Re: [Discuss-gnuradio] Gigabit ethernet

[Stephane Fillod]

Hi David,

On Tue, Nov 23, 2004 at 02:32:40AM -0600, David Carr wrote:
> Has anyone looked into using gigabit ethernet is an interface to 
> software radio hardware?

What a coincidence, I just added a page to the wiki no later than this
week-end. Have a look at http://comsec.com/wiki?USRPnotUSB and let me
know what you think.

REM: btw, how to do we pronounce "USRP"?

> There are some definite benefits:
> -max data rate ~750Mb/s, the rest is lost as signaling overhead
> -can do nifty tricks like multi-casting to multiple hosts or sending 
> different streams to different hosts.  One could then throw multiple 
> machines at high-bandwidth problems
> -gigabit ethernet interfaces are becoming common and cheap
> -hardware is not that expensive (PHY ~$15)
> -can remotely locate hardware

and much more, especially with cable and tapping at the feed point.

> Downsides
> -have to implement PCI or gigabit MAC in FPGA
> -not trivial to implement (either of above)
> -latency (could be mitigated to < 1ms -- comparable to USB)
> 
> It seems like to best approach to this would be to use an external PHY 
> and implement the MAC in the FPGA.  The PHY would use the GMII or RGMII 
> interfaces.  Implementing the MAC is difficult but doable and there 
> exist things like the 10/100 opencores MAC to use as a guide.  

A simplyfied MAC can be implemented, ie. only full-duplex = no
collision, or even, single speed (=no speed negociation), which are quite
complex stuff otherwise. There are PHY that are SGMII, on top of a SerDes link.
I don't know about what's available for MAC as openIP, but I've heard
manufacturers are about to embed GigE MAC (diffused) in their FPGA.

Such a GNU Radio device would need a small flash/eeprom, at least for a
minimal boot to request through BOOTP/TFTP firmware and FPGA bitstream.
A standalone device would be better though.

> How much 
> network stack would we need?  I think that we'd use the jumbo frame size 
> and UDP.

Yes, at GigE speed, jumbo frame (<9.6KiB) has still less latency than regular
100Mbps frames. Talking about protocol, UDP is mostly recommanded and maybe
RTP which comes in handy for timestamps, etc.
FPGA wise, the UDP/RTP is just a context prefixed to the payload, no
clever logic here.


Cheers,
Stephane