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Make the non-scrambled AX.25 decoder more conservative 

In case of scrambling the self synchronizing scrambler ensures
that enough repetitions of the AX.25 flag have been received.
However, this does not hold for the case of non scrambled
transmissions. In this case, we wait for at least two consecutive
AX.25 flags to reduce the false alarms. Experiments have shown
that due to the poor CRC there were many false positive frames.

It is expected however, to miss some transmissions that use only one
AX.25 flag.
This commit is contained in:
Manolis Surligas 2019-10-03 16:54:37 +03:00
parent 040763af11
commit 747195f054
2 changed files with 27 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
include/satnogs/ax25_decoder.h
View File

@ -127,7 +127,7 @@ private:
const bool d_crc_check;
const size_t d_max_frame_len;
decoding_state_t d_state;
uint8_t d_shift_reg;
uint16_t d_shift_reg;
uint8_t d_dec_b;
uint8_t d_prev_bit_nrzi;
size_t d_received_bytes;

33
lib/ax25_decoder.cc
View File

@ -108,7 +108,26 @@ ax25_decoder::_decode(decoder_status_t &status)
case NO_SYNC:
for (size_t i = 0; i < d_bitstream.size(); i++) {
decode_1b(d_bitstream[i]);
if (d_shift_reg == AX25_SYNC_FLAG) {
/*
* In case of scrambling the self synchronizing scrambler ensures
* that enough repetitions of the AX.25 flag have been received.
* However, this does not hold for the case of non scrambled
* transmissions. In this case, we wait for at least two consecutive
* AX.25 flags to reduce the false alarms. Experiments have shown
* that due to the poor CRC there were many false positive frames.
*
* It is expected however, to miss some transmissions that use only one
* AX.25 flag. We believe that such transmissions are yet rare.
*/
bool have_sync = false;
if (d_descramble) {
have_sync = (d_shift_reg >> 8) == AX25_SYNC_FLAG;
}
else {
have_sync = ((d_shift_reg & 0xFF) == AX25_SYNC_FLAG)
&& (d_shift_reg >> 8) == AX25_SYNC_FLAG;
}
if (have_sync) {
d_bitstream.erase(d_bitstream.begin(),
d_bitstream.begin() + i + 1);
/* Increment the number of items read so far */
@ -138,7 +157,7 @@ ax25_decoder::_decode(decoder_status_t &status)
d_decoded_bits++;
if (d_decoded_bits == 8) {
/* Perhaps we are in frame! */
if (d_shift_reg != AX25_SYNC_FLAG) {
if ((d_shift_reg >> 8) != AX25_SYNC_FLAG) {
d_start_idx = i + 1;
enter_decoding_state();
cont = true;
@ -155,7 +174,7 @@ ax25_decoder::_decode(decoder_status_t &status)
case DECODING:
for (size_t i = d_start_idx; i < d_bitstream.size(); i++) {
decode_1b(d_bitstream[i]);
if (d_shift_reg == AX25_SYNC_FLAG) {
if ((d_shift_reg >> 8) == AX25_SYNC_FLAG) {
d_sample_cnt = nitems_read() + i - d_frame_start;
LOG_DEBUG("Found frame end");
if (enter_frame_end(status)) {
@ -169,11 +188,11 @@ ax25_decoder::_decode(decoder_status_t &status)
cont = true;
break;
}
else if ((d_shift_reg & 0xfc) == 0x7c) {
else if (((d_shift_reg >> 8) & 0xfc) == 0x7c) {
/*This was a stuffed bit */
d_dec_b <<= 1;
}
else if ((d_shift_reg & 0xfe) == 0xfe) {
else if (((d_shift_reg >> 8) & 0xfe) == 0xfe) {
LOG_DEBUG("Invalid shift register value %u", d_received_bytes);
reset_state();
cont = true;
@ -246,7 +265,7 @@ ax25_decoder::enter_decoding_state()
* Due to the possibility of bit stuffing on the first byte some special
* handling is necessary
*/
if ((d_shift_reg & 0xfc) == 0x7c) {
if (((d_shift_reg >> 8) & 0xfc) == 0x7c) {
/*This was a stuffed bit */
d_dec_b <<= 1;
d_decoded_bits = 7;
@ -304,7 +323,7 @@ ax25_decoder::decode_1b(uint8_t in)
{
/* In AX.25 the LS bit is sent first */
d_shift_reg = (d_shift_reg >> 1) | (in << 7);
d_shift_reg = (d_shift_reg >> 1) | (in << 15);
d_dec_b = (d_dec_b >> 1) | (in << 7);
}