[Discuss-gnuradio] reducing the latency in tunnel.py

[Jane Chen]

Hi all,

I would like to send a MAC layer control packet every 4 ms from computer A to computer B to tell B when to send data using USRP and GNU Radio.  It is like TDMA and a frame is 4 ms. I am trying to do this work by modifying tunnel.py in the gnuradio-example folder. I removed the carry sensing part and added the control packet in the tunnel.py.

I enabled the real-time scheduling. I set nice= -18, fusb_block_size = 512, fusb_nblocks  = 2, modulation=gmsk, and bitrate =1Mb/sec.
 
OS: 2.6.29.6-217.2.7.fc11.i586
CPU: 2.8 GHz
Gnuradio: (latest development code 3.2.X)

I used two computers and ran the modified tunnel.py on each computer. For the test, in the main_loop (main loop for MAC) of tunnel.py I generated one byte payload ( I sent the one byte payload chr(0x00) for 10 times)  to print the tx time on one computer and rx time on anther computer. I used software to make two computers have the same system time. The tx time (at computer A) – rx time (at computer B) of the payload is about 5ms. I am wondering why it takes so long to send one byte payload and to decode the one byte payload.  Through the test, I think I cannot implement a TDMA that a frame is 4 ms. I have tried what I can do to reduce the (tx time – rx time) value. However, no luck. Could anyone please give me some hints? Should I just give up this way and try to use the in-band singalling? Any suggestions will be greatly appreciated!!


Best.
Jane