US 5,689,518 A · Method and an apparatus to modify CRC in intermediate high speed network nodes

Abstract

A method and an apparatus to calculate in an intermediate node of a communication network, the new Frame Check Sequence (FCS) appended to a data bits message which has been modified in said intermediate network node. The invention is useful for high speed networks where the transit delay needs to be optimized in the network along with the computing resources in the intermediate network nodes in terms of computer cycles and memory size. The invention consists in calculating the difference between the FCS using the difference between the modified fields in the message and the distance in bits between the end of the modified field and the end of the message; the calculation consists in differentiating `short messages` in the data flow and to provide an optimized processing for the short messages, the processing for larger messages being based on this first optimized processing. The calculation of the modified FCS comprises operations on polynomials whose coefficients belong to the Galois's Field and whose degree is limited to the one of the polynomial generator of the corresponding CRC code. The calculations include also look up operations in tables limited in size. The choice between the possible implementations (full software, full hardware and mixed hardware and software with the usage of a Remult operator for the last two) will depend on the kind of the network (Frame Relay or other network) and the capacity of the intermediate network node.

Claims9

Dependent claims

For use at an intermediate node of a telecommunications data network at which input messages are received and may be modified before being forwarded as output messages to other nodes in the network, each message being contained in a frame having a header field, a data field and a CRC code field, a method of calculating a new CRC code for use in an output message, the value of said new CRC code being a function of the CRC code in the input message, modifications made in a modifiable part of the header field at the intermediate node, and the length of the data field, the distance in bits between the end of the modifiable part and the end of the message being defined as a variable m, said method comprising the steps of:

a. calculating the bit by bit difference of the input message and the message once modified in the intermediate network node, said difference being represented by a polynomial B(X), X being the symbolic variable of the polynomial representation and the polynomial coefficients belonging to the modulo 2 Galois's Field.
b. solving the equation m+16=n.sub.0 +N.multidot.n derive integer values for n.sub.0 and n.sub.1 where N is an integer power of the base 2;
c. calculating the difference between the CRC codes of the input and the modified messages using the three values n.sub.0, n.sub.1 and m as input;
and
d. performing a modulo 2 addition of the said difference with the initial message code value to obtain the new message code value.

2 more elements

In an intermediate node of a telecommunication network, an apparatus to calculate the new CRC code of input data bits messages which have been modified;

said CRC code being a function of the modified message field, the initial error code of the message and the distance m between the end of the modified message field and the end of the message, said apparatus comprising;
a. means for calculating the bit by bit difference of the input message and the message once modified in the intermediate node, said difference being represented by a polynomial B(X), X being the symbolic variable of the polynomial representation and with the coefficients belonging to the modulo 2 Galois's Field (302 or 710);
b. means for solving the equation m+16=n.sub.0 30 N.multidot.n, to establish integer values for n.sub.0 and n.sub.1 where N is an integer power of the base 2;
c. means for calculating the difference between the CRC codes of the input and modified messages using the three values n.sub.0, n.sub.1 and m as inputs;
d. means for performing a modulo 2 addition of the said difference with the initial message code value (308) to obtain the new message code value.

2 more elements

Dependencies are inferred from the claim text.

Method and an apparatus to modify CRC in intermediate high speed network nodes

Life of this patent

Method and an apparatus to modify CRC in intermediate high speed network nodes

Life of this patent

Abstract

Claims9