I'm studying the history and technical details about fiber optics and I'm having trouble understanding which are the different layers that exist and have been used over fiber optics.
For example, I see that DWDM is used to increase bandwidth by allowing different wavelengths to coexist in the same light or laser beam. SONET/SDH is the evolution of PDH and Ethernet is also used because of its higher bandwidth. However, I read different sources and while some seem to suggest (or that's what I understand) that SONET and Ethernet were "rivals" and thus they work separately, others show layer diagrams in which Gigabit Ethernet is the layer over SONET/SDH. Also, from what I understand, DWDM works at a lower level than the other two, but often I see comparisons between the three as if they worked in the same layer, especially between SONET and DWDM.
Some links that caused confusion:
http://ecomputernotes.com/computernetworkingnotes/computer-network/physical-layer-interfaces (here, where is WDM used? I see that ehernet can work either over SONET/SDH or stright over fiber, but WDM techniques seem to be the "lower layer" (so I understand the other protocols are not its competitors)
https://community.fs.com/blog/the-relation-and-difference-between-sonetsdh-and-dwdm.html (however here are being compared as if they could work separately)
My questions aiming to clarify my confusion are:
1- Can and does GbEthernet work over SONET/SDH?
2- When GbEthernet is used, is it over DWDM or it has nothing to do with it?
3- Is SONET/SDH DWDM's competitor or do they work together but at different layers?
4- Can these protocols work completely independently? If so, is GbEthernet working directly over fiber?
5- What's it today's most common implementation?