Single-mode vs multimode fibre decides which optic you can use on existing cabling. This guide covers OM3, OM4, OM5 and OS2, and gives reach figures taken from Cisco transceiver data sheets so you can match an optic to the fibre already in your building.
The short answer
- Multimode (OM3, OM4, OM5): 50 µm core, short-wavelength 850 nm optics, typically 100 m to 400 m depending on speed. Cheaper optics, shorter reach.
- Single-mode (OS2): long-wavelength optics, from hundreds of metres to tens of kilometres. The same fibre works across speeds.
- Identify the fibre you have first. Buy the optic second.
What the fibre categories mean
ANSI/TIA-568.3-D adopted the ISO/IEC 11801 names: "OM" for multimode and "OS" for single-mode, as described in Fluke Networks' fibre guide. That guide lists these values for 50 µm fibre:
| Category | Core | Modal bandwidth, 850 nm overfilled (MHz·km) | Effective laser (MHz·km) | 10GBASE-SR (Fluke) |
|---|---|---|---|---|
| OM1 | 62.5 µm | 200 | not specified | 33 m |
| OM2 | 50 µm | 500 | not specified | 82 m |
| OM3 | 50 µm | 1500 | 2000 | 300 m |
| OM4 | 50 µm | 3500 | 4700 | 400 m |
| OM5 | 50 µm | 3500 | 4700 | 400 m |
OM5 has the same 850 nm bandwidth as OM4; its extra specification is for longer wavelengths used by multi-wavelength optics. OS2 is the single-mode category; the Fluke page gives attenuation of 1.0 dB/km (inside plant) and 0.5 dB/km (outside plant) at 1310 and 1550 nm.
Reach by fibre type, from Cisco data sheets
| Optic | Fibre | Reach | Source |
|---|---|---|---|
| 1000BASE-SX (GLC-SX-MMD) | OM1 / OM2 / OM3 / OM4 | 275 m / 550 m / 1000 m / 1000 m | Cisco SFP data sheet |
| 1000BASE-LX/LH (GLC-LH-SMD) | single-mode G.652 | 10 km | same |
| 10GBASE-SR | OM1 / OM2 / OM3 / OM4 | 33 m / 82 m / 300 m / 400 m | Cisco 10G data sheet |
| 10GBASE-LRM | multimode / single-mode | 220 m / 300 m | same |
| 10GBASE-LR | single-mode | 10 km | same |
| 10GBASE-ER | single-mode | up to 40 km | same |
| SFP-25G-SR-S | OM3 / OM4 / OM5 | 70 m / 100 m / 100 m (RS-FEC on the host) | Cisco 25G data sheet |
| SFP-10/25G-CSR-S | OM3 / OM4 / OM5 | 300 m / 400 m / 400 m at 10G; at 25G with RS-FEC the data sheet lists reach depending on fibre quality | same |
| SFP-10/25G-LR-S | single-mode | 10 km | same |
| QSFP-100G-SR4-S | OM3 / OM4 / OM5 | 70 m / 100 m / 100 m | Cisco 100G data sheet |
| QSFP-100G-SR1.2 and QSFP-40/100-SRBD (BiDi) | OM3 / OM4 / OM5 | 70 m / 100 m / 150 m | same |
| QSFP-100G-CWDM4-S | single-mode | 2 km | same |
| QSFP-100G-LR4-S | single-mode | 10 km | same |
Two cautions from the same sheets. The 25G SR figures need RS-FEC on the host port; with BASE-R FEC the 25G CSR module reaches 70 m on OM3 and 100 m on OM4, and with no FEC 30 m and 50 m. Cisco also notes that 10GBASE-ER links over 30 km are engineered links under IEEE 802.3ae.
Connectors and polarity
- LC duplex: two fibres, one transmit and one receive. Used by SFP, SFP+, SFP28 optics and by the duplex 100G types above. Cisco's data sheets specify PC or UPC patch cords; APC is not supported on these modules.
- MPO/MTP: a multi-fibre connector. QSFP-100G-SR4-S uses MPO-12 and QSFP-100G-PSM4-S also uses MPO-12 (single-mode). SR4 and PSM4 are not interchangeable: multimode and single-mode fibre, and different wavelengths.
- Polarity: a duplex link needs each end's transmit to reach the other end's receive; an MPO trunk adds a polarity method that must be consistent across the whole channel. The TIA-568.3 polarity methods were not part of the sources we read, so confirm the method with your cabling supplier or installer.
BiDi options
A BiDi optic carries both directions on one fibre by using two wavelengths, so the two ends are different parts and must be used as a pair. Cisco's 10G sheet lists SFP-10G-BXD-I with SFP-10G-BXU-I (10 km) and SFP-10G-BX40D-I with SFP-10G-BX40U-I (40 km), over a single strand of G.652 single-mode fibre. On multimode, the 100G BiDi types in the table above run over duplex OM3/OM4/OM5. Arista's 100G FAQ notes that SWDM4, SRBD and SR1.2 do not interoperate with each other and must be deployed as matched pairs; check the same for any other vendor.
How to pick an optic for existing cabling
- Identify the fibre: read the cable jacket markings or the installer's test report. A test report gives the real category and length.
- Add the full path length, including patch cords and every panel.
- Compare to the table: pick the optic whose rated reach on that fibre exceeds the path.
- Check the host supports the speed and FEC mode. See SFP vs SFP+ vs SFP28.
- For runs inside a rack or to a neighbouring one, a cable may be simpler: DAC vs AOC vs optics.
- Check compatibility with your exact switch: OEM vs third-party optics.
Cisco parts discussed: SFP-10G-SR, SFP-10G-SR-S, SFP-10G-LRM, SFP-10G-LR, SFP-10G-BXD-I, SFP-10G-BXU-I, SFP-25G-SR-S, SFP-10/25G-CSR-S, QSFP-100G-SR4-S, QSFP-40/100-SRBD, QSFP-100G-LR4-S.
Common mistakes
- Assuming "multimode" means OM4. Older buildings often have OM1 or OM2, where 10GBASE-SR reaches only 33 m or 82 m.
- Quoting reach for the optic without counting patch cords and panels.
- Ordering an SR4 module for a duplex LC plant, or a single-mode module for multimode fibre.
- Ordering one half of a BiDi pair.
- Forgetting the FEC requirement on 25G.
- Using APC patch cords with these modules.
FAQ
Can I use single-mode optics on multimode fibre?
Not as a general rule. Only types the data sheet lists for both fibres, such as 10GBASE-LRM, are specified for it, and OM1/OM2 use needs mode-conditioning in some cases per the data sheet. Check the data sheet for the exact part.
Is OM5 worth it over OM4?
At 850 nm the figures in the table are the same for the optics listed; OM5 helps multi-wavelength BiDi types such as the 100G SR1.2, which Cisco lists at 150 m on OM5 versus 100 m on OM4.
Which is cheaper, single-mode or multimode?
We do not publish price comparisons here; ask us to quote both options.
What to include in your quote request
- Existing fibre type (OM1 to OM5 or OS2) and a test report if you have one
- Total path length including patch cords
- Connector at each end (LC or MPO) and any polarity requirement
- Speed, switch model and software release at both ends
- Whether BiDi or duplex is preferred
- Quantity
Send these through our quote request form.
Tool: look up optics for your switch in the optics compatibility lookup.