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[Discuss-gnuradio] Reading info from pmt_blob

From: Nicolas Cuervo Benavides
Subject: [Discuss-gnuradio] Reading info from pmt_blob
Date: Sat, 14 Mar 2015 22:29:33 +0100

Hello all,

I'm new in GNURadio and right now I am understanding a little bit on how the pmt work and how can it be read.

For that I'm taking this project as a base: https://github.com/bastibl/gr-ieee802-15-4, working on the MAC layer (modifying mac.cc and mac.h so far) and from here I want to extract the sequence number of a frame that I am receiving from the physical layer. When simulating, I see how the frame is created for Tx, as it can be read clear as an output. Appart from other info, it includes the sequence number of the frame and that is my main interest right now. However, in the Rx part, I dont see it as bytes or as hexadecimal values, as it comes in a coding that I'm missing where it comes from and how it is encoded. I tried something like:

pmt::pmt_t received_frame = pmt::make_blob(pmt::blob_data(pmt::cdr(msg)) , data_len )

where msg is what I am receiving. However on the output I see my message in the payload (not encoded = just as It can be written on the message strobe) and a long header which I dont understand so far. I dont see a similarity with the same frame from when it was sent and, therefore, I have not managed how to retreive its sequence number. 

Any help on how to decode this blob and also how to access to a certain part of the frame only (in order to retreive the sequence number) would be appreciated. 

Thanks in advance! 
Nicolás Cuervo 

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