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QSFP28 Transceivers

Enterprise Resource Center • Networking

QSFP28 Transceivers Explained

Understand QSFP28 transceivers for 100 Gigabit Ethernet, including 100GBASE-SR4, LR4, CWDM4 and PSM4 optics, multimode and single-mode fiber, MPO/MTP and LC connectivity, DAC cables, active optical cables and 4x25GbE breakout.

QSFP28 100GbE 100GBASE-SR4 100GBASE-LR4 4x25G Breakout
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100 Gigabit Networking

What Is a QSFP28 Transceiver?

A QSFP28 transceiver is a compact, high-density pluggable network interface commonly used for 100 Gigabit Ethernet. QSFP28 builds on the quad-lane QSFP architecture while supporting substantially higher signaling rates per lane than earlier QSFP+ interfaces.

A typical 100GbE QSFP28 interface combines four high-speed lanes to create a single 100 Gigabit Ethernet connection. On compatible network hardware, those lanes can also support breakout configurations that divide one QSFP28 port into multiple 25GbE connections.

QSFP28 is widely deployed in data-center leaf-and-spine architectures, enterprise core networks, high-performance computing environments, storage networks and other applications requiring high-density 100GbE connectivity.

QSFP28 Quick Reference

QSFP28 at a Glance

Primary Ethernet Speed Commonly associated with 100 Gigabit Ethernet.
Lane Architecture Four high-speed electrical lanes are commonly used for 100GbE.
Common Connectors MPO/MTP or duplex LC depending on the optical specification.
Media Options Multimode fiber, single-mode fiber, DAC and AOC.
Quad-Lane Architecture

How Does QSFP28 Work?

QSFP28 uses four high-speed electrical lanes between the host equipment and transceiver. In a conventional 100GbE configuration, these lanes operate together to provide the aggregate network connection.

The physical transmission method varies by module. SR4 uses parallel multimode optical lanes, PSM4 uses parallel single-mode optical lanes, and LR4 and CWDM4 use multiple optical wavelengths to transport the connection over a pair of single-mode fibers.

QSFP28 interfaces can also support direct-attach copper, active optical cables and, on compatible hardware, breakout into four individual 25GbE connections.

100GbE Optical Standards

Common QSFP28 Transceiver Types

QSFP28 modules are available for different distances, fiber infrastructures and network architectures. Four commonly encountered 100GbE options are SR4, PSM4, CWDM4 and LR4.

Multimode Fiber

100GBASE-SR4

Parallel 850 nm optical lanes over multimode fiber with MPO/MTP connectivity for short data-center links.

Parallel Single Mode

100G PSM4

Parallel single-mode optical connectivity commonly using an MPO/MTP interface for intermediate-reach links.

Single-Mode WDM

100G CWDM4

Multiple optical wavelengths carried across duplex single-mode fiber, commonly using LC connectivity.

Long Reach

100GBASE-LR4

Long-reach 100GbE optics using multiple wavelengths over duplex single-mode fiber with LC connectivity.

Optical Reach

QSFP28 100GbE Optical Comparison

Type Fiber Connector Typical Reach Architecture
100GBASE-SR4 OM3 / OM4 MMF MPO/MTP Up to 100 m on OM4 Parallel optics
100G PSM4 SMF MPO/MTP Up to 500 m Parallel optics
100G CWDM4 SMF Duplex LC Up to 2 km Wavelength multiplexing
100GBASE-LR4 SMF Duplex LC Up to 10 km Wavelength multiplexing

Actual supported distance depends on the exact transceiver, fiber infrastructure and network platform. Always verify manufacturer specifications before deployment.

Parallel Multimode Optics

What Is 100GBASE-SR4?

100GBASE-SR4 is a short-reach 100 Gigabit Ethernet optical interface designed for multimode fiber. It uses four parallel transmit lanes and four parallel receive lanes to carry the 100GbE connection.

SR4 QSFP28 modules commonly operate at approximately 850 nm and use an MPO/MTP multifiber connector. Standard implementations can support up to approximately 100 meters over OM4 multimode fiber.

Because each optical lane corresponds to an individual high-speed channel, supported SR4 implementations are also useful for 100GbE-to-4x25GbE optical breakout.

Long-Reach Single Mode

What Is 100GBASE-LR4?

100GBASE-LR4 is a long-reach 100GbE optical interface designed for single-mode fiber. Rather than using separate physical fibers for each lane, LR4 combines multiple optical wavelengths onto a duplex fiber pair.

QSFP28 LR4 modules commonly use duplex LC connectivity and are designed for distances up to approximately 10 kilometers, making them useful for data-center interconnects, campus links and other longer-distance enterprise applications.

Single-Mode Alternatives

QSFP28 PSM4 vs. CWDM4

Parallel Single Mode

100G PSM4

  • Single-mode fiber
  • Parallel optical lanes
  • MPO/MTP connectivity
  • Commonly supports up to 500 m
  • Can support compatible 4x25G breakout architectures
Wavelength Multiplexing

100G CWDM4

  • Single-mode fiber
  • Multiple optical wavelengths
  • Duplex LC connectivity
  • Commonly supports up to 2 km
  • Uses only two physical fiber strands
Breakout Networking

What Is QSFP28 4x25GbE Breakout?

Certain QSFP28 ports can divide a 100GbE interface into four independent 25GbE connections. This allows one high-density QSFP28 switch port to connect to four compatible SFP28 server, storage or network interfaces.

1 × QSFP28 100GbE
↓
4 × SFP28 25GbE

Breakout support is platform-dependent. The switch, port configuration, operating system or firmware, transceiver or cable assembly and far-end interfaces must all support the intended configuration.

Direct-Attach Copper

What Is a QSFP28 DAC Cable?

A QSFP28 direct-attach copper cable combines QSFP28 interfaces with a fixed Twinax copper cable assembly. DAC is commonly used for short 100GbE connections between nearby switches, servers and other compatible equipment.

Breakout DAC assemblies are also available for supported applications, with a QSFP28 interface at one end and four SFP28 interfaces at the other for 4x25GbE connectivity.

Connection Options

QSFP28 DAC vs. AOC vs. Optical Transceivers

Copper

QSFP28 DAC

Fixed Twinax copper assembly for short, high-speed connections between nearby compatible devices.

Integrated Fiber

QSFP28 AOC

Factory-assembled active optical cable integrating the optical interfaces and fiber into one cable.

Modular Fiber

QSFP28 Optics

Separate optical modules and structured fiber cabling provide modularity and multiple reach options.

Generation Comparison

QSFP+ vs. QSFP28

QSFP+

QSFP+

  • Commonly associated with 40GbE
  • Quad-lane architecture
  • Can support 4x10GbE breakout
  • Optical, DAC and AOC options
QSFP28

QSFP28

  • Commonly associated with 100GbE
  • Higher per-lane signaling rate
  • Can support 4x25GbE breakout
  • Optical, DAC and AOC options
25GbE and 100GbE

SFP28 vs. QSFP28

SFP28

SFP28 is commonly used for a single 25GbE connection and is widely deployed for server, storage and access-layer connectivity.

QSFP28

QSFP28 combines four high-speed lanes and is commonly used for 100GbE switch uplinks, aggregation and core connectivity.

Optical Monitoring

QSFP28 DOM and DDM

Many QSFP28 optical transceivers provide digital diagnostic monitoring capabilities. When supported by the module and host platform, these diagnostics can provide valuable information about transceiver health and optical link conditions.

Temperature Module operating temperature.
Voltage Transceiver supply-voltage data.
TX Power Optical transmit-power information.
RX Power Received optical-power information.
Compatibility Planning

What Should You Check Before Buying QSFP28 Hardware?

Host Port Verify that the switch provides the required QSFP28 interface and operating mode.
Network Speed Confirm native 100GbE or the intended supported breakout mode.
Fiber Type Match multimode or single-mode fiber to the optical module.
Connector Verify MPO/MTP, duplex LC or the required cable assembly.
Reach Select SR4, PSM4, CWDM4, LR4, DAC or AOC according to distance.
Vendor Support Verify module coding, firmware and hardware support.

Physical Fit Does Not Guarantee QSFP28 Compatibility

QSFP-family interfaces use standardized physical dimensions, but a module that physically fits into a port is not automatically guaranteed to operate. Host platforms can impose requirements for supported speeds, transceiver coding, optical standards and cable types.

Before deployment, verify the exact switch model, port capabilities, firmware, operating mode, transceiver specification, fiber infrastructure and breakout requirements.

Troubleshooting

Why Is My QSFP28 Link Not Working?

Unsupported Module

The host may reject a transceiver that is not recognized or supported by the platform.

Wrong Fiber

SR4, PSM4, CWDM4 and LR4 require different fiber and connector configurations.

MPO Polarity

Parallel optical connections require correct fiber polarity and lane mapping.

Dirty Connectors

Contamination can increase optical loss and prevent reliable 100GbE operation.

Breakout Mode

The port may require explicit configuration before it can operate as four 25GbE interfaces.

DAC Compatibility

DAC cable type, length and coding must be supported by the connected equipment.

Frequently Asked Questions

QSFP28 Transceiver FAQ

What is QSFP28?
QSFP28 is a quad-lane pluggable network interface commonly used for 100 Gigabit Ethernet connectivity in enterprise and data-center networks.
What speed is QSFP28?
QSFP28 is primarily associated with 100GbE, although supported speeds and operating modes depend on the host platform and module.
What is 100GBASE-SR4?
100GBASE-SR4 is a short-reach 100GbE optical interface using parallel optical lanes over multimode fiber. QSFP28 SR4 modules commonly use MPO/MTP connectivity.
What is 100GBASE-LR4?
100GBASE-LR4 is a long-reach 100GbE optical interface for single-mode fiber. It commonly uses duplex LC connectivity and supports distances up to approximately 10 kilometers.
What is the difference between QSFP28 SR4 and LR4?
SR4 uses parallel optics over multimode fiber with MPO/MTP connectivity for shorter links. LR4 uses wavelength multiplexing over duplex single-mode fiber with LC connectivity for substantially longer distances.
Can QSFP28 break out to four SFP28 ports?
Yes, supported QSFP28 ports can operate as four 25GbE interfaces using compatible breakout cables or optical configurations. The switch hardware and software must support breakout operation.
What is the difference between QSFP+ and QSFP28?
QSFP+ is commonly associated with 40GbE, while QSFP28 supports higher per-lane signaling rates and is commonly associated with 100GbE.
What is the difference between SFP28 and QSFP28?
SFP28 is commonly used for a single 25GbE connection. QSFP28 combines four high-speed lanes and is commonly used for a 100GbE connection.
Are all QSFP28 modules compatible?
No. Compatibility depends on the host platform, firmware, operating mode, transceiver coding, optical standard, fiber type and cable configuration.
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