OM3 vs OM4: Bandwidth Behind the Distance
Quick answer / TL;DR: Short answer: OM3 is rated at 2000 MHz-km and reaches about 300 m at 10G, 100 m at 40G and 100 m at 100G. OM4 is rated at 4700 MHz-km and reaches about 400 m at 10G and 150 m at 40G and 100G. They use the same connectors and wavelengths, so the only variable is distance. Choose OM4 when runs exceed 300 m at 10G, when 40G or 100G is planned beyond 100 m, or when the cable will be in place for a long time.
Single-mode patch cords: yellow jacket, LC/UPC, OS2 9/125.Both are 50 micron multimode fibre, both use 850 nm VCSELs, and both take the same LC or SC connectors. The difference is effective modal bandwidth, and it shows up only as distance at a given speed.
That makes the choice simple in principle: if the run fits OM3 at the target speed, OM4 buys margin rather than capability. If it does not fit, OM4 buys the link.
What Effective Modal Bandwidth Measures
In a multimode fibre, light travels in several modes that take different paths. Differential mode delay spreads a pulse as it travels, and the bandwidth figure describes how much the pulse can spread before the receiver cannot distinguish symbols.
| Property | OM3 | OM4 |
|---|
| Core and cladding | 50 / 125 micron | 50 / 125 micron |
| Effective modal bandwidth at 850 nm | 2000 MHz-km | 4700 MHz-km |
| 10GBASE-SR distance | 300 m | 400 m |
| 40GBASE-SR4 distance | 100 m | 150 m |
| 100GBASE-SR10 distance | 100 m | 150 m |
| Jacket colour convention | aqua | aqua or violet |
The jacket colour is not a reliable identifier, because OM4 is sold in both aqua and violet. The print on the jacket is the evidence, and it usually states the category and the bandwidth.
Why the Distance Figures Are Conservative
The published distances assume a specified number of mated connectors and splices. Every connection adds loss and every splice adds a small reflection, so a link with many patch panels will not reach the nominal distance even on OM4.
- Count every mated pair in the channel, including patch panels at both ends
- Add margin for ageing and for re-termination after damage
- Use the design loss budget rather than the headline distance
- Test at 850 nm with an OTDR or a light source and power meter
- Remember that 40G and 100G over multimode use parallel optics and MPO connectors
When the Upgrade Pays
| Situation | OM3 | OM4 |
|---|
| Runs under 100 m at 10G | sufficient | sufficient, more margin |
| Runs of 150 to 300 m at 10G | sufficient | sufficient |
| Runs of 300 to 400 m at 10G | insufficient | required |
| 40G or 100G under 100 m | sufficient | sufficient |
| 40G or 100G at 120 to 150 m | insufficient | required |
| New backbone with a 15 year horizon | marginal | preferred |
The cost difference between OM3 and OM4 cable is small relative to installation, which is the same argument that applies to Cat6A. Where the cable is going into a duct or a ceiling, the category decision is worth taking seriously at purchase time because it is expensive to revisit.
Real-World Applications
Data centre row-to-row links
Runs are typically under 100 m, so OM3 is technically sufficient. Most new builds specify OM4 anyway, because the incremental cable cost is minor against the risk of a run that lands at 120 m when a row is added.
Campus backbone between buildings
Runs of 200 to 400 m are common, and that is exactly the band where OM4 earns its place at 10G. For anything longer, single-mode with inexpensive optics is usually the better answer today.
Industrial cabinet with vibration
Category matters less than the cable construction here. Use a robust jacket and strain relief, then apply the OM3 or OM4 distance limit for the length involved rather than the nominal maximum.
Standards and Verification
The categories and the distance tables are defined in fibre and Ethernet standards, which is why the numbers are consistent across vendors.
- ISO/IEC 11801-1: OM3 and OM4 channel classes and their distance support
- TIA-492AAAC and TIA-492AAAD: the OM3 and OM4 fibre specifications
- IEEE 802.3: 10GBASE-SR, 40GBASE-SR4 and 100GBASE-SR10 distance objectives
- IEC 60793-2-10: multimode fibre product specifications
- TIA-568.3-D: fibre cabling performance and test requirements
Verification is a loss and length test against the design budget, not a bandwidth measurement in the field. Record the measured loss, the length and the number of connections, then compare with the budget for the intended speed. A link that passes at 10G but has little margin is a candidate for re-termination before a 40G upgrade, not after.
Related reading: Fiber cable guide: single-mode, multimode and construction — blogs/news/fiber-cable
Related reading: OM3 fiber cable: the 10G workhorse — blogs/news/om3-fiber-cable
Related reading: OM4 fiber cable: 4700 MHz-km and 400 m — blogs/news/om4-fiber-cable
Frequently Asked Questions
Is OM4 better than OM3?
It has higher modal bandwidth, so it supports longer distances at the same speed: about 400 m against 300 m at 10G, and 150 m against 100 m at 40G and 100G. It is not faster in itself.
Can I mix OM3 and OM4 in one link?
Yes, and the link performs to the lower category. Mixing is common during upgrades, but document it, because the distance limit becomes the OM3 figure.
Do OM3 and OM4 use different connectors?
No. Both use LC, SC or MPO connectors and the same 850 nm optics. The difference is in the fibre core profile and bandwidth.
How do I tell OM3 from OM4?
Read the jacket print. Aqua is used for both, and OM4 also appears in violet, so colour alone is not reliable.
Should I install single-mode instead?
For runs beyond about 400 m, or where future speed upgrades are uncertain, single-mode with low-cost optics is often the better long-term choice. OM4 is the practical multimode ceiling.
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