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5.3 ACRONYMS 125In the final step of output processing at ST A , the contents of INF is regarded as abinary number (i A ). This number is fed into a modulo 2 divider (M2D), where it isdivided (mod 2) by a divisor d. The remainder of the division (c A ) is entered into CBof the SU.Suppose now that the SU arrives at ST B , and that the contents of INF and CB are i Band c B . In the initial step of input processing at ST B ,i B is fed into M2D, where it isdivided (mod 2) by the divisor d mentioned above. The division yields a remainder c .Comparison circuit COMP compares c B and c . If there are no errors in INF(i B ¼ i A ), then c ¼ c A . If there also are no errors in CB (c B ¼ c A ), the inputs toCOMP are equal, and its output e ¼ 0. This is taken as the indication that the SUis error-free. Otherwise, the SU is deemed to contain errors and is discarded.Cyclic redundancy checking does not catch all transmission errors but, with aproperly selected divisor d, the probability of an undetected SU error can bemade very low (on the order of 1 in 10 8 ).Modulo 2 Division. Mod 2 arithmetic is binary arithmetic without “carries” or“borrows.” For example, consider the mod 2 subtraction (or addition)101101111001 (or þ )010100The result is obtained by performing “exclusive or” operations on the bits in correspondingpositions of both numbers.Mod 2 division consists of mod 2 subtractions and shifts. The example belowshows the calculation of the remainder c that results from mod 2 division ofi ¼ 1101011 by divisor d ¼ 1011. By long division we have1011 j11010111011 mod 2 subtraction11001011 mod 2 subtraction11111011 mod 2 subtraction10011011 mod 2 subtractionRemainder c: 010For more information on CRC, including probabilities of undetected errors, thereader is referred to [6,7].5.3 ACRONYMSANIANSIAutomatic number identificationAmerican National Standards Institute