The Need for Speed: Why Form Factors Keep Evolving
In the data center, the only constant is the demand for more bandwidth. The leap from 10G to 100G, which felt monumental just a few years ago, is now the established norm. Today, network architects are grappling with the leap to 400G and even 800G.
This relentless pace is not just about the lasers; it’s about the physical “form factor” that houses them. The transceiver module is the heart of the network, and as speeds increase, these modules must evolve.
For the past several years, the QSFP28 has been the undisputed king of 100G. But its reign is now being succeeded by a new, more powerful form factor: the QSFP-DD.
What is the difference between them? Why is QSFP-DD the future? And most importantly, how do you plan a migration path that doesn’t require a “rip and replace” of your entire infrastructure? This guide will break down the critical differences, the powerful advantages of QSFP-DD, and what it means for your network design.
The 100G Standard: What is QSFP28?
QSFP28 is the form factor that made 100G ubiquitous. Let’s break down its name:
- QSFP: Quad Small Form-factor Pluggable. “Quad” means it has 4 electrical lanes.
- 28: Each lane runs at 28 Gbps (25 Gbps data rate + overhead).
- 4 lanes x 25 Gbps = 100 Gbps total.
Anatomy: “Quad” 25G Lanes (4x25G)
The QSFP28 module is a simple, elegant solution. It takes a 100G electrical signal from the switch’s ASIC (the main chip) and converts it into a 100G optical signal. It does this by using four parallel electrical lanes. This is why the most common QSFP28 module, the 100G-SR4, perfectly aligns with the four 25G lanes of the electrical interface, using four fiber pairs to create a 100G link.
Common Applications (SR4, LR4)
The QSFP28 is the housing for all your 100G “flavors”:
- 100G-SR4: For short-reach MPO/Multimode links.
- 100G-LR4: For long-reach LC/Single-Mode links.
- 100G-PSM4: For 500m MPO/Single-Mode links.
The Inherent Limitation: 4 Electrical Lanes
The QSFP28’s greatest strength is also its greatest weakness: it is a 4-lane interface. To get to 400G, you would need 4x100G lanes, or 8x50G lanes. The QSFP28’s 4-lane electrical connector physically cannot support this. A new form factor was needed.
The 400G+ Future: What is QSFP-DD?
QSFP-DD is the form factor that solves the 4-lane problem.
- QSFP: It’s based on the same, familiar QSFP cage size.
- DD: Double Density.
The “DD” Magic: What “Double Density” Really Means
“Double Density” refers to a brilliant piece of engineering. The QSFP-DD connector is physically the same width and depth as a QSFP28, but it’s ever-so-slightly longer (a few millimeters).
The magic is in the electrical contacts.
- A QSFP28 connector has one row of electrical contacts.
- A QSFP-DD connector has two rows of electrical contacts (one is “hidden” just behind the first).
By adding this second row, QSFP-DD doubles the number of electrical lanes from 4 to 8.
Anatomy: 8 Electrical Lanes (8x50G or 8x100G)
This 8-lane interface is the key to the future. It allows for:
- 400G (8 x 50 Gbps): The first generation of 400G optics uses 8 electrical lanes, each running at 50 Gbps (using PAM4 modulation).
- 800G (8 x 100 Gbps): The next generation of 800G optics uses the exact same QSFP-DD form factor, but runs each of the 8 lanes at 100 Gbps.
The Killer Feature: 100% Backward Compatibility
This is the single most important feature for network designers. The QSFP-DD port on a switch is 100% backward compatible with your existing QSFP28 modules.
How? When you plug in a 4-lane QSFP28 module, it only engages with the first row of electrical contacts. The second “Double Density” row simply sits dormant. The switch port instantly recognizes the 4-lane module and operates it as a 100G port.
This means you can buy a new 400G-capable switch today, populate some ports with 400G QSFP-DD modules, and fill the rest with your existing, cheaper 100G QSFP28 modules. As your bandwidth needs grow, you can hot-swap the 100G modules for 400G modules, one at a time, without any downtime. This provides a “pay-as-you-grow” model that is incredibly valuable.
Head-to-Head: QSFP28 vs. QSFP-DD (The Comparison Table)
| Feature | QSFP28 | QSFP-DD |
| Full Name | Quad Small Form-factor Pluggable 28 | Quad Small Form-factor Pluggable Double Density |
| Max Speed | 100G (or 112G) | 400G, 800G, and beyond |
| Electrical Lanes | 4 lanes | 8 lanes |
| Data Rate (per lane) | 28 Gbps (NRZ) | 56 Gbps (PAM4) or 112 Gbps (PAM4) |
| Backward Compatibility | No (Cannot fit in a QSFP+ 40G port) | Yes (Can accept QSFP28 and QSFP+ modules) |
| Connector | Single row of electrical contacts | Double row of electrical contacts |
| Heat Sink | Typically flat or small fins | Typically has a larger, integrated heat sink |
| Power (TDP) | Lower (e.g., 3.5W – 5W) | Higher (e.g., 12W – 15W+) |
| Primary Use | 100G (Leaf, ToR, Spine) | 400G/800G (Spine, Core, DCI, HPC) |
Why QSFP-DD Won the 400G Form Factor War (vs. OSFP, CFP8)
QSFP-DD wasn’t the only 400G contender. The other main player was OSFP (Octal Small Form-factor Pluggable). OSFP is slightly larger and can handle more power, making it a favorite for some (like Arista initially).
However, QSFP-DD won the market for one simple reason: backward compatibility. The ability to re-use your entire inventory of 100G QSFP28 modules was a financial and logistical benefit that OSFP (which is not backward compatible) could not offer. Today, the vast majority of the industry, including major players like Cisco and Intel, have standardized on QSFP-DD as the 400G/800G port of choice.
Practical Applications for Network Design
When to Use QSFP28 (The “Leaf” Workhorse)
For the foreseeable future, QSFP28 remains the king of the “Leaf” or “Top-of-Rack.” 100G is more than enough bandwidth for most server-to-switch connections. It’s cheap, reliable, and low-power. You will continue to buy and deploy QSFP28 modules in high volume for your access layer.
When to Upgrade to QSFP-DD (The “Spine” and Future)
QSFP-DD is the new standard for your Spine and Core. Your Leaf-to-Spine links, which aggregate all the traffic from your racks, are the first place you will deploy 400G.
- A 32-port 400G QSFP-DD switch provides the same bandwidth as a 128-port 100G switch, in a fraction of the space and power.
- It’s also the standard for Data Center Interconnect (DCI), connecting your buildings.
The Power of QSFP-DD Breakout: 4x100G
Just as QSFP28 can “break out” into 4x25G, the QSFP-DD’s 8 lanes can be bifurcated. The most popular use case is 400G-to-4x100G.
Using a 400G-DR4 transceiver and an MPO breakout cable, a single 400G QSFP-DD port can be connected to four separate 100G QSFP28 ports. This gives network architects incredible flexibility, allowing them to connect their new 400G spine to their existing 100G leaf switches without any waste.
The 800G Horizon: The “DD” Legacy Continues
The genius of the QSFP-DD form factor is its longevity. The 8-lane electrical interface is the foundation for 800G. By upgrading the signal-per-lane from 50G (PAM4) to 100G (PAM4), the exact same port can now support an 800G transceiver.
This means the switches you buy today with QSFP-DD ports are not just 400G-capable; they are 800G-ready. This provides a future-proof investment that will last for years.
FAQ: QSFP28 vs. QSFP-DD
Q: Can I plug a QSFP-DD module into a QSFP28 port?
A: No. A QSFP-DD module is slightly longer and has a second row of contacts, so it will not physically fit or function in an older 100G QSFP28-only port.
Q: Can I plug a QSFP28 module into a QSFP-DD port?
A: Yes. This is the key feature. The QSFP-DD port is backward compatible and will accept a QSFP28 or even an older QSFP+ module.
Q: Does QSFP-DD use more power?
A: Yes, significantly more. A 400G module can draw 12-15W, compared to 3-5W for a 100G QSFP28. This is why QSFP-DD modules have large heat sinks and why the switches that house them require more advanced cooling.
Q: Is OSFP dead?
A: Not “dead,” but it has become a niche product. QSFP-DD has won the mainstream data center market due to its backward compatibility.
Conclusion: Planning Your 100G, 400G, and 800G Roadmap
The choice between QSFP28 and QSFP-DD is not a “which is better” choice; it’s a “when and where” choice.
- QSFP28 is your 100G workhorse. You will continue to deploy it for years at the access layer.
- QSFP-DD is your migration path. It’s the new standard for your high-speed 400G spine and 800G core, and its backward compatibility with QSFP28 is the key feature that makes the upgrade financially and logistically possible.
When you design your next network, you will be building a hybrid. You’ll use QSFP-DD ports in your spine to aggregate traffic, and you’ll leverage their backward compatibility to connect to the proven, cost-effective QSFP28 modules in your leaf switches.
PHILISUN offers a complete portfolio of 100G QSFP28 transceivers for your access layer and cutting-edge 400G QSFP-DD transceivers for your core. Contact our experts to build a future-proof network that scales from 100G to 400G and beyond.
