Single-mode vs multimode fiber: OS2, OM3, OM4 and OM5 compared
The core difference in single-mode vs multimode fiber is size: single-mode (OS2) has a 9 µm core that carries one path of laser light for kilometres, while multimode (OM3, OM4, OM5) has a 50 µm core that carries many paths of light for up to a few hundred metres. Multimode optics cost less, single-mode reaches further and scales to 400G and 800G more easily.
That is the short answer. The right choice for a project depends on link lengths, the speeds you expect over the life of the cabling (usually 15 years or more), and how many optics you will buy. Below we compare the fiber grades, the reach you get at each speed, and what to install in a data center, a campus and a building backbone.
Core Size and Light Source
Every fiber has a glass core surrounded by cladding with an outer diameter of 125 µm. The core size is what separates the two families.
- Single-mode fiber has a core of about 9 µm. Light travels in a single mode, so there is almost no modal dispersion, and the signal can run for 10 km, 40 km or more.
- Multimode fiber has a 50 µm core (or 62.5 µm in old OM1). Light travels in many modes that arrive at slightly different times, which limits bandwidth over distance.
The light source follows from the core size. Multimode optics use VCSELs (vertical-cavity surface-emitting lasers) at 850 nm. They are cheap to make and couple easily into a 50 µm core. Single-mode optics use edge-emitting lasers (FP, DFB or EML) at 1310 nm or 1550 nm, which must be aligned precisely to a 9 µm core. That precision is why single-mode transceivers have historically cost more.
The Fiber Grades: OS2, OM3, OM4 and OM5
ISO/IEC 11801 and TIA-568 define the grades. Multimode grades are set by effective modal bandwidth (EMB) at 850 nm; single-mode grades by attenuation and construction.
OS1 and OS2
OS1 was specified for tight-buffered indoor cable. OS2 has a lower attenuation specification (around 0.4 dB/km at 1310 nm) and covers low water peak fiber such as ITU-T G.652.D. In practice, OS2 is what you buy today for both indoor and outdoor runs.
OM1 and OM2
These are legacy grades. OM1 has a 62.5 µm core and OM2 a 50 µm core. They are fine for 1G but run out at 10G (33 m and 82 m respectively) and are not specified for 25G and above.
OM3 and OM4
Both are 50 µm laser-optimised fiber. OM3 has an EMB of 2000 MHz·km and OM4 4700 MHz·km at 850 nm. OM4 adds about 30 to 50 percent more reach at most speeds, and it is the usual multimode choice for new data center cabling.
OM5
OM5 is wideband multimode fiber. It matches OM4 at 850 nm but is also specified from 850 to 953 nm, so it supports short wavelength division multiplexing (SWDM) and BiDi optics that use several wavelengths in that window. With modules such as 100G-SWDM4 and 400G-SR4.2 it gives about 150 m instead of 100 m on OM4. For standard SR4 or SR8 parallel optics, OM5 performs the same as OM4. Its adoption has stayed limited, because many operators that need more than OM4 move straight to single-mode.
Reach by Speed and Fiber Grade
This table uses IEEE 802.3 and MSA reach figures. Real links also have to stay within the optic’s insertion loss budget, so count connectors and splices.
| Standard | OM3 | OM4 | OM5 | OS2 |
|---|---|---|---|---|
| 10GBASE-SR | 300 m | 400 m | 400 m | n/a |
| 25GBASE-SR | 70 m | 100 m | 100 m | n/a |
| 40GBASE-SR4 | 100 m | 150 m | 150 m | n/a |
| 100GBASE-SR4 | 70 m | 100 m | 100 m | n/a |
| 100G-SWDM4 | 75 m | 100 m | 150 m | n/a |
| 400GBASE-SR8 | 70 m | 100 m | 100 m | n/a |
| 400GBASE-SR4.2 (BiDi) | 70 m | 100 m | 150 m | n/a |
| 10GBASE-LR | n/a | n/a | n/a | 10 km |
| 100G-CWDM4 / 100GBASE-FR1 | n/a | n/a | n/a | 2 km |
| 400GBASE-DR4 | n/a | n/a | n/a | 500 m |
| 400GBASE-FR4 / LR4 | n/a | n/a | n/a | 2 km / 10 km |
The pattern is clear. Each jump in multimode lane speed has cut reach: 10G-SR goes 400 m on OM4, 25G-SR only 100 m. Single-mode reach is set by the optic, not the fiber, so the same OS2 cable carries 10G-LR today and 400G-FR4 later.
Colours and Connectors
Jacket colours
TIA-598 sets the usual jacket colours, which makes it easy to spot the fiber type in a rack:
- OS1/OS2 single-mode: yellow
- OM1 and OM2: orange
- OM3: aqua
- OM4: aqua, or Erika violet (common in Europe and some Middle East projects)
- OM5: lime green
Always confirm the grade printed on the jacket. Aqua alone does not tell you whether a cable is OM3 or OM4.
UPC and APC
Connectors come with two end-face polishes. UPC (ultra physical contact) has a slightly domed flat face. APC (angled physical contact) is polished at 8 degrees, which reduces back reflection. Single-mode UPC connectors have blue bodies, APC connectors are green, and multimode connectors are beige, aqua or another grade colour and are not used with APC polish.
APC is standard in FTTH, PON and RF video, and on single-mode MPO connectors for parallel optics such as 400G-DR4. Never mate a UPC connector to an APC connector: the gap causes high loss and can damage both end faces.
Connector types
LC duplex is the default for transceivers like SFP+, SFP28 and duplex QSFP28 optics. SC is common in older panels and FTTH. MPO/MTP carries 8, 12, 16 or 24 fibers for parallel optics; see our MTP/MPO polarity guide for details. You can find all three in our range of fiber patch cords.
Cost: Optics vs Cable
The cost comparison is often misunderstood. Multimode cable is not cheaper than single-mode cable; OS2 is often the same price or lower per metre. The difference is in the optics:
- Multimode optics (SR, SR4, SR8) use VCSELs and have usually been the cheapest way to run 10G to 100G over short distances.
- Single-mode optics used to cost several times more, but the gap has narrowed a lot at 100G and 400G with DR and FR modules made in high volume for cloud data centers.
- Installation costs are similar for both. Single-mode end faces are less tolerant of dirt, so cleaning and inspection matter more.
So the total cost depends on how many optics a link set will consume over its life. A data hall with thousands of short server links may still favour multimode. A backbone with a few dozen links that will be upgraded twice favours single-mode, because the cable stays and only the optics change.
Which to Choose: Data Center, Campus and Building Backbone
Data center
Inside a row, OM4 with MPO trunks works well for 25G, 100G-SR4 and 400G-SR8 links up to 100 m. For new spine and leaf builds aimed at 400G and 800G, many operators now install OS2 with MPO-12 APC for DR4 and DR8 optics, which reach 500 m and break out cleanly to 100G. For in-rack links, DAC and AOC cables avoid structured fiber altogether.
Campus
Between buildings, use OS2. Distances are usually hundreds of metres to several kilometres, outside the reach of any multimode optic at 10G and above. Use outdoor-rated loose tube cable and pull spare fiber counts.
Building backbone
Riser links from the main equipment room to floor IDFs are often under 300 m. OM4 will run 10G-SR at that distance, but not 25G or 100G-SR4. Installing OS2, or a hybrid cable with both OM4 and OS2, keeps the riser useful when floor switches move to 25G or 100G uplinks. Our office cabling guide covers the horizontal side.
Buyer checklist
- Measure the longest link, not the average.
- List the speeds you expect over the cable’s life, not just today’s.
- Count how many optics the design will need: many short links favour multimode, few long links favour single-mode.
- Check what fiber is already installed and its grade.
- Decide UPC or APC and connector type (LC, SC, MPO) before ordering panels and cords.
- Budget for fiber cleaners and an inspection scope.
Our network installation team in Dubai can survey existing cabling and help plan the fiber plant alongside the optics.
Frequently Asked Questions
Can I use a single-mode transceiver on multimode fiber?
Not in a standard link. A single-mode optic launched into a 50 µm multimode core suffers modal dispersion, and a multimode signal cannot couple efficiently into a 9 µm single-mode core. Match the transceiver to the installed fiber type at both ends.
Is OM4 better than OM3?
Yes. OM4 has more than twice the modal bandwidth of OM3 at 850 nm, giving 100 m instead of 70 m for 25G and 100G-SR4, and 400 m instead of 300 m for 10G-SR. For new multimode installations, OM4 is the usual minimum.
Do I need OM5 fiber?
Only if you plan to use multi-wavelength multimode optics such as 100G-SWDM4 or 400G-SR4.2 and need the extra reach to 150 m. For standard SR4 and SR8 parallel optics, OM5 performs the same as OM4.
What colour is single-mode fiber?
Single-mode cable jackets are normally yellow. Single-mode UPC connectors are blue and APC connectors are green, while multimode cables are aqua (OM3, OM4), Erika violet (some OM4), lime green (OM5) or orange (OM1, OM2).
How far can multimode fiber run at 100G?
100GBASE-SR4 reaches 70 m on OM3 and 100 m on OM4. Multi-wavelength optics such as 100G-SWDM4 reach 150 m on OM5. For longer runs at 100G, use single-mode fiber with optics such as 100G-DR, CWDM4 or LR4.
Talk to TIFNEX
Tell us your switch models, link lengths, the fiber already installed and the quantities you need, and our engineers will match optics, patch cords and MPO cabling to your plant, with a quote within 24 hours. We supply projects across the UAE, Egypt and the wider GCC. Get in touch or call/WhatsApp +971 54 300 8970.
