# Modulation packet loss

**URL:** <https://www.ruby-forum.com/t/modulation-packet-loss/227326>\
**Category:** GNU Radio\
**Created:** [March 11, 2013, 4:47pm UTC](https://www.ruby-forum.com/t/modulation-packet-loss/227326 "2013-03-11T16:47:46Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![cghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/898d66/32.png) [@cghost](https://www.ruby-forum.com/u/cghost)\
**Post date:** [March 11, 2013, 4:47pm UTC](https://www.ruby-forum.com/t/modulation-packet-loss/227326/1 "2013-03-11T16:47:46Z")

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Hi,

When doing any type of modulation, I’m losing the last packet or two.  
I’m  
wondering if it’s possible to either get rid of packetization all  
together  
or to output the piece of the last packet received even though it  
doesn’t  
get filled completely. Inside of packet.py, it seems to be stuck in  
here:  
msg = self.\_msgq.delete\_head()

and doesn’t get to call

ok, payload = packet\_utils.unmake\_packet(msg.to\_string(),  
int(msg.arg1()))

Another issue I’ve been having that’s less important is whenever I’m  
using  
it with USRP’s from one computer to another, the beginning chunk of data  
gets lost. This may have to do with some sort of synchronization but I  
would have no clue where to start with that.

Thanks, Patrick

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<div class="post-metadata">

**Author:** ![cghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/898d66/32.png) [@cghost](https://www.ruby-forum.com/u/cghost)\
**Post date:** [March 18, 2013, 2:31pm UTC](https://www.ruby-forum.com/t/modulation-packet-loss/227326/2 "2013-03-18T14:31:57Z")

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On Mon, Mar 11, 2013 at 11:46 AM, Patrick C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> ok, payload = packet\_utils.unmake\_packet(msg.to\_string(), int(msg.arg1()))
> 
> Another issue I’ve been having that’s less important is whenever I’m using  
> it with USRP’s from one computer to another, the beginning chunk of data  
> gets lost. This may have to do with some sort of synchronization but I would  
> have no clue where to start with that.
> 
> Thanks, Patrick

The packetization adds a number of features. You get the access code  
and header to help acquire your data at the receiver, a CRC check to  
see if the packet was received without errors, and other features like  
whitening of the data for better transmit and receive properties.

You could do without the packetizing, though, and just continuously  
transmit a stream of samples if that’s all you need to do. But its  
likely that you’ll need some kind of header to determine starts of  
packets or frames or something like that.

Also, be aware that there is no channel coding in a lot of the digital  
examples. That means that if any bit is incorrect at the receiver, the  
CRC check will fail and the packet will be marked as bad. You would  
want to add some kind of channel coding into your system to help with  
this.

Tom

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<div class="post-metadata">

**Author:** ![cghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/898d66/32.png) [@cghost](https://www.ruby-forum.com/u/cghost)\
**Post date:** [March 20, 2013, 12:08am UTC](https://www.ruby-forum.com/t/modulation-packet-loss/227326/3 "2013-03-20T00:08:30Z")

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I am actually trying to work with the OFDM block and it seems like  
packetization is kind of integrated into the OFDM module. Can you  
confirm  
this? And do you know where I could find channel coding/FEC examples?

Thanks,  
Pat

---

<div class="post-metadata">

**Author:** ![cghost](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/c/898d66/32.png) [@cghost](https://www.ruby-forum.com/u/cghost)\
**Post date:** [March 20, 2013, 2:11pm UTC](https://www.ruby-forum.com/t/modulation-packet-loss/227326/4 "2013-03-20T14:11:39Z")

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On Tue, Mar 19, 2013 at 7:07 PM, Patrick C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> I am actually trying to work with the OFDM block and it seems like  
> packetization is kind of integrated into the OFDM module. Can you confirm  
> this? And do you know where I could find channel coding/FEC examples?
> 
> Thanks,  
> Pat

They are fairly closely tied together, but you should be able to  
separate them.

What version of GNU Radio are you using? If you track the master  
branch on git, we recently updated the OFDM transmitter  
implementation. Martin B. and Ben R. have done a lot of work  
to restructure the code, which might help with what you are  
attempting.

Tom