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

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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: vdvworks.com/LennieLw/fiber.html – smithian May 17 at 15:44
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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. – Dwobin May 27 at 7:08
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There is excellent NANOG presentation (nanog.org/meetings/nanog57/presentations/Monday/…) on the subject. – ytti May 31 at 18:12

9 Answers

up vote 21 down vote accepted

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 more expensive.

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.

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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? – mdpc May 16 at 20:07
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. – netdad May 16 at 20:12
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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. – Ricky Beam May 16 at 22:26
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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 :) – netdad May 17 at 18:04
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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. – ytti May 31 at 18:11
up vote 20 down vote

Multi-mode fiber (MMF) uses a much bigger core and usually uses a longer wavelength of light. Because of this, the optics used in MMF have a higher capability to gather light from the laser. In practical terms, this means the optics are cheaper.

Single-mode fiber (SMF) has much tighter tolerances for optics used. The core is smaller and the laser wavelength is narrower. This means that SMF has the capability for higher bandwidth and much longer distances in transmission.

With 10GB just around the corner for many of my customers, I've started recommending they use SMF everywhere for connections. When installed as part of a project, the extra cost of SMF is negligible compared to MMF. That also means you won't need to rip out your MMF fiber plant to upgrade speeds in a few years.

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Hmm....so theoretical bandwidth on SMF can be much higher than MMF? – mdpc May 16 at 20:00
Oh interesting, I didn't realize that's why most of the 10G Fiber stuff I see is SMF. Definitely makes sense. – Mat Wood May 16 at 20:11
While the cost difference in cabling may be minimal, that's not so true when it comes to the optics. MM SFP's are fairly cheap, while SM is quite a bit more. – Ricky Beam May 16 at 22:20
@mdpc Not necessarily, (this is really a question for the physical stack exchange): Once you have a basic grasp of physics and en.wikipedia.org/wiki/Total_internal_reflection you can then forget it :) This is only good for a basic understand, how the light is truely behaving is rather counter intuitive. Look at waveguide rp-photonics.com/waveguides.html It's more than I can write in this comments box, but the propagation of high and lower order modes effects the modal group rp-photonics.com/group_velocity.html and even in single mode this can have an effect – javano May 17 at 17:54
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@javano, The use of "therefore" is wrong, but it's also it's simply wrong to imply that as a general rule MMF is used with longer wavelengths. While 1310 or 1550 can in principle be used in MMF, and in a few specific applications they are used, as a general rule 850 nm is much more commonly used, and that is a shorter, not a longer one. Furthermore, 850 nm is practically never used with 9 um SMF, because 1) 9 um is not small enough to be a single-mode waveguide for 850 nm, and 2) attenuation losses are lower for 1310 and 1550. – The Photon May 19 at 15:30
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up vote 5 down vote

Singlemode fiber has a smaller core (9 micron), resulting in less light diffraction over distance than multimode fiber (50, 62.5) micron.

The fragility and increased cost to produce singlemode fiber makes it more expensive to use, which is why multimode is typically used when you don't need the distance of singlemode.

Multimode generally has a reach upto ~550 meters, where as singlemode has the potential to reach 10,000 meters (40,000 meters with ER)

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Can SMF and MMF cables be interchangably used within a datacenter when the distance is not the issue? – mdpc May 16 at 20:01
Each complete link (transceiver on both end + fiber connection) has to be the same tech. Transceivers exist for either MMF or SMF and always have to match the fiber. – Mat Wood May 16 at 20:10
Ahhh....so the transceivers are specifically designed to use either MMF or SMF cabling. – mdpc May 16 at 20:12
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Yes, you'll typically see the following letters in transceiver part numbers: LX/LR (Long Haul) == SMF; SX/SR (Short Haul) = MMF – Mat Wood May 16 at 20:33
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Yes and no... you can fire a SM laser into a MM fiber and it may work. (see also: "mode conditioning cable" -- it ensures the laser is fired into the exact center of the fiber) However, a MM laser fired into a SM fiber will not work at all; there won't be enough signal through the very tiny fiber. The optics have to be the same at both ends, of course. – Ricky Beam May 16 at 22:30
up vote 5 down vote

[MMF] longer wavelength (850nm), much wider beam vs. [SMF] short wavelength (1310nm-15??nm), narrow beam.

The key difference that no one has touched is "modal dispersion", which is a fancy term to describe how the light moves through the fiber. This page goes into far more detail. The first picure sums it up... MM is bouncing off the edge of the fiber resulting in a dispersion of light at the far end because the photons that went down the center traveled less distance than those that bounced a lot; a 1ns pulse entering one end does not exit as a clean 1ns pulse at the other end. The effect is greatly diminished in SMF due to the much smaller diameter fiber, and precise injection ("launch") of the laser at the center of the fiber. Dispersion gets worse as distance increases.

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up vote 4 down vote

The key difference between SMF and MMF? distance and cost

Single Mode Fibre has a greater distance potential, and can support runs between 2 meters and 10,000 meters. However, the optics are twice the cost of MMF optics.

One thing to consider with Multimode fiber is the grade of Fiber that is in place. I have spoken to many customers who laid OM1 fiber in the Mid-90's, and now want 10GbE runs over same fiber, and very frustrated to learn that this fiber will only support 10GbE out to 26m.

Gigabit SX-LC Mini-GBIC provides a full-duplex Gigabit solution up to 550 meters over multimode fiber.

  • 2-220 m (62.5 µm core diameter / 160 MHz*km bandwidth)
  • 2-275 m (62.5 µm core diameter / 200 MHz*km bandwidth)
  • 2-500 m (50 µm core diameter / 400 MHz*km bandwidth)
  • 2-550 m (50 µm core diameter / 500 MHz*km bandwidth)

Gigabit LX-LC Mini-GBIC provides a full-duplex Gigabit solution up to 10 km over singlemode fiber, or up to 550 meters over multimode fiber.

  • 2-550 m (multimode 62.5 µm core diameter / 500 MHz*km bandwidth)
  • 2-550 m (multimode 50 µm core diameter / 400 MHz*km bandwidth)
  • 2-550 m (multimode 50 µm core diameter / 500 MHz*km bandwidth)
  • 2-10,000 m (singlemode fiber)

10-GbE SFP+ Short Range supports the 10-Gb SR standard, providing 10-Gb connectivity up to 300 meters over multimode fiber.

  • 2-26 meters (62.5 µm core diameter / 160 MHz*km bandwidth)
  • 2-33 meters (62.5 µm core diameter / 200 MHz*km bandwidth)
  • 2-66 meters (50 µm core diameter / 400 MHz*km bandwidth)
  • 2-82 meters (50 µm core diameter / 500 MHz*km bandwidth)
  • 2-300 meters (50 µm core diameter / 2000 MHz*km bandwidth)

10-GbE SFP+ Long Range supports the 10-Gb LR standard, providing 10-Gb connectivity up to 10 km over singlemode fiber.

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You imply SM can at maximum do 10km, there are optics for +160km, and with pre-amp and proper dispersion compensation you can lot lot more. The price quote is not entirely correct either, in 1GE both optics are <10USD however, 10G SR is 40USD and LR is 100USD, but as time passes the price difference will be reduced. – ytti May 31 at 18:16
@ytti fair points raised. Not every vendor supports those distances, and sometimes its a case of "may work, not supported". Re Price. I'd like to know where you can get an optic for less than $10USD . LIST price is closer to $400 for 1GE SX, and near $800 for 1GE LX. So there is a difference. I'd also hope over time that optics get a lot closer to each other. – Jez Jun 2 at 10:10
Pretty much any major Chinese vendor will do those numbers for you. Gigalight and XGiga spring to mind. Sold by EU or US resellers prices goes to 20-25USD, because we have to cover the labor fee of the US/EU personnel handling the cheap material. – ytti Jun 2 at 10:12
up vote 4 down vote

Here's a few things not yet covered.

  • Using only single mode lets you use one fibre type for everything (fewer optics, cable types to spare)
  • No likelihood of obsolescence (eg, OM3 seemed like it was current for ~3 years)
  • Offers single-pair options for 40 & 100g
  • Offers single-fibre options at 1 & 10g
  • Although the optics are more expensive they're < 2x while offering much longer reach (remember to buy OEM optics, not rebadged)

Of course on the long haul side there are DWDM systems now sending 16Tb through a single, single mode, fibre (in practice as we're always bi-directional that's a 16Tb pair), and by the time you need multiple links beyond a "campus" distance DWDM systems (for < 40kM fibre length CWDM might) make financial sense, with optical amps and dispersion management there are many multi-terabit submarine cable systems running purely optically regenerated for over 10,000kM.

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+1 Finally someone mentioned WDM systems! – javano May 17 at 17:44
up vote 2 down vote

You can reference cable spec for each module. This one is from Cisco SFP+ 10G spec and its support cabling. http://www.cisco.com/en/US/prod/collateral/modules/ps5455/data_sheet_c78-455693.html

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up vote 0 down vote

I've successfully used MMF cable on a 1000Base LX/LH SFP, you need a mode conditioning patch cord. This was an ugly temp fix, once we got the correct SFP everything was back to normal.

Mode conditioning patch cord

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