Alternatives to Co-Packaged Optics for Data Center Bandwidth

The primary alternatives to co-packaged optics (CPO) for scaling data center bandwidth are pluggable optical transceivers, near-package optics (NPO), and advanced electrical signaling techniques such as linear-drive pluggables (LPO) and on-board optics (OBO). These approaches each move the optical engine closer to or further from the switch ASIC in different ways, trading off power efficiency, serviceability, and cost. This article explains how each option works, where it fits, and the key trade-offs to consider when planning a high-bandwidth data center network.

Background: Why Co-Packaged Optics Got Attention

Data center bandwidth demand has grown rapidly due to AI training clusters, high-performance computing, and cloud services. Switch ASICs now support 51.2 Tbps or more of aggregate bandwidth, and the industry is moving toward 102.4 Tbps switches. The problem is that traditional pluggable transceivers, mounted on the switch faceplate, consume significant power and board area. As speeds increase, the electrical signals traveling from the ASIC to the faceplate degrade over the trace length, requiring more equalization and power.

Co-packaged optics (CPO) places the optical engine (the lasers, modulators, and photodetectors) directly on the same package as the switch ASIC, or very close to it. This shortens the electrical trace, reduces signal loss, and cuts power consumption. However, CPO introduces major challenges: the optics are no longer field-replaceable, thermal management becomes harder, and yield issues in the optical engine can ruin an expensive ASIC. These drawbacks have kept CPO from broad deployment, and have pushed the industry to develop less radical alternatives.

Pluggable Transceivers: The Incumbent

The most common alternative is to keep using pluggable transceivers, but improve them to support higher speeds. Current pluggables include QSFP-DD and OSFP form factors, which support 800G per module. The next generation, 1.6T pluggables, are in development. These modules contain the optical engine and the DSP (digital signal processor) that cleans up the electrical signal.

• Pros: Field-replaceable, standardized, easy to upgrade, and widely supported by switch vendors.
• Cons: Higher power consumption per bit, larger physical footprint, and signal integrity limits at very high speeds.

Pluggables remain the default choice for most data centers because they allow incremental upgrades and simplify maintenance. The main drawback is that at 102.4 Tbps switch capacity, the faceplate would need 64 ports of 1.6T, which consumes a huge amount of power and space.

Linear-Drive Pluggables (LPO)

Linear-drive pluggables (LPO) remove the DSP from the optical module. Instead of retiming and re-shaping the electrical signal inside the module, LPO relies on the switch ASIC’s SerDes to drive the optical engine directly. This reduces power consumption and latency, and lowers cost, because the DSP is a significant portion of the module’s cost.

However, LPO requires very careful signal integrity design. The electrical path from the ASIC to the optical engine must be short and low-loss, and the ASIC’s SerDes must be capable of driving the optical modulator without the benefit of a DSP in the module. This limits the reach and performance of LPO, making it more suitable for short-reach links within a data center (e.g., 2-10 meters using multimode fiber or short single-mode links).

Near-Package Optics (NPO)

Near-package optics (NPO) is a middle ground between pluggables and CPO. In NPO, the optical engines are placed on the same board as the switch ASIC, but not on the same package. They are typically mounted adjacent to the ASIC, and connected via a short, high-speed electrical trace or a substrate. This shortens the electrical path compared to pluggables, but keeps the optics replaceable (though not as easily as a faceplate plug).

NPO is sometimes called “co-packaged optics lite” because it offers some of the power and signal integrity benefits of CPO, while avoiding the yield and thermal issues of integrating optics into the ASIC package. However, NPO still requires custom board design and does not allow the same ease of upgrade as pluggables.

On-Board Optics (OBO)

On-board optics (OBO) refers to optical engines that are mounted directly on the printed circuit board, but not necessarily adjacent to the ASIC. This is a broader category that includes both NPO and other configurations where the optical engine is on the board but connected via a longer trace. OBO is often used in switch designs where the optical engine is placed near the edge of the board, with a connector for external fiber.

OBO offers a compromise: the optics are not as close as CPO, but they are closer than faceplate pluggables. This reduces the electrical reach requirement, which is a key bottleneck at high speeds. However, OBO modules are harder to replace than pluggables, and require a service model where the entire switch line card is replaced if an optical engine fails.

Advanced Electrical Signaling: The Enabler

All of these alternatives depend on advances in electrical signaling. The switch ASIC’s SerDes speed is a critical factor. Current SerDes run at 112 Gbps per lane, with 224 Gbps in development. Higher SerDes speeds allow fewer lanes to carry the same bandwidth, which reduces the number of optical engines needed and the power per bit.

Another enabler is the use of advanced modulation formats, such as PAM4 (pulse-amplitude modulation with four levels), which packs two bits per symbol. This doubles the data rate over a single lane compared to NRZ (non-return-to-zero) encoding. Future formats, such as coherent detection for short-reach links, are also being explored to extend reach and capacity.

How to Choose: Key Trade-offs

When deciding between pluggables, LPO, NPO, and CPO, network architects must balance several factors:

• Power efficiency: CPO and NPO generally consume less power per bit than pluggables, because the electrical trace is shorter and the DSP may be eliminated (in LPO) or simplified.
• Serviceability: Pluggables are the easiest to replace; LPO is also pluggable, but NPO and CPO require more complex procedures.
• Cost: Pluggables benefit from economies of scale and mature supply chains. LPO reduces cost by removing the DSP, but may require more expensive ASIC SerDes. NPO and CPO require custom packaging and board design, which is costly at low volumes.
• Signal integrity: At 224 Gbps per lane, the electrical trace length becomes a major constraint. CPO and NPO shorten the trace, enabling higher speeds without exotic equalization.
• Upgrade path: Pluggables allow you to upgrade optics without changing the switch. NPO and CPO are tied to the switch hardware, so upgrades require a full switch replacement.

Common Problems and Misconceptions

A common misconception is that CPO is inevitable because it is the most power-efficient. In practice, the industry is moving toward a mix of solutions. For example, many 51.2 Tbps switches use pluggable optics, and 102.4 Tbps switches may use a combination of pluggables and NPO. The choice depends on the specific deployment: a cloud provider with a large, uniform fleet may accept the serviceability trade-off of NPO to save power, while an enterprise data center may prefer pluggables for flexibility.

Another issue is the thermal management of optical engines. Lasers are sensitive to temperature, and their performance degrades at high temperatures. CPO puts the optical engine next to the hot ASIC, which requires advanced cooling solutions. NPO and pluggables have more thermal headroom, but still need careful airflow design.

Frequently Asked Questions

Q: Is co-packaged optics already in use?
A: CPO is still in early deployment. Some networking vendors have announced CPO-based switches, but they are not yet mainstream. Most data centers still use pluggable modules, with LPO and NPO as emerging options.

Q: What is the difference between LPO and CPO?
A: LPO (linear-drive pluggable) keeps the optical engine in a pluggable module but removes the DSP, relying on the ASIC’s SerDes to drive the optics. CPO (co-packaged optics) places the optical engine directly on the same package as the ASIC. LPO is a form of pluggable, while CPO is a packaging change.

Q: Will pluggable optics become obsolete?
A: Not in the near term. Pluggables are likely to remain the standard for at least the next few generations, because they offer flexibility and serviceability. However, at very high switch capacities (102.4 Tbps and beyond), the faceplate space and power constraints may force some deployments to adopt NPO or CPO.

Q: What is the power savings of CPO compared to pluggables?
A: Industry estimates suggest CPO can reduce optical power consumption by 30-50% compared to pluggables, mainly by eliminating the DSP and shortening electrical traces. However, the actual savings depend on the specific design and operating conditions.

Q: Can LPO achieve the same reach as pluggables with DSP?
A: No. LPO typically supports shorter reaches (up to a few hundred meters) because the signal is not retimed. For longer reaches, a DSP is needed to compensate for dispersion and other impairments. LPO is best suited for intra-data-center links.

Q: What is the role of 224G SerDes in these alternatives?
A: 224G SerDes doubles the per-lane speed, reducing the number of lanes needed for a given bandwidth. This makes it easier to place optical engines close to the ASIC (as in NPO) because fewer electrical traces are needed. It also enables higher-speed pluggables, such as 1.6T modules.

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