It's a textbook scene: two company buildings across a parking lot, some 600 meters of yard between them, and the order to "get 40 gigs from one side to the other". The contractor showed up with a spool of OM4 multimode fiber — "the good one, the thick orange stuff" — and a pair of SR4 modules. Pulled it, spliced it, plugged it in… and the link wouldn't even come up. It's not bad luck or a dead module: at 40G that fiber reaches 150 meters, and the yard is 600. The physics of light doesn't negotiate either.
Picking fiber looks like a specialist's job, but it boils down to three questions with concrete answers: singlemode or multimode? which cable construction for where it runs? and which jacket does the fire code demand? What follows is that decision, in order, with the whys — because a badly chosen fiber link doesn't "run slow", it simply doesn't light up.
Singlemode vs multimode: it's the core diameter
The whole difference starts with one number: the diameter of the core, the glass strand the light travels through. In singlemode (OS1/OS2) that core is a mere ~9 µm — so thin the light can only take one path, a single mode, straight as a ruler. It's driven by a laser at 1310 or 1550 nm, has very low loss, and so it travels kilometers without falling apart.
In multimode the core is far wider: 62.5 µm in the old OM1 and 50 µm in OM2/OM3/OM4/OM5. That big diameter lets light enter at many angles at once — many "modes" — and each ray bounces along a different path inside the glass. It's driven by an LED or VCSEL at 850 nm, cheap optics. The problem shows up far away: the rays that bounce more travel farther than the ones going nearly straight, so they arrive at the far end out of phase. That smearing of the pulse is called modal dispersion, and it's the physical reason multimode is short-legged.
Let's kill a myth once and for all: plenty of people think multimode is "faster" because the cable is thicker and the optics cheaper. It isn't. The thick core is precisely its limit. The thin laser light of singlemode, traveling one single path, is what carries 10, 40, 100 and even 400 Gb/s down the same strand at distances multimode can only dream of. Multimode wins on exactly one turf: short and cheap inside the datacenter or the building.
When to use each? The field rule
The real decision is almost always driven by distance, and second by the link's future.
Choose SINGLEMODE (OS2) when: it's a campus or WAN link, the run exceeds ~550 m, it's a backbone you want future-proof (10G today, 100G tomorrow on the same strand with no re-pull), it goes through long outdoor stretches, or it hands off to a carrier. Choose MULTIMODE (OM3/OM4) when: it's inside a single building or room, the distances are short (datacenter patching, floor links), or when multimode plant already exists and you want to keep using it.
OM/OS grades and real distances (IEEE 802.3)
Each fiber grade has maximum distances that are measured and standardized, not marketing. These are the ones that matter for sizing:
| Fiber | Core | 1G | 10G | 40/100G (SR4/LR4) |
|---|---|---|---|---|
| OM1 | 62.5 µm | 275 m | no | no |
| OM2 | 50 µm | 550 m | ~82 m | no |
| OM3 | 50 µm | — | 300 m | 100 m |
| OM4 | 50 µm | — | 400 m | 150 m |
| OM5 | 50 µm | — | 400 m | 150 m (SWDM) |
| OS2 | 9 µm | 10 km | 10 km | 10 km (40 km ER) |
The thing that jumps out: OM4, the best common multimode, reaches only 150 m at 40/100G. OM3 reaches 100 m. Meanwhile OS2 singlemode does 10 km with standard LR4 optics, and 40 km with ER modules. Between 150 m and 10 km lies a chasm, and that chasm is the whole conversation. OM5 doesn't "go farther" than OM4 on a single color: its trick is supporting SWDM (several wavelengths 850–950 nm on the same strand) to multiply capacity on short runs. You can check your run against these limits in the fiber reach calculator before buying a single meter.
Cable construction: where it runs dictates the structure
The very same glass strand can come built in very different ways depending on the environment. Mixing them up guarantees failure within months.
Tight-buffered: each fiber is coated with a firm plastic layer bonded to the glass. It's flexible, easy to terminate, and survives handling. This is the indoor construction: in-building backbones, patch cords, rack cables.
Loose-tube: the fibers float freely inside tubes filled with gel. That gel blocks moisture, and the slack lets the cable expand and contract with temperature without stretching the glass. This is the outdoor construction: between buildings, buried, aerial. If you see gel, it's meant for outside.
Ribbon: the fibers are bonded into flat ribbons of 12, packing 144, 288 or more fibers into an ultra-thin cable. It lets you fuse 12 at a time and feeds high-density MPO connectors. This is what large datacenters and carrier trunks use.
For hostile outdoors there are two reinforcements worth knowing: armor (a corrugated metal tape under the jacket that stops rodents — yes, rats chew fiber — and crushing), and ADSS (All-Dielectric Self-Supporting), a fully dielectric, self-supporting cable hung between poles with no metal messenger, ideal for aerial runs alongside power lines because it doesn't conduct.
Jacket: the fire rating is code, same as copper
The outer jacket isn't just a color: it defines how safe the cable is in a fire and where the code lets you run it. It's exactly the same logic as in copper.
| Jacket | What it is | Where it goes |
|---|---|---|
| OFNR | Riser (fire-retardant, won't spread between floors) | Vertical shafts, between floors |
| OFNP | Plenum (low smoke/flame emission) | Ceilings and raised floors for air return |
| LSZH | Low smoke, zero halogen | Confined spaces, tunnels, Europe |
| OFNP > OFNR | Plenum works where riser is asked, not the reverse | Substitution rule |
The practical hierarchy: if a space handles air return (the drop ceiling the AC pulls air back through) it demands plenum (OFNP), because a cable burning there spreads toxic smoke across the whole building. Vertical shafts running between floors call for riser (OFNR), which won't let fire climb. Plenum can always replace riser, never the reverse. LSZH is the European and confined-space philosophy: zero halogens, almost no smoke. Fitting a lower-class cable "because it's cheaper" isn't a saving: it's the finding that stalls your project handover.
Connectors: LC, SC and MPO, plus the polish you never mix
The connector is where the light goes in and out, and there are decisions there too. By size and use:
LC: the small one, with an RJ-style latch, almost always duplex (one fiber transmits, another receives). It's today's standard: the sky-high density of SFP+ ports crowned it. SC: the big square push-pull type, rugged, still common on carrier ODFs and GPON gear. MPO/MTP: the parallel connector, packing 12 or 24 fibers into a single face. It's not about more speed per fiber, but about carrying many strands together: it's what feeds 40G and 100G SR4, where the link uses 4 fibers to transmit and 4 to receive in parallel.
Mind simplex vs duplex: simplex is a single fiber (for BiDi or single-fiber); duplex is two, the normal case of a bidirectional link. And the detail that torches projects: the ferrule polish. UPC (Ultra Physical Contact, blue housing) has a flat-polished face. APC (Angled Physical Contact, green housing) is polished at an 8° angle, returning far less reflection and mandatory on sensitive links like GPON and RF video.
Example: 40 Gb/s between two buildings 600 m apart
Back to the opening scene, done right this time, with the standard's numbers:
- The requirement: 40 Gb/s between two buildings 600 m apart, a campus run across the parking lot.
- Test the OM4 multimode option: at 40G (40GBASE-SR4) OM4 reaches 150 m. The run is 600 m → 600 > 150. It doesn't reach, not close: short by a factor of 4.
- What about OM3? Worse: at 40G it reaches 100 m. Ruled out.
- Test OS2 singlemode: at 40G with LR4 optics OS2 reaches 10 km = 10,000 m. The run is 600 m → 600 < 10,000. Plenty to spare (over 16 times the margin).
- It's also outdoor and long: the cable must be loose-tube (with gel, for moisture and temperature), ideally armored against rodents, and spliced to an indoor riser/plenum cable at each entry point.
- Conclusion: pull OS2 singlemode with LR4 modules and LC connectors. It doesn't just solve the 600 m: it leaves the link ready to jump to 100G on the same strand the day it's needed, without touching the yard again.
The field lesson: when the run passes a couple hundred meters or you're going to bump up the speed, the question is no longer "which multimode?", it's "singlemode, for sure". Before buying, verify reach in the fiber reach calculator and close the link with the fiber loss budget to add up connector and splice loss.
Close the link, don't just pull it
Choosing the fiber well is half of it; the other half is making sure the light leaving the transmitter arrives at the receiver with enough power. Each connector adds on the order of 0.3 dB, each splice ~0.1 dB, and each kilometer of OS2 ~0.35 dB at 1310 nm. Add all of that, compare it against the module's power, and you'll see if the link closes with margin. That math is done by the fiber loss budget and the optical Rx margin, and it's worth converting units with the dBm/mW converter so you don't get lost between milliwatts and decibels.
Two useful closers: if the run is very long and you need to squeeze the fiber, wavelength-division multiplexing packs several channels onto one strand — size the grid with the DWDM/CWDM planner. And if the project is delay-sensitive (trading, sync, remote storage), remember light in fiber travels at ~5 µs per kilometer: measure the real delay with the fiber latency calculator. Well-chosen fiber gives no trouble for years; badly chosen fiber won't light up on day one. Choose by distance and by future, and let physics play in your favor.