NPO vs LPO

Near-Packaged Optics (NPO) and Linear Pluggable Optics (LPO) are two important optical interconnect architectures being considered for high-speed AI and data center networks. Both aim to address the power and signal-integrity challenges created by increasing electrical bandwidth, but they solve the problem in very different ways.

LPO keeps the optical interface in a pluggable module while removing the conventional DSP from the module signal path. NPO moves the optical engine much closer to the switch or compute ASIC, significantly shortening the high-speed electrical path on the host board.

The result is an important architectural distinction: LPO mainly changes how a pluggable module processes signals, while NPO mainly changes where the optical engine is physically located. These approaches can overlap in some system designs, but they are not simply two versions of the same technology.

1. What Is NPO?

NPO stands for Near-Packaged Optics. In an NPO architecture, the optical engine is positioned close to the host ASIC or switching chip instead of being located at the front panel through a conventional optical module.

The objective is to shorten the high-speed electrical connection between the ASIC and optical engine. This reduces PCB transmission loss and can improve the electrical margin available to very high-speed interfaces.

2. What Is LPO?

LPO stands for Linear Pluggable Optics. It is a pluggable optical architecture that removes the conventional DSP or retimer function from the optical module signal path.

Instead of performing major signal restoration inside the optical module, the system relies more heavily on the host ASIC or NIC SerDes to handle equalization and signal conditioning.

3. NPO vs LPO at a Glance

4. The Core Difference Between NPO and LPO

The easiest way to understand the difference is to separate signal processing from physical placement.

LPO primarily changes signal-processing architecture by removing the conventional DSP from the optical module. NPO primarily changes the physical location of the optical engine by moving it closer to the ASIC.

This means that NPO and LPO should not always be treated as mutually exclusive concepts. An NPO optical engine can use a specific signal-processing architecture, while LPO defines a different approach to the module signal path.

5. Traditional Pluggable Optical Architecture

A conventional high-speed pluggable architecture can be represented as:

ASIC → PCB Trace → Connector → Optical Module → Fiber

The optical module is located at the front panel. As the host signaling rate increases, the electrical path between the ASIC and module becomes increasingly difficult to manage.

6. LPO Architecture

An LPO system retains the front-panel pluggable structure:

ASIC → Electrical Channel → LPO Module → Fiber

The difference is that the optical module does not use the same conventional DSP or retimer architecture as a standard retimed module.

7. NPO Architecture

An NPO system changes the location of the optical engine:

ASIC → Short Electrical Connection → Optical Engine → Fiber

The optical engine is physically located much closer to the ASIC, reducing the distance over which the high-speed electrical signal must travel on the host board.

8. Electrical Distance Is a Major Factor

At very high signaling rates, electrical distance becomes increasingly important.

Longer PCB traces and connectors introduce insertion loss, reflections, crosstalk, and other signal-integrity impairments. Moving optics closer to the ASIC reduces this electrical distance.

9. LPO Still Has a Host-to-Module Electrical Channel

LPO can reduce module power by removing the DSP, but the optical engine remains in a pluggable module at the front panel.

This means the host ASIC still has to drive a relatively long electrical channel between the ASIC and optical module. The quality of that channel and the capabilities of the host SerDes therefore become critical.

10. NPO Shortens the Electrical Channel

NPO attacks the same signal-integrity problem from another direction.

Instead of asking a high-speed electrical signal to travel from the ASIC toward the front-panel module, the optical engine is positioned close to the ASIC. The electrical path becomes much shorter before the signal is converted to optical form.

11. Why Electrical Loss Matters More at Higher Speeds

As lane rates move from 100G to 200G and beyond, the electrical channel consumes more of the available signal budget.

PCB material, package transitions, connectors, vias, traces, and other interconnect structures can introduce increasing loss at higher frequencies. Reducing the electrical path can therefore become a system-level requirement rather than simply an optimization.

12. NPO and Signal Integrity

NPO can improve signal integrity by significantly reducing the length of the high-speed electrical connection between the host ASIC and optical engine.

This can provide more electrical margin for the SerDes and reduce the amount of equalization required to compensate for a long PCB path.

13. LPO and Signal Integrity

LPO does not eliminate the host-to-module electrical channel. Instead, it uses a linear optical module and relies more heavily on the host transmitter and receiver to compensate for that channel.

The resulting architecture can work effectively when the host SerDes, PCB design, connector, module, and optical link are engineered as a complete system.

14. Host SerDes Requirements

Both NPO and LPO increase the importance of host SerDes capabilities.

In an LPO system, the SerDes needs to compensate for the host-to-module channel. In an NPO system, the electrical channel is much shorter, but the SerDes still needs to operate correctly with the optical engine and associated interconnect.

15. DSP in LPO

The defining characteristic of LPO is the absence of a conventional DSP or retimer in the main optical module signal path.

The host ASIC or NIC can contain sophisticated SerDes equalization and signal-processing functions. Therefore, LPO should not be interpreted as a network architecture with no digital signal processing anywhere in the system.

16. DSP in NPO

NPO does not inherently define one specific DSP architecture.

The primary NPO concept is the placement of the optical engine close to the ASIC. Depending on the implementation, signal processing can be distributed differently between the host ASIC, optical engine, and other system components.

This is an important distinction when comparing NPO with LPO.

17. Power Consumption

Power reduction is one of the main motivations behind both architectures, but the source of the power advantage is different.

LPO reduces module power by removing the conventional DSP or retimer. NPO reduces electrical transmission losses and the power required to drive a longer high-speed electrical channel by moving the optical engine closer to the ASIC.

18. LPO Module Power

Because the conventional module DSP is removed, LPO can significantly reduce the power consumed within the pluggable optical module.

This is particularly attractive in large systems where hundreds or thousands of high-speed optical modules may operate simultaneously.

19. NPO System Power

NPO can improve system-level power efficiency by shortening the electrical path between the ASIC and optical engine.

However, the total system power depends on the optical engine, driver, laser architecture, thermal design, SerDes implementation, PCB, and host ASIC. NPO should therefore be evaluated at system level rather than judged only by the optical engine's power.

20. Latency

LPO can reduce module-added latency because the conventional DSP processing path is removed.

NPO can also reduce electrical propagation distance and signal-processing requirements associated with a long host channel. The actual latency benefit depends on the complete architecture.

21. Why Latency Matters in AI Networks

AI and high-performance computing systems can contain large numbers of interconnected processors and switches.

Lower latency at each interconnect can contribute to faster communication across multi-stage fabrics, although total application performance depends on network topology, switching, congestion, software, and many other factors.

22. Thermal Management

High-speed optical systems generate increasing amounts of heat as bandwidth scales.

Moving the optical engine closer to the ASIC changes the thermal architecture because the optical components are no longer concentrated on the front-panel modules.

23. LPO Thermal Design

LPO can reduce the heat generated inside each pluggable module because the high-power DSP or retimer is removed.

The reduction in module power can simplify the thermal design of dense front-panel optical environments.

24. NPO Thermal Design

NPO creates a different thermal challenge because the optical engines are located close to the ASIC.

The optical engine, switching ASIC, PCB, heat spreader, and cooling system must therefore be designed together. In very high-density systems, the location of the optical engine can become an important thermal engineering constraint.

25. Serviceability

Serviceability is one of the most significant differences between conventional pluggable architectures and near-package architectures.

LPO retains a removable pluggable module, allowing a failed optical module to be replaced independently from the switch board.

A conventional NPO implementation places the optical engine on or near the main board, reducing the simplicity of field replacement.

26. NPO and Modular Design

NPO does not necessarily mean that every optical engine is permanently inaccessible.

Emerging implementations can use modular onboard optical engines or other mechanical approaches to improve serviceability. Some industry discussions also consider pluggable NPO concepts.

Therefore, NPO serviceability depends on the specific system architecture rather than the NPO label alone.

27. LPO Deployment Flexibility

One major benefit of LPO is that it retains the familiar pluggable optical workflow.

Modules can be inserted, removed, replaced, and upgraded without redesigning the host board. This provides significant operational flexibility for data center operators.

28. NPO Deployment Flexibility

NPO provides less flexibility at the traditional module level because the optical engine is much closer to the ASIC.

At the system architecture level, however, NPO can provide greater flexibility in electrical layout because the optical conversion point can be positioned according to the ASIC and board design.

29. Front-Panel Density

LPO still uses front-panel pluggable modules. As switch bandwidth increases, the front panel can become crowded with high-speed optical interfaces.

NPO changes the architecture by moving optical engines away from the front panel. This can help reduce the dependence on a very large number of front-panel optical module positions.

30. Optical Fiber Connection

Moving the optical engine closer to the ASIC does not eliminate the external fiber connection.

NPO still requires optical fibers to leave the system and connect to the external network. The difference is that the electrical-to-optical conversion occurs much closer to the ASIC.

31. LPO Optical Interfaces

LPO can use familiar pluggable optical form factors such as QSFP-family and OSFP-family modules, depending on the specific implementation.

The optical connector can be LC, MPO-family, or another interface according to the selected optical architecture.

32. NPO Optical Interfaces

NPO is not defined by one universal front-panel connector.

The optical engine can be connected to the external fiber system through different optical interfaces or fiber assemblies depending on the board and system architecture.

33. 800G LPO

800G is an important application area for LPO because removing the DSP can reduce power and latency while retaining a pluggable interface.

The LPO MSA has defined 100 Gb/s-per-lane specifications supporting configurations up to 800G, including parallel single-mode architectures.

34. 800G NPO

800G can also be implemented using NPO architectures where the optical engine is placed close to the switching ASIC.

The shorter electrical channel can provide additional signal margin compared with a front-panel optical module, which becomes valuable as electrical signaling rates increase.

35. 1.6T LPO

1.6T LPO is technically more challenging because 200G-per-lane electrical signaling places significantly greater demands on the host-to-module channel.

As lane speeds increase, maintaining a linear pluggable architecture requires careful optimization of the ASIC SerDes, PCB, connectors, module electronics, and optical components.

36. 1.6T NPO

NPO becomes increasingly attractive as electrical lane rates move toward 200G and beyond because the optical engine can be positioned close to the host silicon.

By reducing PCB transmission distance, NPO can help manage the electrical loss and signal-integrity challenges associated with extremely high-speed interfaces.

37. 224G Electrical Signaling

At 224G-class signaling, the electrical channel becomes one of the primary architectural constraints.

The shorter connection offered by near-package approaches can create more favorable conditions for maintaining signal integrity than a long ASIC-to-front-panel module path.

38. 448G and Future Scaling

As signaling approaches 448G-class interfaces, the electrical budget becomes even more restrictive.

Near-package and co-packaged approaches are therefore being explored as ways to bring optical conversion closer to the host silicon and reduce the electrical distance that must be supported at extremely high bandwidths.

39. NPO vs LPO and AI Scale-Up

AI scale-up networks increasingly require communication between GPUs, XPUs, NICs, switches, and other compute components over high-bandwidth links.

LPO can reduce power while preserving pluggable connectivity. NPO can move optical conversion closer to the compute silicon and reduce the electrical limitations of the board-level interconnect.

40. NPO vs LPO: Main Trade-Off

41. NPO vs LPO Cost

LPO has the potential to reduce module cost by removing the DSP and simplifying the module electronics.

NPO can reduce some electrical interconnect requirements, but it introduces greater board-level integration, optical-engine assembly, thermal, testing, and manufacturing considerations.

Therefore, it is difficult to compare NPO and LPO purely by the unit price of an optical component. Total system cost is more meaningful.

42. Manufacturing Complexity

LPO can use an established pluggable module manufacturing model with a changed signal-processing architecture.

NPO requires tighter integration between optical engines, PCB design, ASIC placement, thermal solutions, fiber routing, manufacturing processes, and system testing.

This makes NPO a broader system-engineering project rather than simply a different optical module.

43. NPO vs LPO: Which Architecture Fits Which Scenario?

44. NPO vs LPO and CPO

NPO and LPO are also best understood in relation to CPO.

LPO keeps optics in a pluggable front-panel module. NPO moves optics close to the ASIC but keeps them separate from the ASIC package in the typical near-package concept. CPO integrates the optical engine even more tightly with the switching ASIC package.

This creates a progression in which the optical engine moves progressively closer to the host silicon as electrical bandwidth increases.

45. NPO, LPO and CPO Architecture

46. NPO Is Not Simply a Better LPO

It is misleading to treat NPO as a direct replacement for LPO in every application.

LPO preserves the operational advantages of pluggable optics and can significantly reduce module power without moving the optical engine onto the host board. NPO addresses electrical reach more aggressively but requires greater system-level integration.

47. LPO Is Not Simply a Smaller NPO

LPO and NPO solve different parts of the interconnect problem.

LPO focuses on a linear pluggable signal path and the removal of module DSP. NPO focuses on moving the optical conversion point closer to the host silicon.

This difference becomes especially important when evaluating future 1.6T and 3.2T network architectures.

48. Future Development Direction

As switch bandwidth continues to increase, the industry is evaluating multiple optical architectures rather than relying on one universal solution.

LPO, NPO, CPO, and other approaches can occupy different positions depending on the required bandwidth, electrical reach, thermal environment, service model, and manufacturing capability.

49. NPO vs LPO Selection Factors

When comparing the two architectures, evaluate the following system parameters:

ASIC-to-optics electrical distance, lane rate, host SerDes capability, module power, optical engine power, thermal design, front-panel density, serviceability, manufacturing complexity, fiber routing, interoperability, and future bandwidth requirements.

50. NPO vs LPO Summary

51. Conclusion

NPO and LPO represent two different approaches to solving the scaling challenges of high-speed optical interconnects. LPO maintains a pluggable optical module while removing the conventional DSP from the module signal path, reducing module power and latency and relying more heavily on host SerDes capabilities.

NPO moves the optical engine much closer to the ASIC, significantly shortening the high-speed electrical path. This can improve signal integrity and reduce the electrical signaling burden as lane rates continue to increase.

The trade-off is architectural. LPO preserves the flexibility, replaceability, and deployment model of pluggable optics. NPO requires greater board-level integration and changes the thermal, manufacturing, testing, and maintenance model.

For 800G systems, both architectures can play important roles. As networks move toward 1.6T, 224G-class electrical signaling, and eventually higher lane rates, reducing electrical distance becomes increasingly important, which is why NPO and other near-package architectures are receiving greater attention in AI and high-performance data center networks.

52.NPO vs LPO Q&A