The short answer
The two fiber families are separated by core diameter, which controls how light travels. Single-mode fiber (SMF) has a roughly 9-micron core that supports one propagation mode, eliminating modal dispersion and enabling reach measured in kilometers; it is graded OS1 and OS2 under ISO/IEC 11801, uses 1310nm and 1550nm laser wavelengths, and OS2 loss is specified at 0.4 dB/km or better. Multimode fiber (MMF) uses a wider core that carries many modes, which limits distance but pairs with low-cost 850nm VCSEL transceivers. The multimode grades are OM1 (62.5-micron core), OM2, OM3, OM4, and OM5 (all 50-micron), distinguished by effective modal bandwidth (EMB). Higher OM numbers carry a given data rate farther.
In practice, choose single-mode (OS2) for any run beyond a few hundred meters, for building-to-building backbones, or where you want headroom for future 100G-plus. Choose multimode (OM3/OM4) for short data-center and in-building links where transceiver cost dominates. Real reach limits are set by IEEE 802.3: 10GBASE-SR reaches 300m on OM3 and 400m on OM4; 40/100GBASE-SR4 reaches 100m on OM3 and 150m on OM4; 10GBASE-LR reaches 10km on OS2 single-mode. Connectors are typically LC (small-form-factor duplex) or SC, with legacy ST bayonet still found on multimode. Use fiber when distance exceeds copper's 100m channel limit, for electrical isolation, or for immunity to EMI; use Category copper for short horizontal runs that also carry PoE.
Single-Mode vs Multimode: The Core Decision
The choice between single-mode and multimode is fundamentally a distance-versus-cost tradeoff rooted in optics. Single-mode fiber confines light to a core near 8.3-9.5 microns (nominal 9um), small enough that only the fundamental mode propagates at 1310nm and 1550nm. With one mode, there is no modal dispersion, so the reach limit becomes attenuation and chromatic dispersion, both very low. That is why single-mode carries 10G to 40km (10GBASE-ER) and forms the backbone of carrier and metro networks. Multimode's 50um or 62.5um core admits hundreds of modes that arrive at slightly different times (modal dispersion), capping distance regardless of how strong the signal is.
For a building or campus, the practical rule is: if any single run exceeds roughly 300-400m, or if it crosses between buildings, specify single-mode OS2. OS2 costs the same or less per meter than OM4 today, and the fiber itself will not be the bottleneck for future upgrades; only the transceivers change. Multimode earns its place inside data centers and equipment rooms where links are under 100-150m and the price gap in optics (850nm VCSEL versus 1310nm distributed-feedback lasers) multiplied across hundreds of ports is significant.
A common mistake is over-buying multimode grade for reach it cannot deliver, or mixing 50um and 62.5um fiber in one channel, which causes severe coupling loss. Match the grade to the actual link budget, keep core sizes consistent, and document the plant so future techs know what is installed.
- Single-mode core: ~8.3-9.5um (nominal 9um), one propagation mode, no modal dispersion
- Multimode core: 50um (OM2-OM5) or 62.5um (OM1), hundreds of modes, modal-dispersion limited
- Single-mode wavelengths: 1310nm and 1550nm (laser); multimode: 850nm (VCSEL), some 1300nm
- OS2 attenuation is specified at <=0.4 dB/km; OS1 at <=1.0 dB/km (ISO/IEC 11801)
- Rule of thumb: runs over ~300-400m or building-to-building = single-mode OS2
- Never mix 50um and 62.5um in the same channel; the core mismatch adds large coupling loss
OM and OS Grades: Bandwidth and Attenuation Specs
Multimode grades are defined by effective modal bandwidth (EMB) at 850nm, published in ANSI/TIA-492 and ISO/IEC 11801. OM1 is 62.5/125um with 200 MHz-km overfilled bandwidth. OM2 is 50/125um at 500 MHz-km. OM3 is laser-optimized 50/125um at 2000 MHz-km EMB, and OM4 is 50/125um at 4700 MHz-km EMB. OM5, standardized in TIA-492AAAE, is also 50/125um with the same 4700 MHz-km at 850nm but adds specified performance across 850-953nm to support short-wavelength division multiplexing (SWDM). OM3 and OM4 are typically aqua-jacketed; OM5 is lime green.
Single-mode grades OS1 and OS2 differ mainly in maximum attenuation and cabling construction. OS1 is a tight-buffered indoor cable rated at 1.0 dB/km at both 1310 and 1550nm. OS2 is loose-tube or blown construction rated at 0.4 dB/km, suited to outdoor and long-haul runs. The glass itself is similar; the specification and jacket differ. Because single-mode bandwidth is effectively unlimited over enterprise distances, OS2 does not carry an EMB figure the way multimode does; its limit is loss and dispersion.
When reading a datasheet, confirm both the core/cladding (e.g., 50/125) and the EMB, not just the OM label, since some cables meet only the minimum.
- OM1: 62.5/125um, 200 MHz-km at 850nm, orange jacket (legacy)
- OM2: 50/125um, 500 MHz-km at 850nm, orange jacket
- OM3: 50/125um, 2000 MHz-km EMB, aqua jacket, laser-optimized
- OM4: 50/125um, 4700 MHz-km EMB, aqua (or violet) jacket
- OM5: 50/125um, 4700 MHz-km at 850nm plus 850-953nm SWDM support, lime-green jacket
- OS1: 9/125um, <=1.0 dB/km; OS2: 9/125um, <=0.4 dB/km, yellow jacket
Real Maximum Distances by Data Rate (IEEE 802.3)
Distance limits come from IEEE 802.3 physical-layer specifications, not marketing. At 1 Gigabit, 1000BASE-SX (850nm) reaches 275m on OM1, 550m on OM2/OM3, while 1000BASE-LX (1310nm) reaches 5km on single-mode and 550m on multimode with a mode-conditioning patch cord. At 10 Gigabit, 10GBASE-SR reaches 33m on OM1, 82m on OM2, 300m on OM3, and 400m on OM4/OM5. Single-mode 10GBASE-LR reaches 10km and 10GBASE-ER reaches 40km on OS2.
At 40 and 100 Gigabit using parallel SR4 optics (802.3ba), the reach is 100m on OM3 and 150m on OM4/OM5 for both 40GBASE-SR4 and 100GBASE-SR4. Single-mode versions extend much farther: 40GBASE-LR4 and 100GBASE-LR4 reach 10km on OS2, and ER4 variants reach 40km. OM5's SWDM advantage appears in duplex short-wavelength standards such as 100GBASE-SR1.2, where fewer fibers carry the same rate over similar multimode distances.
These are worst-case guaranteed reaches; a clean, low-loss plant with fusion splices may perform better, but never design past the specified maximum. Always compute a link budget from the transceiver's transmit power, receiver sensitivity, and the connector, splice, and fiber losses in the path.
- 1G: 1000BASE-SX = 550m (OM3); 1000BASE-LX = 5km (OS2 single-mode)
- 10G: 10GBASE-SR = 300m (OM3), 400m (OM4); 10GBASE-LR = 10km (OS2); 10GBASE-ER = 40km
- 40G: 40GBASE-SR4 = 100m (OM3), 150m (OM4); 40GBASE-LR4 = 10km (OS2)
- 100G: 100GBASE-SR4 = 100m (OM3), 150m (OM4); 100GBASE-LR4 = 10km (OS2); ER4 = 40km
- OM1 caps 10G at ~33m; OM2 at ~82m, illustrating why legacy multimode limits upgrades
- Design to the specified worst-case reach and verify with a measured link budget
Connectors, Simplex vs Duplex, and Polarity
Three connector types dominate. LC (Lucent Connector) is a small-form-factor connector with a 1.25mm ferrule and push-pull latch; its compact size makes it the standard on modern SFP/SFP+/QSFP transceivers and high-density patch panels. SC (Subscriber Connector) uses a larger 2.5mm ferrule with a push-pull square body, common on older enterprise gear and some GPON. ST (Straight Tip) is a bayonet-coupling 2.5mm ferrule connector, largely legacy and most often seen on multimode. Higher-density parallel optics (40G/100G SR4) use MPO/MTP multi-fiber connectors carrying 12 or 24 fibers in one ferrule.
Fiber runs are simplex (one strand) or duplex (two strands, one transmit and one receive). Most Ethernet transceivers are duplex, needing a transmit-receive pair; bidirectional (BiDi) optics use one strand with two wavelengths. Ferrule end-faces are polished flat PC, angled UPC (blue), or angled APC (green). APC's 8-degree angle reflects light into the cladding, giving very low return loss for single-mode and analog RF/PON; never mate an APC to a UPC connector, as it damages both and causes high loss.
Polarity matters on duplex and MPO links: transmit on one end must reach receive on the other. Follow TIA-568 Method A, B, or C consistently across the channel.
- LC: 1.25mm ferrule, small-form-factor duplex, standard on SFP+/QSFP optics
- SC: 2.5mm ferrule, push-pull square body, common on legacy enterprise and PON
- ST: 2.5mm bayonet, legacy, mostly multimode
- MPO/MTP: 12- or 24-fiber ribbon connector for 40G/100G SR4 parallel optics
- APC (green, 8-degree angle) gives low return loss; never mate APC to UPC (blue)
- Duplex = separate Tx/Rx strands; BiDi optics run one strand with two wavelengths
When to Use Fiber vs Copper
Copper's hard limit is the 100m channel defined by ANSI/TIA-568 for Category cabling (90m permanent link plus 10m of patch cords), across Cat5e, Cat6, and Cat6A. Beyond that length, or where electromagnetic interference, ground-potential differences, or lightning-prone outdoor spans are concerns, fiber is the correct medium because glass carries no current and is immune to EMI and crosstalk. Fiber is also the choice for high-aggregate backbones (10G to 100G and beyond) and for building-to-building links where copper would create a dangerous bonding path between separate electrical grounds.
Copper still wins for the last 100m to endpoints that also need Power over Ethernet. IEEE 802.3af delivers up to 15.4W at the source (12.95W at the device), 802.3at (PoE+) up to 30W at the source, and 802.3bt (PoE++, Types 3 and 4) up to 60W and 90-100W at the source. Fiber carries no power, so PoE cameras, access points, and phones require copper or a local injector. Category copper is also cheaper to terminate in the field and easier for generalist techs to handle.
A common enterprise design uses single-mode or OM4 fiber for the backbone and riser, then Cat6A copper for horizontal runs to devices, getting distance and bandwidth on the trunk and PoE at the edge.
- Copper channel limit: 100m total (90m permanent link + 10m patch) per TIA-568
- Cat6A supports 10GBASE-T to 100m; Cat6 to ~55m for 10G
- Fiber is immune to EMI, crosstalk, and ground-loop currents; use it between buildings
- PoE is copper-only: 802.3af 15.4W, 802.3at 30W, 802.3bt up to 90-100W at the source (Type 4)
- Use fiber for backbones, distances over 100m, and electrically noisy or outdoor spans
- Typical design: fiber backbone/riser + Cat6A horizontal for PoE endpoints
Frequently asked questions
Can I mix OM3 and OM4 fiber in the same link?
Both are 50/125um, so they mate without the coupling loss you get from mixing 50um and 62.5um. However, the link's reach is governed by the lowest-grade segment: an OM4 patch cord does not extend an OM3 backbone beyond OM3's rated distance. Document each segment and design to the weakest grade in the channel.
Is single-mode fiber harder or more expensive to install than multimode?
The fiber itself is comparable in price to OM4 and often cheaper. The cost difference is in the transceivers: single-mode 1310nm lasers historically cost more than 850nm multimode VCSELs, though the gap has narrowed. Termination requires the same fusion-splicing or connectorization skill for both. Single-mode's tighter 9um core makes clean end-faces and low-loss connections more critical, so contamination and alignment matter more.
What is the maximum distance for 10 Gigabit Ethernet on multimode fiber?
Under IEEE 802.3ae, 10GBASE-SR reaches 300m on OM3 and 400m on OM4 and OM5 at 850nm. On legacy fiber it drops sharply: about 82m on OM2 and only 33m on OM1. For 10G beyond 400m you move to single-mode 10GBASE-LR, which reaches 10km on OS2, or 10GBASE-ER at 40km.
Do I need fiber if my devices only run at 1 Gigabit?
Not for reach alone if every run is under 100m, since Cat5e or Cat6 copper handles 1000BASE-T to 100m and also carries PoE. Choose fiber at 1G when a run exceeds 100m, crosses between buildings, passes through electrically noisy environments, or when you want backbone headroom to upgrade to 10G-plus later without re-pulling cable.
