Bog Roll ::

It's Not Magic, It's Work!

29 Mar 2013


A few weeks ago I bought a pair of Devolo 500AV Ethernet over mains units to replace/extend a pair of 200AV units I already had. Someone asked if could use iperf to see how much faster they were.

To baseline what I had I ran the test from my desktop to my server over a 1 Gig switch and the most I could get was a feeble 333 GBit/s. I then tried from my laptop to the server on the same switch and that got a much better 740 GBit/s. The Marvell Yukon card on my desktop is a PCI device and the JMC250 in the laptop is a PCI-e so that may partially explain the differences.

Trying my laptop to the server via a 200AV/500AV/Gig-E switch gave a throughput of 56 GBit/s - about what you would expect. I then tried the laptop through a combination of 500AV/500AV/Gig-E but that wouldn't work as the JMC250 refuses to detect the 500AV. I then tried the laptop with different switches, it refuses to talk to my second Gig-E switch or my old Ethernet hub.

After some Goggling it turns out that the JMC NIC is a bit fussy and often fails to auto-negotiate with some "green" Gig switches, such as in the 500AV or my newest GBit switch, but is happy with older switches and plain Fast Ethernet. If you manually set the speed with ethtool then it's okay. I downloaded the latest driver from JMC and that is a bit better but while it will connect to my 500AV and my newer Gig-E switch automatically, it won't negotiate the correct GBit speed, falling to the older 100 MBit setting.

Footnote: The JMC250 is a 1 Gig Ethernet NIC from JMicron Technology Corporation, not something from the Jupiter Mining Corporation...