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.\" |
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.\" Copyright 1998 by Luigi Rizzo, Dip. Ingegneria dell'Informazione, |
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.\" Universitaet Berlin. See the source code for copyright details. |
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.\" THERE IS ABSOLUTELY NO WARRANTY FOR THIS SOFTWARE. |
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.\" |
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.Dd July 15, 1998 |
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.Dt FEC 3 |
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.Os |
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.Sh NAME |
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.Nm fec_new, fec_encode, fec_encode, fec_free |
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.Nd An erasure code in GF(2^m) |
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.Sh SYNOPSIS |
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.Fd #include <fec.h> |
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.Ft void * |
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.Fn fec_new "int k" "int n" |
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.Ft void |
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.Fn fec_encode "void *code" "void *data[]" "void *dst" "int i" "int sz" |
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.Ft int |
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.Fn fec_decode "void *code" "void *data[]" "int i[]" "int sz" |
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.Ft void * |
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.Fn fec_free "void *code" |
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.Sh "DESCRIPTION" |
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This library implements a simple (n,k) |
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erasure code based on Vandermonde matrices. |
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The encoder takes |
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.Fa k |
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packets of size |
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.Fa sz |
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each, and is able to produce up to |
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.Fa n |
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different encoded packets, numbered from 0 to n-1, |
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such that any subset of |
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.Fa k |
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of them permits reconstruction of the original data. |
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.Pp |
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The data structures necessary for the encoding/decoding must |
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first be created using calling |
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.Fn fec_new |
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with the desired parameters. The code descriptor returned by the function |
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must be passed to other functions, and destroyed calling |
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.Fn fec_free |
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.Pp |
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Allowed values for k and n depend on a compile-time value |
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of |
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.Fa GF_BITS |
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and must be k <= n <= 2^GF_BITS. |
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Best performance is achieved with GF_BITS=8, although the code supports |
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also GF_BITS=16. |
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.Pp |
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Encoding is done by calling |
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.Fn fec_encode |
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and passing it pointers to the code descriptor, the source and |
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destination data packets, the index of the packet to be produced, |
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and the size of the packet. |
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|
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.Pp Decoding is done calling |
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.Fn fec_decode |
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with pointers to the code, received packets, indexes of received |
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packets, and packet size. Decoding is done in place, possibly |
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shuffling the arrays passed as parameters. Decoding is deterministic |
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as long as the received packets are different. The decoding procedure |
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does some limited testing on this and returns if parameters are |
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invalid. |
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|
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.Sh EXAMPLE |
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.nf |
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#include <fec.h> |
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|
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/* |
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* example of sender code |
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*/ |
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void *code ; |
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int n, k ; |
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|
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void *src[] ; |
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void *pkt ; |
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|
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code = new_code (k, n ); |
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|
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for (i = 0 ; i < k ; i++ ) |
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src[i] = .. pointer to i-th source packet .. |
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for (each packet to transmit) { |
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i = ... index of the packet ; |
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fec_encode(code, src, pkt, i, size) ; |
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.. use packet in pkt |
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} |
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fec_free(code) ; |
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|
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/* |
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* example of receiver code |
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*/ |
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void *code ; |
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int n, k ; |
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|
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void *data[] ; |
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int *ix[] ; |
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|
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code = new_code (k, n ); |
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|
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for (i = 0 ; i < k ; i++ ) { |
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... receive a new packet ... |
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data[i] = .. pointer to i-th source packet .. |
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ix[i] = .. index of i-th source packet .. |
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} |
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fec_decode(code, data, ix, size) ; |
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/* |
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* now data[] has pointers to the source packets |
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*/ |
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|
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.SH BUGS |
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Please direct bug reports to luigi@iet.unipi.it . |
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.Sh "SEE ALSO" |
