By Dustin Guttadauro, Product Line Manager - Telecom & Fiber, Infinite Electronics
As data center networks move toward higher port densities and faster east-west traffic, 100G connectivity has become an important building block for modern spine-and-leaf architectures. Among the optical and direct-attach options available for 100G links, QSFP28 is one of the most widely used form factors.
QSFP28 is designed for 100G applications and can support different physical implementations depending on the required reach, fiber type, connector configuration, and network architecture. For shorter connections, 100G direct attach cables (DACs) and active optical cables (AOCs) can provide a simpler alternative to separate transceivers and patch cables. For longer or more flexible fiber links, pluggable QSFP28 optical transceivers provide greater deployment flexibility.
Choosing between these options requires more than simply matching a 100G port with a 100G module. Engineers need to consider link distance, media, connector type, rack layout, power consumption, cabling density, upgrade plans, and the possibility of breakout connections.
Key Takeaways
- QSFP28 is a major 100G transceiver form factor used in modern data center networks.
- Common 100G optical implementations include SR4, LR4, and CWDM4.
- SR4 is commonly associated with short-reach multimode fiber and parallel optics.
- LR4 and CWDM4 use single-mode fiber and wavelength-based optical transmission.
- 100G DACs are suited to very short copper connections.
- 100G AOCs integrate optical ends with the cable for simpler short-distance deployment.
- QSFP28 can support a 4 × 25G breakout in compatible systems.
- Connector, fiber type, reach, and switch compatibility must be checked before ordering.
- The right 100G solution depends on the physical topology, not simply the advertised data rate.
- For flexible structured-fiber deployments, separate pluggable QSFP28 optics and fiber assemblies can provide greater adaptability.
What Is QSFP28?
QSFP28 stands for Quad Small Form-factor Pluggable 28. It is a compact pluggable transceiver form factor commonly used for 100G Ethernet applications. The "28" refers to the approximate per-lane signaling capability of the electrical interface rather than the total data rate. A typical QSFP28 interface uses four electrical lanes operating at approximately 25G per lane to provide a 100G-class connection.
This four-lane architecture also enables one of the most useful capabilities of QSFP28: breakout connectivity. A 100G QSFP28 port can, in supported configurations, be connected as a 4 × 25G breakout link. This allows one 100G switch port to connect to four 25G interfaces, making QSFP28 useful in leaf-spine architectures where server-facing interfaces may operate at 25G while the uplink operates at 100G. However, breakout support depends on the switch, transceiver, cabling, and software configuration. Engineers should verify compatibility before designing a breakout connection.
Why QSFP28 Is Important for 100G Data Center Uplinks
The move from 10G or 25G access connections to 100G uplinks can significantly increase bandwidth without requiring the same increase in physical port count. For example, a leaf switch may provide multiple 25G server-facing ports while using 100G QSFP28 connections toward spine switches. This creates a practical hierarchy:
Server → 25G → Leaf → 100G → Spine
The exact architecture varies by data center, but the underlying principle is the same: higher-speed uplinks consolidate traffic between network layers. QSFP28 also provides multiple physical implementation choices. Depending on the application, engineers can select a short-reach multimode solution, a longer-reach single-mode solution, or a copper or optical cable assembly for short switch-to-switch connections. That flexibility makes QSFP28 useful across different rack layouts and data center scales.
Common 100G QSFP28 Options
Not every 100G QSFP28 module uses the same optical architecture. Common implementations include 100GBASE-SR4, 100GBASE-LR4, and 100GBASE-CWDM4.
100GBASE-SR4
100GBASE-SR4 is designed primarily for short-reach multimode fiber applications. The "SR" designation indicates short reach, while "4" reflects the parallel-lane optical architecture. SR4 commonly uses an MPO/MTP-style multi-fiber connection and is suited to high-density connections inside a data center where the distance between switches or equipment is relatively short.
Typical applications include:
- Leaf-to-spine connections within the same data hall
- Rack-to-rack backbone connections
- Short switch-to-switch links
- High-density fiber patching
- Structured cabling environments using multimode fiber
Because SR4 uses parallel optics, the fiber plant and connector configuration must be matched correctly.
100GBASE-LR4
100GBASE-LR4 is intended for longer-reach connections than SR4 and typically operates over single-mode fiber. Unlike SR4's parallel multimode approach, LR4 uses multiple optical wavelengths to carry the 100G signal over a duplex single-mode fiber connection. This makes LR4 useful when the link extends beyond the practical range of short-reach multimode deployments.
Potential applications include:
- Longer data hall links
- Data center backbone connections
- Campus data center links
- Longer switch-to-switch connections
- Infrastructure where single-mode fiber is already deployed
The exact supported distance depends on the transceiver specification and the overall link budget.
100GBASE-CWDM4
CWDM4 is another 100G single-mode solution that uses four optical wavelengths. CWDM4 was developed for relatively short data center single-mode links while allowing a duplex fiber architecture rather than the multi-fiber parallel architecture commonly associated with SR4. This can be useful where a data center standardizes on single-mode fiber or where duplex fiber infrastructure is preferred.
QSFP28 Optics vs. 100G AOC vs. 100G DAC
For a new 100G uplink, choosing the transceiver is only part of the decision. Engineers must also determine whether the link should use separate pluggable optics and fiber cabling, an active optical cable, or a direct attach copper cable.
QSFP28 Optical Transceivers
A QSFP28 optical solution normally uses a pluggable transceiver at each end of the connection along with a separate fiber optic cable assembly. This approach provides considerable flexibility. For example, the same basic switch port can support different link architectures by changing the transceiver and fiber assembly to match the required distance and media.
Advantages include:
- Flexible link distances
- Replaceable transceivers
- Compatibility with structured fiber cabling
- Easier adaptation to different rack layouts
- Support for multimode and single-mode architectures
- Better suitability for longer links
The tradeoff is that separate transceivers and fiber assemblies add components that must be specified, installed, cleaned, and maintained.
100G Active Optical Cables
An active optical cable, or AOC, combines optical transceiver electronics with a permanently attached fiber cable. Instead of purchasing separate QSFP28 modules and a fiber assembly, the engineer specifies an integrated cable with compatible ends. AOCs can simplify deployment for short data center links. They are particularly useful when the endpoint devices are close enough that a fixed-length cable is practical.
Advantages include:
- Simple installation
- Integrated transceiver ends
- Reduced connector count
- No separate fiber patch cable required
- Useful for short equipment-to-equipment connections
- Potentially easier cable management
The primary limitation is flexibility. Because the transceiver ends and cable are integrated, replacing one component independently is generally not possible.
100G Direct Attach Copper Cables
A direct attach copper cable, or DAC, uses copper conductors with integrated connectors at both ends. DACs are commonly used for very short connections where low cost and simple installation are priorities.
Typical applications include:
- Same-rack connections
- Adjacent-rack connections
- Short switch-to-switch links
- Top-of-rack to server connections
- Lab and test environments
DACs avoid optical components altogether, which can make them attractive for short links. However, copper cable is heavier and generally less flexible than optical cable. Distance is also significantly more limited than with optical solutions.
QSFP28 vs. AOC vs. DAC: Which Should You Use?
The decision can be simplified by starting with the physical distance and cabling architecture.
Choose QSFP28 optical transceivers when:
- The link requires significant reach.
- You need single-mode or multimode fiber.
- The facility already has structured fiber infrastructure.
- You want the flexibility to replace optics independently.
- Different link distances may be required.
- The network is part of a larger fiber backbone.
Choose a 100G AOC when:
- The link is relatively short.
- A fixed cable length is acceptable.
- You want integrated optical ends.
- Installation simplicity is important.
- Separate patch cables are unnecessary.
Choose a 100G DAC when:
- The connection is very short.
- Equipment is in the same rack or nearby.
- Copper cabling is appropriate.
- Cost and simplicity are important.
- The link does not require optical reach.
There is no universal choice for every 100G connection. The physical layout and future expansion plans should determine the cable type.
4 × 25G Breakout: One QSFP28 Port, Four 25G Connections
One of the major advantages of QSFP28 is its ability to support breakout configurations in compatible systems. A 100G QSFP28 interface can be used as a 4 × 25G breakout connection, allowing a single 100G switch port to connect to four 25G ports.
The basic architecture looks like this:
100G QSFP28 Port → Breakout Cable → 4 × 25G SFP28 Ports
This can be useful when a switch has available 100G uplink capacity, but the downstream devices use 25G interfaces. Breakout can also help network designers use high-density switch ports more efficiently.
However, engineers should confirm all of the following before deployment:
- The switch supports a 4 × 25G breakout.
- The QSFP28 transceiver supports the required breakout architecture.
- The cable has the correct connector configuration.
- The switch operating system supports the breakout mode.
- The four 25G interfaces are compatible with the intended optics or cable.
- The link budget and physical media are appropriate.
Breakout should therefore be treated as a system-level compatibility decision rather than simply a cable selection.
How to Choose the Right 100G QSFP28 Solution?
Before ordering a 100G transceiver or cable, work through the following checklist.
1. Confirm the switch port
Check whether the switch provides:
- 100G QSFP28
- 40G QSFP+
- QSFP-DD
- Another supported high-speed interface
Do not assume that a physically similar cage supports every QSFP-family module.
2. Determine the required distance
Measure the actual cable route rather than simply estimating the distance between racks.
Include:
- Horizontal cable runs
- Vertical pathways
- Patch-panel connections
- Slack loops
- Cross-connects
3. Select the media
Determine whether the link will use:
- Multimode fiber
- Single-mode fiber
- Copper DAC
- Integrated optical AOC
4. Match the connector
For fiber deployments, verify whether the design requires an MPO/MTP connection, duplex LC, or another connector configuration.
5. Check breakout requirements
If the design requires 4 × 25G, verify that the switch, optics, and cable support the intended breakout mode.
6. Verify compatibility
Check:
- Switch manufacturer compatibility
- Part number
- Ethernet standard
- Data rate
- Fiber type
- Reach
- Connector type
- DOM support if required
- Operating temperature
- Coding or interoperability requirements
7. Consider future expansion
A DAC may be suitable for a short connection today, while structured fiber with pluggable optics may provide greater flexibility if the rack layout or network architecture is expected to change.
Building a 100G Uplink Strategy
A 100G uplink should be designed as part of the entire physical layer rather than as an isolated transceiver purchase. For a short connection inside a rack, a DAC may provide a straightforward solution. For a short optical connection between nearby racks, an AOC may reduce installation complexity. For structured cabling, longer links, or infrastructure that needs greater flexibility, separate QSFP28 optical transceivers and fiber cable assemblies may be more appropriate.
The important point is to match the transceiver, cable, connector, fiber type, distance, and switch port as one system. For data center infrastructure planning, explore L-com data center solutions for fiber, cable assembly, and connectivity options.
Frequently Asked Questions (FAQs)
1. What is QSFP28 used for?
QSFP28 is commonly used for 100G networking applications, including data center spine-and-leaf uplinks, switch-to-switch connections, and high-bandwidth aggregation links. It can support different physical implementations depending on the required reach, fiber type, connector configuration, and network architecture.
2. Is QSFP28 the same as 100G?
No. QSFP28 is a transceiver form factor commonly used for 100G applications, not a single optical standard. The “28” refers to the approximate per-lane signaling capability of the electrical interface. A typical QSFP28 interface uses four electrical lanes at approximately 25G each. Because the optical implementation can vary, check the complete module specification rather than treating every QSFP28 module as identical.
3. What is the difference between QSFP28 SR4, LR4, and CWDM4?
SR4 is designed for short-reach multimode fiber and commonly uses parallel optics with an MPO/MTP connection. LR4 and CWDM4 use single-mode fiber with multiple optical wavelengths over a duplex fiber connection. LR4 is intended for longer reach, while CWDM4 was developed for relatively short data center single-mode links.
4. Should I use QSFP28 optics, an AOC, or a DAC for a 100G link?
Start with distance and cabling architecture. Choose QSFP28 optical transceivers for longer links, structured fiber cabling, and flexibility. Choose a 100G AOC for relatively short links where a fixed cable length is acceptable. Choose a 100G DAC for very short connections in the same or adjacent racks, where cost and simplicity matter most.
5. Can a QSFP28 port connect to four 25G ports?
In compatible systems, yes. A 100G QSFP28 port can be used as a 4 × 25G breakout, connecting one 100G switch port to four 25G interfaces. Breakout support depends on the switch, transceiver, cabling, and software configuration, so verify compatibility before deployment.
6. What should I check before ordering a 100G QSFP28 transceiver or cable?
Confirm the switch port type, the actual cable route distance, the media (multimode fiber, single-mode fiber, DAC, or AOC), and the connector type (MPO/MTP or duplex LC). Also verify breakout requirements, switch manufacturer compatibility, reach, and any coding or interoperability requirements.