100G QSFP28 Explained: Which Optical Transceiver Is Right for You?

As the market for 100G transceivers has matured, the QSFP28 has emerged as the networking industry’s preferred form factor. As such, the available options have widely expanded and can seem overwhelming to someone approaching 100G for the first time. This article will explain why there are so many seemingly overlapping options, and how to choose the right transceiver for your particular needs.

QSFP28 Options for Short-Reach Applications

There is only one standards-based solution for short-reach application: 100GBASE-SR4. This transceiver uses multimode fiber to span distances of 70m or 100m on OM3 or OM4 fiber, respectively.

For Breakout Applications

If you need to aggregate 4x25G connections into a single 100G port, 100GBASE-PSMIR4 supports breakout applications up to 2km via MPO-to-LC breakout cables.

For Intermediate Distances

For distances of 2km to 10km, things start to get complicated. QSFP28 transceivers use 4 “lanes” of 25G transmission to total 100G. However, there are two competing wavelength plans serving similar distances: LAN-WDM and CWDM, as pictured below:

LAN-WDM follows the wavelength plan laid out in the earlier CFP MSAs. As such, transceivers that follow this wavelength plan can often interoperate with transceivers in the CFP family (e.g., CFP, CFP2, CFP4) for comparable applications. CWDM emerged later and uses the lowest four CWDM wavelengths (1270, 1290, 1310, and 1330nm). The CWDM wavelength transceivers require forward error correction (FEC) enabled on their switches to transmit over their full rated distances. This has emerged as a popular alternative to LAN-WDM because of its relatively lower cost and the wide availability of FEC.

Selecting the right transceiver depends on the wavelength plan you require and the availability of FEC on your switches, as shown below:

For Long Distances

There are two options to reach distances up to 40km: 100GBASE-4WDM40 and 100GBASE-ER4 Lite. Both of these options use the LAN-WDM wavelength plan, and are therefore interoperable with one another (after proper power adjustments). Both require FEC to reach the full 40km. The only difference is a technical one: FEC is specified as a requirement in the 4WDM40 MSA, and there is no guaranteed distance if FEC is not enabled. For ER4 Lite (a “lite” version of the IEEE standard), it is possible to reach 25-30km without enabling FEC.

If you’d like to explore your options further and learn more about these 100G transceivers, contact us today!