100GBASEER4

100GBASE-ER4 is a 100 Gigabit Ethernet physical layer specification designed for long-reach links over single-mode fiber. Defined in the IEEE 802.3ba standard, it uses four parallel 25 Gbit/s serial lanes that are multiplexed to deliver a single 100 Gbps link. The optical carriers operate in the 1550 nm region, and the four wavelengths are carried over single-mode fiber using wavelength-division multiplexing to achieve the aggregate rate.

The technology relies on four independent optical channels, each carrying its own NRZ data stream. The four channels are transmitted and received on a single fiber pair through WDM, allowing a higher total bitrate without requiring a single very high-speed channel. The typical reach specified for ER4 is up to 40 kilometers on standard single-mode fiber, enabling interconnects between data centers, metro networks, and core infrastructure without repeaters.

Hardware and form factors for 100GBASE-ER4 are commonly implemented in pluggable transceivers such as QSFP28 or CFP/CFP2 modules, depending on vendor and deployment. These modules pair with compatible switches or routers and require SMF with LC connectors and appropriate dispersion management for 1550 nm operation. Deployment considerations include fiber quality, connector integrity, and proper WDM management to separate the four wavelengths, as well as power budgets and link maintenance at long distances.

In the ecosystem, 100GBASE-ER4 sits alongside other 100G variants such as SR4 and LR4, offering a long-reach option that leverages four parallel lanes to reach tens of kilometers over SMF. It remains part of the historical progression toward higher-density, longer-reach Ethernet solutions.