Re: [Discuss-gnuradio] How is it possible that I receive a packet from the same USRP that transmitted it?

[Marcus D. Leech]

On 06/01/2011 03:41 PM, Thomas Arcuri wrote:
> Hello all,
>
> Currently I am testing a modified version of tunnel.py, but I am 
> seeing some strange results. My modifications were essentially a 
> re-factoring to prepare for future additions, however somehow I was 
> able to receive a transmitted packet on the same USRP that transmitted it.
>
> The USRP transmit/receive paths are set-up exactly the same as in the 
> original tunnel.py, the only difference is that I abstracted the 
> tun/tap layer a bit (in order to improve the MAC layer functionality). 
> I did not change the cs_mac loop and I am only using a single USRP 
> with the modified tunnel to flood PING requests. I am using only a 
> single antenna on a WBX d'board.
>
> I know that I am receiving a transmitted packet because in 
> phy_rx_callback() I am using scapy to dump the packet and inspect its 
> contents. This doesn't happen consistently - I am flooding out PING 
> requests but only a relative few are "looped back" (for lack of a 
> better word).
>
> Now, can anyone explain how this could possibly happen?
>
> Thanks,
> Tom
Put an antenna on the RX port, and you'll get much more consistent 
results--that is, you'll receive *ALL* your packets.  Even without an 
antenna,
  there's enough TX energy coupled into the RX port to receive data, 
but not reliably.  The receiver doesn't shut-down during transmit.
  If you want that behaviour, you have to arrange for it yourself, but 
muting the RX side when you transmit, or similar.





--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org