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Introduction
In high-speed networking, every meter of cabling matters. Data centers, HPC clusters, and enterprise fabrics need short-reach, low-latency, and cost-effective connections. That’s where Direct Attach Copper (DAC) twinax cables come in.
DACs are simple, pre-terminated copper cable assemblies with fixed transceiver-like connectors on each end. They eliminate the need for separate optical transceivers and patch cords, offering plug-and-play simplicity, zero power consumption (passive models), and ultra-low latency for connections within and between racks.
This guide covers the essentials:
- What DACs are and where they fit compared to optical and AOC solutions.
- Speed vs. distance limitations across generations.
- Active, passive, breakout, and AEC (Active Electrical Cable) types.
- Design considerations (AWG, airflow, compatibility, coding).
- Best practices for installation and troubleshooting.
- Cost and TCO trade-offs.
800G OSFP DAC Cable
What is a DAC Cable?
A Direct Attach Copper cable is a twinax copper cable with integrated connectors that plug directly into switch, router, or server ports.
- Form factors: most common are SFP+, SFP28, QSFP+, QSFP28, QSFP56, QSFP-DD, and OSFP.
- Lengths: typically 0.5–7 m, depending on speed and gauge.
- Use case: inside racks (server-to-switch) or adjacent racks (ToR-to-ToR).
Advantages at a glance:
- Cost: no optical transceivers required.
- Latency: no optical-electrical conversions, near-zero added delay.
- Power: passive DACs consume ~0 W.
- Simplicity: plug-and-play, no polishing or splicing.
DAC Speed vs. Distance Matrix
As signaling rates increase, maximum copper reach decreases.
Typical DAC Reach by Speed
| Speed | Form Factor | Passive Typical | Passive Max | Active/ACC Typical | Notes |
| 10G | SFP+ | 1–3 m | 5–7 m | 5–10 m | Legacy common |
| 25G | SFP28 | 1–3 m | 3–5 m | 5–7 m | Used in modern ToR |
| 40G | QSFP+ | 1–2 m | 3–5 m | 5–7 m | 4×10G lanes |
| 100G | QSFP28 | 1–2 m | 3 m | 5–7 m | ToR uplinks |
| 200G | QSFP56 | 1–2 m | 3 m | 5–7 m | Requires PAM4 |
| 400G | QSFP-DD / OSFP | 0.5–1 m | 2–3 m | 3–5 m | Often replaced by AEC/AOC |
Tip: Always check the vendor’s datasheet—values above are common but not universal.
DAC vs. AOC vs. Optical Transceivers
| Attribute | DAC | AOC (Active Optical Cable) | Transceivers + Fiber |
| Medium | Twinax copper | Fiber (integrated optics) | Fiber + pluggable optics |
| Power | Passive: ~0 W, Active: <1 W | 1–2+ W | 1–3 W per transceiver |
| Latency | Lowest (no conversion) | Very low | Very low |
| Distance | 0.5–7 m | 3–100 m | 100 m–80 km+ |
| Cable thickness | Thick (AWG24–30) | Thin, flexible | Thin (fiber) |
| EMI immunity | Moderate | Excellent | Excellent |
| Cost | Lowest | Medium | Highest upfront |
| Best use | In-rack / short reach | Row-to-row, EMI-sensitive | Long reach, flexible patching |
DAC Types
Passive DAC
- No active components.
- Cheapest, simplest.
- ≤7 m at low speeds, ≤3 m for 25G/100G+.
Active DAC (ACC)
- Small electronics equalize and condition signals.
- Extends reach to 5–10 m.
- Slightly higher cost and power (<1 W).
AEC (Active Electrical Cable)
- Advanced form of active DAC with retimers/redrivers.
- Supports higher data rates (200G/400G) beyond passive limits.
Breakout DAC
- Fan-out from one high-speed port to multiple lower-speed ports.
Common Breakout Configurations
| High-Speed Port | Breakout | Medium | Max DAC Length | Notes |
| 40G QSFP+ | 4 × 10G SFP+ | DAC/AOC | ≤3–5 m | Classic ToR to servers |
| 100G QSFP28 | 4 × 25G SFP28 | DAC/AOC | ≤3 m | Common in cloud |
| 200G QSFP56 | 4 × 50G SFP56 | DAC/AEC | ≤3 m | PAM4 signaling |
| 400G QSFP-DD | 4 × 100G QSFP28 | DAC/AEC/AOC | ≤2–3 m | Requires switch breakout mode |
Physical Considerations: AWG, Cable Thickness, Airflow
- Wire gauge (AWG):Lower AWG (24/26) = thicker, longer reach, less flexible. Higher AWG (28/30) = thinner, easier routing, shorter reach.
- Cable bundles: large groups of DACs can obstruct airflow in dense racks.
- Bend radius: observe manufacturer’s specs to prevent signal loss.
Compatibility and Coding
- DACs include EEPROMs storing vendor ID and parameters.
- Some switches enforce vendor coding (lock-in).
- Always confirm vendor-approved or coded DACs for critical deployments.
Selection Workflow
Three-step method:
- Distance vs speed: If ≤3 m → Passive DAC; 3–7 m → ACC/AEC; >7 m → AOC/fiber.
- Form factor: SFP+/SFP28/QSFP+/QSFP28/QSFP56/QSFP-DD/OSFP.
- Compatibility: Check coding, FEC, breakout mode, AWG, and airflow.
Checklist Before Purchase
- Required speed (10G / 25G / 100G / 200G / 400G).
- Length vs matrix validated.
- Form factor matches switch/server port.
- Vendor coding compatibility.
- AWG vs routing constraints.
- Breakout mode support if needed.
- FEC/retimer alignment for high speeds.
Installation Best Practices
- Plan routing: separate from power cables to minimize EMI.
- Respect bend radius: no sharp bends or kinks.
- Secure strain relief: use cable ties/clips without crushing.
- Label both ends: especially for breakout assemblies.
- Airflow awareness: avoid blocking fans.
- Test before production: stress test with line-rate traffic, monitor FEC/BER.
Troubleshooting
DAC Issue → Likely Cause → Solution
| Issue | Likely Cause | Fix |
| Link down | Length exceeds spec, coding mismatch, FEC disabled | Shorter DAC, correct vendor-coded cable, enable FEC |
| Flapping | Loose latch, bent cable | Reseat, re-route, replace cable |
| Errors/FEC alarms | EMI, overstressed cable, damaged connector | Separate routing, replace cable |
| Overheating | Bundle blocks airflow | Re-route, add cable management |
| Breakout subports offline | Switch not in breakout mode | Enable breakout mode, confirm transceiver settings |
FAQs
Q1. What is the difference between passive and active DACs?
A: Passive has no electronics, cheapest, shortest reach. Active/ACC adds equalizers for 5–10 m.
Q2. How do DACs differ from AOCs?
A: DAC = copper, short, thick, no power. AOC = fiber, longer, thin, needs power.
Q3. Can DACs be used for long distances?
A: No. Typical passive reach ≤3 m at 25G+/100G speeds. Use AOC/fiber beyond 7–10 m.
Q4. What is a breakout DAC?
A: Fan-out: 1 × high-speed (100G QSFP28) → 4 × lower-speed (25G SFP28).
Q5. Do all vendors’ DACs work with all switches?
A: Not always—check vendor coding/compatibility lists.
Q6. Are DACs lower latency than optics?
A: Yes, DAC avoids optical conversions, but differences vs AOC are minimal in most use cases.
Conclusion
Direct Attach Copper twinax cables remain indispensable in modern data centers. They deliver unmatched cost efficiency, simplicity, and ultra-low latency for short-reach connections.
When used correctly with the right speed-length matching, airflow planning, and vendor validation, DACs are the most economical and reliable choice for in-rack and adjacent-rack links.
For high-speed fabrics, combining DAC (short), AOC (medium), and optics (long) offers a balanced cabling strategy to optimize cost, performance, and scalability.
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