[Discuss-gnuradio] Latency related results

[Per Zetterberg]

Dear All,

I would like to share the following results in case someone is interested.
It is related to USB latency, linux latency and whatever. It is an attempt
to "emulate" a TDMA system. 

I have connected a square-wave to one of the pins of the J101 connector of
two USRPs each equipped with an RFX1800. The two USRPs are polling the pins
for a change of polarity. When this happens one of them starts to transmit a
frame (a frame consists of 40 USRP buffers)  from the USRP and the other is
receiving the data. In fact, the direction is changed in every burst.  The
length of the useful part of the burst and is 4.4ms and the square-wave has
a period of 6*2=12ms (i.e. each polarity lasts for 6ms). 

The figure http://www.ee.kth.se/~perz/usrp/fig1.eps shows a histogram of the
position of the useful burst in the received frame. The variation is about
0.5ms. 

Does anyone know the cause of the variation ? Or any means of reducing it ?

BR/
Per