40GBASE

40GBASE is a family of high‑speed Ethernet standards defined by the IEEE 802.3 specification for transmitting data at 40 gigabits per second over a variety of physical media. The "BASE" designation indicates baseband signaling, meaning the electrical signal occupies the spectrum of the cable rather than being modulated onto a higher frequency carrier.

The 40GBASE family includes several media types to suit different deployment scenarios. Forty-gigabit over fiber strands is categorized as 40GBASE-LR4, which uses four wavelengths in a single multimode fiber each at 10 Gbps, allowing 400 km runs in dense wavelength division multiplexing systems. Shorter‐reach options include 40GBASE-CR4, employing four 10 Gbps lanes over twisted‑pair copper to support up to 15 m in data‑center environments, and 40GBASE-SR4, which uses four 10 Gbps lanes over multimode fiber for distances up to 82 m. For optical single‑mode fiber, 40GBASE‑LR4 and 40GBASE‑SR4 variants are available, with the former reaching 10 km and the latter limited to several hundred meters. A copper fixed‑length cable, 40GBASE‑CR4, provides a cost‑effective, colored cable alternative for rack‑to‑rack interconnects.

These standards rely on 64b/66b line coding and a clock recovery mechanism that ensures high data integrity over long distances. 40GBASE Ethernet typically appears as a link layer that can be aggregated into 100 GbE or 400 GbE by combining multiple ports through techniques such as link aggregation or multi‑path routing.

Since its introduction in 2009, 40GBASE has become a staple in data‑center interconnects, server–switch uplinks, and high‑performance computing clusters. Its adoption enabled the scaling of bandwidth demands for cloud services, virtualization, and big‑data analytics before the subsequent emergence of 100GBASE and 400GBASE technologies.