1. Introduction
To better meet the growing demands for bandwidth in data centers, fiber optic communication is developing towards higher speed. In front of these requirements, QSFP28 performs well, filling the gap from 40Gbps to 100Gbps. Meanwhile, it also lays a foundation for the development of subsequent 400Gbps (QSFP-DD) optical modules, being an indispensable component in high-speed data communication networks.
2. An Overview of the QSFP28 optical module
(1) Definition
The QSFP28 (Quad Small Form-factor Pluggable 28) is a compact, hot-pluggable optical module, with each channel a data speed of 25Gbps and a total speed of 100Gbps. Therefore, the QSFP28 optical module is also called 100G optical module.
(2) Working Principle
Through electro-optical conversion, optical module facilitates data transmission in optical fibers. On this basis, with a combination of modulation technologies of NRZ and PAM4, QSFP28 ensures an efficient data transmission at different speeds, and then enhances the efficiency and speed in total. Additionally, the Digital Diagnostics Monitoring Interface (DDM/MDI) in QSFP28 monitors module’s information in real time, including temperature, voltage, etc. The tool helps network administrators timely detect the module, assuring network stability and reliability.
(3) Compatible Fiber Types
The QSFP28 modules are compatible with single-mode fiber (SMF) and multi-mode fiber (MMF). For SMF, the module usually uses LC connector, suitable for long-distance fiber optic connection, like city-to-city connections. Furthermore, compatible MMF includes OM3 and OM4 standards, while OM3 can transmit most for 100 meters and OM4 for 150 meters.
(4) Support IEEE Standards
| 802.3bj | Defines the electrical characters of 100Gbps Ethernet transmission, used in copper cable connections. |
| 802.3bm | Standardizes the optical interface of 100Gbps Ethernet. |
| 802.3cd | Specifies higher-speed Ethernet protocols, supporting transmission at 400Gbps. |
3. Backward Compatibility of QSFP28: Can QSFP28 Ports Be Used for 10G/25G/40G Data Transmission?
QSFP28 supports a transmission speed of 100Gbps. Moreover, its design also considers the compatibility issues with older optical modules, ensuring a high degree of backward compatibility.
(1) Physical Interface
The slot and connector design of QSFP28 are similar to QSFP+, SFP28, SFP+, thus allowing it to be compatible with existing lower-speed devices.
(2) Speed Downgrading
QSFP28 supports for automatic speed downgrading, adjusting its speed regarded to the requirements of connection devices. For example, take 100Gbps QSFP28 module as an example, it can be downgraded to 40Gbps (QSFP+), 25Gbps (SFP28), 10Gbps (SFP+). The speed downgrading enables a stable transmission performance at a lower speed. Specific downgrading methods can be referred in this article.
（3）Supported Protocols
QSFP28 optical modules support for various communication protocols, including InfiniBand, Ethernet, Guaranteeing its compatibility with different network architectures. These protocols make it suitable for diverse application scenarios, such as data centers, cloud computing and high performance computing (HPC).
The backward compatibility allows QSFP28 to own high flexibility in the process of high-speed upgrading. Not only meets the needs of new technology, but also supports the usage of older devices and lower-speed modules.
4. Advantages of QSFP28
High Bandwidth. QSFP28 supports a data transmission speed of 100Gbps, catering the requirements of high-speed data transmission in data centers and cloud computing.
Low Power Consumption. Through optimized circuit design and high-efficiency components, the module achieved low power consumption of 3.5W to 4W. It lowers energy consumption and reduces cooling costs in data centers.
Compact Size. QSFP28 is designed in a compact size, ideal for high-density network applications. It can also offer more bandwidth in a limited space.
Hardened Reliability. The Digital Diagnostics Monitoring Interface (DDM/MDI) ensures QSFP28 a reliable operation in complex environments, then reducing operational and maintenance costs.
5. Applications of QSFP28
Data Centers. Features of high density and high efficiency make QSFP28 a wide use in data centers.
Cloud Computing and Big Data. Meets the needs of large-scale data storage and processing, and also helps improve the performance and response speed in cloud service.
High Performance Computing. The QSFP28 module provides high-bandwidth and low-latency network connections for super-computing and research network, so ensuring working tasks smoothly proceed.
5G Communication. Supports high-speed data transmission between 5G base stations and core networks. Its high bandwidth and low power consumption guarantee it a nice performance in 5G data transmission.
Enterprise Networks. By offering higher bandwidth and stronger connectivity, QSFP28 provides an efficient and stable network environment for enterprises.
6. SR4 vs LR4 vs SRBD vs PSM4 vs CWDM4 vs SWDM4: How to Choose an Optical QSFP28 module?
The QSFP28 series includes SR4, LR4, SRBD, PSM4, CWDM4, SWDM4, etc. Below is their information.
| QSFP28 | Maximum Transmission Distance | Fiber Types | Applicable Scenarios | Advantages |
| SR4 | 100 meters（OM4） | MMF | Short-distance transmission, such as within data centers. | High bandwidth, low cost. |
| LR4 | 10 miles（SMF） | SMF | Long-distance transmission, such as telecoms and metropolitan area networks. | Long-distance transmission. |
| SRBD | 300 meters（OM3） | MMF | Suitable for short-distance applications requiring fiber source conservation, like data centers and storage networks. | Single-fiber bidirectional, high bandwidth, high-density connection. |
| PSM4 | 2 miles（SMF） | SMF | Shorter long-distance transmission, data centers, super-computing, etc. | High bandwidth, low power consumption, moderate transmission distance. |
| CWDM4 | 2 miles（SMF） | SMF | Long-distance and requiring WDM, like inter-data center links. | Long distance, supports WDM, efficient and high density. |
| SWDM4 | 100 meters（OM4） | MMF | Medium to short-distance high-speed transmission requiring WDM, such as cloud computing. | Medium to short-distance, supports WDM, high bandwidth. |
(1) Transmission Distance
Short distance connection, like within data centers. SR4, SRBD and SWDM4 are ideal choices.
Long distance transmission, such as metropolitan area networks, across data centers. LR4, PSM4 and CWDM4 are more suitable, especially LR4 for distances up to 10 kilometers.
(2) Fiber Types
SMF: LR4, PSM4 and CWDM4.
MMF: SR4, SRBD, SWDM4.
(3) Fiber Resources Conservation
Bidirectional data can be transmitted on an optical fiber, in SRBD modules, saving optical fiber resources.
(4) Network Density and Cost
SR4, SWDM4 have lower costs, suitable for high-density use.
LR4, CWDM4 are higher-cost, better for high-bandwidth and long-distance network environments.
7. Summary
QSFP28 enables users to combine existing devices and new technologies, then upgrade to higher-bandwidth network, without large-scale equipment replacement. It reduces network costs, and enhances the scalability of networks. Further more, QSFP28 plays a crucial role in modern networks, fulfilling the demands of high-speed data transmission.
