| 摘要 |
<p>In the intermediate network nodes of the high speed packet switching networks, when a message is modified, the invention proposes a method and an apparatus to calculate the modified FCS error code. Using the properties of calculations in the Galois Field, the implementation of the invention, specially in the intermediate Frame Relay network nodes, consists in implementing the following steps and m being the distance from the last bit of the modified filed of the message to the Least Significant Bit of the message including the FCS bits: splitting the bit stream representing m into a plurality of bits streams; building a plurality of tables comprising the possible elements of the Galois Field generated by the generator polynomial G of the error code; each table corresponding to all the possible values of the bits streams coming from the splitting of the m bits stream; for each bits field, pointing in each corresponding lookup table to the corresponding element in the Galois Field; starting with the LSB bits fields, performing recurrent multiplications in the Galois Field of a Galois Field element found in a lookup table with the Galois Field element found in the previous lookup table; determining in the data stored to the element in the Galois Field alpha m corresponding to m; Calculating the delta vector resulting in the difference between the initial and the final bits stream messages not including the FCS bits; calculating the remainder in the division by the generator polynomial of the delta vector; if the length of the modified bits field is smaller than the degree of G, the remainder being the delta vector itself; performing a Galois Field multiplication of the result of the previous step with said remainder; adding the result of the previous step with the initial error code, the result being the final error code which is the error code of the modified message. <IMAGE></p> |
