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

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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 the received 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 ≠ 33

Some Issues: