QSFP+ vs QSFP28: Key Differences in Data Rate, Lane Architecture and 40G/100G Applications

1. QSFP+ and QSFP28: Two Generations of High-Speed Connectivity

QSFP+ stands for Quad Small Form-factor Pluggable Plus. It is a four-lane pluggable interface widely deployed for 40 Gigabit Ethernet, high-performance computing and data center interconnections. A typical 40G QSFP+ implementation uses four electrical lanes operating at approximately 10Gb/s each.

QSFP28 is an enhanced member of the same form-factor family. It was developed to support 100Gb/s-class connectivity through four electrical lanes operating at approximately 25Gb/s each. It maintains a compact module footprint while increasing the bandwidth available from each port.

The key architectural distinction is the electrical lane rate. QSFP28 is not simply a QSFP+ module operating at a higher software-configured speed. Its host interface, signal integrity requirements and supported operating modes are designed for the higher data rate.

2. QSFP+ vs QSFP28: Technical Specification Comparison

The lane rates shown above describe common Ethernet implementations. Actual signaling and optical channel arrangements depend on the module standard. A QSFP28 product may use four optical channels, wavelength multiplexing or a single optical wavelength with internal rate conversion.

3. Data Rate and Lane Architecture

The bandwidth difference between QSFP+ and QSFP28 comes from the speed of each electrical lane, not from an increase in the number of lanes.

A conventional 40G QSFP+ interface uses four lanes to deliver a nominal aggregate rate of 40Gb/s. A common 40GbE implementation operates at approximately 10.3125Gb/s per lane to accommodate the encoding overhead.

A conventional 100G QSFP28 interface uses four lanes at approximately 25.78125Gb/s each. This provides the signaling capacity required for 100GbE. Both formats use four transmit lanes and four receive lanes, but QSFP28 carries substantially more data per lane.

This higher lane rate requires the host ASIC, SerDes, PCB traces, connector contacts and transceiver electronics to maintain signal integrity at the intended operating speed. A port designed only for 10G-per-lane signaling cannot automatically operate at the 25G-per-lane rate required by a standard QSFP28 100G module.

4. Physical Dimensions and Mechanical Compatibility

QSFP+ and QSFP28 use a similar QSFP-family physical package. This allows equipment manufacturers to develop high-density platforms around a consistent general module footprint while supporting different generations of electrical performance.

However, similar module dimensions do not establish full compatibility. A host port must support the electrical lane rate, module management interface, power requirements and network protocol of the installed transceiver.

Many QSFP28-equipped switches support selected QSFP+ modules for 40G operation. This is commonly used when a 100G-capable platform also needs to connect to existing 40G equipment. The feature is host-dependent and should be confirmed in the equipment compatibility documentation.

A QSFP+ port designed only for 40G operation should not be assumed to support a 100G QSFP28 module. The cage may look similar, but the host electrical interface may not have the required bandwidth.

5. Electrical Interface and Signal Integrity

QSFP28 operates at a significantly higher electrical lane rate than QSFP+. This increases the importance of high-speed PCB design, insertion loss, crosstalk control, connector performance and signal equalization.

For a 40G QSFP+ interface, the host electrical system is designed around four approximately 10Gb/s lanes in each direction. The transceiver converts these electrical signals into the corresponding optical channels or wavelength-multiplexed optical output.

For a conventional 100G QSFP28 interface, the host supplies four higher-speed electrical lanes. The module then converts the signals into an optical format appropriate to the selected transceiver type.

Not every QSFP28 optical module has four optical wavelengths or four physically separate optical lanes. For example, traditional 100G SR4 modules use four parallel optical lanes, while LR4 modules combine four optical wavelengths over a duplex single-mode fiber connection. Some single-lambda designs use internal gearboxes to convert four electrical lanes into one higher-rate optical channel.

Consequently, the electrical lane architecture and optical lane architecture should be evaluated separately when designing a 100G link.

6. Breakout Connectivity: 40G to 4 × 10G and 100G to 4 × 25G

One practical advantage of both QSFP+ and QSFP28 is their support for multi-lane breakout configurations when the host platform and selected cable or transceiver support channelization.

A compatible 40G QSFP+ port can be divided into four independent 10G connections. This is commonly used to connect a 40G aggregation switch to four 10G server or access ports.

A compatible 100G QSFP28 port can support four 25G connections through an appropriate breakout cable or optical configuration. This allows a 100G switch port to connect to four 25G SFP28 host ports.

Breakout capability depends on more than the transceiver. The switch ASIC, port configuration, firmware, cable wiring and optical lane mapping must support the intended arrangement. Some optical modules also use internal rate conversion, which can affect whether individual lanes can be separated.

7. Optical Types, Wavelengths and Transmission Distance

Both QSFP+ and QSFP28 support different optical configurations for multimode and single-mode fiber. Their transmission distances depend on the optical standard, laser technology, receiver sensitivity and available link budget rather than the form factor alone.

These are representative optical configurations rather than specifications for every product. Extended-reach QSFP+ and QSFP28 modules are also available. Their actual performance must be checked against the individual datasheet and the optical link design.

8. Power Consumption and Thermal Requirements

QSFP28 supports a higher data rate than QSFP+, but module power consumption cannot be determined from the form factor alone. Optical reach, transmitter type, receiver design, DSP or gearbox functions, and internal signal conditioning all influence power requirements.

A short-reach multimode module may have different power characteristics from a long-reach single-mode module in the same form factor. Extended-reach products may require additional optical and electronic functions, making their power requirements higher than those of some short-reach configurations.

For high-density switches, the total power of all installed transceivers can influence chassis cooling and airflow planning. Network designers should verify maximum rated power and operating-temperature limits for each selected module.

9. Network Applications and 40G-to-100G Migration

QSFP+ is widely used in established 40GbE infrastructure, particularly in aggregation switches, data center interconnects and high-performance computing networks. It remains relevant where existing equipment and traffic requirements are built around 40G interfaces.

QSFP28 is commonly deployed in 100GbE leaf-spine networks, data center uplinks, enterprise core networks and high-bandwidth server connections. Its higher port bandwidth allows network operators to increase capacity without requiring a completely different physical module family.

During a 40G-to-100G migration, both module types may coexist in the same network. A 100G QSFP28 switch can connect to 40G equipment through a supported 100G-to-40G network design, such as a compatible multi-rate port or an appropriate optical transport arrangement. The specific port configuration and optical protocol must be verified at both ends.

10. C-LIGHT QSFP+ and QSFP28 Optical Transceiver Solutions

C-LIGHT provides 40G QSFP+ and 100G QSFP28 optical transceivers for data centers, enterprise networks, telecommunications infrastructure and high-performance computing applications. The product families cover different fiber types, optical connectors and transmission distances.

For a suitable replacement or network upgrade, provide the equipment model, port speed, optical reach, fiber type and required connector. This information helps confirm module specifications and host compatibility before deployment.

11. Selecting the Right Module for Your Network

The choice between QSFP+ and QSFP28 should be based on the required bandwidth and the capabilities of the installed network platform. A 40G interface is appropriate for equipment designed around QSFP+ connectivity, while a 100G link requires a host and transceiver combination that supports QSFP28 operation.

Before ordering, verify the host's port speed, transceiver compatibility list, firmware support, required FEC mode, optical interface and module power limit. Matching these parameters is essential for stable operation.

12. Conclusion