In short: for short links inside a data center, 100G SR4 over multimode fiber is the usual choice. For single-mode fiber, CWDM4 or FR covers up to 2 km, LR4 or LR up to 10 km, and ER4 or ZR4 the long distances between sites. The right type depends on the fiber you have and how far the link has to go.
QSFP28 is the standard 100G module. Its many variants look alike, so here is what the letters mean and when to use each one.
How 100G modules carry the data
A QSFP28 module carries 100 Gbit/s in one of three ways, and that is the main difference between the types:
- Four fibers in each direction (SR4, PSM4): four parallel lanes of 25G, each on its own fiber. These modules use an MTP/MPO connector with several fibers in one plug.
- Four colors on one fiber pair (LR4, CWDM4, ER4, ZR4): four lanes on four different wavelengths, combined onto a normal duplex LC fiber pair.
- One lane on one fiber pair (DR, FR, LR): a single 100G signal on one wavelength, also over a duplex LC pair.
The 100G QSFP28 types side by side
| Type | Fiber | Connector | Typical reach | Typical use |
|---|---|---|---|---|
| SR4 | multimode | MTP/MPO | 100 m | within a row or a data hall |
| eSR4 | multimode | MTP/MPO | 300 m | longer multimode runs |
| SR1.2 (BiDi) | multimode | duplex LC | 100 m | reusing existing duplex multimode cabling |
| DR | single-mode | duplex LC | 500 m | inside a data center |
| PSM4 | single-mode | MTP/MPO | up to 2 km | parallel single-mode cabling |
| CWDM4 | single-mode | duplex LC | 2 km | between data halls, campus |
| FR | single-mode | duplex LC | 2 km | between data halls, campus |
| LR4 / LR | single-mode | duplex LC | 10 km | between buildings and sites |
| ER4 Lite / ER4 | single-mode | duplex LC | 30–40 km | metro links |
| ZR4 | single-mode | duplex LC | 80 km | long links between sites |
The distances are typical values; the exact figure for each module is on its product page.
Inside the data center: SR4 and its relatives
SR4 is the most economical 100G optic for short links. It needs multimode fiber with MTP/MPO connectors – four fibers to send and four to receive. If your data center already has duplex LC multimode cabling from 10G times, the BiDi SR1.2 module can run 100G over that same fiber pair.
Up to 2 km: CWDM4, FR, DR and PSM4
For links between data halls or across a campus, CWDM4 and FR modules reach 2 km over a standard duplex single-mode pair. DR covers 500 m on one wavelength, and PSM4 uses parallel single-mode fibers with an MTP/MPO connector. On duplex fiber, CWDM4 and FR are the simplest choice.
10 km and beyond: LR4, ER4, ZR4
LR4 is the classic 100G module for 10 km of single-mode fiber and the most common choice between buildings and sites. For longer metro links, ER4 Lite and ER4 reach 30–40 km, and ZR4 up to 80 km. As with all long-reach optics, choose the type that fits the actual distance.
How to choose
- What fiber is installed? Multimode with MTP/MPO points to SR4, duplex multimode to SR1.2, duplex single-mode to DR, FR, CWDM4 or LR4.
- How far? Pick the shortest type that covers the distance with some reserve.
- Same type on both ends – an LR4 talks to an LR4, not to a CWDM4.
- Coded for your equipment, so the switch accepts it – see our compatibility guide.
New to the module sizes? Start with SFP vs SFP+ vs SFP28 vs QSFP28, and for the fiber question read single-mode vs multimode.
Frequently asked questions
What is the difference between 100G LR4 and CWDM4?
Both use four wavelengths on a duplex single-mode pair. LR4 reaches 10 km; CWDM4 is made for up to 2 km and is typically used inside or between data centers on the same site.
Can a QSFP28 port take a 40G QSFP+ module?
Many QSFP28 ports can run a QSFP+ module at 40G. Check the documentation of your device.
Why do some 100G modules need MTP/MPO connectors?
SR4 and PSM4 send the four lanes over separate fibers, so they need a connector that holds several fibers at once.
