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Re: [Discuss-gnuradio] writing packet payloads to file


From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] writing packet payloads to file
Date: Sat, 24 Nov 2012 10:30:38 -0500

On Wed, Nov 21, 2012 at 2:21 PM, Colleen Josephson <address@hidden> wrote:
> Thanks for the response. I'm not sure I understand how struct.unpack will
> help, since the only data I can get seems to already be a python string. If
> I transmit a packet with "hello world" as my payload, the only thing I ever
> see in the receive script is the python string "hello world". I don't know
> how to access the intermediate binary representation.

Colleen,

Take a look at how we handle this stuff in gr-digital. Specifically,
in gr-digital/python/pkt.py, the method run() of _queue_watcher_thread
extracts the message contents as a string and passes it to
packet_utils.unmake_packet (in packet_utils.py). We have a bunch of
helper functions in there to handle the string data (such as
string_to_hex_list and the like). Finally, the paylad (As a string) is
passed up to the example scripts like
gr-digital/examples/narrowband/benchmark_rx.py where the rx_callback
metho uses struct.unpack to extract the packet number, which is
non-ASCII.

Hopefully, following the way we handle the messages as strings and the
various methods we use to manipulate them here will help you with your
tasks.

Tom



> On Tue, Nov 20, 2012 at 1:07 PM, Tom Rondeau <address@hidden> wrote:
>>
>> On Fri, Nov 16, 2012 at 6:28 PM, Colleen Josephson <address@hidden>
>> wrote:
>> > Hello all,
>> >
>> > I am researching a new coding algorithm. I am transmitting and coding a
>> > text
>> > file using payload_tx.py and then write the binary of the packet payload
>> > to
>> > a file to do offline analysis.
>> >
>> > I am having trouble writing packet payloads to file, however, because
>> > the I
>> > can't seem to find a way to get data out of a gr_message except for the
>> > to_string() method. If I try and convert the string back to binary (my
>> > attempt involved python's binascii module), I get an "Error: illegal
>> > char"
>> > message.
>> >
>> > This makes sense, since the data is still coded, and should not be a
>> > valid
>> > ascii string. But how can I get around this problem and write binary to
>> > file?
>> >
>> > --
>> > Colleen Josephson
>> > http://www.cjosephson.net
>> > Dept. of Electrical Engineering and Computer Science
>> > Massachusetts Institute of Technology
>> > Class of 2013
>>
>>
>> Colleen,
>>
>> Have you tried to use struck.unpack?
>>
>> Tom
>
>
>
>
> --
> Colleen Josephson
> http://www.cjosephson.net
> Dept. of Electrical Engineering and Computer Science
> Massachusetts Institute of Technology
> Class of 2013



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