Re: [Discuss-gnuradio] [USRP] tracking the transmission/receiving process

[Martin Braun]

On 10.09.2015 12:47, Logan wrote:
> Hi Martin,
> 
> Thank you. Now I see how the interaction between the host PC and USRP works.
> Actually, i am mostly interested in the transmission/sensing to/from the
> radio space (i.e. a radio application on PC sends command to the hardware
> for transmission/sensing). To my understanding, the application has put the
> radio parameters into the property tree and messages into the queue. But how
> does the hardware (motherboard&daughterboard) know when to perform the
> transmission/sensing? I know the command is also delivered through the
> Ethernet as you've introduced. I am wondering where does the USRP somehow
> "decode" the command, and realize the intention of the application?

This depends a lot on which device you're using. On the X-Series, the
E310 and also on the B200 series, we use a packet format we call
'CVITA'. This includes flags for packet type (e.g., command vs data),
time stamps and other info.

Say you send a data packet to the X310 radio. Internally, it uses a
crossbar to route the packet to the correct destination (the radio).
There, it is identified as a data packet by its header flags. The radio
is set up to transmit any data packet it receives, so it goes straight
to the DAC from there (with some preprocessing).

On the receive side, you might send a 'transmit command' packet to the
radio. Again, it uses the header flags to identify it as a command; in
this case, the payload defines what happens (e.g.: start transmitting
now). Data packets go back to the host, etc.

You can imagine this is something we could talk about for days, so I'm
going to have to cut it off here -- but don't hesitate to come back if
you have more specific questions.

M