Multi-Mode and Single-Mode Fiber Differences

  • When cabling a network using fibre, what is the difference between single-mode and multi-mode fibre? When should I be using one or the other? Are there compatibility and/or speed concerns with either?

    I had to downvote this question because it shows no research effort. There are literally TONS of first quality reference material on this subject out there. Here's just one:

    One could argue that it's still an interesting question (and answer) to have on this website. Sure, collectively all the answers are on Internet, but the idea is to have them well organised on this website.

  • netdad

    netdad Correct answer

    9 years ago

    Main difference: Singlemode fiber has a lower power loss characteristic than multimode fiber, which means light can travel longer distances through it than it can through multimode fiber. Not surprising, the optics required to drive singlemode fiber are way more expensive, especially considering any varying circumstances.

    When to use each: Both singlemode and modern multimode fiber can handle 10G speeds. The most important thing to consider is the distance requirement. Within a data center, it's typical to use multimode which can get you 300-400 meters. If you have very long runs or are connecting over longer distance, single mode can get you 10km, 40km, 80km, and even farther - you just need to use the appropriate optic for the distance required, and again, the prices go up accordingly.

    Compatibility issues: They are not compatible. You cannot mix multimode and singlemode fiber between two endpoints. The optics are not compatible either.

    There's a lot more to say about fiber, in general, but I hope this answers your immediate question.

    Can you make one port-to-port connection with one multimode cable and another with a singlemode cable assumming distance is not a problem using the same switch-to-switch situation?

    Yes you can do that, and it's typical. All that matters is that the optics on each end of the cable are of the same type.

    Re: compatibility... it's not *supposed* to work, and on paper it never should, but I have seen -- with my own eyes -- an LX (SM) interface link to an SX (MM) interface over a 62.5MM cable! I know... a 1310nm optic linked to a 850nm optic. WTF.

    Yes, I wanted to stay away from discussing that, but you're right, that this can work. The reason for it is that the receivers tend to be wide-spectrum, meaning they can detect light from a much wider spectrum than the sending optics generate. However - I would never recommend trying it unless you really knew exactly what you were doing :)

    Running LR/LX optic on MM fibre will work pretty much as well as SR optic on MM fibre. But running SR/SX on SM fibre will not work, at all, because the light isn't really even entering the fibre in apperciable quantity.

    It's not really about attenuation so much as dispersion. Multimode lets through plenty of light but it smears it in time.

    Sorry, have to downvote - as Peter Green pointed out the main difference is _not_ attenuation but dispersion. Attenuation of MM is somewhat higher than SM but only because the reach is very limited anyway and it doesn't matter. Additionally, you _can_ mix MM and SM under specific circumstances using mode-conditioning patch cords.

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Content dated before 7/24/2021 11:53 AM