ANNEX B - CHECKSUM ALGORITHMS
 
     (This annex is provided for information for implementors  and  is
     not an integral part of the body of the standard.)
 
 
 
     B.1  SYMBOLS
 
     The following symbols are used:
 
        C0  variables used in the algorithms
        C1
 
        i   number (i.e. position) of an octet within  the  TPDU  (see
            12.1)
 
        n   number (i.e. position) of the first octet of the  checksum
            parameter
 
        L   length of the complete TPDU
 
        X   value of the first octet of the checksum parameter
 
        Y   value of the second octet of the checksum parameter.
 
 
 
     B.2  ARITHMETIC CONVENTIONS
 
     Addition is performed in one of the two following modes:
 
        a)  modulo 255 arithmetic;
 
        b)  one's  complement  arithmetic  in  which  if  any  of  the
            variables  has the value minus zero (i.e. 255) it shall be
            regarded as though it was plus zero (i.e. 0).
 
 
 
     B.3  ALGORITHM FOR GENERATING CHECKSUM PARAMETERS
 
 
 
 
 
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     B.3.1  Set up the complete TPDU with the value  of  the  checksum
     parameter field set to zero.
 
 
     B.3.2  Initialize C0 and C1 to zero.
 
 
 
     B.3.3  Process each octet sequentially from i = 1 to L by:
 
        a)  adding the value of the octet to C0; then
 
        b)  adding the value of C0 to C1.
 
 
 
     B.3.4  Calculate X and Y such that
 
        X = -C1 + (L-n).CO
        Y =  C1 - (L-n+1).C0
 
 
 
     B.3.5  Place the values  X  and  Y  in  octets  n  and  (n  +  1)
     respectively.
 
     [A Note describing the above algorithm in  mathematical  notation
     has been omitted from this copy.]
 
 
 
     B.4  ALGORITHM FOR CHECKING CHECKSUM PARAMETERS
 
 
     B.4.1  Initialize C0 and C1 to zero.
 
 
     B.4.2  Process each octet of the TPDU sequentially from i = 1  to
     L by:
 
        a)  adding the value of the octet to C0; then
 
        b)  adding the value of C0 to C1.
 
 
 
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     B.4.3  If, when all the octets have  been  processed,  either  or
     both  of  C0  and  C1  does not have the value zero, the checksum
     formulas in 6.17 have not been satisfied.
 
     NOTE - The nature of  the  algorithm  is  such  that  it  is  not
     necessary to compare explicitly the stored checksum bytes.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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