Re: [Discuss-gnuradio] Final Packet(s) dropped

[John Ackermann N8UR]

Richard Alimi wrote:
> Hello All,
>
> I am currently having a problem with packet transmission similar to the one 
> Tom Rondeau had mentioned in his February 14th 2006 post titled 'Dropped 
> Packets.'  However, there was no general resolution given there.
>
> When I run the dbpsk/dqpsk/gmsk transmitter/receiver under the 
> gnuradio-examples directory (revision 4796), it always seems to drop the last 
> packet, leading me to believe this hasn't been fully fixed.  Using 
> the --discontinuous option produces the same result.  Even adding a 
> time.sleep(1) before fg.wait() doesn't seem to fix it, as suggested by that 
> post.

I am dangerously jumping into a place that I don't understand, but I 
have had a similar problem in a non-gnu-radio data system and it's 
possible, I suppose, that something similar is going on here.

The issue is the timing of the signal to turn the TX off versus the time 
required to get the last data bit actually on the air.  In the AX.25 
packet system in question, the application would unkey the transmitter 
as soon as the last payload bit was sent.  However, there was a 
downstream scrambler that caused a delay of some number of bit times. 
We had to set a parameter to keep the TX keyed for a few milliseconds 
after the end of data in order to allow all the data in the scrambler to 
make it out over the air.  In the AX.25 protocol, that was the "txtail" 
parameter.

Again, I have no idea if this applies to your situation, but it caused 
us some real head scratching until we figured out what was going on.

John