400GBASEFR4

400GBASE-FR4 is an Ethernet physical layer specification that enables 400 Gigabit Ethernet transmission over single-mode fiber optic cables. It is defined by the IEEE 802.3bs standard. The FR4 designation indicates it operates over duplex single-mode fiber (FR) and utilizes four wavelengths (4) for transmission and reception.

This standard employs a parallel optical interface, meaning it divides the 400 Gbps data stream into four lanes, each operating at 100 Gbps. Each of these 100 Gbps lanes is then modulated onto a distinct wavelength of light. Specifically, 400GBASE-FR4 utilizes four non-return-to-zero (NRZ) modulated wavelengths in the 1310 nm window. Two wavelengths are used for transmit and two for receive, resulting in a total of four wavelengths.

The reach of 400GBASE-FR4 is specified for up to 2 kilometers (km) over standard single-mode fiber (SMF). This makes it suitable for data center interconnects and other short-reach, high-bandwidth applications. The transceiver form factor commonly used for 400GBASE-FR4 is the QSFP-DD (Quad Small Form-factor Pluggable Double Density) or OSFP (Octal Small Form-factor Pluggable), which can accommodate the necessary optical components and high density. Its primary advantage is providing a significant increase in bandwidth compared to previous generations of Ethernet over existing fiber infrastructure, albeit with a higher port density requirement.