25 Gigabit Ethernet

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25 Gigabit Ethernet is a standard for Ethernet connectivity in a datacenter environment, developed by IEEE 802.3 task force P802.3by[1] and is available from multiple vendors.

50 Gigabit Ethernet is a proposed standard under development by the IEEE 802.3cd task force.[2]


An industry consortium, 25G Ethernet Consortium,[3] 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,[4][5] and in November 2015, a study group was formed to explore the development of a single-lane 50 Gbit/s standard.[6]

In May 2016, an IEEE 802.3 task force was formed to develop a single-lane 50 GbE standard.[2]

On June 30, 2016, the IEEE 802.3by standard was approved by The IEEE-SA Standards Board.[7]

25 Gigabit Ethernet[edit]

The IEEE 802.3by standard uses technology defined for 100 Gigabit Ethernet implemented as four 25-Gbit/s lanes (IEEE 802.3bj).[8][9] The IEEE 802.3by standard several single-lane variations.[10]

25GBASE-T, a 25 Gbit/s standard over twisted pair, was approved alongside 40GBASE-T within IEEE 802.3bq.[11][12]

25 Gigabit port types
Name Clause (standard) Medium Media Count Gigabaud per lane Reach RS-FEC (802.3by clause 108)
25GBASE-KR-S 111 (802.3by) Electrical backplane 1 25.78125 No
25GBASE-KR 111 (802.3by) Electrical backplane 1 25.78125 Yes
25GBASE-CR-S 110 (802.3by) Twinaxial copper cable 1 25.78125 3 m (in-rack) No
25GBASE-CR 110 (802.3by) Twinaxial copper cable 1 25.78125 5 m (inter-rack) Yes
25GBASE-SR 112 (802.3by) Multi-mode fibre, 850 nm wavelength 1 25.78125 100 m OM4 or 70 m OM3 Yes
25GBASE-T 113 (802.3bq) Cat 8 balanced twisted-pair structured cabling 4 pairs 2.000 30 m No
25GBASE-LR 114 (802.3cc) Single-mode fibre, 1295 to 1325 nm wavelength 1 25.78125 10 km Yes
25GBASE-ER 114 (802.3cc) Single-mode fibre, 1295 to 1310 nm wavelength 1 25.78125 30 km Yes
25GAUI 109A/109B (802.3by) Chip-to-chip/chip-to-module interface 1 25.78125 25 cm

50 Gigabit Ethernet[edit]

The IEEE P802.3cd [2] standard defines a Physical Coding Sublayer (PCS) in Clause 133 which after encoding gives a data-rate of 51.5625 Gbps. 802.3cd also defines an RS-FEC for forward error correction in Clause 134 which after FEC encoding gives a data-rate of 53.125 Gbps. It is not possible to transmit 53.125 Gbps 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 50 Gbps per lane Ethernet. PAM4 encoding for 50G Ethernet is defined in Clause 135 of the 802.3 standard.

Single-lane 50 Gbit/s PHYs[13] defined in IEEE P802.3cd[2]
Name Clause Medium Media Count Gigabaud per lane Modulation Minimum reach
50GBASE-CR 136 (802.3cd) twinaxial copper cable 1 26.5625 PAM4 3 m
50GBASE-KR 137 (802.3cd) electrical backplane 1 26.5625 PAM4 Backplane with a total channel insertion loss of <= 30dB at 13.28125 GHz. 13.28125 GHz is the Nyquist_frequency i.e. half the sampling rate of 26.5625 GHz
50GBASE-SR 138 (802.3cd) multimode fibre 850 nm laser over OM3 or OM4 1 26.5625 PAM4 100 m on OM4, 70 m on OM3
50GBASE-FR 139 (802.3cd) single-mode fibre 1304.5 to 1317.5 nm 1 26.5625 PAM4 2 km
50GBASE-LR 139 (802.3cd) single-mode fibre 1304.5 to 1317.5 nm 1 26.5625 PAM4 10 km
LAUI-2 135B/135C (802.3cd) Chip-to-chip/chip-to-module interface 2 25.78125 NRZ (no FEC) 25 cm
50GAUI-2 135D/135E (802.3cd) Chip-to-chip/chip-to-module interface 2 26.5625 NRZ (FEC encoded) 25 cm
50GAUI-1 135F/135G (802.3cd) Chip-to-chip/chip-to-module interface 1 26.5625 PAM4 (FEC encoded) 25 cm


As of June 2016, 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,[14] and optical transceiver manufacturers have announced 1310 nm "LR" optics intended for reach distances of 2 to 10 km over two strands of standard singlemode fiber,[15] similar to existing 10GBASE-LR optics, as well as 850 nm "SR" optics [16] intended for short reach distances of 100 m over two strands of OM4 multimode fiber,similar to existing 10GBASE-SR optics.

Breakout type direct attach cables also exist which adapt a single QSFP28 format 100GbE port to four independent 25 Gbit/sec SFP28 connections.,[17] the cable length is available in 1-meter to 5-meter.[18]

Server network adapters supported in Linux using a standard PCI-Express x8 interface are available.[19]


In August 2016, Netronome launched the Agilio CX 25GbE smart network adapter (SmartNIC)[20]. In March 2018, Netronome launched the Agilio CX 25GbE SmartNIC in OCP v2.0 form factor[21]. The Agilio SmartNIC platform fully and transparently offloads eBPF, virtual switch, virtual router, and P4-based datapath processing for networking functions such as overlays, security, load balancing and telemetry.

Adapter Agilio CX Agilio CX for OCP v2.0
Ethernet 25 Gbit/s 25Gbit/s
Ports 2 1
Interface PCie Gen3 PCIe 3.0 x8
Form factor PCIe single slot, low-profile OCP form factor (Mezz 2.0)
Connector SFP28 SFP28
Cabling CR Direct Attach Copper

SR & LR Optics

CR Direct Attach Copper

SR & LR Optics

Product Information Agilio CX 25GbE SmartNIC Agilio CX 25GbE SmartNIC for OCP

Mellanox Technologies[edit]

Mellanox was the first company to introduce 25 GbE network adapters to the market and has the biggest market share according to Crehan Research analyst firm with more than 70% for 2016.[citation needed] The Mellanox adapters provide Intelligent RDMA capabilities, and advanced application offloads such as RoCE, OVS, DPDK, NFV and more.

The below table covers the basic information of the latest adapters:

Adapter ConnectX-4 Lx ConnectX-5
Ethernet 25 Gbit/s 25Gbit/s
Ports 1,2 1,2
Interface PCie Gen3 PCIe Gen3, PCIe Gen4
Form Factor PCIe, OCP and OEM specific PCIe, OCP and OEM specific
Connector SFP28 SFP28
Cabling Copper and Fiber Copper and Fiber
Part Number MCX4111A-A, MCX4121A-A MCX512A-ACAT
Product Information http://www.mellanox.com/page/ethernet_cards_overview


In January 2017, Intel launched the XXV-series 25 GbE network adapters.[22] These adapters are mainstream available 802.3by adapters and open 25 GbE for the big public. All three use SFP28-ports and support both CR Direct Attach Copper and SR & LR Optics. They communicate over a PCIe 3.0 x8 bus and come in multiple form factors.

Adapter XXV710-DA1OCP XXV710-DA1 XXV710-DA2
Ethernet 25 Gbit/s
Ports 1 2
Interface PCIe 3.0 x8
Form factor OCP form factor (Mezz 2.0 Type 1) PCIe single slot low-profile
Connector SFP28
Cabling CR Direct Attach Copper

SR & LR Optics

Productinformation Intel Ark Intel Ark Intel Ark

Intel also launched six Direct Attach Copper cables, three SFP28 to SFP28 and three QSFP28 to SFP28, both in lengths of 1, 2 and 3 metres.[23] In addition, they launched a SFP28 SR optical transceiver.[24]

Cable Type Length
Intel XXVDACBL1M SFP28 Twinaxial Cable 1 m
Intel XXVDACBL2M 2 m
Intel XXVDACBL3M 3 m
Intel XXV4DACBL1M QSFP28 to SFP28 Twinaxial Breakout Cable 1 m
Intel XXV4DACBL2M 2 m
Intel XXV4DACBL3M 3 m
Intel E25GSFP28SR SFP28 Optical Module n/a

(max. 100 m)


  1. ^ IEEE 802.3by 25 Gb/s Ethernet Task Force
  2. ^ a b c d IEEE 802.3cd 50 Gb/s, 100 Gb/s, and 200 Gb/s Ethernet Task Force
  3. ^ "25G Ethernet Consortium". Retrieved 2017-09-17. 
  4. ^ Rick Merritt (2014-07-21). "25G Ethernet on Tap at IEEE". Retrieved 2014-09-29. 
  5. ^ "IEEE 802.3 25 Gb/s Ethernet Study Group Public Area". IEEE 802.3. 2014-10-29. Retrieved 2014-12-31. 
  6. ^ "Joint Webpage for IEEE 802.3 50 Gb/s Ethernet Over a Single Lane and Next Generation 100 Gb/s and 200 Gb/s Ethernet Study Group IEEE 802.3 200 Gb/s Ethernet Single-mode Fiber Study Group". Retrieved 2017-09-17. 
  7. ^ "[STDS-802-3-25G] IEEE Std 802.3by-2016 Standard Approved!". 2016-06-30. 
  8. ^ "Overview 25G & 50G Ethernet Specification, Draft 1.4" (PDF). 25G Ethernet Consortium. 2014-09-11. Retrieved 2014-12-31. 
  9. ^ Stephen Hardy (July 23, 2014). "IEEE launches 25 Gigabit Ethernet Study Group". LightWave. Retrieved 2014-09-29. 
  10. ^ "Adopted & Approved Objectives: 25 Gb/s Ethernet over a single lane for server interconnect" (PDF). Retrieved 2017-09-17. 
  11. ^ "IEEE P802.3bq 25G/40GBASE-T Task Force". Retrieved 2016-02-08. 
  12. ^ "Approval of IEEE Std 802.3by-2016, IEEE Std 802.3bq-2016, IEEE Std 802.3bp-2016 and IEEE Std 802.3br-2016". IEEE. 2016-06-30. 
  13. ^ IEEE 802.3cd Task Force Objectives
  14. ^ "SFP28 direct attach". 
  15. ^ "SFP28 optical LR". 
  16. ^ "25G SFP28 SR Transceiver". 
  17. ^ "100GbE QSFP28 to SFP28 breakout cable". 
  18. ^ "100G QSFP28 to 4x25G SFP28 Passive Direct Attach Copper Breakout Cable". 
  19. ^ "25GbE SFP28 based server adapter" (PDF). 
  20. ^ https://www.netronome.com/press-releases/netronome-announces-25gbe-smartnics-with-openstack-firewall-security-and-open-vswitch-acceleration/
  21. ^ https://www.netronome.com/press-releases/netronome-introduces-worlds-smallest-lowest-power-2550gbe-smartnics-based-ocp-v20-mezzanine-specification/
  22. ^ "Intel® Ethernet Network Adapter XXV710". Intel. Retrieved 2017-04-18. 
  23. ^ "Intel® Ethernet SFP28 Twinaxial Cables". Intel. Retrieved 2017-04-18. 
  24. ^ "Intel® Ethernet SFP28 Optics". Intel. Retrieved 2017-04-18. 

External links[edit]