OM4 Fiber Cable: What the Extra Bandwidth Buys
Quick answer / TL;DR: Short answer: OM4 is a 50/125 micron laser-optimised multimode fibre with a higher modal bandwidth than OM3, which extends 10GBASE-SR from 300 to 400 metres and supports 40GBASE-SR4 and 100GBASE-SR10 to about 150 metres. It uses the same connectors and transceivers as OM3 and is backwards compatible. Choose OM4 over OM3 when runs approach or exceed 300 metres at 10G, or when you want headroom for 40G and 100G in the same ducts; choose OM5 only if you need short-wavelength division multiplexing above 850 nm.
Fiber cords: two connector families, two fiber types, three polish options.OM4 is the laser-optimised multimode fibre that sits between OM3 and OM5, and its value is distance rather than speed. It carries the same 10G, 40G and 100G signals as OM3, but for longer runs before the link budget runs out.
The decision between them is usually made once, at design time, and then lives in the building for fifteen years. That makes the reach table and the upgrade path worth understanding before the cable is ordered.
What Distinguishes OM4 From OM3
Both are 50 micron core multimode fibre with an 850 nm optimum. The difference is the effective modal bandwidth: OM4 is specified at roughly 4700 MHz-km against OM3 at 2000 MHz-km, which is a measure of how much the pulse spreads as it travels. Less spread means a higher data rate over the same distance, or the same data rate over a longer one.
| Grade | Core / cladding | Modal bandwidth at 850 nm | 10GBASE-SR reach | 40G / 100G reach | Jacket colour convention |
|---|
| OM1 | 62.5 / 125 | about 200 MHz-km | 33 m | not supported | orange |
| OM2 | 50 / 125 | about 500 MHz-km | 82 m | not supported | orange |
| OM3 | 50 / 125 | about 2000 MHz-km | 300 m | 100 m | aqua |
| OM4 | 50 / 125 | about 4700 MHz-km | 400 m | 150 m | aqua or violet |
| OM5 | 50 / 125 | wideband, 850 to 953 nm | 400 m | 150 m | lime green |
The colour column is a convention, not a specification. Aqua is used for both OM3 and OM4 in many installations, which is why jacket print rather than jacket colour has to be the deciding evidence when a patch field is audited.
The Reach Table in Practice
Distance claims assume a clean link: known connector count, known splice count, and a loss budget that matches the transceiver's sensitivity. Real installations add margin, and the margin is what turns a 400 metre specification into a 320 metre design.
- 10GBASE-SR: 400 m is the specification, design to about 300 m to leave loss budget for connectors and ageing
- 40GBASE-SR4: about 150 m on OM4, against 100 m on OM3, using an MPO connector with 8 or 12 fibres
- 100GBASE-SR10: about 150 m, and increasingly replaced by 100GBASE-SR4 on MPO
- 25GBASE-SR: about 100 m, commonly used in top-of-rack links
- Any duplex link over 400 m: single mode is the correct answer, not a higher multimode grade
The last line matters commercially. There is a tendency to specify OM4 or OM5 for runs where single mode would be cheaper and future-proof, because the transceiver cost difference has narrowed. Above roughly half a kilometre, single mode wins on total cost of ownership in most new builds.
Connectors, Polarity and Loss
OM4 is used with LC duplex connectors for 10G and 25G links, and with MPO connectors for 40G and 100G where several fibres run in parallel. Both bring a discipline problem rather than a fibre problem.
| Element | Typical loss | Where it bites |
|---|
| LC connector pair | 0.3 dB to 0.75 dB | Cumulative over several patch panels |
| MPO connector pair | 0.35 dB to 0.75 dB | One MPO carries 8 to 24 fibres, so one bad ferrule affects several lanes |
| Fusion splice | 0.1 dB to 0.3 dB | Splice count in a long backbone |
| Bend | avoid entirely below minimum radius | Tight bends cause loss that appears only when the link is loaded |
Polarity is the other common failure. An MPO trunk has a defined method (A, B or C) for how the fibres are ordered, and a link built with mismatched methods transmits on fibres the receiver is not listening to. The symptom is a link that is dark even though every individual fibre tests clean.
OM4 Against OM5
OM5 is not simply a faster OM4. It is specified for wideband operation from 850 to 953 nm, which allows short-wavelength division multiplexing to run several channels on one fibre pair. At 850 nm alone its reach is the same as OM4: 400 metres at 10G and about 150 metres at 40G or 100G.
That makes the choice simple. If the design uses standard SR transceivers at 850 nm, OM4 delivers the same reach for less money. OM5 is justified when the plan depends on SWDM4 transceivers to multiply capacity over the existing duplex fibre.
Real-World Applications
Campus backbone between buildings
A 300 metre building-to-building run at 10G sits at the edge of OM3 and inside OM4 with margin. Specify OM4 for the pull, because the cable will outlive the transceivers, and terminate into patch panels at both ends so the transceiver can be changed without re-pulling.
Data hall top-of-rack links
Short 25G and 100G links over OM4 are the standard pattern, usually with MPO trunks between rows. The design work is in polarity and in documenting which trunk maps to which port, not in the fibre grade.
Industrial and harsh environments
Where vibration and temperature swing are present, the jacket and the connector body matter more than the grade. An OM4 with a tight-buffered construction and a robust connector survives where a standard patch cable fails, and the reach advantage is irrelevant at 30 metres.
Upgrading an OM3 installation
OM4 is backwards compatible and uses the same connectors, so a mixed patch field works. Label the grade at every panel: an OM3 patch cord inserted into an OM4 link shortens the reach to the OM3 figure, which is the kind of fault that only appears when a link is pushed to its limit.
Standards and Verification
Multimode grades are defined by bandwidth and geometry specifications rather than by marketing tiers, and the test methods are published.
- ISO/IEC 11801-1 and TIA-568.3-D: multimode fibre grades, reach tables and channel requirements
- IEC 60793-2-10: the fibre type specification for category A1 multimode fibres, including OM3, OM4 and OM5
- IEC 61300-3-4: attenuation measurement method for fibre optic components
- IEC 61754 series: fibre optic connector interfaces, including LC and MPO
- IEEE 802.3: the 10GBASE-SR, 40GBASE-SR4, 100GBASE-SR4 and 25GBASE-SR physical layer specifications
- TIA-568.3-D annexes on MPO polarity methods A, B and C
Verification has two parts. Measure each fibre with an optical loss test set against the calculated loss budget, not against a generic threshold, and record the result per fibre rather than per cable. Then verify polarity on any MPO link by confirming that the transmit path arrives at the expected receive position, which a per-fibre loss test alone will not catch.
Related reading: OM3 fiber cable: the 300 m grade — blogs/news/om3-fiber-cable
Related reading: OM5 fiber cable: wideband multimode — blogs/news/om5-fiber-cable
Related reading: Multimode fiber cable: grades, reach and construction — blogs/news/multimode-fiber-cable
Frequently Asked Questions
What is the difference between OM3 and OM4?
Bandwidth, and therefore reach. OM4 has roughly 4700 MHz-km of modal bandwidth against OM3's 2000, which extends 10GBASE-SR from 300 to 400 metres and 40G or 100G links from 100 to about 150 metres. The connectors and transceivers are the same.
Is OM4 better than single mode?
No, they solve different problems. OM4 is cheaper per metre and per transceiver for runs under a few hundred metres. Beyond roughly 400 to 500 metres, single mode is the correct and usually cheaper overall choice.
How far can OM4 run at 100G?
About 150 metres with 100GBASE-SR10 or SR4, assuming a clean link with a normal connector count. Design to less than the specification to leave loss budget for splices, connectors and ageing.
What colour is OM4 cable?
The convention is aqua or violet, but aqua is also used for OM3, so colour is not reliable evidence. Read the jacket print, which states the grade.
Can I mix OM3 and OM4 patch cords?
They are physically compatible and the link will work, but the reach becomes that of the lowest grade in the channel. Label every panel so a mixed patch field does not silently shorten a long link.
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