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:

  1. 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?

  2. 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.)

  3. 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.)

2 Answers 2


To elaborate on Ron's answer:

  1. What you have is Bi-directional fiber modules, and they use one wave-length for TX and one for RX. The downside is that they come in pairs of upstream and downstream modules, since the wavelengths need to be switched in the downstream module. This is a downside since you now need to stock 2 different spare parts instead of one. Upside is that they use half the amount of fiber strands. They can also be quite costly, off-brand they are usually 3-5 times more expensive than their regular two-strand counterparts.

  2. I've never seen or heard of a BiDi module for MMF. If you need more capacity, I'd say invest in new SMF fiber runs instead of esoteric modules.

  3. Check the mantle of the MMF cable, it'll usually tell you how old the cable is. If it's older than about 2005, I'd say they pulled MMF to save money. Or they just spec'd it because of old habits.

About CWDM: CWDM uses optics to merge and split apart (mux and de-mux) several different wavelengths. This way, you can cram 8 signals into the same pair of fiber, and have them enter and exit anywhere along the whole length of a fiber pair by installing add-drop multiplexers. The modules for CWDM has a narrower bandwidth than normal modules to allow for more signals, and they come in 8 different wavelength versions. CWDM saves you a lot when you need a lot of bandwidth and don't want to buy a lot of dark fiber, but it's quite costly when it comes to the muxers and all the different modules you need to stock as spares.

  1. No, that is not the same as CWDM.
  2. Most fiber is connected with separate Tx and Rx fibers. There is nothing unusual about the MMF connections.
  3. MMF used to be cheaper, the fiber transceivers still are, and it was pretty common to install it in a building and SMF between buildings.

You may find yourself limited by what you can do with OM1 in the future. As you get faster connections (think 10 Gbps) you will find the distance is limited (~33 meters/100 feet at 10 Gbps for OM1).

  • Other than no one making MM simplex transceivers, there's no technical reason it can't be done.
    – Ricky
    Feb 19, 2016 at 21:12
  • I actually priced both MMF and SMF transceivers yesterday, and off-brand they are identical in price.
    – Stuggi
    Nov 19, 2018 at 12:38

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