Method and an apparatus for modifying a FCS
AssigneeInternational Business Machines Corporation
InventorRene Glaise
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Abstract
In intermediate network nodes of high speed packet switching networks, when a message is modified, the invention proposes a method and an apparatus for calculating a 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, is reduced to a plurality of simple well known operators (90, 100, 120, 30); the implementation of the invention allows the usage of a plurality of small lookup tables (60, 70, 80, 110) as desired and thus avoiding large amounts of storage resources.
Claims14
A method to calculate a modified Frame Check Sequence (FCS) character for a bit stream message (M) conveyed in a packet switching network using a FCS error code based on a generator polynomial G generating a Galois Field for which the root is .alpha., knowing that one field has been modified in said messages;
- m being the distance from the last bit of the modified field of the message to the last least significant bit of the message including the FCS bits;
- said method comprising the following steps:
- storing the elements of the Galois Field corresponding to said FCS error code;
- determining the element in the Galois Field .alpha..sup.m corresponding to m in the stored data;
- calculating the delta vector resulting in the difference between the initial and the final bit 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.
An apparatus for calculating a modified Frame Check Sequence (FCS) character for a bit stream messages (M) conveyed in a high speed packet switching network using an FCS error code based on a generator polynomial G generating a Galois Field for which the root is .alpha., knowing that one bit stream field has been modified in said messages;
- m being the distance from the last bit of the modified field of the message to the last least significant bit of the message including the FCS bits;
- said apparatus comprising:
- means for storing the elements of the Galois Field corresponding to said FCS error code;
- means for determining, in the data stored, the element in the Galois Field .alpha..sup.m corresponding to m;
- means for XORing the two bit streams corresponding to the initial message and the modified message said messages not including the FCS fields, the result being a delta vector;
- means for calculating the remainder in the division by the generator polynomial of said delta vector;
- if the length of the modified bits field is smaller than the degree of G, the remainder being the delta vector itself;
- means for performing a Galois Field multiplication of the result of the previous step with said remainder;
- means for 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.
Dependencies are inferred from the claim text.