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Re: [Discuss-gnuradio] USRP2 with a Switch

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] USRP2 with a Switch
Date: Tue, 23 Mar 2010 14:32:43 -0700
User-agent: Mutt/1.5.20 (2009-08-17)

On Tue, Mar 23, 2010 at 09:54:37AM -0700, ValentinG wrote:
> Dear all,
> We have 12 USRP2 boards which we would like to synchronise and run in a
> phase array system as an expansion of the 4 element system we currently have
> running using the VRT branch of GNURadio.
> Currently we use a 4 port PCIe Gb Ethernet Card to connect the USRP2 boards
> to the host computer. Our Communications group suggested that we could use
> an Ethernet switch to support 12 boards instead of trying to find a computer
> we can stuff that many Ethernet cards in to.
> Data from all 4 boards should get streamed into a data buffer on the switch
> and then be accessed sequentially by the host computer at a rate to avoid
> the over-run of the buffers. Switches work on the ethernet layer and hence
> the protocol should be compatible.
> We have read about some problems on the forums regarding using a switch for
> the USRP2's. Does anybody know if it is a realistic task to be able to use a
> switch to stream data in from multiple boards in this manner and how one
> might start in creating a system which can do this? Does a new protocol for
> sending packets to the computer need to be created. If so then where is the
> current protocol specified?
> Best regards,
> Valentin.

Are you transmitting or just receiving?

In an Rx only application, the switch will probably work out OK.
If you attempt to transmit, and the transmit data flow is not somehow
controlled on the host you'll probably have a problem with the switch.

If you read and understand the gigabit ethernet specification on pause
frames and asymmetric flow control, you'll have the background details
to reason about what you're trying to do.


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