The message size is limited by the packet framer and deframer, which is
modified from the packet framer/deframer found in GNU Radio.  I believe
that the limit is 4096 bytes.  However, the laws of practical wireless
comms may limit the system to smaller packet sizes.  The more bits you
have, the more likely you'll see a bit error in the frame, causing the
deframer to detect a bad CRC and discard.  Smaller packets = smaller target?

The heartbeat block simply generates a message with the specified key/value
at the specified period.  In the case of pre-cog, this block just produces
a strobe to make the work function run via the mostly useless "ctrl_in"
port.  Or, you may use it to generate a periodic broadcast message if you
connect it to the application input port.  The scheduler will not run the
work function unless a message arrives at an input port.  The work function
must run periodically to support the stop-and-wait ARQ functionality.

-John


On Thu, Jul 18, 2013 at 6:36 PM, GNU radio <gnuradio.g...@gmail.com> wrote:

> Hello
>         I am trying to send low rate data through pre-cog simple trx
> example. I have difficulty understanding the function of the heart beat
> block. If I decrease the rate (period) of the heart beat block, I will be
> able to send more messages. Which means more data.
>
> Does any one know how much should be the size of a single message?
>
> Regards
>
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>
>
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