sebastian
/
gr-satnogs
mirror of https://gitlab.com/LongHairedHacker/gr-satnogs.git
1
0
Fork 0
Code Issues Releases Wiki Activity
gr-satnogs/lib/ax25_decoder.cc

331 lines
8.4 KiB
C++
Raw Blame History

/* -*- c++ -*- */
/*
* gr-satnogs: SatNOGS GNU Radio Out-Of-Tree Module
*
* Copyright (C) 2019, Libre Space Foundation <http://libre.space>
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif
#include <gnuradio/io_signature.h>
#include <satnogs/ax25_decoder.h>
#include <satnogs/ax25.h>
#include <satnogs/metadata.h>
namespace gr {
namespace satnogs {
decoder::decoder_sptr
ax25_decoder::make(const std::string &addr, uint8_t ssid, bool promisc,
bool descramble, bool crc_check, size_t max_frame_len)
{
return decoder::decoder_sptr(
new ax25_decoder(addr, ssid, promisc, descramble, crc_check,
max_frame_len));
}
ax25_decoder::ax25_decoder(const std::string &addr, uint8_t ssid, bool promisc,
bool descramble, bool crc_check, size_t max_frame_len) :
decoder(sizeof(uint8_t), 2 * max_frame_len * 8),
d_promisc(promisc),
d_descramble(descramble),
d_crc_check(crc_check),
d_max_frame_len(max_frame_len),
d_state(NO_SYNC),
d_shift_reg(0x0),
d_dec_b(0x0),
d_prev_bit_nrzi(0),
d_received_bytes(0),
d_decoded_bits(0),
d_lfsr(0x21, 0x0, 16),
d_frame_buffer(
new uint8_t[max_frame_len + AX25_MAX_ADDR_LEN + AX25_MAX_CTRL_LEN
+ sizeof(uint16_t)]),
d_start_idx(0),
d_frame_start(0),
d_sample_cnt(0)
{
}
decoder_status_t
ax25_decoder::decode(const void *in, int len)
{
const uint8_t *input = (const uint8_t *) in;
decoder_status_t status;
if (d_descramble) {
for (int i = 0; i < len; i++) {
/* Perform NRZI decoding */
uint8_t b = (~((input[i] - d_prev_bit_nrzi) % 2)) & 0x1;
d_prev_bit_nrzi = input[i];
b = d_lfsr.next_bit_descramble(b);
d_bitstream.push_back(b);
}
}
else {
for (int i = 0; i < len; i++) {
/* Perform NRZI decoding */
uint8_t b = (~((input[i] - d_prev_bit_nrzi) % 2)) & 0x1;
d_prev_bit_nrzi = input[i];
d_bitstream.push_back(b);
}
}
/*
* The decoder always consumes all the available input bits and stores them
* internally
*/
_decode(status);
status.consumed = len;
return status;
}
void
ax25_decoder::reset()
{
reset_state();
}
bool
ax25_decoder::_decode(decoder_status_t &status)
{
while (1) {
bool cont = false;
switch (d_state) {
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) {
d_bitstream.erase(d_bitstream.begin(),
d_bitstream.begin() + i + 1);
/* Increment the number of items read so far */
incr_nitems_read(i);
enter_sync_state();
/* Mark possible start of the frame */
d_frame_start = nitems_read();
d_start_idx = 0;
cont = true;
break;
}
}
if (cont) {
continue;
}
incr_nitems_read(d_bitstream.size());
d_bitstream.clear();
return false;
case IN_SYNC:
/*
* Most of the transmitters repeat several times the AX.25 SYNC
* In case of G3RUH this is mandatory to allow the self synchronizing
* scrambler to settle
*/
for (size_t i = d_start_idx; i < d_bitstream.size(); i++) {
decode_1b(d_bitstream[i]);
d_decoded_bits++;
if (d_decoded_bits == 8) {
/* Perhaps we are in frame! */
if (d_shift_reg != AX25_SYNC_FLAG) {
d_start_idx = i + 1;
enter_decoding_state();
cont = true;
break;
}
d_decoded_bits = 0;
}
}
if (cont) {
continue;
}
d_start_idx = d_bitstream.size();
return false;
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) {
d_sample_cnt = nitems_read() + i - d_frame_start;
LOG_DEBUG("Found frame end");
if (enter_frame_end(status)) {
d_bitstream.erase(d_bitstream.begin(),
d_bitstream.begin() + i + 1);
/* Increment the number of items read so far */
incr_nitems_read(i);
d_start_idx = d_bitstream.size();
return true;
}
cont = true;
break;
}
else if ((d_shift_reg & 0xfc) == 0x7c) {
/*This was a stuffed bit */
d_dec_b <<= 1;
}
else if ((d_shift_reg & 0xfe) == 0xfe) {
LOG_DEBUG("Invalid shift register value %u", d_received_bytes);
reset_state();
cont = true;
break;
}
else {
d_decoded_bits++;
if (d_decoded_bits == 8) {
d_frame_buffer[d_received_bytes++] = d_dec_b;
d_decoded_bits = 0;
/*Check if the frame limit was reached */
if (d_received_bytes >= d_max_frame_len) {
LOG_DEBUG("Wrong size");
reset_state();
cont = true;
break;
}
}
}
}
if (cont) {
continue;
}
d_start_idx = d_bitstream.size();
return false;
default:
LOG_ERROR("Invalid decoding state");
reset_state();
return false;
}
}
}
ax25_decoder::~ax25_decoder()
{
delete[] d_frame_buffer;
}
void
ax25_decoder::reset_state()
{
d_state = NO_SYNC;
d_dec_b = 0x0;
d_shift_reg = 0x0;
d_decoded_bits = 0;
d_received_bytes = 0;
}
void
ax25_decoder::enter_sync_state()
{
d_state = IN_SYNC;
d_dec_b = 0x0;
d_shift_reg = 0x0;
d_decoded_bits = 0;
d_received_bytes = 0;
}
void
ax25_decoder::enter_decoding_state()
{
uint8_t tmp;
d_state = DECODING;
d_decoded_bits = 0;
d_received_bytes = 0;
/*
* Due to the possibility of bit stuffing on the first byte some special
* handling is necessary
*/
if ((d_shift_reg & 0xfc) == 0x7c) {
/*This was a stuffed bit */
d_dec_b <<= 1;
d_decoded_bits = 7;
}
else {
d_frame_buffer[0] = d_dec_b;
d_decoded_bits = 0;
d_received_bytes = 1;
}
}
bool
ax25_decoder::enter_frame_end(decoder_status_t &status)
{
uint16_t fcs;
uint16_t recv_fcs = 0x0;
/* First check if the size of the frame is valid */
if (d_received_bytes < AX25_MIN_ADDR_LEN + sizeof(uint16_t)) {
reset_state();
return false;
}
/*
* Check if the frame is correct using the FCS field
* Using this field also try to correct up to 2 error bits
*/
if (frame_check()) {
metadata::add_pdu(status.data, d_frame_buffer,
d_received_bytes - sizeof(uint16_t));
metadata::add_time_iso8601(status.data);
metadata::add_crc_valid(status.data, true);
metadata::add_sample_start(status.data, d_frame_start);
metadata::add_sample_cnt(status.data, d_sample_cnt);
status.decode_success = true;
reset_state();
return true;
}
else if (!d_crc_check) {
metadata::add_pdu(status.data, d_frame_buffer,
d_received_bytes - sizeof(uint16_t));
metadata::add_time_iso8601(status.data);
metadata::add_crc_valid(status.data, false);
status.decode_success = true;
LOG_DEBUG("Wrong crc");
reset_state();
return false;
}
return false;
}
inline void
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_dec_b = (d_dec_b >> 1) | (in << 7);
}
bool
ax25_decoder::frame_check()
{
uint16_t fcs;
uint16_t recv_fcs = 0x0;
uint8_t orig_byte;
/* Check if the frame is correct using the FCS field */
fcs = ax25_fcs(d_frame_buffer, d_received_bytes - sizeof(uint16_t));
recv_fcs = (((uint16_t) d_frame_buffer[d_received_bytes - 1]) << 8)
| d_frame_buffer[d_received_bytes - 2];
if (fcs == recv_fcs) {
return true;
}
return false;
}
} /* namespace satnogs */
} /* namespace gr */
Reference in New Issue View Git Blame Copy Permalink