What Is a 25G SFP28 Optical Transceiver?

25G SFP28 optical transceiver is a compact pluggable optical module designed to provide 25Gb/s Ethernet connectivity over fiber. It converts electrical signals from a switch, server, router, or network interface into optical signals for transmission and converts received optical signals back into electrical signals.

SFP28 is one of the most widely used form factors for 25G Ethernet. It has a similar physical size to SFP and SFP+ modules, allowing high port density while supporting a significantly higher data rate than traditional 10G SFP+ interfaces.

25G SFP28 optical transceivers are commonly deployed in data center server connections, leaf-spine networks, enterprise networks, 5G transport infrastructure, and other applications requiring more bandwidth than 10G while maintaining a compact pluggable interface.

1. What Does SFP28 Mean?

SFP28 refers to a Small Form-factor Pluggable module designed for 25G-class Ethernet and related high-speed applications. The name describes the form factor rather than a single optical reach or wavelength.

Different SFP28 modules can have very different optical specifications. For example, a 25GBASE-SR SFP28 module normally uses 850 nm multimode optics for short-distance transmission, while a 25GBASE-LR SFP28 module typically uses 1310 nm single-mode optics for 10 km-class links.

Therefore, "SFP28" should be understood as the module form factor, while terms such as SR, LR, ER, and BiDi identify specific optical implementations.

2. How Does a 25G SFP28 Optical Transceiver Work?

A 25G SFP28 transceiver performs electrical-to-optical and optical-to-electrical conversion.

On the transmit side, the host switch or server sends a high-speed electrical signal to the module. The module drives an optical transmitter, such as a VCSEL or laser source, to generate the corresponding optical signal.

On the receive side, a photodetector converts the incoming optical signal into an electrical signal. Internal electronics recover and condition the signal before delivering it to the host equipment.

The basic transmission path is:

Switch / Server → Electrical Interface → SFP28 Transceiver → Optical Fiber → SFP28 Transceiver → Electrical Interface → Network Device

3. What Data Rate Does a 25G SFP28 Support?

A 25G SFP28 optical transceiver is designed around a nominal 25Gb/s data rate. In Ethernet applications, 25GBASE-R technologies use a single high-speed serial lane rather than the multiple lower-rate lanes used by earlier 40G and 100G architectures.

The actual line rate is higher than the useful payload rate because Ethernet transmission includes coding and overhead. For example, 25G Ethernet implementations based on 64B/66B coding operate at a line rate above 25 GBd.

25G provides approximately 2.5 times the nominal interface bandwidth of 10G Ethernet while retaining a similarly compact SFP-class module footprint.

4. Is 25G SFP28 the Same as 25G Ethernet?

No. 25G Ethernet describes the networking technology and interface speed, while SFP28 describes a common physical module form factor.

A 25G Ethernet port can use different physical connectivity options, including SFP28 optical transceivers, passive direct-attach copper cables, or active optical cables. Likewise, an SFP28 module can support different optical specifications depending on its design.

5. What Are the Main Types of 25G SFP28 Optical Transceivers?

The exact distance and optical specifications depend on the specific module implementation, fiber quality, connector losses, and host requirements.

6. What Is a 25GBASE-SR SFP28 Transceiver?

25GBASE-SR is a short-reach 25G optical interface designed for multimode fiber. It commonly operates at 850 nm and is primarily used for short links inside data centers.

Typical implementations support approximately 70 meters over OM3 and 100 meters over OM4. The module normally uses a duplex LC connector.

Because SR modules use multimode fiber and relatively short optical paths, they are commonly deployed for server-to-switch and switch-to-switch connections within the same data center environment.

7. What Is a 25GBASE-CSR SFP28 Transceiver?

25GBASE-CSR is a longer-reach multimode solution designed to extend the reach of 850 nm 25G connectivity beyond conventional SR implementations.

Depending on the fiber grade and implementation, CSR-class modules can support several hundred meters. This makes them useful for larger data center environments where multimode infrastructure is already installed but link distances exceed typical SR requirements.

The available reach should always be checked against the exact fiber type and modal bandwidth specified by the module manufacturer.

8. What Is a 25GBASE-LR SFP28 Transceiver?

25GBASE-LR is a long-reach 25G optical interface designed for single-mode fiber. It typically operates around 1310 nm and supports a nominal transmission distance of up to 10 km.

LR modules usually use duplex LC connectors and are suitable for longer data center links, enterprise networks, campus networks, and telecom-related applications.

Compared with 25G-SR, LR changes both the fiber type and optical architecture, making it suitable for distances that are well beyond conventional multimode data center links.

9. What Is a 25GBASE-ER SFP28 Transceiver?

25GBASE-ER is an extended-reach 25G optical solution designed for single-mode fiber. Typical implementations operate around the 1310 nm region and target approximately 40 km-class links.

ER modules generally require a higher optical link budget than SR or LR modules because of the significantly longer fiber path. They may also involve more stringent optical power, receiver sensitivity, and FEC requirements.

ER is commonly considered when a 25G link must extend well beyond the 10 km range of conventional LR optics.

10. What Is a 25G BiDi SFP28 Transceiver?

A 25G BiDi SFP28 transceiver uses bidirectional transmission over a single optical fiber. Instead of using separate fibers for transmit and receive directions, it uses different wavelengths for the two directions.

A BiDi connection therefore normally requires a matched pair of modules, such as one upstream wavelength module and one downstream wavelength module.

11. What Wavelengths Are Used by 25G SFP28?

The wavelength depends on the optical type.

The use of 850 nm for short multimode links and approximately 1310 nm for many single-mode 25G links follows the optical characteristics of the fiber and required transmission distance.

12. What Fiber Is Used with 25G SFP28?

25G SFP28 modules use either multimode fiber or single-mode fiber depending on the reach.

Multimode Fiber: OM3, OM4, and OM5 fiber are commonly associated with 850 nm short-reach and extended multimode 25G applications.

Single-Mode Fiber: OS2 or G.652-class single-mode fiber is commonly used for LR, ER, and BiDi 25G links.

Fiber selection must match the transceiver specification. Connecting an SR module to a single-mode link, for example, does not turn it into an LR module and may result in an unsupported or nonfunctional link.

13. Does 25G SFP28 Use NRZ or PAM4?

Traditional 25G Ethernet SFP28 optical interfaces are generally based on NRZ signaling. NRZ uses two signal levels and carries one bit per symbol.

This differs from newer 50G, 100G, 200G, and 400G architectures where PAM4 is increasingly used to increase the number of bits transmitted per symbol.

The simpler NRZ approach helps make 25G SFP28 a practical and mature technology for a wide range of data center and network applications.

14. Does 25G SFP28 Require FEC?

FEC requirements depend on the specific transceiver, cable type, host equipment, and link configuration. Some 25G optical modules can operate under conditions where FEC is not required, while particular reaches or implementations may require host-side FEC for full specified performance.

For example, some 25G SR implementations specify RS-FEC on the host ports, while other 25G configurations can have different FEC requirements. Direct-attach copper cables also have length-dependent FEC requirements in some implementations.

For deployment, the module data sheet and switch or NIC compatibility documentation should be checked together rather than assuming that all 25G SFP28 links use the same FEC configuration.

15. How Much Power Does a 25G SFP28 Use?

25G SFP28 modules are generally designed for relatively low power consumption compared with newer 100G, 400G, and 800G optical modules.

Actual power varies according to the optical architecture, transmitter technology, transmission distance, operating temperature, and module design. For example, a Cisco 25G SFP portfolio lists maximum power figures around 1.2 W for SR and CSR optical modules, about 1.3 W for LR modules, and higher values for certain extended-reach and BiDi implementations.

Low module power is one reason SFP28 remains attractive for high-port-density systems where many 25G interfaces operate simultaneously.

16. What Is the Difference Between SFP+, SFP28, and SFP56?

SFP+, SFP28, and SFP56 share a similar compact pluggable form factor family, but they target different generations of electrical interface speeds.

Although the mechanical dimensions are closely related, the host electrical interface and signal requirements are different. A device supporting one generation should not automatically be assumed to support every other SFP-family module.

17. Where Are 25G SFP28 Optical Transceivers Used?

Data Center Server Connectivity: 25G SFP28 is widely used to connect high-performance servers to top-of-rack switches, providing more bandwidth than 10G while maintaining a compact interface.

Leaf-Spine Networks: 25G can be deployed for server-facing connections and selected switch-to-switch links in modern data center architectures.

Enterprise Networks: Organizations upgrading from 10G can use 25G to increase interface bandwidth without immediately moving to larger 100G-class interfaces.

5G Networks: 25G optical interfaces can be used in telecom equipment and transport systems where compact high-speed optical connectivity is required.

Storage and High-Performance Computing: 25G connectivity can support servers, storage systems, and compute clusters that require more throughput than conventional 10G networking.

18. Can 25G SFP28 Be Used for 10G Applications?

Some SFP28 modules are designed as dual-rate 10G/25G products and can operate on both speeds when supported by the host platform. However, not every 25G SFP28 module is automatically backward compatible with 10G.

For example, the 10/25GBASE-LR SFP28 product category is explicitly designed for both 10G and 25G operation. Compatibility depends on the optical module, switch port, firmware, coding, and host configuration.

Therefore, a 25G SFP28 module should be selected according to the supported speed modes of both ends of the link.

19. How Do You Choose the Right 25G SFP28 Transceiver?

The correct 25G SFP28 module should be selected based on the complete link rather than the 25G data rate alone.

Other important parameters include the exact switch or NIC model, module coding, wavelength, connector type, fiber type, link distance, optical power budget, operating temperature, FEC requirements, and supported data rates.

C-LIGHT provides 25G SFP28 optical connectivity for data center, telecom, enterprise, and industrial networking applications, with options covering different wavelengths, fiber types, transmission distances, temperature grades, and equipment compatibility requirements.

20. Conclusion

A 25G SFP28 optical transceiver is a compact pluggable module that provides 25Gb/s-class network connectivity over optical fiber. SFP28 is the physical form factor, while specific optical variants such as 25G-SR, 25G-CSR, 25G-LR, 25G-ER, and 25G-BiDi define the transmission method and reach.

For short data center connections, 850 nm multimode SR optics are commonly used. For longer links, 1310 nm single-mode LR and ER solutions provide substantially greater reach, while BiDi modules can reduce the required fiber count by transmitting both directions over one fiber.

When deploying 25G SFP28, network designers should consider transmission distance, MMF or SMF, wavelength, connector, host compatibility, FEC, power consumption, temperature range, and optical budget together. Proper matching of these parameters helps ensure stable 25G network operation and efficient use of existing fiber infrastructure.

21.25G SFP28 Optical Transceiver Q&A