1. Understanding 800G QSFP-DD and OSFP-RHS

QSFP-DD and OSFP-RHS are two different mechanical approaches to 800G pluggable optical connectivity. Their electrical interfaces can deliver the same nominal bandwidth, but their host cages and cooling arrangements are different.

The term RHS stands for Riding Heat Sink. It describes the thermal and mechanical arrangement, not a different transmission-speed category. OSFP-RHS modules can support different data-rate generations, including 800G and 1.6T implementations, depending on the specific module specification.

2. Main Technical Differences

The most significant difference is the cooling arrangement. QSFP-DD uses its own module and host thermal design, while OSFP-RHS removes the integrated module heatsink and transfers the thermal interface to a riding heatsink in the host system.

3. Electrical Architecture: Eight 100G-Class PAM4 Lanes

Both 800G QSFP-DD and 800G OSFP-RHS commonly use eight electrical lanes operating at 100G-class PAM4 signaling. The eight lanes provide an aggregate host bandwidth of 800Gb/s.

For QSFP-DD800, the MSA defines an eight-lane electrical interface supporting up to 800Gb/s aggregate bandwidth. Its electrical operation supports up to 112Gb/s per lane using PAM4 signaling. The OSFP MSA defines the corresponding OSFP800 electrical generation, and OSFP-RHS uses the same electrical and management interface as its matching OSFP variant.

PAM4 uses four signal levels to encode two bits per symbol. Compared with NRZ, it enables more data to be carried within a given symbol-rate range, but it also increases signal-integrity requirements for the host ASIC, PCB traces, connector and module electronics.

FEC, DSP implementation and the selected Ethernet or InfiniBand mode must also match the intended system configuration. Although both form factors can provide 800G bandwidth, the host platform must support the correct electrical interface and module management behavior.

4. OSFP-RHS Riding Heatsink Design

The defining feature of OSFP-RHS is its external riding-heatsink arrangement. Unlike standard OSFP modules with an integrated heatsink, an OSFP-RHS module has no integrated heatsink on its top surface. The host cage provides a riding heatsink that makes thermal contact with the module.

The OSFP MSA specifies an OSFP-RHS module height of approximately 9.5mm without the heatsink. The riding heatsink is mounted as part of the host-side mechanical and thermal structure. This separates the transceiver's mechanical package from the main heatsink assembly.

This approach is particularly useful where the equipment manufacturer wants to integrate transceiver cooling with a shared thermal system. Depending on the platform, the riding heatsink may work with a cold plate or another dedicated cooling assembly.

OSFP-RHS should not automatically be described as a liquid-cooled transceiver. RHS defines the heatsink arrangement. Whether the complete system uses liquid cooling, a cold plate or another thermal method depends on the host equipment design.

5. Module Dimensions and Port Density

QSFP-DD800 has a compact QSFP-derived mechanical structure. This helps equipment manufacturers arrange high-speed ports within a relatively limited front-panel area.

OSFP-RHS follows the wider OSFP mechanical family, but removes the integrated module heatsink and uses a host-mounted riding heatsink. The lower module height does not make it a QSFP-DD module: the width, connector position, forward stop and cage geometry remain part of the OSFP-RHS mechanical design.

Port density cannot be judged from module dimensions alone. Cage spacing, PCB routing, airflow, power delivery, riding-heatsink geometry and maintenance access all influence the number of usable ports in a switch.

6. Host Compatibility and Physical Interchangeability

800G QSFP-DD and OSFP-RHS are physically different form factors. A QSFP-DD800 module cannot be installed in an OSFP-RHS cage, and an OSFP-RHS module cannot be installed in a QSFP-DD800 port.

There is also a second compatibility distinction within the OSFP family. Standard OSFP modules with integrated heatsinks and OSFP-RHS modules use different cage geometries and mechanical keying. The OSFP MSA specifies that standard OSFP and OSFP-RHS are not mechanically interchangeable.

The OSFP-RHS and standard OSFP share the corresponding OSFP-family electrical and management interface, but their physical cooling and cage arrangements differ. Electrical similarity does not establish mechanical interchangeability.

Optical link interoperability is a separate matter. An 800G QSFP-DD module at one end and an 800G OSFP-RHS module at the other can communicate if both support the same optical media type, wavelength arrangement, reach and transmission standard. Each module must still be installed in its own compatible host port.

7. Optical Configurations and Transmission Distance

The form factor does not determine the transmission distance of an 800G module. Both QSFP-DD and OSFP-RHS product families can offer parallel-fiber, wavelength-multiplexed and coherent configurations, depending on the design.

Short-reach multimode links commonly use 850nm optics. Single-mode configurations include DR4, FR4 and LR4 designs, while coherent modules support much longer distances for data center interconnect and optical transport applications.

These are representative reach categories. Exact reach, wavelength, optical lane mapping, connector type and loss budget must be confirmed using the specific transceiver datasheet and applicable optical standard.

8. C-LIGHT 800G QSFP-DD Optical Transceiver Options

C-LIGHT provides 800G QSFP-DD optical modules for high-density data center, cloud computing and AI networking applications. The product family includes different reach configurations for multimode and single-mode fiber infrastructure.

9. C-LIGHT 800G OSFP-RHS Optical Transceiver Options

C-LIGHT's 800G OSFP product family includes selected OSFP-RHS configurations for host systems that use riding-heatsink cooling. These models cover short-, medium- and longer-reach data center connections.

When comparing these product families, note that the listed 2x400G designation describes the product's optical or port configuration. It should not be confused with the eight-lane host electrical interface used for the 800G module.

10. Which Form Factor Is Suitable for Different Network Platforms?

The choice between QSFP-DD800 and OSFP800-RHS depends mainly on the switch architecture and cooling infrastructure. The optical reach alone does not determine the preferred form factor.

For a conventional switch platform already designed around QSFP-DD800, selecting OSFP-RHS is not a simple module replacement. Conversely, an OSFP-RHS platform cannot accept a QSFP-DD module without a different host interface design.

11. Important Checks Before Selecting an 800G Module

Before deploying 800G QSFP-DD or OSFP-RHS transceivers, engineers should verify the following technical requirements:

• Host interface: Confirm whether the switch, router, NIC or DPU uses QSFP-DD800 or OSFP-RHS ports.

• Electrical signaling: Verify eight 100G-class PAM4 lanes and the platform's supported SerDes capabilities.

• Cage and mechanical design: Check the exact module type, cage geometry, keying and insertion requirements.

• Thermal interface: Confirm whether the platform uses a module heatsink or a host-mounted riding heatsink.

• Cooling method: Verify airflow, cold-plate contact and operating case-temperature limits.

• Optical standard: Match SR4, DR4, FR4, LR4 or coherent specifications at both link endpoints.

• Connector and fiber: Check MPO, LC or CS connector type, fiber category, polarity and optical loss budget.

• Power and management: Confirm module power limits, CMIS support, firmware and platform qualification.

For an 800G link, modules at opposite ends do not have to use the same physical form factor, but their optical transmission specifications must match. Host compatibility and optical interoperability should therefore be evaluated separately.

12. Conclusion