SR4 vs LR4 vs ER4 vs ZR4: Which 100G QSFP28 Module Should You Really Use?

■ 100G QSFP28 Single-Fiber Optics: Choosing the Right Module for Your Network (And Why Cabling Matters More Than You Think)

Let me tell you a quick story.

A data center manager in Chicago once called us in a panic. His new 100G backbone was failing — randomly. No alarms, no clear errors. Just sluggish performance and dropped links. He had spent big money on high-end switches. Top-tier 100G QSFP28 modules. Everything should have worked.

But it didn’t.

After a site visit, we found the culprit: he used OM3 multimode fiber for a 10km LR4 link.

That’s like putting racing tires on a tractor and expecting Formula 1 speed.

This happens more than you think.

When upgrading to 100G, people focus on the shiny new gear — the switches, the modules — but forget the foundation: the cable plant.

So today, let’s break down four key 100G QSFP28 single-fiber modules:
SR4, LR4, ER4, and ZR4 — what they do, where they work, and how to avoid costly mistakes.

■ 1. QSFP28-100G-SR4 – The Short-Range Workhorse

Best for: Data center racks, intra-building links
Max distance: 100m (OM3), 300m (OM4)
Fiber type: Multimode (OM3/OM4), MPO-12 connector
Standard: 100GBASE-SR4

The SR4 is the most common 100G module in data centers.

It uses four lanes of 25G NRZ over 850nm VCSEL lasers. Each lane sends light through one fiber, so you need 8 fibers total (4 Tx, 4 Rx) in an MPO ribbon cable.

Think of it like a 4-lane highway — all lanes moving traffic at once.

✅ Pros:
– Low cost
– Low power (~3.5W)
– High reliability with VCSEL tech
– Supports DDM (Digital Diagnostics Monitoring)

⚠️ Watch out:
– Only works with multimode fiber
– OM3 supports 100m, OM4 supports 300m — no more
– MPO connectors must be clean and aligned

📌 Real-world tip:
If you’re connecting two racks in the same room, SR4 is perfect. But don’t try to stretch it beyond 300m. Physics says no.

■ 2. QSFP28-100G-LR4 – The City Connector

Best for: Campus links, metro networks, DCI up to 10km
Max distance: 10km (some up to 20km)
Fiber type: Single-mode (OS2), duplex LC
Standard: 100GBASE-LR4

Now we move to single-mode fiber.

LR4 uses Wavelength Division Multiplexing (WDM). Instead of four fibers, it sends four wavelengths (1295, 1300, 1305, 1310nm) over one fiber pair (Tx and Rx).

Each lane runs at 25.78 Gbps NRZ. They’re combined (muxed) into one stream.

At the other end, they’re split (demuxed) back into four.

✅ Pros:
– Only needs 2 fibers (LC duplex)
– Up to 10km reach
– DDM support via I2C
– RoHS compliant

⚠️ Common mistake:
Using old or low-quality OS2 fiber. Microbends or splices with loss >0.2dB can kill LR4 links.

📌 Data point:
A 2022 study by the Uptime Institute found that over 30% of 100G link failures in metro networks were due to poor fiber quality — not module defects.

✅ Pro tip: Always test fiber with an OTDR before deployment.

■ 3. QSFP28-100G-ER4 – The Long-Distance Player

Best for: Long-haul DCI, telecom backbones
Max distance: Up to 40km
Fiber type: Single-mode (OS2), duplex LC
Standard: 100GBASE-ER4

ER4 is like LR4’s tougher cousin.

It uses EML lasers (Electro-absorption Modulated Lasers) and APD receivers — more sensitive, better for long distances.

Same WDM setup: 4 wavelengths, 25.78 Gbps each, over one fiber pair.

But it runs hotter (~5W) and costs more.

✅ Pros:
– 40km reach
– APD receivers detect weaker signals
– Full DDM support
– Industrial temp range options

⚠️ Warning:
ER4 modules are optical power hungry. They can overload receivers if used with short fiber and no attenuation.

■ 4. QSFP28-100G-ZR4 – The Longest Reach (80km+)

Best for: WAN, cross-city links, carrier networks
Max distance: 80km+
Fiber type: Single-mode (OS2), duplex LC
Standard: OIF-100G-ZR (not IEEE)

ZR4 is not a standard Ethernet module. It follows OIF implementation agreements, which means better efficiency and longer reach.

It uses coherent optics in some cases, but most ZR4 modules today are direct-detect with SOA amplifiers.

Key tech:
– SOA (Semiconductor Optical Amplifier) boosts weak signals
– LAN-WDM wavelengths (same as LR4/ER4)
– Tight dispersion tolerance

✅ Pros:
– Over 80km reach
– Lower cost than coherent 100G
– Plug-and-play with compatible switches
– DDM and alarm pins (RxLOS, TxDisable)

⚠️ Reality check:
ZR4 is sensitive to chromatic dispersion and polarization mode dispersion (PMD).

Old fiber plants (especially pre-2005) may not support it.

📌 Fact:
According to OIF testing, ZR4 requires PMD < 10 ps/√km and CD < 600 ps/nm for 80km.

If your fiber doesn’t meet this, your link will flicker — or fail.

✅ Advice: Test dispersion before deploying ZR4.

■ 100G QSFP28 Optical Module Comparison Table Single-Mode Fiber​

• OM3/OM4: Laser-optimized multimode fiber for 850nm transmission.
• G.652.D: Standard single-mode fiber, most widely used in metro and data center interconnects.
• G.654.E: Low-loss, large effective area single-mode fiber, optimized for long-haul and high-power transmission (e.g., ZR4, coherent systems).

🧠 Q&A: Common Questions (and Mistakes) About 100G QSFP28

Q: Can I use an LR4 module in an SR4 port?
A: No. SR4 uses MPO and multimode. LR4 uses LC and single-mode. Physically and optically incompatible.

Q: Why is my 100G link only running at 4x25G?
A: That’s normal. 100G modules use 4 lanes. The “100G” is the total, not per lane.

Q: Can I mix ER4 and ZR4 in the same link?
A: No. Different power levels, sensitivity, and standards. Even if they light up, performance will be unstable.

Q: Do I need active cooling for ZR4?
A: Not usually. But ZR4 runs hotter (~4.5W). Ensure good airflow in high-density racks.

Q: Is OM5 fiber better for SR4?
A: Slightly. OM5 supports SWDM, but SR4 doesn’t use it. For SR4, OM4 is fine.

Q: Can I use LR4 for 20km?
A: Some vendors offer 20km LR4. But standard is 10km. Check datasheet. At 20km, you’re pushing limits — test carefully.

🔑 Key Takeaways: The 100G Deployment Checklist

1. Know your distance. Pick the module that matches — no overreach.
2. Check your fiber type. MMF vs SMF is not optional.
3. Test your cable plant. Clean connectors, check loss, OTDR if >10km.
4. Respect power levels. Use attenuators for short ER/ZR links.
5. Don’t assume compatibility. MSA helps, but not all modules work in all switches.

Final Thought: Infrastructure Is Everything

You can buy the best car in the world.

But if the road is full of potholes, you’re not going anywhere fast.

The same is true for 100G networks.

SR4, LR4, ER4, ZR4 — each has its place.

But none will save you from dirty connectors, mismatched fiber, or poor planning.

So before you order that shiny new module, ask yourself:

“Do I *really* know what’s in the patch panel?”

Because in networking, the foundation decides the future.

Need help choosing the right 100G module? We’ve tested thousands. Drop us a line — no sales pitch, just straight advice.