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Re: [Discuss-gnuradio] Timestamps, data rates

From: Juha Vierinen
Subject: Re: [Discuss-gnuradio] Timestamps, data rates
Date: Thu, 16 Apr 2009 15:13:51 +0000

> I wouldn't expect the time stamps in the streams to start at the same
> point (the start_rx_streaming command doesn't accept a "start at
> time T argument).  I would expect that there is a constant delta_t
> between subsequent frames, and that that delta_t would be the same for
> both of the USRP2s.

How accurate is sync_to_pps? Can you do MIMO with just sync_to_pps
(assuming your cables are equal length)?

Off topic: We have been playing around with my USRP2 and it seems very
nice. I noticed that even though the FAQ strongly discourages
connecting the USRP2 to your main network, it does actually work. We
have a fairly large LAN and I assume that the USRP2 could possibly
flood the network when doing 25 MHz. Is there any other reason for not
just putting the USRP2 in your LAN?


> At -d 4 you are likely to be getting overruns (indicated by an 'S'
> (sequence error) on stderr).  That is, the host can't keep up.
> Are both USRP2's connected to the same host?
> If so, are they each connected to their own dedicated ethernet port,
> or are they connected to a switch that feeds a single ethernet port?
> If both USRP2s are connected to a single machine, and even if they are
> connected to dedicated ethernet ports, I'd be pleasantly surprised if
> you could run -d 8 on both of them.  I doubt the problem is in the
> USRP2 firmware; the overhead of the host code and lack of sufficient
> horsepower on the host is almost certainly the bottleneck.
>>  My plan has been to setup buffers to align samples based on timestamp -
>>  but with such variations it looks to me like it could be quite
>> difficult to implement. Or at the very least, I'd need very large
>> buffers, and could end up needing/wanting to change their size often.
>>  Any suggestions on how to best deal with this? Should I be looking into
>> the USRP2 firmware code to try to get some more predictable behavior out
>> of it?
>>  Thanks,
>>   Doug
> Doug, assuming that the host can keep up without overruns, aligning
> samples from the two USRP2s should be no big deal.  I wouldn't expect
> that you need more than 200 ms of buffer.  If the host isn't keeping
> up, all bets are off.
> I'd start with something like -d 64 of each of them, get that working,
> then start reducing the decimation rate until you get failures.  Then
> go after those.
> Eric
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