We have a few fiber optic connections between our buildings, and admittedly we haven't a bloody clue about them (other than "all buildings have SMF, except one that has MMF because reasons, and we're not 100% sure it's actually MMF because $BOSS[-1] was too smart for his own good and left exactly zero documentation"). Yeah, it's that kind of place.
So even after reading the NANOG "Fiber 101" .pdf I still remain confused about several things:
The SMF connections use a single bidirectional fiber, and have transceivers with "1310 / 1550 nm" written on them – is that the same thing as CWDM?
The MMF connections are apparently OM1, and use two separate fibers per connection.
So a) generally, does MMF hardware always use separate Rx & Tx fibers, or can a single bidirectional fiber be used with MMF?
And b) if that's possible, can our old OM1 fibers be used as four bidirectional links instead of two paired unidirectional ones? (Merely curious.)
What are the possible reasons MMF would have been used for this particular link? (Our guess is "it's only 300 meters away so MMF was cheaper"... no, we can't call and ask.)