1 files changed, 224 insertions, 0 deletions
diff --git a/gsmstack/cch.c b/gsmstack/cch.c
new file mode 100644
index 0000000..d3a2619
--- /dev/null
+++ b/gsmstack/cch.c
@@ -0,0 +1,224 @@
+/* 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;
+}