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/* This file was taken from gsm-tvoid */
#include "system.h"
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include <math.h>
#include "burst_types.h"
#include "cch.h"
#include "conv.h"
#include "fire_crc.h"
/*
* GSM SACCH -- Slow Associated Control Channel
*
* These messages are encoded exactly the same as on the BCCH.
* (Broadcast Control Channel.)
*
* Input: 184 bits
*
* 1. Add parity and flushing bits. (Output 184 + 40 + 4 = 228 bit)
* 2. Convolutional encode. (Output 228 * 2 = 456 bit)
* 3. Interleave. (Output 456 bit)
* 4. Map on bursts. (4 x 156 bit bursts with each 2x57 bit content data)
*/
/*
* Parity (FIRE) for the GSM SACCH channel.
*
* g(x) = (x^23 + 1)(x^17 + x^3 + 1)
* = x^40 + x^26 + x^23 + x^17 + x^3 + 1
*/
static const unsigned char parity_polynomial[PARITY_SIZE + 1] = {
1, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 1, 0,
0, 1, 0, 0, 0, 0, 0, 1,
0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 1, 0, 0,
1
};
// remainder after dividing data polynomial by g(x)
static const unsigned char parity_remainder[PARITY_SIZE] = {
1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1
};
/*
static void parity_encode(unsigned char *d, unsigned char *p) {
int i;
unsigned char buf[DATA_BLOCK_SIZE + PARITY_SIZE], *q;
memcpy(buf, d, DATA_BLOCK_SIZE);
memset(buf + DATA_BLOCK_SIZE, 0, PARITY_SIZE);
for(q = buf; q < buf + DATA_BLOCK_SIZE; q++)
if(*q)
for(i = 0; i < PARITY_SIZE + 1; i++)
q[i] ^= parity_polynomial[i];
for(i = 0; i < PARITY_SIZE; i++)
p[i] = !buf[DATA_BLOCK_SIZE + i];
}
*/
static int parity_check(unsigned char *d) {
unsigned int i;
unsigned char buf[DATA_BLOCK_SIZE + PARITY_SIZE], *q;
memcpy(buf, d, DATA_BLOCK_SIZE + PARITY_SIZE);
for(q = buf; q < buf + DATA_BLOCK_SIZE; q++)
if(*q)
for(i = 0; i < PARITY_SIZE + 1; i++)
q[i] ^= parity_polynomial[i];
return memcmp(buf + DATA_BLOCK_SIZE, parity_remainder, PARITY_SIZE);
}
static void decode_burstmap(unsigned char *iBLOCK, unsigned char *eBLOCK,
unsigned char *hl, unsigned char *hn) {
int j;
for(j = 0; j < 57; j++) {
iBLOCK[j] = eBLOCK[j];
iBLOCK[j + 57] = eBLOCK[j + 59];
}
*hl = eBLOCK[57];
*hn = eBLOCK[58];
}
/*
* Transmitted bits are sent least-significant first.
*/
static int compress_bits(unsigned char *dbuf, unsigned int dbuf_len,
unsigned char *sbuf, unsigned int sbuf_len) {
unsigned int i, j, c, pos = 0;
if(dbuf_len < ((sbuf_len + 7) >> 3))
return -1;
for(i = 0; i < sbuf_len; i += 8) {
for(j = 0, c = 0; (j < 8) && (i + j < sbuf_len); j++)
c |= (!!sbuf[i + j]) << j;
dbuf[pos++] = c & 0xff;
}
return pos;
}
#if 0
int get_ns_l3_len(unsigned char *data, unsigned int datalen) {
if((data[0] & 3) != 1) {
fprintf(stderr, "error: get_ns_l3_len: pseudo-length reserved "
"bits bad (%2.2x)\n", data[0] & 3);
return -1;
}
return (data[0] >> 2);
}
#endif
/*
* decode_cch
*
* Decode a "common" control channel. Most control channels use
* the same burst, interleave, Viterbi and parity configuration.
* The documentation for the control channels defines SACCH first
* and then just keeps referring to that.
*
* The current (investigated) list is as follows:
*
* BCCH Norm
* BCCH Ext
* PCH
* AGCH
* CBCH (SDCCH/4)
* CBCH (SDCCH/8)
* SDCCH/4
* SACCH/C4
* SDCCH/8
* SACCH/C8
*
* We provide two functions, one for where all four bursts are
* contiguous, and one where they aren't.
*/
static unsigned char *decode_cch(GS_CTX *ctx, unsigned char *burst,
unsigned int *datalen)
{
int errors, len, data_size;
unsigned char conv_data[CONV_SIZE], iBLOCK[BLOCKS][iBLOCK_SIZE],
hl, hn, decoded_data[PARITY_OUTPUT_SIZE];
FC_CTX fc_ctx;
data_size = sizeof ctx->msg;
if (datalen)
*datalen = 0;
// unmap the bursts
decode_burstmap(iBLOCK[0], burst, &hl, &hn); // XXX ignore stealing bits
decode_burstmap(iBLOCK[1], burst + 116, &hl, &hn);
decode_burstmap(iBLOCK[2], burst + 116 * 2, &hl, &hn);
decode_burstmap(iBLOCK[3], burst + 116 * 3, &hl, &hn);
// remove interleave
interleave_decode(&ctx->interleave_ctx, conv_data, (unsigned char *)iBLOCK);
// Viterbi decode
errors = conv_decode(decoded_data, conv_data, CONV_INPUT_SIZE_CCH);
if (errors) {
DEBUGF("conv_decode: %d\n", errors);
return NULL;
}
// check parity
// If parity check error detected try to fix it.
if (parity_check(decoded_data))
{
FC_init(&fc_ctx, 40, 184);
unsigned char crc_result[224];
if (FC_check_crc(&fc_ctx, decoded_data, crc_result) == 0)
{
errors = -1;
DEBUGF("error: sacch: parity error (%d)\n", errors);
return NULL;
} else {
DEBUGF("Successfully corrected parity bits!\n");
memcpy(decoded_data, crc_result, sizeof crc_result);
errors = 0;
}
}
if((len = compress_bits(ctx->msg, data_size, decoded_data,
DATA_BLOCK_SIZE)) < 0) {
fprintf(stderr, "error: compress_bits\n");
return NULL;
}
if(len < data_size) {
fprintf(stderr, "error: buf too small (%d < %d)\n",
sizeof(ctx->msg), len);
return NULL;
}
if(datalen)
*datalen = (unsigned int)len;
return ctx->msg;
}
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