Ethernet up to 100Mbps uses only 2 of the 4 twisted wires in an ethernet cable while gigabit is using all of them. Connecting an LED to one of the gigabit-only pairs should therefore make it easy to detect a gigabit transmission.
Well, actually not. Problem is, that PoE may or may not be used, the ethernet data is AC while an "this is gigabit"-led is DC, ethernet has no fixed current and I have no clue what I'm talking about.
I really ain't versed in electronics. A rectifier between ethernet and the LED would solve the AC problem, but this can't be the whole solution. Ethernet has no fixed voltage as described in here. Is there a module that counters fluctuation in a way that the LED gets the power it needs? Even with the low energy from a ethernet cable? If yes, would this work with PoE injection on the unused wirepairs as well?
Background: An intrusion detection system based on Microsofts ATA has to have a physical connection to the network so I've built a network tap with a maximum throughput of 100Mbps. I forward only two of the wirepairs, forcing any network clients to use a maximum of 100Mbps. This may obviously cause a giant bottleneck, but while not preventable, I would at least like to know when this is a problem. A litte status LED would be nice, flickering upon gigabit data connection.
First post and glad about criticism. Thanks