Ha! As I remember, each end had an independent clock and a little bit of buffer to smooth out any variance. And it mostly worked!

The word "plesiochronous" is burned into my brain as a result of the few times that it didn't, though...

My career began in the late 90s telecoms boom, which was a gloriously chaotic period in retrospect. It seemed so very obvious at the time that "ethernet for everything" was what we should be working towards, but the legacy telcos didn't arrive at the same conclusion until well after the crash - so there were lots of opportunities for smaller players to undercut them.

"Never bet against Ethernet." Bob Metcalf, probably