[jech]

> how does [100BASE-TX] save power vs [1000BASE-T] running at low throughput?

100BASE-TX uses just two pairs (lanes), one for sending and one for receiving. 1000BASE-T uses all four pairs, for both sending and receiving. Therefore, a 100BASE-TX interface that's only receiving needs to power up one pair. A 1000BASE-T interface needs to power all four pairs all the time.

I recall reading about some extensions that allow switching off some of the pairs some of the time ("Green Ethernet"), but I think that they require support on both sides of the link, and I'm not sure if they are widely deployed.

[userbinator]

100M also needs much less signal processing, and at lower speed, than 1G.

10M is even simpler, to the point that even a fast MCU can bit-bang it.

[rincebrain]

My only annoyance with "Green Ethernet" things is that often they seem to work poorly.

The dedicated machine I still keep around for Windows things has two onboard 2.5GbE ports. It will apparently sometimes, even with all power saving features turned off, randomly negotiate down to 100 mbit if I leave the machine alone for a bit, and then stay at that speed forever unless I manually reset the link after wondering why transferring large amounts of data is bottlenecking severely.