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Introduction
As businesses, cloud providers, and telecom operators continue to face exploding traffic demands, 100G networking has become the new standard. The QSFP28 port is at the heart of this transition. Compact, powerful, and flexible, it enables 100G Ethernet links using four 25G lanes.
QSFP28 ports are now widely deployed in data centers, AI/HPC clusters, telecom backbones, and enterprise networks. They combine high bandwidth with backward compatibility, making them the go-to interface for 100G networking. This article explains what a QSFP28 port is, its features, cabling options, interoperability with SFP, and how to troubleshoot common issues.
Overview of QSFP28 Port
What is a QSFP28 Port?
QSFP28 stands for Quad Small Form-factor Pluggable 28. It is a compact, hot-swappable transceiver interface that supports up to 100 Gbps.
- Architecture: 4 electrical lanes × 25 Gbps each = 100 Gbps.
- Hot-swappable: Replace modules without shutting down devices.
- Backward compatible: Works with QSFP+ (40G) equipment when supported by the switch.
- Applications: High-speed Ethernet, InfiniBand, data center interconnect (DCI).
Key Features of QSFP28
QSFP28 vs QSFP+
| Aspect | QSFP+ (40G) | QSFP28 (100G) |
| Max speed | 40 Gbps (4×10G) | 100 Gbps (4×25G) |
| Electrical lanes | 4 | 4 |
| Modulation | NRZ (10G per lane) | NRZ/PAM4 (25G per lane) |
| Connector type | MPO-12 / LC (some optics) | MPO-12 / LC duplex |
| Backward compatibility | — | Compatible with QSFP+ (40G) |
| Typical application | Legacy DC fabrics | Modern 100G leaf–spine |
👉 QSFP28 essentially quadruples the throughput per lane compared to QSFP+, enabling 100G networks with minimal increase in footprint.
How QSFP28 Ports Work?
When connected:
- Electrical-to-optical conversion occurs in the QSFP28 module.
- Optical signals travel over fiber (single-mode or multimode).
- At the other end, signals are converted back to electrical.
- Each of the four lanes carries 25G, aggregated into 100G.
QSFP28 also supports breakout modes, such as:
- 4×25G SFP28 (one QSFP28 port split into four SFP28 ports).
- 4×10G SFP+ (legacy support for older infrastructure).
Cabling & Connectivity
QSFP28 ports support different media depending on distance, cost, and environment.
AOC vs DAC
| Aspect | DAC (Direct Attach Copper) | AOC (Active Optical Cable) |
| Medium | Copper twinax | Fiber with integrated optics |
| Reach | 1–7m (passive), up to 15m (active) | 30–100m (sometimes 150m) |
| Power | 0W (passive) / 1–2W (active) | ~1–2W |
| Weight | Heavy, thick | Light, flexible |
| Cost | Lowest | More expensive than DAC but cheaper than optics |
| Best use case | In-rack server ↔ ToR switch | Rack-to-rack, medium-range links |
Single-Mode vs Multimode Fiber
| Aspect | Single-Mode (SMF) | Multimode (MMF) |
| Core size | ~9µm | 50µm / 62.5µm |
| Typical reach | Up to 10–40km | 100–300m |
| Cost (cabling) | Higher | Lower |
| Applications | DCI, metro, backbone | Data center short links |
👉 Rule of thumb:
- Use DAC for in-rack, cheapest and lowest latency.
- Use AOC or MMF for short/medium rack-to-rack links.
- Use SMF for long-haul DCI and backbones.
QSFP28 Interoperability with SFP
QSFP28 ports can interoperate with lower-speed devices using adapters or breakout cables:
- QSFP28 → 4×SFP28: Creates four 25G links.
- QSFP28 → 4×SFP+: Legacy 10G compatibility.
Compatibility considerations:
- The maximum speed is limited by the slower device (e.g., SFP28).
- Fiber type (SMF vs MMF) and wavelength must match.
- Not all QSFP28 → SFP adapters support every module; always check vendor compatibility matrices.
Use case examples:
- Data centers consolidating ports with QSFP28 but still connecting to 25G servers.
- Telecom operators upgrading to 100G backbones while maintaining 25G access.
Use Cases of QSFP28 Ports
- Data centersLeaf–spine fabrics at 100G. 4×25G server breakout connections.
- EnterprisesCore upgrades from 40G to 100G. Future-proofing edge-to-core interconnects.
- TelecomsMetro and long-haul links using SMF modules. 100G DCI (data center interconnect).
- AI/HPC clustersHigh-throughput GPU-to-switch links.
Troubleshooting and Maintenance
Common issues and best practices:
- Connectivity problems: Check cable type (DAC vs AOC vs fiber). Verify correct port configuration (speed/duplex).
- Hardware failures: Inspect QSFP28 modules for physical damage. Use diagnostic tools to check optical power and temperature.
- Firmware compatibility: Update switch/router firmware to support new QSFP28 modules. Confirm vendor coding if using third-party optics.
FAQs
Q1: What is a QSFP28 port?
A: A hot-swappable 100G optical/electrical transceiver interface with four 25G lanes.
Q2: How does QSFP28 differ from QSFP+?
A: QSFP+ = 40G (4×10G), QSFP28 = 100G (4×25G).
Q3: Can QSFP28 ports connect to SFP devices?
A: Yes, via breakout cables or adapters (e.g., 4×25G SFP28).
Q4: What types of cables work with QSFP28?
A: DAC, AOC, multimode fiber, single-mode fiber.
Q5: What’s the max reach of QSFP28 ports?
- DAC: ≤7m passive, ≤15m active.
- AOC: ≤100m.
- SMF optics: up to 40km.
Q6: Does QSFP28 support breakout to 4×10G?
A: Yes, one 100G port can split into four 10G SFP+ ports.
Q7: Where are QSFP28 ports used?
A: Data centers, telecom backbones, AI/HPC, enterprise cores.
Conclusion
QSFP28 ports are the workhorses of 100G networking.
- Provide high-density, high-bandwidth connectivity in compact form factors.
- Support multiple cabling types (DAC, AOC, SMF, MMF).
- Offer backward compatibility with SFP28/SFP+.
- Power today’s data centers, telecom networks, and AI clusters.
For organizations upgrading from 40G or preparing for 200G/400G, QSFP28 ports provide a scalable, cost-efficient stepping stone.
👉 Deployment tip: Ensure end-to-end compatibility (switch ↔ NIC ↔ transceiver ↔ cable). Trusted providers like network-switch.com supply validated QSFP28 solutions for reliable 100G performance.
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