---
title: "CWDM4 vs PSM4 for 100G: Why the Four-Wavelength Decision Matters More Than You Think"
type: comparison
target_audience: technical
score: 9/10
---

The 100G QSFP28 market bifurcated cleanly along two lines when IEEE 802.3bm ratified in 2015: CWDM4 and PSM4. Both deliver 4x25G lanes over SMF to 500m, both land at roughly the same optical reach, and at a glance both seem interchangeable for the same cabling run. They are not. The decision between them compounds across thousands of ports in a real data center build, and getting it wrong means either pulling fiber or throwing away optics, neither of which is cheap.

PSM4 — Parallel Single Mode 4-lane — is conceptually the simplest architecture imaginable. Four 25G lanes each travel over a separate single-mode fiber at approximately 1310nm (the individual lane wavelengths are not tightly controlled since they don't need to be wavelength-division multiplexed), with all four using NRZ modulation at 25.78125 Gbps per lane. The connector is an MPO-12 (the outer 4 fibers on each side unused), which means every PSM4 link consumes eight fiber strands. This is the critical arithmetic: a 48-port leaf switch with PSM4 uplinks requires 192 individual fibers just for the uplinks. In a spine-leaf fabric with 10,000 server-facing 25G ports and 400 100G uplinks, PSM4 alone demands 3,200 strands of single-mode fiber between the layers. The Senko MPO connectors on production PSM4 modules — as used in Innolight TR-FC13J-NCD or Flexoptix P.10741 — have a mechanical life of roughly 500 insertion cycles before ferrule wear degrades the contact geometry enough to affect loss budget.

CWDM4 takes those same four 25G lanes and wavelength-division multiplexes them onto two fibers using four distinct center wavelengths: 1271nm, 1291nm, 1311nm, and 1331nm, with 20nm channel spacing. The two fibers are LC-duplex, which is the same connector your existing 10G and 40G plant almost certainly uses. The mux/demux is done with thin-film filter arrays inside the module itself. Each lane has its own CDR (Clock and Data Recovery) circuit, which is why CWDM4 modules burn approximately 3.5W versus PSM4's 2.5W — an additional 1W per module that, across a 10,000-port fabric, adds up to 10kW of additional cooling load. Flexoptix P.10733 and the Finisar FTLC1152RDNM are representative production examples. The CDR also introduces approximately 100ps of additional lane-to-lane deskew processing, though this is irrelevant for Ethernet since 802.3bm Clause 87 allows up to 120ns of skew between lanes.

The cost differential has narrowed considerably from 2017 highs when CWDM4 modules cost nearly three times PSM4, but a material gap remains. In volume pricing as of early 2026, compatible CWDM4 QSFP28 modules from a quality vendor like Flexoptix or ProLabs land at approximately €180-220 per unit, while PSM4 equivalents are €120-150. On a 400-port spine layer that is a €24,000 to €28,000 difference just in optics. That number must be weighed against fiber plant cost: an 8-fiber MPO trunk cable costs roughly 40% more than a 2-fiber LC-duplex equivalent for the same run length, and MPO cassettes for breakout add another €15-25 per port of termination cost. The crossover point where PSM4's cheaper optics are eaten by higher fiber plant costs typically occurs around the 200-300 port threshold for new greenfield builds where fiber is being installed anyway.

For brownfield environments, CWDM4 almost always wins on economics even at its optics premium. Any data center built after 2010 has LC-duplex SMF infrastructure to every cabinet. Pulling new 8-fiber MPO trunks to replace 2-fiber LC runs costs €8-15 per meter in installation labor plus materials, so a 50-meter average run to 400 switch ports is €160,000-300,000 in fiber plant costs before a single PSM4 module is purchased. The CWDM4 optics premium of €70 per module times 400 modules is €28,000 — a trivial fraction.

The interoperability risk that gets overlooked in vendor comparisons is connector polarity. PSM4 uses Type B MPO polarity (per TIA-568-C.3), meaning the fiber labeled 1 at one end connects to fiber 1 at the other. A Type A MPO cassette — the most commonly pre-installed type in legacy data centers — crosses the fibers, which will work fine for 40G QSFP+ where both ends use MPO, but PSM4 QSFP28 requires methodical polarity management. Plugging a PSM4 module into an incorrectly polarized MPO plant is a non-obvious failure: the module will power on, DOM will show nominal TX power on all four lanes at the transmitting end, but the far end will show either zero RX power or a scrambled fiber-to-lane mapping that produces persistent bit errors. Field engineers unfamiliar with PSM4 will spend 45 minutes inspecting the optics before realizing the MPO cassette orientation is wrong.

Platform support nuances also favor CWDM4 in heterogeneous environments. Cisco Nexus 9332C and 93180YC-FX both support CWDM4 and PSM4, but the 9200 series requires a firmware upgrade to enable PSM4 auto-negotiation correctly, and Juniper QFX5120-48Y had a known bug in Junos 20.2R1 where PSM4 modules would intermittently fail to come up after a port flap until the bug was addressed in 20.2R3. CWDM4 with its LC-duplex interface is electrically and mechanically simpler from the platform's perspective — the transceiver looks and behaves more like a conventional duplex interface, which means fewer edge cases in NOS port drivers.

The decision framework is straightforward once you quantify the numbers. For new hyperscale builds where leaf-to-spine cabling is being installed from scratch, PSM4 saves real money at scale when the fabric exceeds roughly 500 ports per tier. For enterprise data centers operating on existing LC-duplex SMF plant, any calculation that ends with pulling and replacing fiber plant for PSM4 should be rejected — CWDM4 at its optics premium is the rational choice. For inter-building runs where the fiber plant is OS2 single-mode but the connectors are already MPO for 40G migration, PSM4 is worth evaluating only if you have verified Type B polarity throughout. Mixed environments — where some switches use CWDM4 and some PSM4 — require optical-to-electrical breakout panels at the connection point, since you cannot directly couple a CWDM4 module to a PSM4 module regardless of the fiber plant. These modules are not optically compatible, full stop.

One final consideration: CWDM4 gives you a more credible upgrade path to 400G CWDM4 (100G per lane, 4 lanes on the same 1271/1291/1311/1331nm wavelength plan per IEEE 802.3bs Clause 87), meaning your fiber plant investment carries forward. PSM4 fiber infrastructure does the same job for 400G-DR4 (IEEE 802.3bs Clause 124), but DR4 requires OS2 with 0.2dB/km loss specification and highly polished MPO connectors, not the generic OM3/OM4 that 40G PSM4 sometimes ran on with margin to spare. If your 10-year fiber plant investment needs to justify both present-day 100G and future 400G density, the wavelength route with LC-duplex is the lower-risk architectural bet.
