100G QSFP28 Optical Transceivers: SR4, CWDM4, LR4 and DWDM
QSFP28 is the 100G hot-pluggable form factor most carrier and data-centre platforms use: four 25G lanes in one cage, electrically compatible with the QSFP+ cage it replaced. The cage is the same across every variant below — what differs is the optics inside, and that is what decides whether a link works at 100 m or 80 km.
Which variant fits which link
- SR4 — 850 nm over multimode fibre, short reach inside a rack or a small data hall. Cheapest per port; needs MMF, not SMF.
- CWDM4 — four wavelengths multiplexed onto one duplex single-mode pair, typically to 2 km. This is the workhorse for 100G between buildings on a campus where you do not want to spend a DWDM mux on four lambdas.
- LR4 / ER4 — single-mode, longer reach, again as four lanes over one duplex pair. ER4 is the long-haul variant of the same idea.
- DWDM — fixed or tunable channels on the ITU C-band grid, used where the link feeds a multiplexer and the wavelength has to match the channel plan. Buying here means knowing your grid position, not just your reach.
What to check before ordering
- Reach and fibre type first. A 2 km CWDM4 module on multimode fibre is a dead link, and the reverse wastes money.
- Whether the platform accepts third-party optics. Some switches accept any coded module; others check a vendor code and refuse the rest. Tell us the switch model and we will tell you which coding that platform expects.
- Optical budget, not just nominal reach. A 10 km rated pair on a 9 km span with many patch points can still be out of budget.
- Connector and polarity. Duplex LC is the norm, but breakout and CS/SN variants exist and are not interchangeable with the patch panels you may already have.
- DOM/DDM support if you monitor optical levels — worth confirming rather than assuming.
- DAC/AOC instead? Inside one rack, a direct-attach cable is usually cheaper and lower loss than two modules plus a patch lead.
We hold 100G optics across these variants; state the module you are replacing or the platform it goes into, and we will quote the exact part with its reach, coding and condition.