The short answer
Single-mode fibre has a small core that carries one mode of light, so it can run for kilometres with little dispersion. It is the default for access networks, FTTH, metro rings, campus backbones and anything that leaves the building. Multimode fibre has a much larger 50 um core that carries many modes; it works with lower-cost 850 nm optics over short runs and is used inside buildings and data centres. Both share the same 125 um cladding and, in the reference construction on our optical fibre page, a 242 um coating, which is why they look identical once cabled. Only the jacket colour, the print and the datasheet tell them apart.
A practical rule the desk uses: if the link is longer than a building or a data hall, or if nobody can say how long the network will stay in service, plan single-mode. If the link is short, the optics are already chosen and they are 850 nm multimode, buy the multimode grade that those optics were specified for.
What OS2, OM3, OM4 and OM5 actually mean
The OS and OM names come from cabling standards (ISO/IEC 11801, later adopted by TIA-568) and describe cabled fibre performance classes. The fibre itself is specified by other documents: ITU-T G.652.D or G.657 for single-mode, and the IEC 60793-2-10 and TIA-492 families for multimode. A buyer should put both on the RFQ: the cabling class the designer asked for and the fibre standard the plant will test against.
| Class | Fibre type | Core / cladding | Key performance figure | Typical use |
|---|---|---|---|---|
| OS2 | Single-mode, usually G.652.D or G.657.A | Core per datasheet (MFD typically 8.6-9.2 um) / 125 um | Cabled loss per G.652 tables or datasheet | FTTH, outdoor, campus, long runs |
| OM3 | Multimode, laser-optimised | 50 / 125 um | EMB at least 2,000 MHz.km at 850 nm | Short data centre and building links |
| OM4 | Multimode, laser-optimised | 50 / 125 um | EMB at least 4,700 MHz.km at 850 nm | Data centres; 10 Gb/s commonly quoted to 400 m (IEEE 10GBASE-SR) |
| OM5 | Wideband multimode | 50 / 125 um | OM4 performance plus 2,470 MHz.km EMB at 953 nm | Short-wave wavelength multiplexing (SWDM) optics |
EMB is effective modal bandwidth, the figure that limits how far a given multimode optic can reach. OM4 is the usual starting point for new data centre multimode. OM3 and OM4 share the 50/125 geometry and can be connected to each other, but the link must then be designed to the lower grade. Older 62.5 um OM1 and OM2 links still exist in buildings; matching those is a site survey question, not a catalogue choice.
Loss and distance: what can be written down
Buyers often ask for a single attenuation figure. The honest answer depends on whether you mean bare fibre or cabled fibre, and on which standard edition. For single-mode, the cabled fibre limits in G.652 tables are 0.4 dB/km from 1310 to 1625 nm and 0.3 dB/km at 1550 nm; the product value for a given fibre is on its datasheet. For multimode, older TIA-568 planning values for cabled fibre are 3.5 dB/km at 850 nm and 1.5 dB/km at 1300 nm; use the cable maker's specification where available. Loss is rarely what limits a multimode link, though. Bandwidth and the optic's reach are, which is why OM grades are defined by EMB rather than by dB.
The fibre attenuation guide shows how to build a simple loss budget from those values, and the G.652.D vs G.657 guide covers the single-mode grades in more depth.
Cost: fibre is cheap, optics are not
Per metre, multimode cable is not cheaper than single-mode, and in many constructions it costs more. The saving with multimode sits in the transceivers: 850 nm multimode optics have historically been cheaper than single-mode optics at the same data rate. That saving is real inside a data hall with thousands of short links. It disappears once links get longer than the multimode reach, and it reverses when a site has to be re-cabled because the optics moved on.
For a brand owner or distributor stocking patch cords and indoor cable, the commercial question is different: which fibre types do your customers order, in which jacket colours, and in which lengths. The desk usually suggests a small range, for example G.657.A2 single-mode plus OM4, rather than every grade, because each fibre type multiplies the stock list across connector types, lengths and jacket ratings.
Testing and identification differences
The two families are also tested differently. Single-mode cable is usually checked at 1310 and 1550 nm, and outdoor drums are shipped with OTDR traces so the buyer can see the loss along the length and find any point defect. Multimode cable is checked at 850 and 1300 nm; for laser-optimised grades the bandwidth class is a property of the fibre, so the evidence is the fibre maker's grade on the test report rather than a figure measured on each drum. The OTDR test report guide lists what to ask for before shipment.
Identification matters as much as testing, because a mixed warehouse is where mistakes happen. Individual fibres follow the TIA-598 colour order whichever type they are, so the fibre colour says nothing about the grade. Jacket colour often signals fibre type by industry convention, for example yellow for single-mode and aqua for OM3 or OM4, but conventions vary between markets and private label ranges. The desk writes the jacket colour and the full print legend, including fibre type and core count, into the spec sheet and checks it on the pre-shipment sample.
How to specify it on an RFQ
- Fibre type and standard: G.652.D, G.657.A1 or A2, or OM3, OM4 or OM5 to the named multimode standard.
- Coating: 242 um standard or 200 um reduced-diameter, which matters for high-count and micro cables.
- Cable construction: tight buffer or loose tube, indoor or outdoor, and the jacket rating, such as a CPR class declared on the DoP or an NEC Article 770 rating.
- Jacket colour and print: including any private label print.
- Test records: attenuation per fibre at the wavelengths you need, and OTDR traces for outdoor drums.
- Quantity and packing: total length and drum length, commonly 2 km or 4 km for outdoor cable.
Indoor cables are on the indoor fibre cables page; for data hall projects, the data centre application page shows how the fibre choice interacts with LSZH jackets and copper cabling. When the brief is ready, the quote form takes the same fields in order.