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NRCube Technology Library · Volume 1

SFP, SFP28 and QSFP28

A practical introduction to three established pluggable form factors, the network roles they serve and the checks that matter before selection.

Explore the form factors Browse by form factor

Why pluggable interfaces matter

The port remains. The physical connection can change.

Pluggable transceivers separated media choice from the switching or routing platform. Instead of dedicating hardware to one cable type or reach, operators could select a module for the required speed, fiber, connector and distance.

That flexibility improved hardware reuse, inventory planning and migration. The form factor, however, is only the physical starting point—it does not by itself establish speed, wavelength, reach or platform compatibility.

At a glance

Three form factors, three common network roles.

Form factor	Typical Ethernet role	Lane model	Common deployment focus
SFP	1G-centric access and uplinks	Single lane	Enterprise, campus, industrial and service-provider access
SFP28	25G server and access connectivity	Single 25G lane	Servers, storage and leaf access
QSFP28	100G fabric and aggregation	Four 25G lanes	Data centers, cloud, storage and high-density networks

These are typical roles, not universal compatibility rules. Exact platform and port documentation remains authoritative.

Fiber-media comparison

Single-mode and multimode optics solve different physical paths.

SMF and MMF are fiber-media choices, not SFP form factors. Either may be implemented in an SFP-family module when the host, optical specification and cabling support it. Use this comparison for orientation, then confirm the exact product specification and installed fiber grade.

Selection factor	Single-mode fiber (SMF) optic	Multimode fiber (MMF) optic
Typical wavelength families	Commonly 1310 nm or 1550 nm; wavelength-multiplexed and BiDi products use other defined wavelengths.	Commonly 850 nm; some multimode technologies use additional short-wavelength bands.
Practical reach profile	From short in-building links to metro and extended-distance classes. Exact reach can range from hundreds of metres to 100 km or more, depending on the optical specification and link budget.	Primarily short-reach links, commonly tens to hundreds of metres. Supported distance depends on data rate, fiber grade, connector path and the exact optic.
Common transmitter technology	FP, DFB, EML and other architectures may be used depending on speed, reach and wavelength.	VCSEL is common in short-reach multimode products.
Common connector patterns	LC duplex for many serial or wavelength-multiplexed links; MPO/MTP for parallel single-mode technologies.	LC duplex for many serial links; MPO/MTP for parallel multimode technologies.
Typical deployment focus	Campus backbones, data-center interconnect, service-provider access, metro and longer routed paths.	In-rack, row, data-hall and building links where compatible multimode infrastructure is installed.
Relative cost tendency	Often higher for comparable data rates because of the optical design, but pricing varies materially by technology, reach and volume.	Often economical for supported short-reach applications, subject to the installed MMF plant and transceiver generation.

Selection sequenceEndpoint platform→Port mode and speed→Installed SMF or MMF→Connector and routed distance→Qualified optic

The ranges above describe common industry patterns, not guaranteed limits. Optical power budget, attenuation, connector loss, polarity, software support and both endpoint requirements remain part of the final selection.

1G-centric

SFP

Established access and uplink connectivity

SFP made the physical interface modular: the same equipment port could use an optical or copper module chosen for the actual link. It remains widely deployed across enterprise, campus, industrial, public-sector and service-provider networks.

Where it is commonly used

Campus and building uplinks
Access-layer switching
Industrial and utility networks
Customer-edge connectivity

Representative link types

1000BASE-SX · MMF · LC
1000BASE-LX · commonly SMF · LC
1000BASE-ZX · extended SMF links
Copper SFP · RJ45 Ethernet

25G

SFP28

Compact server, storage and leaf access

SFP28 retains the compact SFP footprint while carrying a single 25G electrical lane. It became an important server-access interface because it improved bandwidth density and provided a natural building block for 100G systems.

Where it is commonly used

Server and hypervisor links
Storage connectivity
Leaf-switch access
AI support and storage networks

Representative link types

SR · short-reach MMF
LR and ER · longer SMF paths
BiDi · wavelength-paired single-fiber links
DAC and AOC · fixed equipment links

100G

QSFP28

High-density fabric and aggregation connectivity

QSFP28 combines four 25G electrical lanes into a 100G interface. It is widely used across data-center fabrics, cloud infrastructure, storage networks, service-provider systems and supporting layers of AI infrastructure.

Where it is commonly used

Leaf-to-spine fabrics
Cloud and storage interconnects
Data-center aggregation
100G native and breakout links

Representative link types

SR4 · parallel MMF · MPO
LR4 · duplex SMF · LC
CWDM4 · duplex-fiber 100G
ER4 · extended reach after budget review

Why 25G changed the progression

A more efficient step between 10G and 100G.

SFP28 gave servers and storage a 25G single-lane interface without moving to a larger module. Four 25G lanes also underpin the common QSFP28 100G architecture.

4 × 25G=100G

This relationship makes port mode and lane architecture important even when the two endpoints operate at different aggregate speeds.

Migration questions worth answering

Can the existing MMF or SMF plant be reused?
Does the host support the intended native or breakout mode?
Will the access layer move from 10G to 25G?
Is a later migration to 100G expected?
Will connector or polarity choices constrain that migration?

Connector and infrastructure choices

The cabling decision can outlast the transceiver.

LC duplex

Common with duplex optical technologies such as many SX, LX, LR, CWDM4 and LR4 links. Confirm the required fiber type and wavelength.

MPO / MTP

Common with parallel optics such as SR4. Fiber count, polarity, keying and patching design must match the application.

RJ45

Used by copper SFP modules. Confirm the port's support, cable category, reach, power and thermal constraints.

Do not select by connector alone. The same connector can appear across different speeds, wavelengths, media and optical technologies.

QSFP28 breakout

One 100G host port can serve four 25G endpoints—when the platform supports it.

QSFP28 · 100G→4 × SFP28 · 25G

Validate before ordering

Host breakout support
Port configuration
Electrical lane mapping
Cable or optical topology
Fiber count and MPO polarity
Far-end platform support

Frequent misunderstandings

Form factor is not a complete specification.

SFP

Not every SFP is optical; copper Ethernet modules also use the form factor.

Connector

LC or MPO describes the physical interface, not compatibility.

Fiber

An MMF technology should not be assumed suitable for an SMF path, or vice versa.

25G

SFP28 is not merely a faster 10G optic; the host electrical interface must support 25G.

100G

QSFP28 technologies are not interchangeable across SR4, LR4, CWDM4 and ER4 infrastructure.

Breakout

A four-lane module does not guarantee the platform exposes a supported 4 × 25G mode.

NRCube review guidance

Start with the equipment and the physical path.

A successful selection begins with both platforms, port capabilities, installed media and routed distance. The product name comes later.

The transceiver enables the connection; it does not define the complete solution.

Information NRCube typically reviews

Exact endpoint platforms
Port rate and supported modes
Fiber or copper infrastructure
Connector and routed distance
Optical technology and link budget
Breakout requirements
Coding and compatibility expectations
Future migration plans

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Volume 1 summary

A compact progression from established access to high-density 100G.

SFPMature, flexible 1G-centric connectivity

SFP2825G server, storage and modern access links

QSFP28High-density 100G native and breakout connectivity

Browse NRCube form factors Read the selection guide