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Re: [Discuss-gnuradio] Question Regarding GMSK2 examples

From: Michael Dickens
Subject: Re: [Discuss-gnuradio] Question Regarding GMSK2 examples
Date: Mon, 1 May 2006 21:42:01 -0400

On Monday, May 1, 2006, at 08:21 PM, Elaine Garbarine wrote:
As a related question, does rx_callback only exist if the packet is received
properly (i.e. without errors)?

In essence, yes: the rx_callback will only be called when a packet is received without (*) errors. See .../gnuradio-core/src/lib/general/gr_packet_sink.cc and it's work() method and finite-state-machine-ish code if you really want the in-depth story.

(*) This happens when enough of the sync bits are correct (either the user-supplied value or the default of 12 bits wrong out of 64) and the immediately-following 32-bit header is correct (meaning that the first 16 bits and second 16 bits are identical [correct or not], and that the payload length [the integer value of either of the first or second 16 bits] is <= 4096). Note that the "function" 'slice()' could more reasonably be called "hard_decode_bit()" since that's what it does.

ps> As a general question regarding the gr_packet_sink, why does this have to do it's output via a gr_message inserted into a queue? Why not stream the output to the next block in the flow graph style? Is this one of the "message versus stream" issues that BBN's proposal will address? Or is it something to do with carrier recovery and such? Or was the carrier issue why the gmsk2_mod_pkt required queued packets and not a stream?

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