Direct connection of optical modules and connection to switches

Direct-connection optical modules enable high-speed, low-latency links between switches using either optical transceivers or integrated optical cables like DAC and AOC.

Overview of Direct-Connection Optical Modules

Direct-connection optical modules are used to interconnect switches in data centers, particularly between Leaf and Spine layers or Top-of-Rack (ToR) and Leaf switches. These modules convert electrical signals from the switch into optical signals for transmission over fiber, and then back into electrical signals at the receiving end, enabling high-speed communication over short to medium distances .

Common Types

1. Optical Modules (SFP/SFP+/QSFP/QSFP-DD)
   • Hot-swappable transceivers that convert electrical signals to optical signals.
   • Support multimode or single-mode fiber, with distances ranging from a few meters to several kilometers.
   • Often used in Leaf-Spine interconnects and longer intra-data center links .
2. Direct Attach Cables (DAC)
   • Copper-based cables with integrated connectors, available in passive or active forms.
   • Cost-effective, low-latency solution for very short distances (typically <10 meters).
   • Passive DACs do not require signal conversion, while active DACs include signal conditioning for slightly longer reach .
3. Active Optical Cables (AOC)
   • Optical cables with integrated transceivers at both ends.
   • Lightweight, flexible, and immune to electromagnetic interference.
   • Support longer distances than DACs (up to 300 meters) and are suitable for high-density deployments .

Emerging Technology: Optical Circuit Switches (OCS)

Optical Circuit Switches provide direct optical paths at the physical layer, bypassing intermediate packet processing. This approach offers ultra-low latency, non-blocking bandwidth, and energy efficiency, making it ideal for AI training clusters and high-performance computing networks. OCS maintains the optical signal throughout the switch, avoiding optical-electrical-optical (O-E-O) conversion, and dynamically reconfigures connections between fiber ports .

Key Considerations

• Distance and Reach: DAC is ideal for very short links (<10 m), AOC for medium distances (up to 300 m), and optical modules for longer intra-data center links.
• Latency: DAC provides the lowest latency, followed by AOC and optical modules with DSP-based transceivers.
• Cost: DAC is the most cost-effective, AOC is more expensive due to integrated optical transceivers, and high-speed optical modules with DSP chips are the costliest.
• Flexibility: AOC and optical modules allow customizable lengths and support high-speed upgrades, while DAC is limited in length and flexibility.
• Deployment Scenario: DAC and AOC are commonly used for ToR-to-Leaf connections, while optical modules and OCS are preferred for Leaf-to-Spine or high-bandwidth reconfigurable links .

Conclusion

Direct-connection optical modules, including DAC, AOC, and optical transceivers, are essential for high-speed, low-latency switch interconnections in modern data centers. Emerging OCS technology further enhances network efficiency by providing dynamic, all-optical paths, complementing traditional electrical switching for high-performance workloads. Choosing the right solution depends on distance, latency, cost, and network architecture requirements.