25 Gigabit Ethernet

An industry consortium, 25G Ethernet Consortium, was formed by Arista, Broadcom, Google, Mellanox Technologies and Microsoft in July 2014 to support the specification of single-lane 25-Gbit/s Ethernet and dual-lane 50-Gbit/s Ethernet technology. The 25G Ethernet Consortium specification draft was completed in September 2015 and uses technology from IEEE Std. 802.3ba and IEEE Std. 802.3bj. In November 2014, an IEEE 802.3 task force was formed to develop a single-lane 25-Gbit/s standard, In May 2016, an IEEE 802.3 task force was formed to develop a single-lane 50 Gigabit Ethernet standard. On November 12, 2018, the IEEE P802.3cn Task Force started working to define PHY supporting 50-Gbit/s operation over at least 40 km of SMF. The IEEE 802.3cd standard was approved on December 5, 2018. On December 20, 2019, the IEEE 802.3cn standard was published. On April 6, 2020, 25 Gigabit Ethernet Consortium rebranded to Ethernet Technology Consortium, and announced an 800 Gigabit Ethernet (GbE) specification. On June 4, 2020, the IEEE approved IEEE 802.3ca, which allows for symmetric or asymmetric operation with downstream speeds of 25 or , and upstream speeds of 10, 25, or over passive optical networks. == 25 Gigabit Ethernet ==

The IEEE 802.3by standard uses technology defined for 100 Gigabit Ethernet implemented as four 25-Gbit/s lanes (IEEE 802.3bj). ; : 25GBASE-T, a 25-Gbit/s standard over twisted pair, was approved alongside 40GBASE-T within IEEE 802.3bq. == Forward Error Correction ==

All fibre and twisted pair versions of 25 Gigabit Ethernet are required to support Reed-Solomon Forward Error Correction, often abbreviated RS-FEC, defined in clause 108 of the IEEE 802.3 standard. This also applies to 25GBASE-CR but not to 25GBASE-CR-S, both of which are variants used in DAC cables. 25GBASE-CR as well as 25GBASE-CR-S are required to support Fire-Code FEC (BASE-R FEC, also FC-FEC, defined in clause 74 of IEEE 802.3). While RS-FEC has to be supported for the mentioned 25 G versions, clause 108 also mandates that it has to be possible to turn FEC off, which makes it possible to not use FEC if desired. For an Ethernet link to form, the interfaces involved must use the same type of FEC or no FEC. == 50 Gigabit Ethernet ==

The IEEE standard defines a Physical Coding Sublayer (PCS) in Clause 133, which, after encoding, gives a data rate of . 802.3cd also defines an RS-FEC for forward error correction in Clause 134, which, after FEC encoding, gives a data rate of . It is not possible to transmit over an electrical interface while maintaining suitable signal integrity, so four-level pulse-amplitude modulation (PAM4) is used to map pairs of bits into a single symbol. This leads to an overall baud rate of 26.5625 GBd for per lane Ethernet. PAM4 encoding for 50G Ethernet is defined in Clause 135 of the 802.3 standard. == Availability ==

, 25 Gigabit Ethernet equipment is available on the market using the SFP28 and QSFP28 transceiver form factors. Direct attach SFP28-to-SFP28 copper cables in 1-, 2-, 3- and 5-meter lengths are available from several manufacturers, and optical transceiver manufacturers have announced 1310 nm "LR" optics intended for reach distances of 2 to 10 km over two strands of standard single-mode fiber, similar to existing 10GBASE-LR optics, as well as 850 nm "SR" optics intended for short reach distances of 100 m over two strands of OM4 multimode fiber, similar to existing 10GBASE-SR optics. == See also ==