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[Discuss-gnuradio] Several basic questions

From: Teun
Subject: [Discuss-gnuradio] Several basic questions
Date: Mon, 11 Jun 2007 00:54:11 -0700 (PDT)

Hi Guys,

I tried searching the forum, but I still got some problems which I don't
fully understand.

To my knowledge, the receiving ADC Rate is 64 MSamples/second, at a
granuality of 14 bits/sample. This would make it possible to have a total
system bandwith of 32 Mhz (Nyquist), which is enough for example 802.11
standards. However, samples are transfered to the PC, using the USB bus. The
usual I/O handling of the USB bus is 16 bits I and 16 bits Q signals, which
means that you require 4 bytes per complex sample. The raw data rate for the
USB is 480Mbit/s, but due to overhead it is possible to get only about
320Mbit/s, which is about 40Mbyte/s. Summing everything up, it is possible
to transfer 40M/4 = 10Msamples/second over the USB bus, which means that you
can only check a system with a bandwith of approximately 10Mhz. Am I right
about this? If this is true, the limiting factor is the USB bus, not the
ADC's on the USRP. Is it possible to use some sort of 1Gbit ethernet
connection for the USRP? I know this wouldn't help that much, but It would
make it possible to achieve the 802.11 standards (Channels of 20Mhz). Is
this possible or are other things like CPU speed/PC memory becoming an

Another question (I'm pretty new to GNU Radio / USRP). Within our group we
are looking for a testbed to test MESH protocols (MAC level). We are
especially interested in congestion control and throughput measurements
(Hopping between nodes). Therefor a basic, maybe stupid, question. If I'm
creating some interleaving node (A node which is just receiving messages and
forwarding them), what is the minimal time for the USRP to switch between
receiving mode and transmitting mode? (RFX2400) Couldn't find anything about
this in documentation.

If anyone could help clearify these things, that would be great.



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