Three 100G parts, three different fibre plans
CWDM4, PSM4 and DR4 all carry 100G over single-mode fibre, and all three fit the same QSFP28 cage, which is why they are so often confused in a bill of materials. They are not interchangeable. Each one makes a different assumption about the fibre plant: how many strands are available, whether a mux can sit in the link, and whether the next step is a 400G port that will want the same cabling.
This guide sets out what each part does, what it costs in cabling and patch panels, and how the choice carries into 400G and 800G. Rate families are listed on the 100G QSFP28 and 400G QSFP-DD and OSFP pages, and the wavelength options are compared on the CWDM, DWDM and SWDM4 page.
How each part uses the fibre
CWDM4 carries 100G as four 25G lanes on four coarse wavelengths between 1271 nm and 1331 nm, multiplexed inside the module onto one duplex pair. The mux and demux are part of the module, so the link looks like an ordinary duplex single-mode link, which is why it fits existing dark fibre so easily.
PSM4 carries four 25G lanes in parallel, all at 1310 nm, each on its own fibre, over an eight-fibre MPO connector. There is no wavelength plan and no mux: four strands go one way and four the other. It reaches further than a duplex grey part and tolerates a plant that already has MPO trunks.
DR4 also uses four parallel 25G lanes over eight-fibre MPO, but the lanes are tuned for a shorter, tighter budget, which is why DR4 and the newer DR4+ class are common at 500 m to 2 km on structured cabling. DR1 takes the opposite route: one 100G lane at 1310 nm on a duplex pair, no breakout at all.
CWDM4, PSM4 and DR4 at a glance
| Module | Lanes | Media / wavelength | Reach class | Where it usually lands |
|---|
| 100G CWDM4 | 4 x 25G | SMF duplex, 1271 to 1331 nm | 2 km class | Existing duplex single-mode links between racks or buildings |
| 100G PSM4 | 4 x 25G | SMF, 8-fibre MPO, 1310 nm | 500 m to 2 km class | Trunks already cabled with MPO and no spare duplex pair |
| 100G DR1 | 1 x 100G PAM4 | SMF duplex, 1310 nm | 500 m class | Single-lane links and simpler patch records |
| 100G DR4 and DR4+ | 4 x 25G | SMF, 8-fibre MPO, 1310 nm | 500 m to 2 km class | Structured cabling with an upgrade path to 400G |
| 100G FR4 | 4 x 25G | SMF duplex, LAN-WDM around 1310 nm | 2 km class | Duplex campus runs at the same reach as CWDM4 |
| 400G DR4 and DR4+ | 4 x 100G PAM4 | SMF, 8-fibre MPO, 1310 nm | 500 m to 2 km class | The same MPO trunk running four times the rate |
| 400G FR4 and LR4 | 4 x 100G PAM4 | SMF duplex, CWDM or LAN-WDM | 2 to 10 km class | Duplex runs where MPO trunks are not available |
Reach classes are the standard ones for each family and the working limit is the loss measured through the real patch panels. One worked example of the parallel route is the APX-QSFP-DD-400G-2KM-DR4P part, and the duplex side is covered by the rate pages above.
Fibre count, patch panels and the cost that is easy to miss
The headline difference is fibre count. A CWDM4 link uses one duplex pair. A PSM4 or DR4 link uses eight strands, so it needs an MPO trunk and an MPO to LC or MPO to MPO patch arrangement at both ends. On a campus with spare duplex fibre the CWDM4 route is cheaper and simpler; in a rack that was cabled with MPO trunks, a parallel part avoids a whole new cable pull.
Count the strands before the optics, and count them for the growth case, not just the first link. A trunk that will carry ten 400G links needs eighty strands on the parallel route, against twenty on the duplex route. Where the duct is already full, that difference decides the cable plan.
Breakout, sparing and mixed cages
Parallel parts breakout cleanly: a 400G DR4 port can feed four 100G DR1 links, or a 100G PSM4 port four 25G links, on the same MPO trunk. Duplex WDM parts do not break out without an external mux and a wavelength plan, so a fabric that expects to step down rates at some ports is easier to run on the parallel route.
Sparing follows the same split. Parallel parts are held per rate and per lane count, and one spare of the higher rate covers the breakout positions beneath it; duplex WDM parts are held per rate and per grid, and the spare has to match the wavelength plan of the link.
How to choose, in order
- Count spare strands per link and the strands a growth case would need.
- Check what is already cabled: duplex SMF, MPO trunk, or both.
- Measure the route including patch panels and set the reach class from the result.
- Decide whether any port will break out to a lower rate later.
- Pick the duplex WDM part or the parallel part, then fix the connector and polarity.
- Set the spare rule and the patch record before the first link is turned up.
Frequently asked questions
Can CWDM4 and PSM4 be patched into each other?
No. One expects a duplex pair carrying four multiplexed wavelengths and the other eight parallel fibres. They are not interchangeable even at the same rate.
Is CWDM4 or FR4 the better duplex choice?
Both reach about 2 km on a duplex pair. Pick on the wavelength plan and on what the far end already runs, not on the label alone.
Does a DR4 link need a wavelength plan?
No, all four lanes sit at 1310 nm on separate fibres, so there is no channel assignment. What it needs is the correct MPO polarity and a clean fibre count.
Will 100G DR4 cabling carry 400G later?
Often yes, when the trunk is eight-fibre MPO and the loss budget suits the 400G DR4 class. Measure the loss before committing, since the tighter budget is the deciding factor.
What do you need to quote these parts?
The link list with rate, measured distance and loss; the cable already installed; the connector type; whether a breakout is planned; and the temperature at each end.
Send the link list with the measured distances, the cable type in place and whether any port will break out later, and we come back with the duplex or parallel part, the reach class and the connector per link. Related reading: the 100G QSFP28 and 400G families, the wavelength comparison, and all optics selection guides.
