Benchmark_xx Packets

GNU Radio
1

Hi,

I am planning to modify benchmark_xx to send and receive custom packets
(similar to 802.11 packets).

I have been rooting around these two files. I would like to know: in
benchmark_rx how does the radio “know” when it has received a packet
(and call rx_callback). rx_callback is called by pkt.py I believe,
demod_pkts. I looked at demod_pkts and I am still unclear how demod_pkts
realizes it has a packet on its hands?

Thanking you in advance.

2

On Mon, Mar 09, 2009 at 11:29:54PM +0100, William Sherman wrote:

Thanking you in advance.

If you look at demod_pkts in pkt.py you’ll see that it connects
together the chosen demodulator (passed in),
gr.correlate_access_code_bb and a gr_framer_sink_1.

gr_correlate_access_code_bb slides the given access code (usually an
M-sequence) across the demodulated bits, and produces two bits of
output for each input bit: the original bit, and a flag bit:

[from gr_correlate_access_code_bb.h]

/*!
 * \param access_code is represented with 1 byte per bit, e.g.,

“010101010111000100”
* \param threshold maximum number of bits that may be wrong
*/
gr_correlate_access_code_bb_sptr
gr_make_correlate_access_code_bb (const std::string &access_code,
int threshold);

/*!
 * \brief Examine input for specified access code, one bit at a

time.
* \ingroup synch
*
* input: stream of bits, 1 bit per input byte (data in LSB)
* output: stream of bits, 2 bits per output byte (data in LSB, flag
in next higher bit)
*
* Each output byte contains two valid bits, the data bit, and the
* flag bit. The LSB (bit 0) is the data bit, and is the original
* input data, delayed 64 bits. Bit 1 is the
* flag bit and is 1 if the corresponding data bit is the first data
* bit following the access code. Otherwise the flag bit is 0.
*/

gr_framer_sync_1 reads the output from gr_correlate_access_code_bb and
builds packets which it inserts into a message queue.

[from gr_framer_sync_1.h]

gr_framer_sink_1_sptr
gr_make_framer_sink_1 (gr_msg_queue_sptr target_queue);

/*!
 * \brief Given a stream of bits and access_code flags, assemble

packets.
* \ingroup sink
*
* input: stream of bytes from gr_correlate_access_code_bb
* output: none. Pushes assembled packet into target queue
*
* The framer expects a fixed length header of 2 16-bit shorts
* containing the payload length, followed by the payload. If the
* 2 16-bit shorts are not identical, this packet is ignored.
Better
* algs are welcome.
*
* The input data consists of bytes that have two bits used.
* Bit 0, the LSB, contains the data bit.
* Bit 1 if set, indicates that the corresponding bit is the
* the first bit of the packet. That is, this bit is the first
* one after the access code.
*/
class gr_framer_sink_1 : public gr_sync_block

Did this help?

Eric