Heath/FX.25

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While discussing internet access via HF Packet Radio on the forums, we discovered this...

FX.25 Forward Error Correction Extension to AX.25 Link Protocol (PDF)

How It Works

It takes an AX.25 packet and adds forward error correction to it. The resultant packet doesn't confuse the far end as it has been designed to be compatible with AX.25. After the FX.25 has been decoded, it is passed to the FX.25 decoder which turns it to an AX.25 packet.

This is slightly counter intuitive because you'd think it would be like this...

AX.25 <-> FX.25 <-> Radio <-> Radio <-> FX.25 <-> AX.25

But it is actually like this...

AX.25 <-> FX.25 <-> Radio <-> Radio <-> AX.25 <-> FX.25

This extension would make HF Packet Radio A LOT more useful.

Hacking

It doesn't look that hard as the protocol has a really clever design. There are a few problems though:

  • Hasn't been implemented on Linux
  • All the parameters seem to be hardcoded

Source Code:

I think the relevant code that needs to be hacked is ax25.c in this: net-tools

FX-25 Patch

This patch gets the extractor to compile. GCC spits the worrisome warning:

FX-25_extract.c:817: warning: integer constant is too large for ‘long’ type

Usage is: patch -p0 < FX-25.patch

--- FX-25_extract.c.orig        2007-10-23 14:00:12.000000000 -0500
+++ FX-25_extract.c     2008-06-04 03:51:50.000000000 -0500
@@ -84,6 +84,24 @@
 //#include "char.h"
 #define DTYPE unsigned char

+  unsigned char     out_buffer = 0;
+  int           out_bits = 0;
+  int           consec_ones = 0;
+  unsigned char     out_byte;
+  unsigned char     block[BLOCKSIZE];
+  int           i;
+  unsigned char     mstate, phase;
+  unsigned char     sample_char;
+  unsigned char     fault;
+  unsigned int      dbuffer, bytecount;
+  unsigned char     derrors;
+  int           erasures;
+
+  int           in_size;
+  unsigned        *in_buf;
+       int argc;
+       char ** argv;
+
 /* Reed-Solomon codec control block */
 struct rs {
   unsigned int mm;              /* Bits per symbol */
@@ -795,8 +813,8 @@
 //
 /***************************************************************************/
 char find_corr_tag (unsigned char newbyte, char threshold) {
-
-       static unsigned long long const tag = INT64_C(0x3E2F538ADFB74DB7);        // fixed correlation tag value
+
+       static unsigned long long const tag = 0x3E2F538ADFB74DB7;               // fixed correlation tag value
        int     i, j;
        static unsigned long long       ltemp1 = 0;  // this one needs to be persistent
        unsigned long long                      ltemp2, tag_temp;
@@ -845,8 +863,13 @@
 // Process data LSb first
 //
 //
-int main(int argc, char **argv)
+int main(int argc2, char **argv2)
 {
+
+       argc = argc2;
+       argv = argv2;
+
+/*
        unsigned char                   out_buffer = 0;
        int                                             out_bits = 0;
        int                                             consec_ones = 0;
@@ -862,6 +885,7 @@

        int                                             in_size;
        unsigned                                *in_buf;
+*/

        struct  rs *rs;
        int     index;
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