Data Communications Networking Devices - 4th Ed.pdf

56 _______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSFigure 1.29InputMultisection shift used for ITU CRC computation ‡ exclusive-OR gate.Output00 001 110 111 0The operation of an exclusive-OR gate results in the output of a 0 if the inputsare both 0 or both 1. If the inputs differ,the output of the exclusive-OR gate is a 1.Once all of the bits in a data block entered the shift register the transmitterretrieves the contents of the shift register and transmits the 16 bits as a two-byteCRC. This two-byte CRC is commonly referred to as the block check charactersBCC) in communications literature. Even though a CRC-32 uses a four-byteCRC,that CRC is also commonly referred to as a BCC. After transmitting theCRC the transmitter initializes the shift register positions to zero prior to sendingbits from the next block into the register. At the opposite end of the transmissionlink the receiving device applies the incoming bit stream to point X of a similarmultisection shift register. Once the receiver's CRC is computed it is thenextracted from the shift register and compared to the CRC appended to thetransmitted data block. If the two CRCs match the data block is considered to bereceived without error.Although hardware-based CRC computations are still normally used withsynchronous transmission,almost all such computations are performed bysoftware using the microprocessor of personal computers. To calculate the 16-bit CRC the message bits are considered to be the coef®cients of a polynomial.This message polynomial is ®rst multiplied by X 16 and then divided by thegenerator polynomial …X 16 ‡ X 12 ‡ X 5 ‡ 1† using modulo 2 arithmetic. Theremainder after the division is the desired CRC.International transmissionDue to the growth in international communications,one frequently encounteredtransmission problem is the employment of dissimilar CRC generating polynomials.This typically occurs when an organization in the United States attemptsto communicate with a computer system in Europe or a European organizationattempts to transmit to a computer system located in the United States.