Short answer: optical switching is not universally faster or cheaper than Ethernet. Optical circuit switching (OCS) is compelling when an AI cluster has large, predictable flows, needs direct paths between disaggregated resources, or must adapt its topology quickly. Electrical packet switching remains the safer baseline for bursty and mixed traffic because it offers mature buffering, routing, interoperability, and operations. For most new installations, a workload-aware hybrid—electrical switching for general packet traffic with optical paths for stable, high-volume flows—is the most defensible design.
A 2025 modeled disaggregated-AI study reported 3 microseconds over 62 m for PCIe-over-optical and a 48.3% reduction in backpropagation completion time compared with PCIe connected through an Ethernet switch. In the same modeled architecture, optical networking reduced cost by 31.6% and power by 12% versus a Leaf-Spine Ethernet design. Those are scenario results, not guarantees for every cluster; fiber length, interface bandwidth, topology, component prices, and control software determine the outcome.
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What optical and electrical switching actually do
Optical circuit switching (OCS)
OCS is a Layer 1 circuit-switched technology. Instead of examining every packet and forwarding it through an electrical fabric, an optical switch establishes a physical light path between ports. The path can remain in place for the duration of a large transfer, then be torn down or reassigned when the traffic pattern changes. A controller therefore changes the cluster’s physical connectivity rather than simply selecting a different packet queue.
This direct path can remove electrical forwarding stages and their buffers from a data-intensive transfer. It also lets an operator reconfigure a topology around the current job: accelerators can be connected directly to memory pools, storage, or another accelerator group, and links can be rerouted around a failed component. The trade-off is that a circuit takes time to establish and is inefficient when flows are tiny, short-lived, or highly unpredictable.
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Electrical packet switching (EPS)
Ethernet and other electrical packet fabrics forward traffic hop by hop. Switches inspect packet headers, buffer contention, and use routing and congestion-control mechanisms to share links among many simultaneous flows. A packet can take a different path from the next packet, so the network does not need a separate setup phase for each conversation.
That flexibility makes EPS the normal baseline for management traffic, storage protocols, service-to-service calls, and workloads whose destinations change rapidly. It also brings mature telemetry, replacement parts, standards, and operational procedures. The costs are electrical serialization and forwarding power, queueing under congestion, and more hops between resources that are not physically adjacent.
Where co-packaged optics fits
Co-packaged optics (CPO) places optical engines close to, or in the same package as, the switching ASIC. CPO is not itself a packet- or circuit-switching protocol: it is a way to implement the links used by either architecture. Its objective is to replace long, power-hungry electrical connections on the board with shorter electrical reaches and optical transmission. The approach can lower interconnect power at high bandwidth, but it changes thermal design, fiber attachment, service procedures, and component qualification.
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The clearest numbers available for this comparison come from a 2025 modeled study of disaggregated AI data centers beyond the rack scale. The authors evaluated a PCIe-over-optical (PO) design against PCIe connected through an Ethernet switch and compared an optical architecture with a Leaf-Spine Ethernet solution. The paper is available from the Optica Publishing Group.
| Metric | Reported result | How to interpret it |
|---|---|---|
| Latency | 3 microseconds over 62 m | Modeled end-to-end result for the paper’s PCIe-over-optical path and stated fiber distance; not a universal OCS latency. |
| Backpropagation completion time | 48.3% lower than PCIe with an Ethernet switch | Result for the study’s disaggregated-AI workload and modeled interface and topology assumptions. |
| Network cost | 31.6% lower than Leaf-Spine Ethernet | Modeled capital and component comparison in that architecture, not a guaranteed purchase-price saving. |
| Network power | 12% lower than Leaf-Spine Ethernet | Modeled consumption for the same scenario; cooling, optics, and facility overhead can change the real result. |
The paper’s authors summarize the modeled comparison by stating that “the optical network architecture with the PO scheme saves 31.6% in cost and 12% in power consumption, respectively, compared with the Leaf-Spine with Ethernet solution.” Treat those percentages as sensitivity points for your own bill of materials and traffic matrix, not as a product specification.
A separate 2024 study examined expansion of regional data-center interconnects (DCIs) under simulated settings. Its optimization results found OCS expansion more cost-efficient than electrical packet-switching (EPS) expansion across the scenarios tested. The authors wrote, “Our results suggest that OCS-based expansion is more cost-efficient regardless of the settings of scenarios.” The study is simulation-based rather than a field benchmark; its methods and assumptions are described in the Journal of Lightwave Technology paper.
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Performance: when is optical faster?
Latency and tail latency
An established optical circuit avoids packet-processing hops once the circuit is active. For a long accelerator-to-memory transfer, that can reduce median latency and, more importantly, tail latency caused by queue buildup in intermediate electrical switches. The 3-microsecond, 62-meter result demonstrates what a short, direct PCIe-over-optical path can look like in a modeled design.
OCS does not make every operation faster. Circuit setup, controller messaging, and optical settling add a fixed delay before data can flow. If an application sends thousands of small messages to changing destinations, setup overhead can exceed the forwarding work an electrical switch would have done. A useful evaluation therefore measures flow completion time at the 99th or 99.9th percentile, not only an unloaded link’s propagation time.
Job completion and collective operations
AI training repeatedly moves large tensors and performs collective operations such as all-reduce. When those transfers are scheduled and occupy links for a meaningful interval, direct optical paths can reduce contention and the number of forwarding stages. In the 2025 model, this effect produced a 48.3% lower backpropagation completion time than PCIe with an Ethernet switch.
The percentage cannot be transferred directly to a different cluster. It depends on how much of the training step uses disaggregated resources, whether the optical controller can establish circuits before the collective begins, the available interface bandwidth, and whether the electrical baseline was provisioned at the same oversubscription. Benchmark the complete training step, including circuit setup and teardown, synchronization, retries, and straggler behavior.
Bandwidth density and radix
Optical links carry high aggregate bandwidth over fiber and can span rows or rooms without the reach limits of copper traces. An optical fabric can also provide a large port radix without paying for an electrical gearbox at every intermediate hop. Those advantages matter when a cluster must connect many accelerator trays to shared pools.
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Traffic predictability and burst tolerance
OCS rewards traffic that is large, repeated, and predictable. A scheduler can reserve a path for a known all-reduce phase or for a resource-disaggregation transaction, then reuse that path for the expected duration. EPS is better at statistical multiplexing: it absorbs short bursts from many tenants without reserving a circuit for each one.
Measure flow-size distribution, inter-arrival times, destination churn, and the percentage of bytes carried by the largest flows. A cluster in which a small number of elephant flows dominate the byte count may benefit from optical bypass paths while leaving mice flows on Ethernet. A cluster dominated by short, bursty RPC-like traffic usually gains less from circuit switching.
Cost and power: what changes the equation
Capital cost
Optical equipment can remove electrical switching stages and reduce the number of hops required, but it adds optical switch fabrics, transceivers, fiber assemblies, patch panels, and control hardware. Electrical switches benefit from a broad commodity ecosystem and familiar spares. The least expensive design depends on port count, reach, oversubscription, optics type, and the service life you assume.
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Use the 31.6% modeled reduction from the 2025 PO study as a scenario to reproduce, not a discount to apply to a quotation. Build two bills of materials with the same delivered bandwidth and redundancy. Include optics, fiber, breakout assemblies, licenses, controllers, rack space, installation, and three-to-five-year replacement inventory. The 2024 DCI optimization result supports considering OCS for planned expansion, but it does not replace a site-specific cost model.
Switching and cooling power
Optical forwarding can avoid electrical SerDes, buffering, and packet-processing work on selected paths. The modeled PO architecture used 12% less network power than its Leaf-Spine Ethernet comparison. Facility power may fall by less than that percentage because optics, control servers, transceivers, and cooling remain in the design.
Record power at the port and system levels under realistic utilization. Include the electrical switches that remain in a hybrid fabric, optical transceiver draw at the intended reach and speed, controller servers, and the cooling overhead represented by your facility’s PUE. A lower switch nameplate does not automatically mean a lower cluster bill if optical modules run hot or require additional airflow.
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Cabling and lifecycle expense
Fiber can simplify long-reach links and support future speed upgrades, but every additional fiber pair and transceiver is a failure and handling point. Document polarity, cleaning requirements, bend-radius limits, connector types, and spare strategy. Electrical copper may be simpler inside a rack, while optical cabling is often more practical between racks and rows.
Lifecycle cost also includes technician training, optical-power testing, firmware support, controller upgrades, and the time needed to replace a failed module. Price the labor required to isolate a bad fiber or optical engine, not only the component itself.
Reconfiguration, resilience, and operations
Topology adaptation
Because OCS changes physical paths, software can create a topology that follows the current job placement. A scheduler might connect accelerator groups directly to a memory pool for one phase and redirect those ports to storage or another accelerator group for the next. This is especially useful in disaggregated designs where compute, memory, and accelerators are separate resource pools.
The control plane must know the workload schedule, reserve non-conflicting paths, and confirm that the optical state matches the intended topology. Define a maximum path-setup time and test what happens when a job starts before the circuit is ready. If the controller cannot react within the application’s phase boundaries, the theoretical topology gain will not appear in job completion time.
Failure handling
An optical circuit can bypass a failed link or switch by establishing a different physical route, which is a resilience advantage when spare capacity exists. Recovery still depends on detecting the fault, computing a valid path, moving the circuit, and ensuring that endpoints tolerate the interruption. Keep an electrical path available for control traffic and for workloads that cannot pause during reconfiguration.
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Serviceability and observability
EPS operations teams commonly have mature counters for drops, queue depth, congestion, and route changes. OCS requires equivalent visibility into circuit state, port-to-port connectivity, optical power, setup failures, stale reservations, and controller decisions. Integrate those signals into the same incident system before production.
Maintain a deterministic fallback procedure: identify the affected circuit, drain or pause the workload, establish an alternate path, and return traffic to the normal schedule. Without documented rollback, an optical failure can become an application outage even when spare physical capacity exists.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Co-packaged optics: should an AI data center adopt it?
CPO can reduce the distance that high-speed electrical signals travel between a switch ASIC and its optical engine. That is attractive as lane rates and port counts rise, because insertion loss and equalization consume power and board area. A lower electrical-interconnect budget can improve the switch’s total energy per bit and make very high-radix designs feasible.
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Deployment constraints are substantial:
- Thermals: optical engines and the switching ASIC share a dense thermal envelope. Heatsink, airflow, and liquid-cooling designs must be validated at sustained load.
- Reliability: optical devices, lasers, and attachment interfaces have different aging and failure behavior from the ASIC. Qualification must cover temperature cycling, vibration, and continuous high-power operation.
- Fiber attachment: fibers are attached close to the package, so routing, strain relief, bend radius, and connector cleanliness become part of board and rack design.
- Serviceability: a replaceable front-panel module can be swapped without removing the switch. A co-packaged engine may require a larger assembly replacement or specialized field procedure.
- Standards and supply: interoperability between switch silicon, optical engines, management interfaces, and fiber assemblies must be established before committing to a vendor-specific package.
The trade-off between external and integrated light sources is examined in the IEEE study “External vs. Integrated Light Sources for Intra-Data Center Co-Packaged Optical Interfaces”. Use that work to frame reliability and packaging questions, then validate the answers against the exact CPO implementation and service model you plan to deploy.
Optical versus electrical switching: decision matrix
| Decision axis | Optical circuit switching | Electrical packet switching | Practical design implication |
|---|---|---|---|
| Latency and tail behavior | Very low forwarding delay after a circuit is active; less queueing on the direct path. | Hop and queueing delay vary with congestion. | Use OCS for sustained critical flows; retain EPS where burst tolerance matters. |
| Traffic shape | Best for large, scheduled, repeated flows. | Best for short, bursty, mixed flows. | Classify traffic by flow size and destination churn before choosing. |
| Reconfiguration | Changes physical connectivity under controller control. | Changes routes and queues without rebuilding links. | OCS needs a scheduler and setup-time budget; EPS needs congestion control. |
| Failure recovery | Can create an alternate physical path when spare links exist. | Reroutes packets using mature protocols and telemetry. | Test both restoration time and application-visible interruption. |
| Bandwidth and radix | High aggregate bandwidth and long-reach fiber options. | Broad selection of standardized switch port speeds and topologies. | Compare delivered bisection bandwidth, not nominal port count. |
| Power | Can reduce electrical forwarding and interconnect power on selected paths. | Consumes power in SerDes, buffers, packet processing, and multiple hops. | Model optics, controllers, cooling, and residual EPS in a hybrid design. |
| Capital cost | Potentially lower in disaggregated or expansion scenarios, with specialized optical components. | Predictable commodity procurement and a large installed base. | Build equal-bandwidth bills of materials and include lifecycle labor. |
| Control-plane maturity | Requires optical topology orchestration and circuit-aware scheduling. | Mature routing, monitoring, and automation ecosystems. | Assess your team’s ability to operate optical state, not just install it. |
| Interoperability | Depends on optical switch, transceiver, fiber, and controller compatibility. | Strong Ethernet interoperability across suppliers. | Define standards and qualification tests before a purchase. |
| Serviceability | Fiber cleanliness, optical power, and attachment procedures are critical. | Familiar module replacement and packet-level diagnostics. | Budget training, spares, and optical test equipment. |
| Migration | Often introduced as an optical tier or bypass for selected flows. | Provides the stable underlay and fallback path. | Plan a hybrid transition rather than a one-time fabric replacement. |
How to choose an architecture, step by step
- Document the workload mix. Export flow records from representative training, inference, storage, and management jobs. Record bytes per flow, duration, fan-out, synchronization phases, and destination changes.
- Set measurable targets. Specify acceptable median and tail latency, job-completion time, restoration time after a link failure, and maximum power per delivered terabit. Include the circuit-setup interval in the latency budget.
- Map resource disaggregation. Identify which accelerator, memory, storage, and PCIe resources must communicate across racks or rows. OCS is most valuable where a direct path removes repeated electrical hops.
- Size both fabrics at equal service levels. Use the same bisection bandwidth, redundancy, oversubscription, reach, and growth reserve in the optical and electrical bills of materials. Count transceivers, fiber, patching, controllers, licenses, and spares.
- Model the control plane. Demonstrate path computation, reservation, conflict handling, setup and teardown, telemetry, and rollback. Inject stale state and controller failure to verify that EPS control traffic and a safe fallback remain available.
- Run an application-level pilot. Measure complete training steps and inference batches, not only link latency. Compare the 99th-percentile flow completion time, collective efficiency, accelerator utilization, and energy per job.
- Choose the split. Keep management, short-flow, and unpredictable traffic on EPS. Place stable elephant flows or disaggregated PCIe paths on OCS when the measured benefit exceeds setup and operational overhead.
- Set migration gates. Require interoperability testing, optical-power margins, spare-part availability, documented cleaning and repair procedures, and a proven rollback before expanding beyond the pilot.
When each approach is the better fit
Choose an optical tier when
- Large collective or memory-access flows dominate the byte count and have predictable phases.
- Compute, memory, or accelerator pools are disaggregated beyond a rack and electrical hop count is a measurable bottleneck.
- The scheduler can reserve circuits in advance and the controller can meet the application’s setup-time budget.
- You need topology adaptation or alternate physical paths for planned expansion and have staff able to operate optical telemetry.
Stay primarily electrical when
- Traffic is dominated by short, bursty, multi-tenant flows with rapidly changing destinations.
- Interoperability, standardized spares, and familiar packet-level diagnostics outweigh the potential optical efficiency.
- The cluster cannot tolerate circuit-setup interruptions and has no reliable optical control plane.
- Your topology is small enough that additional electrical hops do not materially affect job completion time.
Use a hybrid fabric when
- Training has a few repeatable elephant-flow phases alongside ordinary Ethernet services.
- You need an EPS underlay for control, fallback, and best-effort traffic while OCS handles selected high-volume paths.
- You want to introduce optical switching incrementally and compare measured job, power, and operational results before broad adoption.
Bottom-line recommendation
Do not frame the decision as “optical versus Ethernet” in the abstract. Start with the traffic matrix and the topology your AI jobs actually create. OCS can deliver direct low-latency paths, faster disaggregated workloads, and lower modeled cost and power when flows are large and predictable; EPS remains the flexible, interoperable foundation for bursty traffic and operational safety. A hybrid design, validated with application-level measurements and a complete lifecycle cost model, is the prudent default. The 2025 modeled results and the 2024 DCI simulations justify a serious optical evaluation, but neither establishes a universal percentage for every AI data center.
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