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?
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.
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.
[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 picture 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.
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.
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.
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)
Singlemode fiber often costs less than multimode fiber. When building a 1Gb fiber network that I wanted to be able to go to 10Gb or faster on eventually, I found that the savings on cost of fiber (about half-price) for singlemode .vs. OM3 or OM4 multimode more than covered the (35% or so with adequate shopping) increase in cost for (brand-new) SFPs at that time - and did not limit my link lengths as OM3 or OM4 did. Nor is it limited to 10Gb. I also gained a huge loss budget, since a singlemode LX set is designed to go 10Km (but also works fine at 5m.)
Conventional wisdom has not caught up with that particular reality. If you are willing to look at used ex-fiberchannel SFPs (I was, but I didn't use them in the economic justification phase), the price of singlemode 1Gbe drops through the floor.
My preference is to ONLY use singlemode (and, inside buildings, 5e copper) at this point in time. I'll be doing a 25m singlemode fiber sometime soon, becasue it's a between-buildings connection, and I have been eliminating copper between building connections and having a lot less trouble with thunderstorms as a direct result of that. Under different constraints, I would probably consider using OM3/4 multimode for short, easily replaced runs inside buildings or within a server room; under the actual constraints I have to operate in (low budget) 10Gbe is still some time in the future - but I can go there on the cabling I have, when I can afford to. In my opinion, any multimode fiber should be considered as a short-term purchase with the progression of data rates. Just ask anyone with a 62.5 multimode plant (100Mbit, 2Km, IIRC) in place - the singlemode that could have been installed at the same time still works at today's data rates. OM1 is obsolete. OM2 is obsolete. OM3 is darn near obsolete.
If you have budget for and need for 10Gb short connections, fiber appears preferable to 10Gb copper, and the economics at last check still supported multimode. Keep an eye on those economics though, as history suggests that the price premium for singlemode will drop.
Multimode fiber has a larger core, this makes the tolerances easier for termination and makes the transciever hardware cheaper but it allows multiple modes of propagation resulting in a time-smearing of the received signal. There are tricks to reduce this time-smearing resulting in a proliferation of different grades of multimode fiber.
So the traditional wisdom was you would use multimode within a buliding or campus and singlemode for longer distance links between multiple sites.
However that traditional wisdom has become more and more questionable. Newer multimode standards have become increasingly demanding about the type and lengths of the cables making it very difficult to future proof a multimode cable plant. This can happen even while the speed stays the same, for example early 10 gigabit stuff was often LX4 which is relatively tolerant while modern 10 gigabit multimode stuff is usually the far less tolerant SR.
Meanwhile existing single mode fiber runs, at least at campus distances have remained compatible with new standards that have come along.
So my advice would be, if it's a short-term installation that will be ripped out for unrelated reasons before any speed upgrade is needed then consider both singlemode and multimode and go for the more economical option. If it's a long term installation that will need speed upgrades over time then go for singlemode.
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
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.
From physical point of view the dispersion is one of the main reasons for lower speed for multimode fibers.
At longer distances dispersion makes the optical pulses broader, more blurred, noisy, not clear, distinguishable. And dispersion is higher for multimode fibers compared to single mode fibers. There are different kind of dispersion. Single mode fibers also have dispersion but much lower.
Multimode fiber is a fiber with a "large" core (50-60µ), this mean that the light can follow multiples ways (i.e. modes) to go through the fiber. My analogy is that you are in a cathedral, and you speak with a person at the opposite of the cathedral. There will be lots of echoes and it will be really difficult to hear him.
Single Mode fiber have a really smaller core (9µ). So the light can follow only 1 track (ie mode). Now imagine you speak to the same person but instead of a cathedral, you speak through a small pipe, sound we be clear and sharp (it's not obvious but try it!).
Now try to put the pipe at the end of the cathedral : you will hear almost nothing : mixing multimode and single mode is not a good idea, even if it can work on specific conditions (very small distances).
Due to smaller core, single mode fiber and transceivers are a little complex to make. But SM price tend to get closer of MM price.
Concerning performances, This more complex than previous answers stated :
- On one hand, with single mode, having a sharp and clear signal make communicaiton a little easier to perform. This is why most people say that SM offer better performances for a specified fiber length.
- On the other hand, there are now products allowing to isolate each mode of multi-mode fiber individually (it's called Multi-Plane Light Conversion). It's like you divide the cathedral in dozen of small pipes.
So nowadays, datarate performances records are performed on multimode fiber...
Single Mode Fiber, SMF, is typically used for cable runs measured in miles, or kilometers. The laser-based optics are relatively expensive. SMF is typically used by carriers and on very large campuses, industrial plants.
Multimode Fiber, MMF, is used for cable runs measured in hundreds of feet or meters. The LED-based optics are relatively affordable. MMF is typically used in multistory buildings and on smaller campuses such as high schools.