Fiber Optic Cable Types: A Practical Guide for SMB and Campus Networks

Fiber backbone design used to be a niche topic for service providers and large data centres. Today it shows up in almost every serious SMB and campus network: uplinks between wiring closets, links to server rooms, building-to-building runs and connections to ISP demarcation points. The part that often slows projects down is not the switch port or transceiver, but a simple question: which fiber cable type is actually the right one here?

This guide looks at fiber optic cable types the way a system integrator or IT manager would: not as a catalogue of codes, but as a set of design decisions. We focus on the combinations that really appear in structured cabling projects, and leave the exotic variants to specialist designs. If you are already familiar with structured cabling basics, you can treat this article as the “fiber chapter” of your playbook.

1. Three Dimensions of Fiber Cable Choice

When someone asks “what fiber optic cable types do you have?”, they are usually mixing three different dimensions into one question. Breaking them apart makes projects much easier to reason about:

1) Transmission mode and core size. This is the familiar single-mode vs multimode distinction: OS2 for long-reach, OM3/OM4 for shorter, high-bandwidth links inside a building or campus. It determines how the light travels and which transceivers you can use.

2) Cable construction. Tight-buffered vs loose-tube, simplex vs duplex vs breakout, indoor vs outdoor, armoured or non-armoured. This is about how the fibers are packaged and how they behave during pulling, termination and everyday handling.

3) Jacket material and rating. OFNR, OFNP, OFN and LSZH markings are not speed grades; they are fire-safety and smoke-toxicity ratings that determine where the cable may legally be installed.

Once you see fiber choices on these three axes, the catalogue turns from a long list of part numbers into a small number of patterns that repeat across projects.

2. Single-Mode vs Multimode: OS2, OM3 and OM4 in Plain Language

The most visible split between fiber types is mode. At a physical level it is about how many “paths” light can take through the core of the fiber. In network design terms it becomes a trade-off between reach, bandwidth, optics price and how much legacy equipment you need to support.

2.1 Single-Mode OS2 – The Long-Reach Workhorse

Single-mode fibers have a very small core (around 9 µm). Light essentially travels in a single path, which keeps dispersion low and allows the signal to survive over many kilometres. Modern outdoor and campus single-mode is almost always OS2, optimised for long-wavelength operation around 1310/1550 nm.

In structured cabling projects you typically see OS2 in three situations: building-to-building runs on a campus, long risers between equipment rooms, and links to carrier demarcation points. It is also the default whenever you expect to migrate from 1G to 10G, 40G or 100G over longer distances, because the same OS2 plant can be re-used with different optics generations.

2.2 Multimode OM3/OM4 – High Bandwidth Inside the Building

Multimode cores are much larger (50 µm in modern systems), allowing multiple light paths. That makes the optics cheaper and easier to couple, but introduces distance limits at higher speeds due to modal dispersion. The OM classification tells you how much bandwidth the fiber can carry over a standard link length.

For current SMB and campus projects the realistic choices are OM3 and OM4. OM1/OM2 still exist in older buildings but are not recommended for new installations. OM3 supports 10G to 300 m at 850 nm and can handle 40G/100G over shorter distances. OM4 stretches the reach for 10G to around 400 m and gives more comfortable margins for 40G and 100G in data rooms.

In practice, the choice is usually simple: use OS2 whenever you are crossing car parks or open ground, and use OM3/OM4 where both ends sit inside the same building or in adjacent buildings within a few hundred metres. Many SMBs standardise on OS2 for all new outside plant and on OM4 for new data rooms to keep the optics options open.

3. Cable Construction: Tight-Buffered, Loose-Tube and Common Formats

Once you know whether the link will be single-mode or multimode, the next question is how the fibers are packaged. The mechanical construction determines how well the cable survives pulling, bending, moisture and incidental abuse.

3.1 Tight-Buffered Indoor Cables

Tight-buffered fiber places a 900 µm buffer around each individual fiber. Several of these buffered fibers are then bundled together under a common jacket, often with aramid yarn for strength. These cables are easy to strip and terminate directly into connectors or indoor splice trays, which is why you see them in risers, equipment rooms and indoor patching.

Typical formats include simplex (one fiber), duplex (two fibers joined together) and distribution cables with 4, 6, 12 or more fibers in one sheath. For short indoor runs, duplex tight-buffered cable feeding LC connectors on a patch panel is still the most common pattern.

3.2 Loose-Tube Outdoor and Universal Cables

Loose-tube designs use gel-filled or dry tubes that contain several bare fibers each. These tubes are then stranded around a central strength member and protected by an outer jacket, sometimes with armour. The design keeps moisture away from the glass and allows the cable to tolerate temperature extremes and mechanical stress, which is exactly what you need outdoors or in ducts.

When you specify a campus backbone between buildings, you are usually looking at OS2 loose-tube cable, often with armouring where there is a risk of rodent damage or accidental digging. Termination is typically via splice trays into pigtails and patch panels rather than direct connectorisation of the loose tubes.

3.3 Breakout and Fan-Out Cables

Breakout cables sit somewhere between the two extremes. Each fiber is given its own secondary jacket so it can be treated almost like a tiny individual cable. A breakout cable with 12 fibers, for example, can be split in a cabinet so that each core can be terminated on a separate connector without additional protection tubes.

This format is useful in smaller data rooms where you are feeding multiple devices from one trunk, but do not want to manage a full blown splice enclosure or a large patch field. In higher-density environments the same role is often played by pre-terminated MPO/MTP trunks, which are covered in a separate design guide.

4. Jacket Ratings: OFNR, OFNP and LSZH in Practice

The strings of letters printed on the cable jacket are often where non-specialists lose confidence. They are not arbitrary; they encode how the cable behaves in a fire and which building spaces it may legally occupy.

In North American practice the key markings are OFN, OFNR and OFNP. In other regions you will also see LSZH referenced as a requirement. The table below summarises what each label implies.

The important point is that these ratings are not interchangeable: you cannot substitute an OFNR cable wherever you feel like it if the local code requires OFNP for plenum spaces. In many SMB buildings the practical rule is simple: use OFNR for vertical shafts and OFNP for any run passing through a ceiling that is used as a return-air plenum. When in doubt, check the local electrical code or work with an installer who regularly signs off on inspection.

LSZH is often specified in regions where smoke toxicity is a bigger concern than strict plenum definitions. It is not a drop-in replacement for OFNR/OFNP in every jurisdiction, but it does indicate a jacket that produces less corrosive and opaque smoke when exposed to fire.

5. Matching Fiber Types to Common SMB and Campus Scenarios

With the building blocks covered, the real design work is to match them to actual links. In day-to-day projects the same scenarios appear again and again. Using them as templates saves time during design reviews and RFQs.

5.1 Building-to-Building Campus Backbones

For campus links that cross open ground or run through external ducts, OS2 single-mode in a loose-tube outdoor or universal cable is the default. It gives you the reach to handle future speed increases and tolerates temperature swings and moisture. Where there is a risk of mechanical damage or rodents, armoured versions are worth the extra cost.

Termination usually happens on fiber patch panels inside each building, with a short run of fiber patch cords to switches or routers. Choosing LC connectors for single-mode makes it easier to mix and match transceivers as the network evolves.

5.2 Vertical Risers Inside a Building

When you are linking an equipment room to multiple floor distributors, both OS2 and OM3/OM4 can make sense. If the building is likely to carry mostly 1G and 10G traffic over the next decade and the risers are only tens of metres, multimode OM4 is often enough and keeps optics cost low. If you expect to carry higher-speed uplinks or aggregate traffic from several floors, single-mode OS2 risers may be a safer long-term choice.

In either case the cable should have an OFNR rating at minimum. Many designers standardise on tight-buffered distribution cables here because they are straightforward to route and terminate in indoor hardware.

5.3 Short Links Inside Data Rooms

In server rooms and aggregation closets the fibres are short and the density is high. Here the fight is less about reach and more about patching convenience and panel density. OM4 multimode with LC or MPO connectivity is common for 10G/40G equipment rows, often using pre-terminated trunks and cassettes.

If you are planning a small data room for an SMB, you may keep things simple with duplex OM4 LC links between switches and servers. As the environment grows you can layer MPO trunks and cassettes on top of the same basic cabling choices.

5.4 Connections to ISP Demarcation Points

When connecting your internal plant to a service provider hand-off, OS2 single-mode is almost always the right answer. Providers design their access equipment around single-mode optics, and staying aligned avoids surprises later. It also lets you re-use the same access fiber if you change bandwidth tiers or carriers in the future.

6. Common Mistakes When Working with Fiber Cable Types

Even experienced teams occasionally trip over the same issues. The patterns below show up repeatedly in as-built documentation and post-installation troubleshooting.

One mistake is mixing fiber types within what everyone assumes is a single, homogeneous plant. For example, a riser might be built on OM4 but patch cords are sourced from an old stock of OM2. The result works at 1G and may even pass basic light tests, but fails intermittently at 10G. Keeping a clear record of which fiber type is used on each link and standardising patch-cord ordering prevent this class of problem.

Another is under-estimating the importance of jacket rating. Using an indoor OFNR cable in an outdoor duct or through a plenum ceiling may work electrically on day one, but it creates compliance issues and raises questions for insurers and inspectors. The extra effort to align each run with the correct OFNR/OFNP or LSZH rating is far cheaper than re-pulling cable after an audit.

Finally, documentation for fiber plants is often an afterthought. Unlike copper, where you can sometimes rely on simple continuity testing and visual tracing, fiber links often disappear into splice trays and ducts. A simple single-line diagram that lists which fiber type, construction and jacket rating is used on each route can save many hours when something needs to be upgraded or repaired years later.

7. Bringing It All Together

From a distance the world of fiber optic cable types can look overwhelming. Once you break it down into mode, construction and jacket rating, the design space for SMB and campus networks becomes much smaller and more manageable. OS2 single-mode for campus and carrier edges, OM3/OM4 for indoor links, tight-buffered for risers, loose-tube for outdoors, and OFNR/OFNP or LSZH selected according to the spaces the cable traverses – those patterns cover the vast majority of real projects.

Treat your fiber plant as part of the long-lived infrastructure of the building, not as an accessory to this year’s switches. If you do, the choices you make today on fiber types will still look reasonable when the next generation of optics and applications arrives.

FAQ: Fiber Optic Cable Types

Is OS2 always better than OM3 or OM4?

OS2 single-mode offers much longer reach and flexibility for future upgrades, but the optics are usually more expensive. Inside a building, where distances are short and link counts are high, OM3/OM4 multimode can still be more economical while meeting all bandwidth needs.

Should I still install OM3 or go straight to OM4?

For new projects many integrators standardise on OM4 because the price difference to OM3 has narrowed and OM4 gives more margin for higher speeds. If your design is very conservative and will never go beyond 10G, OM3 remains acceptable, but OM4 is a safer default.

Can I mix different multimode types in one link?

Mixing multimode types in the same channel is not recommended. The weakest section will dictate performance, and mixing OM2 with OM3 or OM4 can lead to unpredictable behaviour at higher speeds. It is better to keep each link on a single, documented multimode type and standardise patch cords to match.

When is LSZH more appropriate than OFNR/OFNP?

LSZH jackets are useful in enclosed or high-occupancy areas where smoke toxicity is a concern, such as tunnels, transport hubs or some public venues. Depending on local regulations they may be used alongside or instead of OFNR/OFNP, but they are not automatically interchangeable with North American riser and plenum ratings.