Data Integrity The Checksum Procedure When transmitting data using contactless technology it is very likely that interference will be encountered, causing undesired changes to the transmitted data and thus leading to transmission errors (Figure ). A checksum can be used to recognise transmission errors and initiate corrective measures, for example the retransmission of the erroneous data blocks. The most common checksum procedures are parity checks, XOR sum and CRC. | 7 RFID Handbook Fundamentals and Applications in Contactless Smart Cards and Identification Second Edition Klaus Finkenzeller Copyright 2003 John Wiley Sons Ltd. ISBN 0-470-84402-7 Data Integrity The Checksum Procedure When transmitting data using contactless technology it is very likely that interference will be encountered causing undesired changes to the transmitted data and thus leading to transmission errors Figure . A checksum can be used to recognise transmission errors and initiate corrective measures for example the retransmission of the erroneous data blocks. The most common checksum procedures are parity checks XOR sum and CRC. Parity checking The parity check is a very simple and therefore a very popular checksum procedure. In this procedure a parity bit is incorporated into each byte and transmitted with it with the result that 9 bits are sent for every byte. Before data transfer takes place a decision needs to be made as to whether to check for odd or even parity to ensure that the sender and receiver both check according to the same method. The value of the parity bit is set such that if odd parity is used an odd number of the nine bits have the value 1 and if even parity is used an even number of bits have the value 1. The even parity bit can also be interpreted as the horizontal checksum modulo 2 of the data bit. This horizontal checksum also permits the calculation of the exclusive OR logic gating XOR logic gating of the data bits. However the simplicity of this method is balanced by its poor error recognition Pein 1996 . An odd number of inverted bits 1 3 5 . will always be detected but if there is an even number of inverted bits 2 4 6 . the errors cancel each other out and the parity bit will appear to be correct. Example Using odd parity the number E5h has the binary representation 1110 0101 p 0. A parity generator for even parity can be realised by the XOR logic gating of all the data bits in a byte Tietze and Schenk 1985 . The .
