bit. So that the sum of the bits is always odd (or even). If the convention is agreed to by the sender and receiver, as long as there is only a one bit error, the error can be detected.

Example:

Symbol	ASCII	ASCII+odd parity	Bit Count
B	1000010	11000010	3
C	1000011	01000011	3

Issues

A single bit error can be detected but it cannot be corrected.
Parity Checks do not work if there are are an even number of errors

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CheckSum Error Detection:

A slightly more robust technique is called the Checksum.Assume that the message is a sequence of bytes. Before transmission, a Checksum byte or fixed number of bytes is computed and appended to the message. The receiver performs the same calculation on the received message minus the Checksum byte or bytes. This result is checked against thereceived Checksum and if they agree then it is assumed that the message is error free.Example: The Checksum will be one byte. The Checksum calculation will be, Sum the bytes in the message mod 256.For example, writing the bytes in decimal form:
The Message 5 9 17 2

With Checksum 5 9 17 2 33

Message Received 7 9 18 2 33

5 + 9 + 17 +2 = 33
but
7 + 9 + 18 +2 = 36 ≠ 33Some Issues:

The Message	5 9 17 2
With Checksum	5 9 17 2 33
Message Received	7 9 18 2 33