Example: ASCII code

• The ASCII code is really a combined source and channel code.
• It provides for encoding of 128 characters.
• $\log_2 128 = 7$ bits are required for that purpose.
• The 8-th bit in an ASCII code word is used for error detection.
• An error detection scheme called parity checking is used.
• The term parity is used to indicate if the number of 1's in an ASCII code word is even or odd.
• In a communication system, it is agreed between receiver and transmitter if even parity or odd parity is to be used.
• If even parity is used the transmitter sets the 8-th bit of an ASCII word such that there are 0, 2, 4, 6, or 8 occurences of 1's.
• Example:
  • lower case a is encode as 61x (or 110 0001).
  • By convention, bit 1 is on the rightand bit 7 is on the left.
  • With even parity, a '1' is added on the left to yield 4 1's.
  • Hence the codeword 1110 0001 would be transmitted for character 'a'.
• The receiver can determine that an error occured by checking the parity of a received code word.
• Clearly, any error involving a single bit can be detected (but not corrected).
• To recover from an error, the receiver would request retransmission of erroneous code words.