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Choosing an optical transceiver is a matching problem before it is a purchasing decision. The switch port sets the electrical lane count and the per-lane signaling rate; the fiber plant sets the reach and the connector; and the two together narrow the choice to a small set of standard form factors. This guide walks through the pluggable form factors from 1.25G up to 800G, how each maps to a switch cage, and how to read the reach codes that appear in module names.
Read the Port Before the Module
Every pluggable port is defined by three numbers: how many electrical lanes it exposes, the signaling rate of each lane, and how much power the cage can dissipate. A 100G port built from four 25G NRZ lanes (QSFP28) is not electrically the same as a 100G port that runs 100G PAM4 on every lane. The physical footprint can be identical while the electrical interface is not, so the first step is always to confirm the host lane structure and PAM4 support, not just the cage type.
Once the lane structure is known, the aggregate rate falls out of it. One lane at 25G is a 25G port; four lanes at 50G is a 200G port; eight lanes at 50G is a 400G port. PAM4 encodes two bits per symbol instead of one, which is why the same lane count can carry two generations of speed. Higher-rate links also add forward error correction, standardised as KP4 RS(544,514) FEC for 400G and 800G Ethernet, which is negotiated rather than chosen by the installer.
Form Factor and Speed Reference
| Form factor | Electrical lanes | Per-lane rate | Aggregate rate | Typical reach classes | Common connector |
|---|---|---|---|---|---|
| SFP | 1 | 1.25G | 1.25G to 4.25G | SR 100m to ZR 80km | LC or RJ45 |
| SFP+ | 1 | 10G NRZ | 10G | SR 300m, LR 10km, ER 40km | LC |
| SFP28 | 1 | 25G NRZ | 25G | SR 100m, LR 10km, ER 40km | LC |
| SFP56 | 1 | 50G PAM4 | 50G | SR 100m, LR 10km | LC |
| QSFP+ | 4 | 10G NRZ | 40G | SR4 100m, LR4 10km | MPO or LC |
| QSFP28 | 4 | 25G NRZ | 100G | SR4 100m, DR 500m, LR4 10km, ER4 40km | MPO or LC |
| QSFP56 | 4 | 50G PAM4 | 200G | SR4 100m, FR4 2km, LR4 10km | MPO or LC |
| QSFP-DD | 8 | 50G or 100G PAM4 | 400G or 800G | SR8 100m, DR4 500m, FR4 2km, LR4 10km | MPO or LC |
| QSFP112 | 4 | 100G PAM4 | 400G | SR4 50m, DR4 500m, FR4 2km, LR4 10km | MPO or LC |
| OSFP | 8 | 50G or 100G PAM4 | 400G or 800G | SR8 100m, DR8 500m, FR8 2km, LR8 10km | MPO or LC |
The SFP family is single-lane: SFP for 1.25G, SFP+ for 10G, SFP28 for 25G and SFP56 for 50G. The QSFP family is four-lane, and the double-density and octal families are eight-lane. QSFP112 is the exception that proves the rule: it keeps the four-lane QSFP footprint but raises each lane to 100G PAM4, reaching 400G.
Reach Codes: SR, DR, FR, LR, ER and ZR
Module names carry the reach in a short code. SR is short reach over multimode fiber, usually around 100m on OM4. DR is parallel single-mode at 500m. FR is 2km over a duplex single-mode pair. LR extends to 10km, ER to 30 to 40km, and ZR to 80km. At the highest rates, ZR-class links generally move to coherent optics rather than direct-detect PAM4.
The reach code also implies a connector. Parallel standards such as SR4, DR4 and DR8 use multi-fiber MPO or MTP connectors, while duplex FR and LR standards use LC. Wherever possible, choose the code that fits the fiber you have already installed; changing the fiber plant is almost always the larger project.
Breakout and Backward Compatibility
Higher-lane modules can fan out to lower-rate ports. A 400G QSFP-DD can break out to four 100G or two 200G links, and an 800G module can break out to two 400G, four 200G or eight 100G links. Breakout is usually a cabling and configuration choice rather than a different module, but it depends on the host supporting the split.
Mechanical compatibility is not electrical compatibility. QSFP-DD slots accept QSFP28 and QSFP56 modules mechanically, and QSFP112 fits QSFP cages, but each combination still requires the host to support that lane rate and modulation. OSFP uses its own larger cage and its own ecosystem. Modules are designed to interoperate with major networking platforms; confirm the host interface and firmware before ordering.
How Apex Helps
Bring us your switch models, port types, fiber distances and connector plans, and we will map them to one consistent transceiver list across your fabric. We supply matched optics and cabling as a single bill of materials so that the lane rates and reach classes line up end to end. Send your requirement list and we will quote within hours.
