100G remains a widely deployed interface for data center interconnects, campus networks, and network upgrades. Yet selecting a 100G QSFP28 transceiver is not simply a matter of choosing the longest available reach.

PSM4, CWDM4, and LR4 all operate over single-mode fiber, but they differ substantially in optical architecture, connector type, fiber utilization, reach, and deployment economics. The right choice depends on the actual link rather than the headline distance alone.

This guide compares the three options from an engineering selection perspective, focusing on where each architecture fits and what should be checked before deployment.

Where the Three 100G QSFP28 Options Fit

The three solutions address different combinations of reach and cabling infrastructure:

Where PSM4 Makes Sense

100G QSFP28 PSM4 uses four parallel 25G optical lanes over single-mode fiber. Four fibers carry transmit signals and four carry receive signals, typically through an MPO/MTP interface.

Choose PSM4 when parallel optics match the cabling plan

Its architecture is well suited to short-reach, high-density links where parallel single-mode infrastructure is already available.

Breakout is a practical advantage

Because the 100G interface is built from four independent 25G lanes, PSM4 can be useful in 100G-to-4×25G connectivity when the host equipment and breakout design support it.

Check fiber consumption before choosing it

PSM4 uses more fiber cores than duplex-LC WDM solutions. In facilities standardized on duplex LC, the cabling change may outweigh savings at the transceiver level.

Where CWDM4 Fits Better

100G QSFP28 CWDM4 carries four 25G lanes on separate CWDM wavelengths and multiplexes them onto a duplex single-mode fiber pair. This allows 100G transmission through a familiar Duplex LC interface while extending reach beyond typical short-reach parallel-optics deployments.

Use existing duplex fiber more efficiently

CWDM4 requires only one transmit and one receive fiber, making it attractive where fiber availability is constrained or existing LC cabling should be retained.

The 2 km class addresses many campus-scale links

It can cover connections between data halls, buildings, or nearby facilities without moving directly to a 10 km-class optic.

It is often the middle ground between PSM4 and LR4

Where PSM4 consumes too much fiber and LR4 provides unnecessary reach, CWDM4 can offer a practical balance of cabling simplicity, reach, power, and cost.

When LR4 Is the Better Choice

100G QSFP28 LR4 uses four LAN-WDM optical lanes over duplex single-mode fiber and is designed for links up to 10 km. It is the more appropriate option when the physical path or link budget exceeds the practical range of CWDM4.

Select LR4 for genuinely longer links

Typical use cases include cross-campus connections, longer data center interconnects, and network segments where additional optical budget is required.

Do not select reach in isolation

Connector loss, patch panels, splices, fiber attenuation, engineering margin, and the exact transmitter/receiver specifications all affect whether a link will close.

Avoid unnecessary overprovisioning

For a short, straightforward link, paying for 10 km-class optics may add cost and power without improving the application.

PSM4 vs CWDM4 vs LR4: A Practical Selection Path

A useful selection sequence is:

1. Start with the required reach and calculate the actual link budget rather than relying only on nominal distance.
2. Check the installed fiber plant: MPO/MTP parallel cabling favors PSM4, while duplex LC infrastructure naturally favors CWDM4 or LR4.
3. Confirm whether 100G-to-4×25G breakout is required. If so, a parallel-optics architecture may be preferable.
4. Compare total deployment cost, including transceivers, fiber count, patching, cabling changes, power, and future expansion—not module price alone.

Three Common Deployment Scenarios

ETERN Optoelectronics 100G QSFP28 Solutions

ETERN Optoelectronics provides a range of 100G QSFP28 optical transceivers for different fiber architectures and transmission distances, including PSM4, CWDM4, LR4, SR4, and other 100G connectivity options.

For an actual deployment, module selection should be validated against the target switch or router, fiber type, connector infrastructure, required reach, link budget, FEC requirements where applicable, and the specifications of the selected transceiver.