100G QSFP28 Selection Guide: SR4 vs LR4 vs CWDM4 vs DWDM4

A practical selection guide for 100G QSFP28 optics: SR4 vs PSM4 vs CWDM4 vs LR4 vs DWDM4 compared by wavelength plan, fiber type, reach, FEC assumptions, and cost tier — with a decision flow for real deployments.

Module Selection Guides · Updated 2026-10-24 · 4 min read

The short answer

Pick by distance and fiber plant first — wavelength plan, laser type, and cost follow from that decision:

*Reach figures assume RS-FEC enabled where the module spec requires it (CWDM4, PSM4). All five are 4 × 25.78125 Gbps electrical lanes in the same QSFP28 cage — the differences are entirely optical.

Related transceiver specifications are catalogued on Canjintech's product pages

SR4 uses four VCSEL pairs at 850 nm over parallel ribbon fiber — 8 fibers in an MPO-12 connector (4 TX + 4 RX).

• Reach: 70 m on OM3, 100 m on OM4. That covers in-rack, adjacent-rack, and short row-to-row runs.
• Power and cost: the lowest of the 100G family; no TE coolers, no DFB lasers, just VCSELs and photodiodes.
• Watch out for: MPO polarity (Type A/B/C methods), ribbon cable handling, and cassette insertion loss. Most "SR4 doesn't link" tickets are a polarity or contamination problem, not a module fault.
• Breakout: 100G SR4 ↔ 4 × 25G SFP28 SR is the standard pattern for feeding 25G servers from a 100G ToR.

CWDM4: the volume default — and why

CWDM4 packs four 25G lanes onto four coarse-WDM wavelengths (1271/1291/1311/1331 nm) over one duplex LC pair of single-mode fiber.

• Reach: 2 km with FEC — enough for the overwhelming majority of intra-campus datacenter links, which is why CWDM4 became the volume 100G module worldwide.
• Cost: uncooled DFB lasers keep it dramatically cheaper than LR4; expect roughly 2–3× SR4 pricing.
• Assumption: the CWDM4 MSA was written assuming RS-FEC at both ends. It is a "with-FEC" spec, not a defect.
• CLR4: the 10 km, tighter-margin variant of the same CWDM wavelength plan, promoted by the CLR4 Alliance for operators who want CWDM4 economics at LR4-class distance.

LR4: IEEE-grade margin for 10 km

100GBASE-LR4 (IEEE 802.3) uses the LAN-WDM grid — four wavelengths around 1296–1310 nm with tighter channel spacing and tighter laser tolerance than CWDM.

• Reach: 10 km per the IEEE spec, without relying on FEC.
• When to choose it: campus runs between buildings, metro-edge, older switch platforms with mandatory-FEC disabled, or anywhere you want the standards-defined margin rather than an MSA "assumes FEC" shortcut.
• Trade-off: LAN-WDM DFBs cost more than CWDM uncooled parts; some LR4 SKUs ship with a TEC for wavelength stability at temperature extremes.

DWDM4: for plants that already speak DWDM

DWDM4 places its four lanes on the ITU-T 100 GHz DWDM grid in the C-band.

• Why it exists: the signals can pass through DWDM muxes, filters, and amplifiers — so a DWDM4 link can coexist with, or ride over, an existing DWDM line system.
• Cost: TEC-cooled DWDM lasers make it the most expensive module in this table. Choose it when the fiber route is already DWDM infrastructure, not for plain dark-fiber runs — CWDM4 or LR4 will do that job for less.

FEC and mixed-vendor realities

Practical checks that prevent most field issues: confirm FEC is enabled on both switch ports, read DDM (Tx/Rx power, temperature) after link-up, and keep MPO/APC vs UPC polish consistent with the cassette plant.

Decision flow

flowchart TD
    A[100G link needed] --> B{Distance?}
    B -- under 100 m, MMF in place --> C[SR4]
    B -- 100 m to 2 km, SMF --> D[CWDM4]
    B -- 2 km to 10 km --> E{FEC available?}
    E -- yes --> F[CLR4 or LR4]
    E -- no --> G[LR4]
    B -- DWDM line system on path --> H[DWDM4]
    C --> I[Enable FEC + check MPO polarity]
    D --> I
    F --> I
    G --> I
    H --> I

Bottom line

• ≤100 m over existing MMF: SR4 — cheapest, coolest, breakout-friendly.
• 100 m–2 km over SMF: CWDM4 — the volume default for a reason.
• Up to 10 km or no-FEC plants: LR4.
• DWDM plant coexistence: DWDM4.

Match both ends to the same module type, keep FEC on where the spec assumes it, and use DDM to catch plant problems before they become tickets.

Compiled from IEEE 802.3 specifications, CWDM4/PSM4 MSA public materials, and common operator practice; original synthesis, no reproduction.

FAQ

Can I replace CWDM4 with LR4 freely?

No. They use different wavelength plans (CWDM 1271–1331 nm vs LAN-WDM ≈1296–1310 nm). Two modules of the same type must be used on both ends of a link. Physically they share the duplex-LC SMF plant, but the optics will not link across types.

Does CWDM4 work without FEC?

The CWDM4 MSA spec assumes RS-FEC (KP4/KR4 FEC) is enabled. Most switches enable it by default for these rates, but if FEC is off, reach and margin degrade sharply. Verify FEC status in the switch before blaming the module.

Is SR4 compatible with 25G SFP28 ports?

Yes, via breakout: a 100G SR4 in a QSFP28 port breaks out to 4 × 25G SFP28 SR over an MPO-to-4×duplex-LC harness. This is the standard way to light 25G server ports from a 100G uplink.

How far does 100G run on OM4 multimode fiber?

100G SR4 reaches 100 m on OM4 (70 m on OM3). Beyond that, you need single-mode modules — CWDM4 is the usual next step, reaching 2 km over the same duplex LC patching style.