SFP28 (25 Gbps)
Direct Answer
SFP28 (25G Small Form-Factor Pluggable) is a transceiver standard defined under SFF-8402 and IEEE 802.3by. Using a single 25.78125 Gbps NRZ electrical lane within the standard SFP physical cage size, it provides high-density 25 Gigabit Ethernet and 32G Fibre Channel server-to-switch interconnects.
Source: SFF Committee SFF-8402 / IEEE 802.3by (25GBASE-R)
Specs
| Standard / MSA | SFF-8402 / SFF-8432 / IEEE 802.3by |
| Form Factor | SFP28 |
| Electrical Channels | 1-lane (25.78125 Gbps NRZ) |
| Max Data Rate (Ethernet) | 25.78125 Gbps |
| Max Data Rate (Fibre Channel) | 32 Gbps |
| Optical Connector | Duplex LC |
| Direct Attach Copper (DAC) | Up to 3m passive (5m with RS-FEC) |
| Forward Error Correction (FEC) | Clause 74 (BASE-R) / Clause 108 (RS-FEC) |
| Backward Compatibility | SFP+ (10 Gbps) in multi-rate ports |
The 25G Single-Lane Standard
SFP28 was created to streamline datacenter economics. In older 40G systems (4x10G), quad-lane cabling created architectural complexity. Because 100G Ethernet uses four 25 Gbps lanes (4x25G), single-lane SFP28 became the ideal native server port speed, matching the internal ASIC pipeline of 100G top-of-rack switches.
Forward Error Correction (FEC) Requirements
At 25 Gbps NRZ, high-frequency electrical loss over copper increases significantly. IEEE 802.3by introduced mandatory Reed-Solomon Forward Error Correction (RS-FEC, Clause 108) or FireCode FEC (Clause 74) to maintain error-free transmission across passive DAC copper cables up to 3–5 meters.