Re: [Discuss-gnuradio] MAC layer development and USRP2

[Jeff Brower]

Charles-

>> I would tend to blame Linux and buffering more than GbE itself (MAC + PHY).  
>> Here is an interesting doc where the
>> researchers were asking similar questions:
>>
>>  http://www.hep.man.ac.uk/u/rich/atlas/docs/atlas_net_note_draft5.pdf
>>
>> I'm not sure yet how much buffering is done in the USRP2 firmware but we 
>> hope to know shortly as a couple of our
>> guys
>> are in the process of taking apart the logic, pulling out non-GbE related 
>> sections, and rebuilding.
>>
>> -Jeff
>
> I glanced over the document briefly and was wondering if your analysis
> of the linux issue was because of this document, or a separate source.
> I'm only asking because the document is 10 years old and is using
> RedHat 5 and Pentium 2s. I would assume the linux kernel support for
> GigE has improved since then.

Which part of the Linux issue... sustained throughput or latency?  I wouldn't 
be surprised to find that latency hasn't
improved substantially because it's not a priority for server software.  Even 
VoIP applications are not concerned
about a 1 msec improvement... whereas that makes or breaks a wireless MAC.

What I found interesting in that particular document is the authors were 
careful not to speculate and to use a logic
analyzer to make exact measurements.  For me the key figures are GbE (MAC + 
PHY) and PCI latencies, which are likely
not too reducible.

-Jeff