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Introduction
The explosive growth of cloud computing, AI, 5G, and high-performance computing (HPC) is driving networks to move from 100G to 200G and 400G—and soon to 800G. To meet this demand, network operators need optical modules that deliver higher bandwidth, better density, and backward compatibility with existing infrastructure.
One of the most important solutions is QSFP28-DD (Quad Small Form-factor Pluggable 28 Double Density). It doubles the capacity of the familiar QSFP28 form factor, enabling 200G and 400G speeds while remaining compatible with legacy QSFP28 modules.
This guide explains what QSFP28-DD is, how it compares to QSFP28 and QSFP56, its key features, benefits, and use cases, and how to choose the right form factor for your network.
Overview of QSFP28-DD
What is QSFP28-DD?
QSFP28-DD stands for Quad Small Form-factor Pluggable 28, Double Density.
- Electrical lanes: 8 instead of 4 (QSFP28).
- Speeds: 200G (8×25G NRZ) or 400G (8×50G PAM4).
- Connector: 38 pins (vs 20 pins in QSFP28).
- Size: Same external form factor as QSFP28, but with added density.
- Compatibility: QSFP-DD ports can accept QSFP28 (100G) and QSFP56 (200G) modules, protecting prior investments.
In short, QSFP28-DD offers double the bandwidth in the same footprint as QSFP28.
QSFP28 vs QSFP28-DD vs QSFP56
| Aspect | QSFP28 | QSFP28-DD | QSFP56 |
| Max Speed | 100G (4×25G NRZ) | 200G (8×25G NRZ) / 400G (8×50G PAM4) | 200G (4×50G PAM4) |
| Electrical Lanes | 4 | 8 | 4 |
| Connector Pins | 20 | 38 | 20 |
| Backward Compat. | QSFP+ | QSFP28/56 | QSFP28 |
| Power Use | ~3.5W | ~4–7W | ~4–6W |
| Applications | 100G DC fabrics | 200G/400G fabrics, AI/HPC, DCI | 200G Ethernet, HPC |
👉 Key takeaway:
- QSFP28 is the 100G standard.
- QSFP28-DD scales to 200G/400G while preserving backward compatibility.
- QSFP56 offers 200G but lacks the density and scalability of QSFP-DD.
Key Features of QSFP28-DD
- High Data RateSupports 200G and 400G Ethernet, with PAM4 modulation doubling per-lane throughput.
- Enhanced Density“Double density” design doubles the number of lanes. Maximizes switch front-panel ports in 1U or 2U form factors.
- Backward CompatibilityQSFP-DD ports accept QSFP28 and QSFP56 modules. Simplifies migration from 100G to 400G.
- Thermal and Power EfficiencyAdvanced heat dissipation designs for high-performance modules. Lower watts-per-Gbps compared to earlier optics.
- Versatile Media SupportWorks with both single-mode fiber (long reach) and multimode fiber (short reach).
Benefits of QSFP28-DD in Practice
Higher Data Rates and Bandwidth
- Enables 200G and 400G Ethernet, supporting bandwidth-heavy workloads like AI training, virtualization, and big data.
Density and Scalability
- More ports per switch → higher network capacity in the same rack space.
- Ideal for leaf-spine architectures in hyperscale data centers.
Efficiency
- Better watts-per-Gbps ratio than QSFP28.
- Optimized airflow and thermal management reduce cooling costs.
Flexible Operation (2×100G Mode)
- QSFP28-DD can operate as 2×100G links.
- Supports incremental upgrades where 400G is not yet required.
Breakout Options
- QSFP28-DD breakout cables allow 4×25G SFP28 or 4×10G SFP+ connections.
- Perfect for mixed environments with legacy gear.
Applications of QSFP28-DD
- Hyperscale data centers: 400G leaf-spine interconnects.
- Cloud computing: Bandwidth for virtualization and distributed apps.
- AI and HPC clusters: High-throughput GPU-to-GPU and GPU-to-switch links.
- Telecom & DCI: Metro and long-haul connections.
- Enterprises: Future-proofing for 200G/400G-ready networks.
Compatibility and Interoperability
- QSFP28-DD modules comply with QSFP-DD MSA (Multi-Source Agreement).
- Supports IEEE 802.3bs/cd Ethernet standards.
- Interoperable across vendors when MSA-compliant.
- Backward compatible with QSFP28 and QSFP56, ensuring smooth migration.
How to Choose?
| Requirement | Recommended Form Factor | Why |
| 100G leaf-spine fabric | QSFP28 | Mature, cost-effective. |
| 200G short-term upgrade | QSFP56 | 200G PAM4, simple migration. |
| 200G with future 400G path | QSFP28-DD | Backward compatible, scalable. |
| Hyperscale AI/HPC clusters | QSFP28-DD | Supports 400G with high density. |
| Long-haul DCI | QSFP28-DD optics | Single-mode modules support 10km+. |
FAQs
Q1: What is QSFP28-DD?
A: A double-density QSFP module supporting 200G/400G with 8 electrical lanes.
Q2: How does it differ from QSFP28?
A: QSFP28 = 100G, 4 lanes. QSFP28-DD = 200G/400G, 8 lanes, backward compatible.
Q3: Is QSFP28-DD backward compatible?
A: Yes. QSFP-DD ports can accept QSFP28 and QSFP56 modules.
Q4: What modulation does QSFP28-DD use?
A: NRZ (25G per lane) and PAM4 (50G per lane).
Q5: Can QSFP28-DD operate as 2×100G?
A: Yes, it supports dual 100G mode for flexible upgrades.
Q6: What are typical applications?
A: Data centers, AI/HPC, telecom DCI, enterprises preparing for 400G.
Q7: How does QSFP28-DD compare to QSFP56?
A: QSFP56 = 200G, but QSFP28-DD scales to 400G and is denser.
Conclusion
The QSFP28-DD form factor is a cornerstone of next-generation networking.
- 200G/400G ready, with PAM4 for higher efficiency.
- Double density design increases switch port capacity.
- Backward compatibility protects prior QSFP28/56 investments.
- Flexible operation: breakout support and 2×100G mode.
For enterprises, cloud providers, and AI/HPC operators, QSFP28-DD offers the ideal balance of scalability, efficiency, and future-proofing.
👉 Deployment tip: Always ensure end-to-end compatibility—NICs, switches, transceivers, and cables must align. Vendors like network-switch.com provide validated QSFP28-DD solutions for seamless 200G/400G upgrades.
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