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Re: [Discuss-gnuradio] Gigabit ethernet

From: John Gilmore
Subject: Re: [Discuss-gnuradio] Gigabit ethernet
Date: Tue, 23 Nov 2004 01:13:26 -0800

> -max data rate ~750Mb/s, the rest is lost as signaling overhead

Actually it's better than that.  With 1500 byte packet sizes the
Ethernet overhead is only a few percent, so well driven hardware runs
well over 950 Mbit/sec.  IP and UDP would eat a bit more of that,
but it's only 28 out of 1500 bytes (another 2%).

GigE always runs full duplex, which is a big advantage over USB2.  You
get the same bandwidth simultaneously in each direction (2 Gbit/sec
total) and they don't interfere with each other on the wire.

I'd skip using jumbo frames, since they're nonstandard and might not
work through various switches or networks.  See:


> -gigabit ethernet interfaces are becoming common and cheap

True.  But they don't exist on very many laptops, and it would be a
lot more work to make it ramp down to do the 100 Mbit Ethernet PHY
when plugged into one.  (The USRP still won't run on USB1, only USB2;
Eric says it's too much work and he wants to clean that up later, or
preferably never.)  I've only seen GigE on Apple laptops so far,
though I haven't looked very hard.

> -can remotely locate hardware

Well, if you have a gigabit fiber between the hardware and the PC...
or a 100m coax cable.  But indeed 100m is further than USB cables go.

We did look at GigE.  I think the big kicker was (1) we didn't expect
GigE on laptops as soon as USB2 on laptops, and (2) a very handy
PHY/controller chip was available for USB2.

But feel free to start hacking on one yourself, it would be useful!
Make it take the same daughterboards as the USRP...


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