On 06/10/2013 01:17 PM, Josh Blum wrote:
On 06/10/2013 09:43 AM, Sean Nowlan wrote:
Do late packets always get dropped by the USRP? What happens if its buffers get
filled up with samples, all of which are late?
The stream args have a policy parameter. Also, these args can be set
from a parameter in the USRP GRC blocks, as well as the constructor for
the gr-uhd blocks themselves.
This should be helpful:
http://files.ettus.com/uhd_docs/doxygen/html/structuhd_1_1stream__args__t.html#a4463f2eec2cc7ee70f84baacbb26e1ef
Ok, makes sense. So lets say I scheduled 20 minutes of bursts (1 second
period with 50/50 duty cycle) and I started the flowgraph 10 minutes
late. With the "next_burst" policy, could I rely on the USRP to
dutifully drop all late bursts? Are the packets dropped in the Ethernet
buffer or does the TX sample buffer fill up first? If it's the latter
case, my scenario implies that the TX buffer will have to be filled and
emptied multiple times until there are no more late packets, and normal
transmissions begin. This would happen at the sample rate times the
number of samples sent.
I suppose I'd also want to implement a message handler to drop the flood
of "L" symbols before they get piped to STDERR/STDOUT.
Do I have the right understanding of this process? Thanks!
--sean
P.S. -- Copying to USRP list since this could be relevant to people there.
-josh
"Marcus D. Leech" <mle...@ripnet.com> wrote:
L = late packet, there was a time on the packet which was> time on
device when
There are two different "cases" for late packets happening.
The first is that you haven't sent your packet far enough in advance to
account for latency variations on the host. Unfortunately, on a
general-purpose
OS like Windows or Linux, latency variability can be extreme, and for
long-running flow-graphs you might need to develop a good model to determine
what the worst-case is and account for that.
The second is that the clock on the USRP and the clock on the host will
tend to drift apart over time, particularly if both of them are "free
running".
So when you schedule timed bursts far enough in advance during the
start of a "session", it's entirely possible that after quite some time, the
two clocks have drifted apart unfavourably in terms of allowing you
to schedule things far enough in advance, relative to the USRP clock.
PC clocks are *terrible* by themselves. They'll drift significant
fractions of a second on a daily basis without any outside steering.
The USRP
clock, even free-running, is typically much, much better.
--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
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