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Re: [Discuss-gnuradio] Timestamps, data rates

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] Timestamps, data rates
Date: Thu, 16 Apr 2009 08:34:25 -0700

On Thu, 2009-04-16 at 15:13 +0000, Juha Vierinen wrote:

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

Yes.  Two USRP2s synced to the same 10 MHz reference and PPS will have
sampling times accurate to the PPS arrival time, and will have precisely
100M samples per second.

> Is there any other reason for not just putting the USRP2 in your LAN?

Aside from bringing your network to its knees, the USRP2 receive side
does not implement any sort of flow control, and has a fairly small
packet buffer.  So, especially at higher data rates, it is possible for
side traffic to cause missing packets at the host PC.

Similar issues exist on the transmit side.

That being said, the USRP2 will otherwise work just fine sitting on a
switch and not being directly connected to the host.  In fact, it is
possible to use it at low data rates with a host PC that does not have
GbE ports by plugging both the USRP2 and the PC into a switch that does
100/1000 auto-sense.

Finally, the USRP2 uses Ethernet directly and does not use IP, so it
must be in the same layer 2 broadcast domain as the host PC.

So it is possible, but not recommended.


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