[Discuss-gnuradio] USRP2 xmit buffering

[Sharif Shaher]

Hello,

I am using a USRP2 to transmit signals and I am interested in knowing
if anyone has any idea as to how the buffering is handled as it relates
to the call to u2->tx_16sc....when I call this function it usually 
returns within
a couple of microseconds.  This is much quicker then the time required to
actually transmit the data.  For example, using an interpolation rate of 
16 yields
a transmit time for a buffer of size 371 samples of about 59.3 usec.  Can
someone explain where the data is stored during the xmit.  Also, would
someone be able to estimate how long after this call is made does the first
sample actually get transmitted?  I am trying to understand the 
variability (in
the USRP2 or if the buffering is done on the PC then on the PC) as it 
relates
to transmitting and buffering in general.  If the data is stored on the 
usrp2, how
much storage is available or how many calls to tx_16sc can I make before the
buffer is full?

Also, related, if instead I specify the metadata-md to transmit at a 
known time
(meta data or md is the last argument to tx_16sc), where is the data stored
after I make this call and it returns.   If on the usrp2, how much 
storage is
available?

Any comments greatly appreciated.

Thanks,
Sharif