Re: [Discuss-gnuradio] usrp usb buffer minimum requirement

[Eric Blossom]

On Thu, Aug 27, 2009 at 12:46:13PM +0200, Mattias Kjellsson wrote:
> adib_sairi wrote:
>> Dear All,
>> As i being experimenting with USRP and GNU Radio, i notice that some of the
>> last packet of my data that i receive is keep inside the USB buffer. this
>> data will be flush out to my PC only if i transmit (receive) new data. i
>> believe that this is due to the minimum byte data needed by the USB buffer
>> (fusb_block_size - 512). so even though there is some receive packet inside
>> the USRP, it won't be send to the PC until it is a group of 512 byte of
>> data. am i correct?

Yes.

>> so at the transmitter, i should send an extra 512byte of data for each of my
>> last packet that is transmitted for one session..    
> I think there is more to it than just the usrp and usb. Today I have  
> been working on a test- case
> where I have the following flow- graph (in c++ if that might matter)
>
> file_source-> "some blocks" -> file_sink
>
> If the file- sink was not exactly 8k = 8192 bytes large the whole flow-  
> graph gave unpredictable output in terms of size of the output. Note  
> that I used "repeat = false" in the file- source.
>
> I'm not sure if it's the number of items in the source or the size of  
> it, since it in my case is the same (using a bit- file- source).

The length of the output should be reproducible from run to run.
(Assuming you let it run to completion and aren't ^C-ing it.)
If it's not, there's a bug somewhere (unless you're deliberately
introducing some random input to the system somewhere).

> Hope this helps (and that someone else has more input)
> //Mattias

Eric