Hey, I'm currently working on an open-source reimplementation of the FasTrak protocol using GNU Radio and the Ettus E100. What I have so far (currently non-functioning) is available here: https://github.com/Meyermagic/OpenFasTrak
Anyway, I've run into a couple of problems, most probably related to my complete lack of DSP experience, but I thought I'd try making a mailing list post anyway. First, I currently have the E100 with a Class-4 MicroSD card, which should guarantee sustained 4MB/s write speed. My first problem comes from the fact that the bitrate of the FasTrak communications is 300kbps, and with manchester encoding on the bits, sampling at 600kHz should get me exactly one sample per half-bit. I initially tried recording data at 915MHz carrier with a 64MHz sample rate, and simply storing it to a file, in order to analyze it later (and extract the "agency code" I need to communicate with a FasTrak transponder). Unfortunately, this would theoretically require a 512MB/s sustained write speed, so I got frequent overrun errors (it took me some time of trying to analyze the incomplete stored data to realize the printed "O"s were on stderr, and were significant). To compensate for this, I tried capturing at an offset carrier frequency, bandpassing for the real carrier, demodulating it, and thresholding the data, all before writing to a named pipe. On the other side of the pipe, I had a C program reading and run-length-encoding the data, outputting to a real file. Each half-bit being ~107 samples at 64 MHz sample rate (which comes to a bit over 4MB/s if I write 1 byte for every half-bit), I figured, with buffered i/o, I would be able to receive the whole message. Unfortunately, this didn't seem to be true, as I again got overrun errors, and found mostly runs of 0s in the output file. Finally, I realized that since the reader to transponder polling message is so short (around 786.3us, including the transponder's response packet), and there are only 2^16 possible Agency Codes, I could try every possible agency code in under a minute. I wrote some simple C programs for this purpose, and chained them together in a shell script to output a run-length-encoded version of the data to send. Testing the decoding program on my desktop, I got hopeful results: [ifx@melancholy tests]$ time ./rle2samples fuzzmessage.rle.dat /dev/null real 0m41.390s user 0m41.217s sys 0m0.077s On the E100, though, root@usrp-e1xx:~/New# time -p ./rle2samples fuzzmessage.rle.dat /dev/null real 1044.22 user 36.43 sys 1006.12 So, this time, I got underrun errors when I tried to send the data. rle2samples.c available here, for reference: https://gist.github.com/1118976 Can you guys offer any help with this situation? -Meyer S. Jacobs _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
