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QSFP Cables Explained: Types, DAC vs AOC, and Data Center Use Cases

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Network Switches
IT Hardware Experts
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Introduction

In modern data centers and enterprise networks, Quad Small Form-factor Pluggable (QSFP) cables are everywhere. They connect servers, switches, and storage at speeds from 40G to 400G and beyond.

Thanks to their four-lane parallel architecture, QSFP cables combine high bandwidth with compact design, helping IT teams build dense, scalable, and efficient infrastructures.

But QSFP is not a single thing, there are multiple generations (QSFP+, QSFP28, QSFP56, QSFP-DD), different physical media (copper vs fiber), and multiple deployment options (DAC vs AOC). Picking the wrong type can lead to compatibility issues, wasted budget, or poor performance.

This guide explains everything you need to know about QSFP cables: their types, how they work, when to choose DAC vs AOC, and how to ensure compatibility in your environment.

QSFP Cables

What is a QSFP cable and how does it work?

QSFP cable basics

QSFP stands for Quad Small Form-factor Pluggable. Each QSFP port uses four data lanes, supporting aggregate speeds:

  • QSFP+ = 4×10G → 40G.
  • QSFP28 = 4×25G → 100G.
  • QSFP56 = 4×50G → 200G.
  • QSFP-DD = 8×50G PAM4 → 400G (and 8×100G PAM4 → 800G in next-gen).

QSFP cables can be:

  • Copper-based (DAC): Transmit electrical signals directly over twinax conductors.
  • Fiber-based (AOC): Convert electrical to optical, transmit via fiber, then back to electrical.

Both are plug-and-play, no separate optical transceivers required.

Types of QSFP Cables

Generations of QSFP Cables

Type Speed Typical Reach Connector Style Notes
QSFP+ 40G ≤10km (fiber), ≤7m (copper DAC) MPO-12 (fiber), QSFP+ (copper) First generation, 4×10G.
QSFP28 100G ≤40km (fiber), ≤7m (DAC) LC duplex (fiber), QSFP28 (copper) Most common today for 100G DC links.
QSFP56 200G ≤10km (fiber) LC duplex or MPO-12 Doubles lane rate to 50G.
QSFP-DD 400G ≤10km (fiber), ≤3m (DAC passive) MPO-16, LC/MDC/CS Double density, backward compatible with QSFP28.

👉 Rule of thumb:

  • QSFP28 dominates 100G.
  • QSFP-DD is the standard for 400G and the transition to 800G.

DAC vs AOC: which QSFP cable should you choose?

QSFP cables come in two main media options:

  • DAC (Direct Attach Copper): Uses twinax copper cable.
  • AOC (Active Optical Cable): Uses fiber with integrated optics.

 QSFP DAC vs AOC

Aspect DAC (Copper) AOC (Fiber)
Reach ≤7m passive, ≤15m active 30–100m (sometimes up to 150m)
Weight Heavy, stiff Light, thin, flexible
EMI immunity Good with shielding Excellent (fiber is immune)
Power 0W (passive), ~1–2W (active) ~1–2W (integrated optics)
Cost Lowest (short links) Higher than DAC, lower than optics
Best Use Case In-rack server ↔ ToR switch Row-to-row, medium interconnects

👉 Use DAC inside racks (shortest, cheapest, lowest power).
👉 Use AOC between racks (longer but still cheaper than optics).
👉 Use discrete optics + patch cords for long distances.

Applications of QSFP Cables

QSFP cables are widely deployed across:

  • Data centers: Server-to-ToR switch connections. Leaf-to-spine fabrics at 40G/100G/400G. AI/HPC clusters requiring GPU interconnects.
  • Telecom networks: Metro and backbone optical links. DWDM systems at 100G/200G/400G.
  • Enterprise networks: High-bandwidth campus backbones. Storage networks and converged infrastructures.
Applications of QSFP Cables

Technical Considerations

When selecting QSFP cables, evaluate:

  1. Data rate: 40G, 100G, 200G, or 400G?
  2. Cable length: DAC ≤7m passive, ≤15m active. AOC ≤100m. Optics + patch cords for longer.
  3. Connector type: MPO vs LC vs MDC/CS.
  4. Signal integrity: Check insertion loss and crosstalk values.
  5. Compatibility: Cisco, Arista, Mellanox/NVIDIA often require coded cables.
  6. Environment: High-EMI areas → AOC or fiber.

Vendor Compatibility

For Cisco environments:

  • Use the Cisco transceiver compatibility matrix to confirm supported QSFP cables.
  • Look for Cisco-certified DACs and AOCs.
  • Ensure firmware/software versions align with QSFP cable coding.

Many third-party vendors also offer Cisco-compatible QSFP cables at lower cost, but validation is key.

How to Install and Maintain QSFP Cables?

Installation steps:

  1. Inspect ports for dust/debris.
  2. Align QSFP connector and gently insert.
  3. Listen for the latch click to secure.
  4. Power devices and check link LEDs.

Maintenance tips:

  • Regularly clean connectors with lint-free wipes and IPA.
  • Avoid tight bends (respect minimum bend radius).
  • Keep spares stored in dust-free bags.
  • Replace damaged cables immediately.

Case Study: Cisco QSFP-H40G-CU1M

The Cisco QSFP-H40G-CU1M is a 40GBASE-CR4 passive DAC cable:

  • Speed: 40Gbps over copper twinax.
  • Length: 1 meter.
  • Power: Passive, 0W.
  • Use case: High-density Cisco data center interconnects.
  • Benefits: Low cost, plug-and-play, compact design.

Feedback: IT teams report easy deployment and reliable short-range performance, especially in Meraki and Cisco enterprise setups.

cisco qsfp case

FAQs

Q1: What does QSFP stand for?
A: Quad Small Form-factor Pluggable.

Q2: What does “QSFP28” mean?
A: It refers to 100G QSFP with 4×25G lanes.

Q3: What’s the difference between QSFP DAC and AOC?
A: DAC = copper, short distance. AOC = fiber, medium distance.

Q4: What is the maximum length of QSFP DAC cables?
A: ~7m passive, ~15m active.

Q5: What’s QSFP-DD?
A: Double density QSFP supporting 400G (8 lanes).

Q6: Are QSFP cables compatible with Cisco?
A: Yes, if certified or vendor-coded for Cisco.

Q7: What does “30AWG” mean in QSFP DAC?
A: It refers to the cable wire gauge: thinner, more flexible, shorter max reach.

Q8: When should I use QSFP vs SFP?
A: Use QSFP for 40G+ speeds; SFP/SFP+ for 1G/10G access links.

Conclusion

QSFP cables are the backbone of 40G, 100G, 200G, and 400G networking in modern data centers:

  • QSFP+ (40G): Entry-level.
  • QSFP28 (100G): Common standard.
  • QSFP56 (200G): Higher lane speeds.
  • QSFP-DD (400G): Double density, future-proof.

DAC is best for in-rack, AOC for row-to-row, and optical transceivers for long-haul.

👉 Always ensure end-to-end consistency - NICs, switches, QSFP cables, and optics must match. Vendors like network-switch.com provide validated solutions for 100G/200G/400G deployments.

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