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Re: [Discuss-gnuradio] about USRP2_MIN_RX_SAMPLES

From: Mattias Kjellsson
Subject: Re: [Discuss-gnuradio] about USRP2_MIN_RX_SAMPLES
Date: Tue, 15 Sep 2009 17:29:08 +0200
User-agent: Thunderbird (X11/20090817)

Ulrika Uppman wrote:
Hi all!
I ran into the constant USRP2_MIN_RX_SAMPLES = 371 defined in usrp2_base.h, but I don't get what it is used for (it comes with the comment "BIG ASS FIXME: get from lower layer MTU calculation").
Does anyone know anything about this constant and what it affects? ... what is 
depending on that it has the value 371?

(I got seg faults when trying to run less than 371 items out of a USRP2 source 
block, I believe this constant has something to do with that perhaps?)

I think it had something to do with 371items*4bytes = 1484 bytes. I think it was excluding the header, which I can't remember the exact size of on top of my head. But I have a feeling that the size of a packet was 1498 bytes or so. I have a feeling that I sent a code- snippet to Eric Blossom last summer which read/re- wrote the MTU of the NIC, but as he pointed out, the code should work on a whole bunch of platforms and systems. The code should work in most cases, since MTU=1500 is more or less standard, except some odd cards which have 1400 bytes MTU.

I think it was set to 371 in order to minimize overhead.


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