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Introduction: Why talk about AOCs?
As data center speeds grow from 10G → 25G → 100G → 400G → 800G, the choices for server-to-switch interconnects become more complex. Today, IT teams typically pick between three options:
- DAC (Direct Attach Copper) – cheapest, short-distance.
- AOC (Active Optical Cable) – medium-distance, lightweight.
- Optical transceivers + patch cords – flexible, long-distance.
Active Optical Cables sit right in the middle. They combine the lightweight nature of fiber optics with the plug-and-play convenience of DAC. AOCs are widely used for rack-to-rack links and AI/HPC clusters, where distances are too long for DAC but too short to justify expensive optical transceivers.
AOC Cables Overview
What is an Active Optical Cable (AOC)?
An AOC is a pre-assembled, fixed-length cable that integrates:
- Optical fiber as the transmission medium.
- Integrated optical engines in the connectors that convert electrical signals to light and back.
- Standard connectors (SFP+, SFP28, QSFP+, QSFP28, QSFP-DD, OSFP).
From the outside, an AOC looks similar to a DAC: a cable with transceiver-style ends. The difference is that inside the ends, there are chips that perform electrical-to-optical conversion, and the cable itself is fiber instead of copper.
Speeds supported: 10G, 25G, 40G, 100G, 200G, 400G, and now 800G AOCs are available.
How does an AOC work?
The signal path of an AOC is simple:
- Electrical input from the NIC or switch is fed into the AOC connector.
- A tiny laser converts the electrical signal to optical signal.
- The optical signal travels through the fiber cable inside the AOC.
- At the other end, a photodiode converts the optical signal back to electrical signal.
Because the signal travels as light:
- AOCs are immune to electromagnetic interference (EMI).
- They can run longer than copper DAC cables.
- The cable is lighter and thinner, which improves rack airflow.
Advantages of AOCs
| Aspect | AOC Benefit |
| Distance | Longer reach than DAC (up to 30–100m). |
| Weight | Light and thin → easier routing in racks. |
| EMI Immunity | Fiber transmission is immune to interference. |
| Power | Lower power draw than discrete optical modules. |
| Flexibility | Better bend radius and easier cable management. |
| Cost | Cheaper than separate optics for medium distances. |
Limitations of AOCs
| Aspect | Limitation |
| Distance | Limited to ~100m. Cannot replace long-haul optics. |
| Fixed Design | Integrated → cannot replace just the cable or the ends. Entire AOC must be swapped. |
| Compatibility | Vendor-specific coding may limit interoperability. |
| Cost vs DAC | More expensive than passive DAC copper. |
| Durability | Fiber core is more fragile than copper under stress. |
Applications of AOCs in Data Centers
- Server ↔ ToR switch: When DAC length is insufficient (>7m).
- Leaf ↔ Spine (row-to-row): 25–100m interconnects.
- High-density racks: Thin AOCs improve airflow and reduce cable bulk.
- AI/HPC clusters: AOCs provide low-latency interconnects at 100G/400G speeds where medium distances are common.
AOC vs DAC vs Optical Modules
| Aspect | DAC (Copper) | AOC (Fiber) | Optical Modules + Patch |
| Reach | ≤7m (passive), ≤15m (active) | 30–100m | 100m–10km+ |
| Weight | Heavy copper | Light fiber | Light fiber |
| Cost | Lowest | Medium | Highest |
| EMI Immunity | Good (shielded) | Excellent (immune) | Excellent (immune) |
| Power | 0–2W | ~1–2W | 2–5W per module |
| Flexibility | Thick, stiff cables | Thin, flexible | Flexible (fiber patch cords) |
| Typical Use | In-rack, short runs | Rack-to-rack, pod-to-pod | Leaf–spine, long-haul, DCI |
👉 In practice:
- DAC = in-rack, cheapest.
- AOC = row-to-row, mid-range.
- Optics = long-reach, most flexible but costly.
Deployment Considerations
When selecting an AOC, consider:
- Connector type: Match your NIC/switch ports (SFP28, QSFP28, QSFP-DD, OSFP).
- Length: If >15m and <100m, AOC is usually best.
- Vendor compatibility: Some switches require coded AOCs (Cisco, Arista, Mellanox).
- Power budget: AOCs draw ~1–2W, check switch/NIC specs.
- Cabling strategy: Mix DAC (short), AOC (medium), and optics (long).
Future Outlook
- 400G/800G AOCs are already shipping for AI/HPC clusters.
- As data rates rise, copper DAC reach gets shorter, making AOC even more valuable.
- New form factors like QSFP-DD800 and OSFP are enabling next-gen AOCs.
- In high-density AI training pods, expect a mix: DAC inside racks, AOC between racks, optics for leaf–spine.
FAQs
Q1: How is an AOC different from optics + patch cords?
A: AOCs are pre-assembled, fixed cables. Optics + patch cords are modular and flexible but cost more.
Q2: What’s the maximum reach of AOCs?
A: Typically 30–100m, depending on the data rate and vendor.
Q3: Can AOCs support 400G/800G?
A: Yes. QSFP-DD and OSFP AOCs are already available at 400G, and 800G OSFP AOCs are emerging.
Q4: Are AOCs hot-pluggable?
A: Yes. They function like transceivers with fixed fiber attached.
Q5: Which vendors support AOCs?
A: Most major switch and NIC vendors (Cisco, Arista, Mellanox/NVIDIA, Juniper) offer AOC-compatible ports.
Q6: Why are AOCs better than DAC for dense racks?
A: They are thinner, lighter, and improve airflow.
Q7: How much power do AOCs save vs optics?
A: Typically 1–2W vs 3–5W per end for optics. Savings add up across thousands of ports.
Q8: Should SMBs use AOCs?
A: Often no. SMBs typically use DACs (cheaper, shorter). AOCs are more relevant in enterprise and hyperscale DCs.
Conclusion
Active Optical Cables (AOCs) are the middle ground interconnect in data centers:
- Longer reach than DAC (30–100m).
- Cheaper and lower power than separate optical modules.
- Lightweight and immune to EMI, making them great for dense racks and AI clusters.
The right deployment strategy uses all three:
- DAC for in-rack,
- AOC for row-to-row,
- Optics for leaf–spine and DCI.
👉 To ensure consistent performance, always source end-to-end solutions (switches, NICs, cables) from trusted vendors like network-switch.com, where DACs, AOCs, and optics are tested for compatibility.
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