High-performance computing (HPC), modern AI training clusters, and large-scale simulation environments increasingly rely on InfiniBand HDR (200G) networks to interconnect GPUs, compute nodes, and storage systems. As HDR continues to dominate latency-critical and bandwidth-intensive environments, choosing the correct physical-layer cabling — AOC (Active Optical Cable) or DAC (Direct Attach Copper) — becomes essential for balancing performance, reliability, and total cost of ownership (TCO).
This guide provides an engineering-level comparison of 200G InfiniBand AOC vs DAC cables, including physical-layer behaviour, reach limitations, BER performance, thermal considerations, deployment scenarios, and a complete TCO breakdown.
Understanding InfiniBand HDR Cabling Options
InfiniBand HDR operates at 4× 50G PAM4 lanes, where each lane requires extremely low jitter, stable optical/electrical characteristics, and strict loss budgets. Even minor cable impairments — skew, crosstalk, attenuation, and reflections — can collapse the PAM4 eye diagram.
Understanding the difference between DAC and AOC cabling helps ensure the physical layer is engineered correctly.
Defining DAC (Passive and Active Copper)
DAC (Direct Attach Copper) is the simplest and lowest-latency interconnect option.
Passive DAC (pDAC)
- Twinax copper with no electronics
- Minimal latency (<2 ns)
- Lowest cost option
- Limited reach due to attenuation and crosstalk
- Typical maximum length for 200G HDR: 1–2 m
At 25–26.5 GHz Nyquist rates, copper loss becomes substantial, resulting in eye closure that the switch or NIC may not recover from.
Active DAC (aDAC)
Active DAC incorporates signal conditioning, including:
- CTLE (Continuous-Time Linear Equaliser)
- Retimers (occasionally used, more expensive)
- Adaptive equalization
This extends usable reach to 3–5 m, depending on the host device output.
Despite improvements, aDAC still suffers from:
- Heavier weight
- Bulkier installation
- Higher EMI sensitivity
Defining AOC (Active Optical Cable)
AOC, unlike DAC, converts the electrical HDR signal to optical via VCSEL-based optical engines and reconverts it at the far end.
AOC includes:
- VCSEL laser drivers
- Photodiode receivers
- DSP for PAM4 equalization and FEC optimization
- Ultra-low-loss multimode fiber
Typical 200G HDR AOC features:
- Reach of 30–100 m
- Lighter and thinner cable footprint
- No EMI issues
- Very stable BER (10⁻¹⁵ to 10⁻¹⁸, depending on FEC mode)
AOC is the dominant choice for:
- GPU/CPU cross-row links
- Leaf–spine InfiniBand connections
- Distributed DGX/HGX cluster wiring
Key InfiniBand HDR Network Requirements
InfiniBand HDR cabling must satisfy several technical constraints:
1. Insertion Loss Budget
HDR has one of the tightest electrical budgets among networking interfaces.
- DAC introduces significant attenuation per meter.
- AOC introduces no practical optical loss at short distances.
2. Crosstalk and Near-End Reflections (NEXT/Return Loss)
Copper behaves poorly in dense bundles; optics do not.
3. BER and FEC Stability
DAC relies heavily on host-side FEC margins.
AOC delivers more stable BER performance in long deployments.
4. Thermal and Airflow Impacts
Thick DAC bundles impede airflow in HPC racks; AOC does not.
5. Weight and Bend Radius
- DAC is heavy and rigid.
- AOC is lightweight and flexible.
Performance Comparison: AOC vs DAC
Below is an engineering-level comparison based on physical-layer behaviour.
Reach and Distance Limitations
| Cable Type | Typical HDR Reach | Extended HDR Reach | Limitations |
| Passive DAC | 1–2 m | — | High loss, crosstalk, copper attenuation |
| Active DAC | 3 m | 4–5 m (best case) | Still bulky, affected by EMI |
| AOC | 30 m | 100 m | Essentially unlimited in rack-scale deployments |
Signal Integrity and Error Rate
DAC
DAC’s twinax copper introduces:
- Skin effect loss
- Dielectric loss
- Impedance discontinuities
- Crosstalk between pairs
- Return loss from connectors
These negatively affect the PAM4 eye diagram, making the host FEC work harder to maintain link stability.
AOC
Active Optical Cable(AOC) converts the signal to light, which gives:
- Wide signal margin
- Minimal jitter
- No EMI
- Very clean eye diagrams
- Significantly lower BER over time
AOC is therefore preferred for:
- Mission-critical workloads
- Large-scale GPU clusters
- HPC environments requiring zero downtime
Power Consumption Differences
DAC (pDAC) has negligible power consumption.
Active DAC consumes slightly more (<0.3–0.5 W per end).
AOC typically consumes 1–2 W per end because of optical engines and DSP.
However, in the overall TCO of an HPC cluster, this power difference is negligible compared with performance and reliability improvements.
Comprehensive Cost Analysis (TCO)
Selecting the correct cable type affects capital expenditures (CapEx) and operational expenditures (OpEx).
Initial Unit Cost Comparison
| Cable Type | Approx Relative Cost | Notes |
| Passive DAC | Lowest | Ideal for ≤2 m server-to-switch |
| Active DAC | Low–Medium | More expensive than DAC but cheaper than AOC |
| AOC | Medium | Higher component cost (optics + DSP) |
However, cables are a tiny fraction of the total cost of an HPC node or GPU cluster.
Installation and Management Costs
DAC Installation Challenges
- Heavy and rigid; difficult to route
- Requires more time for cable dressing
- Thick copper bundles limit airflow
- Higher risk of human-induced bending damage
AOC Installation Advantages
- Thin and flexible
- Simplifies cable routing
- Reduces rack congestion
- Improves cooling efficiency
When scaled across 10–30 racks, AOC reduces installation time dramatically.
Long-Term Reliability and Replacement Costs
DAC
- More susceptible to mechanical stress
- Signal integrity degrades in dense bundles
- Exposure to EMI in GPU clusters
AOC
- Very stable optical path
- No electrical interference
- Lower rate of failure over multi-year operation
Over a 3–5 year cluster lifespan, AOC offers the lowest effective TCO.
Selecting the Optimal Cable Based on Application
When to Deploy DAC for 200G HDR
Use DAC when:
- The distance is ≤1–2 m (passive DAC)
- Cabling is inside the same rack
- Minimal airflow impact is acceptable
- You require the lowest upfront cost
- Latency must be absolutely minimized (<2 ns)
Ideal uses:
- Switch-to-server in the same chassis
- Direct short-reach GPU/CPU connections
- Lab test setups
For longer runs, DAC becomes impractical.
When AOC is the Superior Choice
Choose AOC when you need:
- 3–100 m reach
- Low BER
- Reduced EMI sensitivity
- Lightweight cabling
- Improved airflow and thermal performance
- Long-term reliability
- Clean cable management
AOC is the industry standard for:
- GPU cluster leaf–spine architecture
- Multi-rack DGX/HGX deployments
- HPC and AI supercomputers
- University research clusters
- Cloud-scale distributed compute systems
Environmental Factors (Weight, Bend Radius, EMI)
Weight
- DAC: Heavy (up to 10× heavier than AOC at long distances)
- AOC: Extremely light
Bend Radius
- DAC: Limited (risk of deformation)
- AOC: Small bend radius; ideal for tightly packed racks
EMI
- DAC: Vulnerable, especially near power supplies or GPU cages
- AOC: Immune
This makes AOC the clear winner for complex, high-density layouts.
PHILISUN’s High-Reliability 200G InfiniBand Solutions
PHILISUN provides complete interconnect solutions engineered for InfiniBand HDR environments:
Certified AOC and DAC for HDR
- Low-loss optical engines
- HDR-compliant copper cable assemblies
- Extremely low BER
- 100% link validation in production
- Highly stable PAM4 performance
Custom Length and Low Lead Time Options
PHILISUN supports:
- Custom cable lengths
- OEM labeling
- Fast lead time manufacturing
- Highly reliable QA/validation
- High-volume production for hyperscale/HPC deployments
This ensures seamless integration into InfiniBand HDR and NDR-ready systems.
Conclusion
Selecting between AOC and DAC for 200G InfiniBand HDR comes down to balancing reach, signal integrity, rack layout, and long-term reliability:
- Choose DAC
When distances are ≤1–2 m and cost must be minimized.
- Choose AOC
For anything beyond 2 m, or when deploying in dense GPU/HPC racks where reliability, airflow, and cable flexibility matter.
In real-world HPC and AI cluster deployments, AOC consistently delivers the best TCO, the most stable BER, easier installation, and better physical-layer performance. DAC remains important for very short intra-rack links, but AOC is the backbone of modern InfiniBand HDR networks.
PHILISUN’s full range of HDR-ready AOC and DAC solutions provides reliable, validated connectivity across all HPC, data center, and AI supercomputing environments.
FAQ: 200G InfiniBand AOC vs DAC
1. What is the main difference between 200G InfiniBand AOC and DAC?
DAC uses copper twinax and is suited for 1–3 m distances, while AOC uses optical fiber, supports 30–100 m, and offers better BER and airflow in HPC racks.
2. Which cable type is better for multi-rack 200G HDR deployments?
AOC is strongly preferred for multi-rack or cross-row HDR links due to low weight, no EMI, and stable PAM4 performance.
3. Does AOC have higher power consumption than DAC?
Yes, AOC consumes more power (1–2 W per end), but the difference is negligible in a large HPC cluster compared to performance and reliability benefits.
4. Is passive DAC reliable for 200G InfiniBand?
Yes, but only at very short distances (≤1–2 m). Beyond that, copper attenuation and crosstalk degrade PAM4 stability.
5. Can AOC replace DAC in all HDR applications?
Yes — except for ultra-short runs where DAC offers the lowest cost and lowest latency.
6. Does cable weight matter in HDR networks?
Absolutely. Heavy DAC bundles obstruct airflow, increase installation difficulty, and reduce rack cooling efficiency. AOC solves all of these issues.
7. What is the recommended cable type for HDR leaf–spine?
AOC, typically 3–30 m depending on rack layout.
8. Does PHILISUN offer HDR-certified AOC and DAC cables?
Yes. PHILISUN provides validated 200G InfiniBand AOC and DAC solutions with low BER, custom lengths, and fast lead times.
9. How does EMI influence HDR cable choice?
DAC is sensitive to EMI, which can destabilize PAM4. AOC is immune, making it preferable for GPU-dense environments.
10. What’s the lowest TCO option for 200G HDR?
For ≤2 m: DAC is the lowest cost.
For ≥3 m: AOC provides the lowest long-term TCO due to improved reliability and airflow.
