As data centers evolve toward higher bandwidth and longer reach, 100G Ethernet has become a crucial standard for backbone and cloud network infrastructures. Among the many 100G optical transceiver types, the QSFP28 form factor is the most common thanks to its compact size, low power consumption, and scalability. However, not all 100G QSFP28 modules are identical. Variants such as SR4, PSM4, CWDM4, LR4, and ER4 serve very different transmission needs, mainly determined by distance, fiber type, wavelength management, and cost.
This article explains how these modules differ in design and use, helping network planners choose the most suitable option.
Overview of the QSFP28 Form Factor
QSFP28 (Quad Small Form-factor Pluggable 28 Gbps) is the 100G generation of the QSFP family, operating with four electrical lanes, each carrying 25 Gbps. These lanes can be transmitted optically through various encoding and wavelength strategies, which lead to the distinct module families discussed below.
Physically, all these modules share the same mechanical connector and electrical interface, but their optical characteristics—such as wavelength plan, number of fibers, and maximum reach—differ significantly. This allows network architects to mix and match modules according to distance and cabling type while maintaining electrical compatibility.
QSFP28 SR4 for Short-Reach Connections
The 100G QSFP28 SR4 is optimized for short-reach multimode fiber (MMF) applications, typically within data centers. It uses four parallel 850 nm optical lanes to send and receive signals simultaneously over eight fibers in total (four transmit, four receive).
The key features include:
- Maximum distance: up to 100 m on OM4 (or 70 m on OM3) multimode fiber.
- Interface: MPO‑12 connector.
- Advantages: lowest cost per port, low latency, and minimal power consumption (about 3 W or less).
- Common use: server-to-switch or leaf-spine aggregation in large data centers.
The 100GBASE-SR4 module’s main drawback is its high fiber count—because each 100G link consumes eight fibers, total cabling density increases quickly. For large-scale deployments, this can become a cost factor.
QSFP28 PSM4 for Cost‑Efficient Longer Distances on Single‑Mode Fiber
The PSM4 (Parallel Single‑Mode 4‑lane) standard was created as an economical way to extend reach beyond the SR4 module’s limit. Like SR4, it transmits four parallel lanes, but each uses single‑mode fiber (SMF) at a wavelength of 1310 nm.
Key specifications:
- Reach: up to 500 m over SMF.
- Connector: MPO‑12 (same physical structure as SR4 but with single‑mode fiber).
- Wavelengths: all four lanes operate at the same 1310 nm wavelength.
- Cost-performance balance: cheaper than CWDM4 or LR4 for distances below 500 m to 2 km.
Because PSM4 still relies on parallel fiber connections—eight fibers total—it can increase fiber utilization compared to SR4 but at a higher per‑fiber cost due to SMF cabling. It fits well for data center interconnects across multiple halls or nearby buildings where extended reach is required but cost control is critical.
QSFP28 CWDM4 Balancing Reach and Fiber Efficiency
The CWDM4 (Coarse Wavelength Division Multiplexing) variant solves the cabling density issue by multiplexing four optical wavelengths (1271, 1291, 1311, and 1331 nm) onto a single pair of fibers. It maintains the same 4×25 Gbps logic as SR4 and PSM4 but reduces the physical fiber count dramatically.
Essential details:
- Reach: up to 2 km over SMF.
- Connector: duplex LC.
- Advantages: lower fiber cost than PSM4; moderate module price; compact link design.
- Applications: inter‑rack or short campus links where duplex SMF is already installed.
CWDM4 achieves a strong compromise between performance and cost, making it one of the most popular options in modern hyperscale networks. The main trade‑off is slightly higher module complexity and power consumption due to the integrated multiplexing optics.
QSFP28 LR4 for Long‑Reach Data Center Interconnects
For longer interconnects—spanning several kilometers—the LR4 (Long Reach 4‑lane) type is the standard choice. It also uses wavelength division multiplexing, transmitting four 25 Gbps channels on separate wavelengths within the 1310 nm band but designed for up to 10 km reach over single‑mode fiber.
Features and benefits:
- Reach: up to 10 km over duplex SMF.
- Connector: duplex LC.
- Compliance: IEEE 802.3ba 100GBASE‑LR4.
- Typical uses: campus networks, metro access nodes, and long data center interconnects.
LR4 modules cost more than CWDM4 or PSM4 due to tighter wavelength control and more advanced optical components. However, their 10 km span covers nearly all typical enterprise and metropolitan data center needs, ensuring reliability over longer distances.
Extended Range QSFP28 ER4 for Metro or Long‑Haul Links
When network architects need connections far beyond 10 km, the ER4 (Extended Reach) module type becomes essential. Based on similar WDM optics as LR4 but with higher optical power and receiver sensitivity, ER4 modules can drive signals up to 30 km over standard single‑mode fiber.
Relevant specifications:
- Reach: up to 30 km.
- Connector: duplex LC.
- Optical type: 4×25 Gbps WDM in the 1310 nm window, following the 100GBASE‑ER4 standard.
- Use cases: metro aggregation, telecom backbone, or dark‑fiber inter‑data‑center connections.
The extra distance capability requires more powerful lasers and better quality optics, resulting in higher price and power consumption. Therefore, ER4 modules are primarily used in specialized long‑haul or carrier environments rather than within a single campus.
Comparison Summary
| Module Type | Fiber Type | Wavelength | Max Distance | Connector | Typical Application |
| SR4 | MMF (OM3/OM4) | 850 nm | 70–100 m | MPO‑12 | Short‑reach data center links |
| PSM4 | SMF | 1310 nm × 4 lanes | 500 m | MPO‑12 | Cost‑effective medium reach |
| CWDM4 | SMF | 1271–1331 nm (4 λ) | 2 km | LC duplex | Inter‑rack or campus |
| LR4 | SMF | 1295–1310 nm (4 λ) | 10 km | LC duplex | Metro or long data center links |
| ER4 | SMF | 1295–1310 nm (4 λ, higher power) | 30 km | LC duplex | Metro‑area networks |
From the table, we can see that distance and fiber count are the primary differentiators among these modules, followed closely by wavelength management and price. SR4 and PSM4 use parallel optics, while CWDM4, LR4, and ER4 adopt wavelength multiplexing to reduce cabling complexity.
Choosing the Right Module
Selecting the ideal 100G transceiver depends on network topology, cabling infrastructure, and budget.
- SR4 suits environments where fiber runs are short and multimode infrastructure already exists.
- PSM4 provides a bridge between SR4’s short reach and CWDM4’s duplex efficiency when single‑mode fiber is available.
- CWDM4 becomes the best overall compromise for 100G up to 2 km links.
- LR4 is the standard for long data center or campus connections up to 10 km.
- ER4 targets specialized networks where spans reach 30 km or more.
Additionally, optical power budgets, cabling polarity, connector types, and future scalability (e.g., migration to 400G) should be part of the planning process.
Conclusion
The 100G QSFP28 module family provides a flexible range of solutions from short‑reach parallel multimode to long‑distance wavelength‑division single‑mode transmission. Understanding each type’s reach, fiber requirements, and cost implications helps organizations deploy high‑speed infrastructure efficiently and sustainably. As data demands continue to grow, the careful matching of module type to network design remains a cornerstone of scalable 100G Ethernet deployment.
