From the interface designation ER, which stands for Extended Reach, it is clear that it is an interface that operates with an extra-long wavelength of 1,550 nm in the third optical window. In addition, the 40GBase-ER4, which has four lanes, can bridge distances of up to 40 km. It is described in IEEE 802.3ba.
The 40GBase-ER4 interface operates with 64B66B coding and single mode fibers. The wavelength range at 1,300 nm with four wavelengths is used as the optical window. The light signals are generated by VCSEL lasers. Transmission takes place at a data rate of 10 Gbit/s. The 64B66B coding results in a baud rate of 10.3125 GBaud. 40GBsae-ER4 uses the QSFP module as optical transceiver.