If you work as a system integrator, low-voltage contractor, or in-house IT, the question “Which fiber should we pull?” comes up in almost every project. Vendors throw OS2, OM3, OM4, SR, LR at you, while you’re juggling four real constraints: distance, speed, budget, and future upgrades.

1. When to use singlemode vs multimode (plain-language version)

If you just want a rule of thumb before we dive into theory, start here:

• Pick OS2 singlemode when:
  • Links run between buildings, across a campus, or through risers and ducts.
  • The backbone will likely move from 1G/10G to 25G/40G/100G over the next years.
  • Pulling new fiber later would be painful or disruptive (ceilings opened, ducts full, approvals needed).
• Pick OM3 / OM4 multimode when:
  • Most links sit inside one data hall or wiring closet, typically under 100–150 m.
  • You want to keep CAPEX low by using cheaper SR optics instead of LR optics.
  • You’re mainly running 1G/10G today and can live with some rework when the next upgrade cycle comes.

The rest of this article just gives you a cleaner way to explain that decision to your customer, your boss, or your future self.

2. Basics: what is singlemode and what is multimode?

2.1 Singlemode fiber (OS2)

Singlemode fiber has a core of about 9 µm and only allows one propagation mode of light.

• Typical standard: OS2
• Typical wavelengths: 1310 nm and 1550 nm
• Very low modal dispersion, so it can push high speeds over long distances.
• The fiber itself is not expensive; the cost lives in the LR/ER optics.

You can think of singlemode as: “long distance, high speed, upgrade-friendly – optics cost more, but the glass rarely limits you.”

2.2 Multimode fiber (OM3 / OM4 / OM5)

Multimode fiber uses a larger 50 µm core and supports many propagation modes.

• Common standards: OM3, OM4, OM5
• Typical wavelength: 850 nm with SR optics
• More modal dispersion, so distance is limited, but SR optics are cheap and widely available.

Very rough, real-world numbers:

• OM3: about 300 m at 10G, around 100 m at 40/100G SR4.
• OM4: about 400–550 m at 10G, around 150 m at 40/100G SR4.
• OM5: niche, optimized for SWDM in certain high-density DC designs.

Multimode is basically: “short reach, optics are cheap, great inside a room or a hall – just don’t ask it to cross the campus.”

3. Singlemode vs multimode at a glance

Parameter	OS2 singlemode	OM3 multimode	OM4 multimode
Core size	~9 µm	50 µm	50 µm
Typical wavelength	1310 / 1550 nm	850 nm	850 nm
10G typical reach	10 km+ (10G LR)	~300 m (10G SR)	~400–550 m (10G SR)
40/100G typical reach	10–40 km (LR4/ER)	~100 m (SR4)	~150 m (SR4)
Cable cost	Similar to OM3/OM4	Low	Slightly higher than OM3
Optics cost	Higher (LR/ER)	Low (SR)	Low (SR)
Typical use	Campus & building backbones, DC interconnects	Short-reach DC links, SMB server rooms	Medium DCs, in-building backbones where distance > 100 m

From this table you can see why most designs end up as a mix: OS2 for long backbones, OM3/OM4 inside the room.

4. Scenario-based guidance: choose like you are on site

4.1 Office and campus networks

Typical environment:

• One main equipment room (MDF) and several floor closets (IDFs).
• Several buildings across a campus or industrial park.
• Today mostly 1G/10G, with 25G/40G being discussed for “later”.

Practical approach:

• Between buildings and in risers: pull OS2 singlemode as the backbone. It’s the cable you really don’t want to replace later.
• Inside the building, IDF → MDF (tens of meters):
  • If distance is safely under 100–150 m and budget is tight, OM4 + SR optics is efficient.
  • If closets are far apart or you know a 40G/100G backbone is coming, it’s often simpler to stay on OS2 here as well.

In many campus projects, the best compromise is: OS2 for all risers and building-to-building runs, OM4 only inside rooms where you control distance.

4.2 Data centers and server rooms

Typical environment:

• Top-of-rack (ToR) or end-of-row (EoR) switches.
• Rack-to-rack links mostly 5–30 m.
• Mix of 10G/25G/40G/100G today, with higher speeds on the roadmap.

Practical approach:

• Inside the rack or to the next rack:
  • 10G/25G: use DAC direct attach copper or very short OM3/OM4 + SR.
  • 40G/100G: short OM4 + SR4 links work well up to ~100–150 m.
• Row-to-row, still within one hall: OM4 is usually the sweet spot.
• Between halls or data centers: this is singlemode territory; use OS2 with LR/ER optics.

Here the logic is simple: optics volume is huge, so cheap SR optics + OM3/OM4 win for short runs, and OS2 is reserved for the few long links.

4.3 Building risers, FTTx and FTTR

Typical environment:

• Vertical risers feeding multiple floors.
• Floor distributors feeding rooms, ONUs or APs.

Practical approach:

• Use OS2 singlemode for risers and any cable you’ll hide in shafts or conduits.
• Use OS2 or OM3/OM4 on each floor depending on distance and the hardware you standardize on.

Once the shaft is closed, nobody wants to reopen it. That’s why most modern FTTH/FTTR and multi-tenant building designs go singlemode in the backbone from day one.

5. Don’t just compare cable price: where the real money goes

It’s tempting to ask “which cable is cheaper per meter?”. In real projects, most of the cost sits somewhere else:

• Optics (SR vs LR)
• Installation and rework
• Downtime and risk when you touch live infrastructure

A more useful way to think about it:

• For very short links inside racks or rows, it makes little sense to pull singlemode and pay for LR optics. DAC, AOC, or OM3/OM4 with SR optics will usually win on total cost.
• For backbones in risers, tunnels and outdoor ducts, the ability to upgrade from 10G to 40/100G by “just changing optics” often outweighs the extra cost of LR modules today.
• A balanced design is often: OS2 where re-pulling is painful, OM4 where it’s easy to touch later.

A simple question you can ask for each link is: “If I had to replace this cable in five years, how painful would it be?” The more painful the answer, the stronger the case for singlemode.

6. Step-by-step fiber selection checklist

Use this checklist whenever you design a new fiber link. It’s simple to explain to clients as well.

1. Measure distance.
   • < 30 m → think DAC / AOC / short OM3/OM4.
   • 30–150 m → both OM4 and OS2 are on the table.
   • > 150 m → usually OS2 singlemode is the safer choice.
2. Check current speed and the 3–5 year roadmap.
   • Staying at 1G/10G with limited growth → multimode is often fine.
   • Backbones that will carry 25G/40G/100G → favor singlemode.
3. Look at the environment.
   • Outdoor, between buildings, in ducts or shafts → OS2.
   • Inside racks and rooms → OM3/OM4, DAC and AOC give you flexibility.
4. Consider the optics you already own or plan to buy.
   • Lots of SR ports or optics already in place → keep using OM3/OM4 where it fits.
   • New deployments with unified optics strategy → standardizing on LR + OS2 can simplify stock.
5. Ask yourself about rework.
   • If re-pulling in that path would be a nightmare → lean towards OS2.
   • If it’s easy to access and change later → OM4 can save budget today.

If you want to turn this into a bill of materials (BOM), you just map each segment in your floor plan to one of the choices above and pick the corresponding cable type, connector, and length.

7. Where AMPCOM fits into this picture

Once you’ve decided on singlemode vs multimode for each part of the design, you still need the actual components to make it work and keep it tidy. That’s where AMPCOM’s structured cabling portfolio comes in.

• OS2 singlemode patch cords and trunks for campus and building backbones.
• OM3/OM4 LC and MPO patch cords and pre-terminated trunks for data halls and equipment rooms.
• High-speed DAC and AOC assemblies for short-reach 10G/25G/40G/100G server and switch links.
• Fiber patch panels and terminal boxes to keep racks clean and make moves, adds and changes safer.

If you have a floor plan or a rough description of your topology, you can share the distance, speed and environment for each hop. We can help you translate that into a concrete OS2/OM4 + DAC/AOC BOM and a neat rack-level layout that you can hand straight to your customer or installation team.

8. Quick FAQ

Can I splice singlemode and multimode fibers directly?

No. They use different core sizes and optics. Directly splicing or patching SMF to MMF is not supported. You need proper media conversion or modules designed for that purpose.

Can I run 10G over older OM2 multimode?

For very short distances it may work, but it’s not something you want to design into a new project. For new 10G deployments, OM3 or OM4 is the reasonable starting point.

Is OM5 worth it for most SMB and campus projects?

In most SMB networks and typical campus backbones, OM3/OM4 or OS2 already cover the needs. OM5 mainly makes sense in specialized data centers using SWDM optics and very high fiber-core utilization.