How to Choose the 100G QSFP28 Optical Transceivers?

NADDOD Edward Jan 17, 2023

As people’s demand for bandwidth is getting higher and higher, 100G networks have developed rapidly. The 100G optical module is an important part of the 100G network. There are multiple standards and packaging types. Each optical module has its own merits. You can choose the appropriate optical module according to your actual needs.

100G Optical Module Standard
There are two key organizations for the definition of optical modules, namely IEEE and MSA (MultiSource Agreement, multi-source agreement), which complement each other and learn from each other.

IEEE is the Institute of Electrical and Electronics Engineers, and 802.3 is a working group under IEEE. Many 10G, 40G, 100G, and 400G optical module standards are proposed by the IEEE802.3 working group.

MSA is a multi-vendor specification. Compared with IEEE, it is a non-official non-official organization. Different MSA protocols will be formed for different optical module standards. It can be understood that it is the behavior of enterprise alliances in the industry. In addition to defining the structural package of optical modules (including dimensions, electrical connectors, pin assignments, etc.), MSA also defines electrical interfaces and optical interfaces, thereby forming a complete optical module standard.

IEEE and MSA have formulated multiple standards for 100G optical transceivers, the mainstream of which are the following 6 standards.

100G Optical Module

100G Short-distance Module Selection: 100GBASE-SR4 Becomes the Mainstream
The 100G short-distance optical module standards mainly include 100GBASE-SR10 and 100GBASE-SR4. The 100GBASE-SR10 standard uses 10 x 10Gbps parallel channels to achieve 100Gbps point-to-point transmission. The rate of electrical signals is 10G, and the rate of optical signals is also 10G. It adopts NRZ modulation and 64B/66B encoding.

Connection 1

As the electrical interface rate of the switch ASIC chip increases from 10G bps to 25G bps, the electrical interface standard is upgraded from CAUI-10 (10 channels x 10Gbps) to CAUI-4 (4 channels x 25Gbps), and the channels are reduced from SR10 parallel 10 channels to With 4 channels in parallel, the number of components of the optical module can be reduced, the cost can be reduced, the size of the module can be reduced, and the power consumption can be reduced.

Connection 2

The reduction in the size of the optical module enables the switch to provide a higher density of 100G interfaces per 1U of space. Based on the above advantages, 100GBASE-SR4 has replaced 100GBASE-SR10 as the current mainstream 100G short-distance optical module standard.

100G medium and long-distance module selection: LR4, CWDM4 and PSM4 each have their own advantages and disadvantages

1. With the 100GBASE-LR4 standard, why do we need to formulate the 100G CWDM4 standard?

For the internal interconnection scenarios of large data centers, the distance supported by 100GBASE-SR4 is too short to meet all interconnection requirements, while the cost of 100GBASE-LR4 is too high. Therefore, MSA has brought a mid-distance interconnection solution to the market, and PSM4 and CWDM4 are the products of this revolution.

Although the transmission distance of 100GBASE-LR4 completely covers CWDM4, in the scenario of 2km transmission, the cost of CWDM4 solution is lower and more competitive.
The working principle of 100GBASE-LR4 is shown in the figure above. The working principle of 100G CWDM4 is shown in the figure below.

Connetion 3

The working principle of 100GBASE-LR4 is shown in the figure above. The working principle of 100G CWDM4 is shown in the figure below.

Connection 4

The working principles of LR4 and CWDM4 are similar. They both use optical devices MUX and DEMUX to multiplex 4 parallel 25G channels into a 100G optical fiber link. However, there are several differences between the two

The optical MUX/DEMUX device used by LR4 is more expensive: CWDM4 defines a CWDM interval of 20nm, because the wavelength temperature drift characteristic of the laser is about 0.08nm/°C, and the wavelength change in the working range of 0~70°C is about 5.6nm , the channel itself should also leave some isolation strips. LR4 defines the LAN-WDM interval of 4.5nm. The larger the channel spacing, the lower the requirements for optical MUX/DEMUX devices, which can save costs.

The laser used by LR4 is more expensive and consumes more power: CWDM4 uses DML (Direct Modulated Laser, directly modulated laser), while LR4 uses EML (Electro-absorption Modulated Laser, electro-absorption modulated laser). DML is a single laser, and EML is two devices, one is DML, the other is EAM modulator. Together they are called EML.

LR4 needs an additional TEC (Thermo Electric Cooler semiconductor thermoelectric cooler): because the interval between adjacent channels of LR4 is only 4.5nm) The laser needs to be placed on the TEC for temperature control. The TEC Driver chip needs to be placed on the circuit, and the Laser must also be integrated into the TEC material. In this way, compared with CWDM4, the cost of LR4 has increased.

Based on the above three points, the 100G CWDM4 standard well complements the gap caused by the high cost of 100GBASE-LR4 within 2km.

2. Compared with 100G CWDM4, what are the advantages and disadvantages of 100G PSM4?

The 100G PSM4 specification defines a point-to-point 100Gbps link of 8 single-mode fibers (4 transmit and 4 receive), with each channel transmitting at a rate of 25Gbps. Each signal direction uses four independent channels of the same wavelength. Therefore, two transceivers usually communicate through an 8-fiber MTP/MPO single-mode patch cord. The transmission distance of PSM4 is 500 meters, and 100G PSM4 produced by some manufacturers can transmit 2km (Nadod 100G PSM4 optical module supports 2km transmission distance).

Although the PSM4 optical module runs on single-mode fiber, its wiring structure is the same as that of the QSFP28 SR4 optical module. Without changing the existing wiring structure, the use of QSFP28 PSM4 optical modules can save conversion costs between multi-mode and single-mode.


Due to the use of wavelength division multiplexers, the cost of CWDM4 optical modules is higher than that of PSM4 optical modules. However, CWDM4 only needs two single-mode fibers for sending and receiving bidirectionally, which is far less than the eight single-mode fibers of PSM4. , the total cost of the PSM4 program rises very quickly. In practical applications, it is necessary to decide whether to use PSM4 or CWDM4 based on the interconnection distance.

Link Distance


In the 100G data center, the following standards can be referred to for the selection of 100G optical modules:

selection of 100G optical modules

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