25G SFP28 Optical Modules for 5G Fronthaul Connectivity

1. 25G SFP28 in the 5G Fronthaul Architecture

Fronthaul connects radio units to baseband processing or distributed processing equipment. In a traditional architecture, the radio unit and baseband unit exchange radio-related data through CPRI interfaces. In more distributed 5G architectures, eCPRI and Ethernet-based transport allow radio functions to be divided across network elements with greater flexibility.

The fronthaul optical link must carry the required traffic while maintaining the latency, synchronization and availability characteristics of the radio network. A 25G SFP28 transceiver provides the optical-electrical conversion needed to transport these signals across fiber between compatible network interfaces.

Typical connection points include:

• Radio Unit (RU): Connects the radio equipment to the fronthaul fiber network.

• Distributed Unit (DU): Receives and processes traffic transported from radio units in split-RAN architectures.

• Centralized Unit (CU): Connects higher-layer radio functions to the wider transport and core network.

• Fronthaul aggregation switch: Aggregates multiple radio-site connections and forwards traffic toward processing equipment.

• Optical distribution infrastructure: Provides the fiber paths, patch panels and intermediate connections between network locations.

2. 25G SFP28 Data Rates and Electrical Interfaces

SFP28 is a single-lane pluggable transceiver form factor designed for 25G-class serial interfaces. A common 25GbE line rate is 25.78125 Gb/s, corresponding to the 25GBASE Ethernet physical layer. Certain CPRI and eCPRI implementations use different line rates, including 24.33024 Gb/s in specific radio transport configurations.

This distinction is important when selecting an optical module. A transceiver advertised as 25G does not necessarily support every 25G-class protocol rate. The host equipment, module electrical interface, clocking behavior and supported line-rate configuration must be checked together.

IEEE 802.3by and IEEE 802.3cc define 25GbE physical-layer specifications for different optical media. CPRI and eCPRI compatibility involves additional protocol and equipment requirements. Ethernet compliance alone should not be treated as proof of compatibility with every radio fronthaul interface.

3. Optical Module Types for 5G Fronthaul

Fiber distance and infrastructure conditions determine which optical configuration is appropriate. Short links inside a radio site may use multimode optics, while connections between distributed radio locations and aggregation facilities generally rely on single-mode fiber. Where fiber resources are limited, BiDi or wavelength-division multiplexing modules can increase the number of optical links supported by the installed infrastructure.

Reach values are not interchangeable across optical types. For example, the reach of an 850nm SR module depends on the multimode fiber category and channel characteristics. A 1310nm LR module is designed for single-mode fiber and a different optical link budget. The final distance must follow the specific transceiver datasheet and the applicable interface specification.

4. 1310nm Single-Mode SFP28 for 10km Fronthaul Links

1310nm 25G SFP28 LR modules are widely suited to point-to-point fronthaul links using duplex single-mode fiber. A typical implementation uses a DFB laser transmitter and PIN photodetector receiver, with duplex LC optical interfaces and a nominal transmission distance of up to 10km under the specified link conditions.

The 1310nm optical window provides a practical solution for access and metropolitan fiber routes. It supports direct connections between compatible radio equipment and aggregation platforms without requiring wavelength multiplexing equipment for a basic point-to-point link.

Key specifications to verify include:

• 25G-class optical data rate and supported protocol modes.

• 1310nm transmitter wavelength and receiver operating range.

• DFB laser and PIN receiver characteristics, where specified.

• Minimum and maximum transmitter output power.

• Receiver sensitivity and overload limits.

• Duplex LC connector and single-mode fiber compatibility.

• Operating temperature range and module power consumption.

• CDR functionality, diagnostic support and host compatibility.

For outdoor radio installations, industrial-temperature variants may be required. The temperature rating must be evaluated against the equipment enclosure and actual operating conditions rather than selected solely from the network's nominal ambient temperature.

5. 25G BiDi SFP28 for Fiber Resource Optimization

In some 5G deployments, available fiber cores are limited or the cost of installing additional fiber is high. A 25G BiDi SFP28 solution can transmit and receive signals over a single strand of single-mode fiber by using separate wavelengths for each direction.

A BiDi link requires two complementary transceivers. One module transmits on wavelength A and receives on wavelength B, while the remote module transmits on wavelength B and receives on wavelength A. The wavelength pairing must be correct at both ends.

BiDi modules can reduce fiber consumption for point-to-point links, but their optical budget, wavelength compatibility and equipment support must be verified. They are particularly useful where existing fiber routes have limited spare cores or where new cable installation is difficult.

6. CWDM and LWDM for Multi-Channel Fronthaul

When several radio units need to share a fiber route, wavelength-division multiplexing can carry multiple optical channels over the same fiber infrastructure. CWDM and LWDM 25G SFP28 modules use assigned wavelengths to separate individual links, allowing multiple channels to coexist through compatible multiplexers and demultiplexers.

CWDM uses a wider wavelength spacing, while LWDM uses a more closely spaced wavelength plan. The actual channel count and supported reach depend on the selected wavelength grid, transceiver characteristics, optical filtering and system loss.

WDM designs require careful channel planning. Transmit wavelength accuracy, channel isolation, insertion loss, optical power levels and receiver sensitivity all influence the performance of the combined system. Modules should be selected as part of the complete optical link rather than as isolated components.

7. Low Latency and Signal Integrity in 5G Fronthaul

Fronthaul latency affects the timing relationship between radio functions and the processing equipment. The complete transport path must meet the latency requirements of the selected radio architecture and functional split. Fiber propagation, switching, packet processing, buffering and intermediate transport equipment all contribute to end-to-end delay.

At the optical interface, signal integrity depends on the transmitter, receiver, electrical host interface and optical channel. Some 25G SFP28 modules incorporate clock and data recovery (CDR) circuits to retime signals and improve the electrical or optical interface performance within the module's specified operating conditions.

Important design factors include:

• CDR behavior: Confirm whether the module uses transmitter-side and receiver-side CDR and whether the operating mode matches the host interface.

• Signal quality: Check transmitter characteristics, receiver sensitivity and the applicable eye-mask or bit-error-rate requirements.

• FEC configuration: Verify whether the interface requires or supports forward error correction and whether both link endpoints use compatible settings.

• Fiber condition: Excessive attenuation, reflections, connector contamination and bending can degrade link performance.

• Timing architecture: SyncE and IEEE 1588 PTP support depends on the complete network equipment and transport design, not simply on the optical transceiver.

8. Optical Link Budget and Fronthaul Distance

The optical power budget determines whether the received signal remains within the transceiver's supported operating range. For a basic point-to-point link, the available power budget is calculated from the minimum transmitter output power and the receiver sensitivity.

Available Optical Power Budget (dB) = Minimum Tx Output Power (dBm) − Receiver Sensitivity (dBm)

The total link loss must account for fiber attenuation, connector insertion loss, splice loss, patch panels and other passive components. An engineering margin should also be reserved for installation variation, maintenance and aging.

A nominal 10km reach rating does not guarantee that every 10km route will operate successfully. The actual fiber attenuation, number of connectors, splice quality, optical reflections and module specifications must be considered together. For longer engineered links, dispersion and other transmission penalties may also require evaluation.

9. Temperature, Power Consumption and Outdoor Deployment

5G radio equipment is frequently installed in outdoor cabinets, remote shelters and locations exposed to changing environmental conditions. Optical modules used in these environments must operate within the temperature and power limits of the host platform.

Commercial-temperature modules are generally intended for controlled equipment environments. Industrial-temperature variants are available for applications requiring a wider operating case-temperature range, commonly specified as -40°C to +85°C for certain product designs. The actual rating must always be confirmed from the individual datasheet.

Deployment checks should include:

• Operating case-temperature range under the expected cabinet conditions.

• Maximum power consumption and host-port power capability.

• Airflow and thermal management around densely populated optical ports.

• Module temperature monitoring through supported DDM functions.

• Mechanical retention and connector protection during field installation.

• Environmental requirements of the complete radio or transport equipment.

10. DDM Monitoring and Optical Fault Diagnosis

Digital Diagnostic Monitoring provides real-time operating information from supported SFP28 modules. Network operators can use these readings to monitor optical performance and identify abnormal operating conditions across distributed radio sites.

DDM is particularly useful for troubleshooting intermittent fronthaul links. A declining Rx power trend may indicate connector contamination, fiber damage or a change in the optical path. Abnormal temperature or voltage readings may point toward host equipment or environmental issues.

Diagnostic readings should be interpreted alongside interface alarms, error counters, BER measurements and physical fiber testing. DDM accuracy and alarm thresholds vary by module design, so reported values should be evaluated against the relevant product specifications.

11. Compatibility and Interoperability Requirements

A 25G SFP28 module must match both the optical requirements of the link and the electrical interface of the host equipment. Physical insertion into an SFP28 port does not guarantee that the module will be recognized or operate at the intended rate.

• Host interface: Verify the supported SFP28 port speed and electrical interface.

• Protocol rate: Confirm the exact Ethernet, CPRI or eCPRI rate required by the application.

• Optical standard: Match the module to the relevant SR, LR, BiDi, CWDM or LWDM specification.

• Wavelength pairing: Ensure complementary wavelengths for BiDi and correct channel assignments for WDM systems.

• Connector and fiber: Check LC interface type, fiber category and polarity requirements.

• Power and thermal limits: Confirm that the host can supply the module's required power and maintain suitable cooling.

• Management interface: Verify EEPROM identification, DDM support and host monitoring compatibility.

• FEC and CDR settings: Confirm that both link endpoints support compatible signal processing and error-correction configurations.

Interoperability testing should include the actual radio, switch or transport platform used in deployment. This is especially important for multi-rate modules and specialized fronthaul interfaces, where host firmware and port configuration can affect link establishment.

12. C-LIGHT 25G SFP28 Optical Connectivity Solutions

C-LIGHT provides optical transceiver solutions for telecom access, wireless transport and high-speed Ethernet applications. A 25G SFP28 configuration can be selected according to the required data rate, fiber type, transmission distance, wavelength plan, connector and host-platform requirements.

13. Engineering Priorities for 5G Fronthaul Expansion

As 5G radio deployments expand, fronthaul networks must accommodate higher traffic capacity, more distributed radio locations and increasingly complex fiber infrastructure. 25G SFP28 provides a practical optical interface for many 25G-class links, while BiDi and WDM configurations help operators make more efficient use of existing fiber resources.

Network planning should begin with the radio interface requirements and the exact line rate, followed by fiber-route assessment, optical budget calculation and transceiver compatibility testing. Temperature range, power consumption, diagnostic functions and maintenance procedures should be included in the equipment specification.

A properly engineered 25G optical link supports reliable fronthaul connectivity and provides a scalable transport foundation for distributed 5G radio access networks.

14. Conclusion

25G SFP28 optical modules play an important role in 5G fronthaul by providing compact, high-speed fiber connectivity between radio equipment, distributed processing resources and aggregation networks. SR, LR, BiDi, CWDM and LWDM configurations address different fiber distances and infrastructure conditions.

Successful deployment depends on matching the transceiver to the exact CPRI or eCPRI rate, host electrical interface, optical wavelength, fiber route and link budget. With suitable module selection and end-to-end compatibility verification, 25G SFP28 connectivity can support the bandwidth and operational requirements of modern 5G transport networks.