What is the difference between 100G QSFP28 SR4/LR4/PSM4/CWDM4/ER4?
903 2021-12-30
Ⅰ. What is a QSFP28 optical module?
The 100G QSFP28 optical module is suitable for 100G Ethernet, EDR InfiniBand and 32G Fibre Channel, and usually has exactly the same volume and panel density as 40G QSFP+. 40G QSFP+ uses 4* 10Gbps channels, and 100G QSFP28 module is similar to it, using 4 high-speed differential signal channels with data rate In addition, the QSFP28 has an upgraded electrical interface capable of supporting signals up to 28 Gbps. QSFP28 SR4, QSFP28 LR4, QSFP28 PSM4, QSFP28 CWDM4, QSFP28 ER4, compare with other package types of 100G optical modules (CFP/CFP2/CFP4), 100G QSFP28 offers higher density, lower power consumption and significant price advantages.


Ⅱ. The difference between 100G QSFP28 optical modules
After understanding the basic concept of 100G QSFP28, this section will elaborate on the differences of 100G QSFP28 modules in terms of standard, laser type, transmission medium and distance.
1、Standard of 100G QSFP28 optical module
The standards for 100G QSFP28 optical modules are mainly defined by two major organizations, IEEE and MSA, among which IEEE has defined the standards for QSFP28 SR4, QSFP28 LR4, and QSFP28 ER4.
"QSFP" indicates the package of optical module; 
"28" indicates the maximum transmission rate of 28Gbps per channel; 
"SR" indicates short distance (up to 100m), 
"LR" indicates long distance (up to 10km), 
"ER" means extended distance (up to 40km);
"4" means the module has 4 channels. 
For example, QSFP28 LR4 means that the 100G long-range module can transmit 100G signals using four wavelengths up to 10km.
100GBASE-SR4 and 100GBASE-LR4 are the two most common 100G port specifications defined by IEEE, but QSFP28 SR4 is too short to meet all connectivity requirements, and QSFP28 LR4 is too costly for large data centers. Good thing MSA brought a new solution to the medium-distance transmission market: the QSFP28 PSM4 and CWDM4 standards were born. Although QSFP28 LR4 covers the transmission distance of QSFP28 CWDM4, the cost of CWDM4 is far more affordable and competitive than LR4 for transmission over a distance of 2km.

2、100G QSFP28 laser type
Lasers play a huge role in 100G QSFP28 optical modules. there are five common types of lasers: VCSEL, FP, DFB, DML and EML. these different types of lasers have different operating wavelengths, operating modes and application environments. 
vcsel lasers have small size, low power consumption, easy integration, low cost and high coupling efficiency with multimode fiber, and are commonly used in QSFP28 SR4 modules. QSFP28 LR4 and ER4 modules are used for long-distance transmission (10km or 40km) and require lasers with large eye opening, low dispersion, high extinction ratio and long transmission distance, and EML lasers meet all the requirements perfectly. DML lasers can achieve signal modulation by modulating the laser injection current. Since the magnitude of the injection current changes the refractive index of the active region of the laser, it causes a wavelength drift and thus a dispersion. Therefore, DML is not suitable for long distance transmission and cannot achieve high speed signal modulation, but is suitable for QSFP28 PSM4 and QSFP28 CWDM4 modules with a transmission distance of 500m or 2km.
3、100G QSFP28 transmission medium and distance

The 100G QSFP28 optical modules are available with duplex LC or MTP/MPO-12 connectors, with transmission distances ranging from 70m to 40km. The transmission distance of these modules determines their application scenarios, and all QSFP28 modules can be used for direct and interconnected enterprise networks or data centers. The following table shows the differences between them in detail.

Ⅲ. How to choose 100G QSFP28 optical module?
After introducing different kinds of 100G QSFP28 modules, I believe you have a basic understanding of these modules, let‘s talk about how to choose 100G QSFP28 modules.
1. When the transmission distance is 5m-100m, you are recommended to choose QSFP28 SR4 optical module, which can transmit 70m on OM3 fiber and 100m on OM4 fiber.

2. When the transmission distance is 100m-2km, you can choose QSFP28 PSM4 or QSFP28 CWDM4 modules. CWDM4 is generally more expensive than PSM4, but CWDM4 only needs 2 single-mode fibers for bidirectional transmission, while PSM4 needs 8 fibers, so the overall cost may be more expensive with QSFP28 PSM4. Therefore, in practical applications, the specific transmission distance needs to be taken into account before deciding which module is appropriate to choose.

3. When the transmission distance is 10km-40km, choose either QSFP28 LR4 or QSFP28 ER4, the former can transmit up to 10km, the latter can transmit up to 40km.

Conclusion
Do you know how to choose them?