Under 100 Gigabit Ethernet, there are several 100GbE interfaces for fiber, one of which is 100GBase-ER4, which can bridge distances of up to 40 km. However, this distance assumes that the implementations work with an optical amplifier.
In the ER4 extension, the" ER" means Extended Reach and stands for the four wavelengths in the second optical window. The interface uses 64B66B coding and the number 4 states that the transmission is switched a duplex monomode fiber using DWDM technology over four DWDM paths.
Wavelength division multiplexing can be done at the wavelengths of 1,295 nm, 1,300 nm, 1,305 nm, and 1,310 nm with a wavelength spacing of 5 nm, as in the 100GBase-LR4 interface, but it can also be done using DWDM at the wavelength of 1,550 nm. The effective data rate per DWDM path is 25 Gbit/s, resulting in a baud rate of 25.78125 Gbit/s for 64B66B coding. Technically, the 100GBase-ER4 interface is implemented with an optical transceiver in a CFP module.