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Redwood City Police <br /> Department System PerFormance <br /> �; <br /> Once the algorithm has reconstructed all correctable errors,it <br /> reviews the message for remaining errors. This final enor <br /> detectioa algorithm ensures that only valid messages are presented <br /> . to system users by utilizing a Cyclical Redundancy Check (CRC). <br /> RD-LAP's unique 32 bit CRC (Cyclical Redundancy Check), <br /> appended to the end of each data message,provides excellent <br /> protection against the possibility of receiving an undetected eiror. <br /> An upper liaut is approached when RF data terminals are operating <br /> in areas where the raw,uncorrected,bit error rate(BER) is very <br /> high(greater than 10'2). This Iimit has been approximated using <br /> the 2-P equation,where p is the number of bits in the CRC code. <br /> The upper limit of probability that an etror has gone undetected is <br /> approximately 2.3*10'1�. This means that roughly 1 message in <br /> 4.3 billion may be presented to a user with an undetected error. <br /> Only messages that are not correct initiate an automatic <br /> retransmission algorithm. The high level of error correction <br /> inherent in the RD-LAP protocol reduces the number of inessages <br /> that require retransmission. This effecrively enables reliable data <br /> communica.tions in RF fringes on the first transmission. <br /> . Motorola's R.D-LAP protocol uses the addition of an optimum <br /> number of redundant protection bits to the message data to enhance <br /> the power of this encoding scheme. The number and location of <br /> the protection bits to be duplicated in the message is determined by <br /> a convolutional algorithm. Once applied,this scheme provides the <br /> enhanced ezror detection and correction capabilities. <br /> Finally, after the CRC and protection bits have been added to the <br /> message,RD-LAP incoiporates data interleaving to eusure RF <br /> fades do not corrupt neighboring bits from the original message. � <br /> Directly preceding transmission,the bits are arraaged ia a large <br /> table with formatted rows and columns. As they aze placed in the <br /> table,the bits are placed in order across the rows,returning to the <br /> beginning of the next row as one becomes full. When the bits are <br /> takea out of this table,they are selected by column beginning with <br /> the top bit and working down. This interleaving Pmcess is <br /> depicted in the following diagram:. <br /> QMOTOROLA , 3 - 2 <br /> . _ _ . _ _ <br />