100G QSFP28 transceiver types explained: SR4, LR4, CWDM4 and more

A 100G QSFP28 transceiver carries 100 Gbps through four electrical lanes of 25G, and the optical type you pick (SR4, CWDM4, LR4 and so on) decides the fiber, the connector and the reach. In short: SR4 for up to 100 m on multimode, DR or PSM4 for 500 m on single-mode, CWDM4 or FR for 2 km, LR4 for 10 km, and ER4 or ZR4 beyond that.

The names look similar on a price list, but the modules are not interchangeable. Two ends of a link must use the same optical type, the fiber plant must match, and the switch has to support the module’s FEC and power needs. This guide walks through each type, what it plugs into, and how to choose.

How QSFP28 Works

QSFP28 is the 100G member of the QSFP family. The host side is always 4 x 25G NRZ (strictly 25.78 Gbps per lane). What happens on the optical side varies: some modules send four lanes over four fiber pairs, some multiplex four wavelengths onto one fiber pair, and the newer single-lambda types use a DSP to convert the four lanes into one 100G PAM4 signal.

That gives three broad families:

• Parallel optics (SR4, PSM4): four transmit fibers and four receive fibers on an MPO connector.
• Wavelength division multiplexing (CWDM4, LR4, ER4, ZR4, SWDM4): four wavelengths on a duplex LC pair.
• Single-lambda PAM4 (100G-DR, 100G-FR, 100G-LR): one 100G wavelength on a duplex LC pair.

QSFP28 ports usually accept 40G QSFP+ modules too, and a QSFP28 module will plug into most QSFP56 and QSFP-DD ports. For the wider family history, see our QSFP transceivers guide.

Multimode Types: SR4, BiDi and SWDM4

100GBASE-SR4

SR4 is the most common 100G optic inside data center rows. It uses four 850 nm VCSEL lanes on eight fibers of an MPO-12 connector (fibers 1 to 4 transmit, 9 to 12 receive, the middle four are dark). IEEE reach is 70 m on OM3 and 100 m on OM4. Some vendors sell “eSR4” or extended versions rated further on OM4, but those are outside the IEEE spec and need matching modules at both ends.

SR4 needs RS-FEC (IEEE Clause 91) enabled on the switch port. If one side has FEC off, the link will usually stay down or flap.

100G BiDi (SR1.2 / SRBD)

BiDi modules run 100G over a single duplex LC pair of multimode fiber, using two wavelengths (around 850 nm and 910 nm) in each direction. Typical reach is 70 m on OM3 and 100 m on OM4, with a little more on OM5. The main reason to buy BiDi is to reuse existing duplex LC multimode cabling that was installed for 10G or 40G BiDi, without pulling new MPO trunks.

100G-SWDM4

SWDM4 is an MSA module that multiplexes four wavelengths between 850 nm and 940 nm onto one duplex LC pair. It reaches about 75 m on OM3, 100 m on OM4 and 150 m on OM5. It is less common than SR4 or BiDi, and both ends must be SWDM4.

Single-Mode Types: PSM4, CWDM4, DR and FR

100G-PSM4

PSM4 is the single-mode cousin of SR4: four parallel 1310 nm lanes on eight fibers of an MPO-12 connector, rated for 500 m. It was popular as a low-cost single-mode option in large data centers. Because it uses parallel fibers, it can also be broken out to four 25G single-mode links where the platform supports it.

100G-CWDM4

CWDM4 multiplexes four wavelengths (1271, 1291, 1311 and 1331 nm) onto a duplex LC pair of single-mode fiber, with a reach of 2 km. It is the usual choice for 100G between rooms, halls or nearby buildings on a campus. Like SR4, it is designed to run with RS-FEC on the host. A close relative, CLR4, uses the same wavelength grid and reach.

100G-DR and 100G-FR (single lambda)

These newer modules send one 100G PAM4 wavelength at 1310 nm over a duplex LC pair. 100GBASE-DR reaches 500 m and 100GBASE-FR1 reaches 2 km. The 100G Lambda MSA also defines 100G-LR1 for 10 km.

The big advantage is interoperability with 400G. A 400G-DR4 port can break out to four 100G-DR modules, and a 400G-XDR4 (also sold as DR4+) can break out to four 100G-FR modules. If you are building a fabric that will move to 400G, single-lambda 100G optics are the better long-term choice than CWDM4. Note that a 100G-FR and a CWDM4 cannot talk to each other, even though both reach 2 km on LC duplex.

Long-Reach Types: LR4, ER4 and ZR4

100GBASE-LR4

LR4 uses four LAN-WDM wavelengths close together around 1295 to 1310 nm, on a duplex LC pair of single-mode fiber, for 10 km. It does not need FEC on the host, which makes it a safe choice between buildings, to a carrier meet-me room, or across a metro link on dark fiber.

100GBASE-ER4 and ER4 Lite

ER4 uses the same LAN-WDM wavelengths with a more sensitive receiver for 40 km. Many QSFP28 versions are sold as ER4 Lite: they reach roughly 30 km without FEC and 40 km with RS-FEC enabled on the host. Check the data sheet and your switch’s FEC settings before ordering.

100G ZR4

ZR4 targets around 80 km. It is not an IEEE standard, so specifications vary by maker. ZR4 modules draw more power than SR4 or LR4, so confirm that the switch port supports the higher power class. On links this long, also check the optical budget against measured fiber loss, and use attenuators if the span is short enough to overload the receiver.

100G QSFP28 Transceiver Comparison

For very short links inside a rack, a QSFP28 DAC cable or AOC is usually cheaper and lower power than two optics and a patch cord.

Breaking Out 100G to 4 x 25G

Many switches let you split one QSFP28 port into four independent 25G interfaces. This is how 100G leaf switches connect to servers with 25G NICs. Only parallel types can do it, because each of the four lanes needs its own fiber pair:

• SR4 to 4 x 25G-SR: an MPO-12 female to 4 x LC duplex harness connects the QSFP28 to four SFP28 25GBASE-SR modules.
• PSM4 to 4 x 25G single-mode: possible on some platforms with an MPO to LC harness, but check both the switch and the far-end SFP28 optics first.
• Copper and AOC breakouts: QSFP28 to 4 x SFP28 breakout cables are the simplest option for in-rack server connections.

The switch port must be set to breakout mode (often called 4x25G) in the configuration, and the 25G side normally needs matching FEC settings. WDM types like CWDM4 and LR4 cannot be broken out. For more on this, read our breakout cables guide.

What About CFP, CFP2 and CFP4?

Before QSFP28 matured, 100G optics came in the CFP family. The original CFP is a large module, and CFP2 and CFP4 are roughly half and a quarter of that size. You still find them on older routers and transport line cards, and CFP2 lives on in coherent CFP2-DCO modules for long-haul and DWDM work.

For new switch and router ports at 100G, QSFP28 has taken over because of its size and power. If you are replacing optics in older CFP-based equipment, match the exact form factor and type (for example CFP2 LR4) to the existing line card.

How to Choose a 100G QSFP28 Transceiver

Start with distance and the fiber that is already in the ground, then check what the far end and the switch support.

Quick rules

• Same rack or next rack: DAC or AOC.
• Within a row or hall up to 100 m on OM4 MPO trunks: SR4.
• Existing duplex LC multimode only: BiDi.
• Up to 500 m single-mode, with a plan to move to 400G: 100G-DR.
• Up to 2 km single-mode: CWDM4 or 100G-FR (pick one and stay consistent).
• Building to building or metro up to 10 km: LR4.
• 30 to 80 km: ER4 Lite or ZR4, after checking measured fiber loss.

Pre-order checklist

1. Switch or router model and line card, so the optic can be coded for the platform.
2. Optic type at the far end. Both ends must match.
3. Fiber type and grade (OM3, OM4, OS2) and the connector on the patch panel.
4. Measured or estimated link length and loss.
5. FEC setting on both ports (SR4 and CWDM4 need RS-FEC).
6. Whether any port will run in 4 x 25G breakout mode.
7. Patch cords: MTP/MPO cables for SR4 and PSM4, LC duplex for the rest. For parallel types, order female MPO cords with the correct polarity.

TIFNEX stocks the full range of 100G QSFP28 transceivers, coded for Cisco, Juniper, Arista, Dell, HPE, NVIDIA/Mellanox and other platforms and tested before shipping. Teams planning a cloud or data center build in the UAE or Saudi Arabia can send us a port map and we will match optics to each link.

Frequently Asked Questions

What is the difference between 100G SR4 and 100G LR4?

100GBASE-SR4 runs over multimode fiber with an MPO-12 connector and reaches 70 m on OM3 or 100 m on OM4. 100GBASE-LR4 runs over single-mode fiber with an LC duplex connector and reaches 10 km. They cannot be connected to each other.

Can a 100G CWDM4 connect to a 100G LR4?

No. CWDM4 uses four wavelengths spaced 20 nm apart from 1271 to 1331 nm, while LR4 uses four closely spaced LAN-WDM wavelengths around 1295 to 1310 nm. Both ends of a link must use the same optical type.

Can I use a 100G QSFP28 transceiver in a 40G QSFP+ port?

Generally no, because a QSFP+ port runs its lanes at 10G and cannot drive a 25G-per-lane module. The reverse often works: many QSFP28 ports accept 40G QSFP+ optics, but check the switch documentation.

Which 100G QSFP28 types support 4 x 25G breakout?

Parallel types such as 100GBASE-SR4 and, on some platforms, 100G-PSM4 can be split into four 25G links using an MPO to 4 x LC duplex harness. WDM types such as CWDM4, LR4 and ER4 cannot be broken out, and the switch port must support breakout mode.

Should I choose 100G CWDM4 or 100G-FR for 2 km links?

Both reach 2 km on single-mode LC duplex, but they do not interoperate. 100G-FR uses a single 100G wavelength and can connect to a 400G-XDR4 port in breakout, so it is the better choice if you plan to move to 400G; CWDM4 is fine for matching an existing installed base.

Talk to TIFNEX

Send us your switch or NIC models, link distances, fiber type and quantities, and our engineers will recommend the right QSFP28 optics and cabling, with a quote within 24 hours. We deliver across the UAE, Egypt and the wider GCC. Contact TIFNEX or message us on WhatsApp at +971 54 300 8970.