An overview of 400G Optical Transceiver
?Optical transceivers are key devices to realize optical network interconnection in data center. As the number and density of ports goes on, data centers have to take the cost of optical modules into consideration since it is expected to account for nearly half of the cost of optical networks in IDC. At present, the newly-built data centers of major Internet companies have generally applied 100G interconnection technology, making 400G Ethernet likely to be commercialized on a large scale in the next 1-2 years. This article will discuss about the classification, technical features of earlier and current 400G optics as well as its development forecast.
1. Classifications of 400G Optical Module
A few methods can help you classify the different types of 400G optical transceivers. By the wavelength, 400G optical modules include the solutions based on Multi-mode Cable(MM) and Single-mode Cable(SM). In terms of signal modulation methods, they are divided into NRZ and PAM4 modulation transceivers(Most of the current 400G modules apply PAM4 technique). Besides, you may be familiar with 400G SR, DR, FR and LR, which is classified according to the modules’ transmission distance. According to different form factor, there are 400G CDFP optical modules,400G CFP8, 400G OSFP and 400G QSFP-DD for high port-density data center switches etc. The figure below highlights the technical indicators of different 400G optical modules.
? Technical Indicators for Different 400G Optical Modules
2. Features of Earlier 400G Optical Modules
400G optical modules at its inception adopted 16-channel 25Gbps NRZ modulation technology and used CDFP or CFP8 form factor to support the 400G Ethernet data connection among switches and routers in data center, for example, 400G-SR16. The advantage is that it can capitalize on the mature 25G NRZ technology on 100G optical modules. However, such 400G optical modules require16 channels for parallel signal transmission, and consume more power during operation. Also, the 400G CDFP or CFP8 modules are large in size, making them hardly meet the demand for data center applications.
3.Features of Current 400G Optical Modules
In the current 400G optical module, 8 channels of 53Gbps PAM4 (400G-SR8, FR8, LR8) or 4 channels of 106Gbps PAM4 (such as 400GR4, FR4, LR4) are used to achieve 400G signal transmission on the optical port; on the electrical port, 8 channels of 53Gbps PAM4 electrical signals is transmitted. These modules are usually in OSFP or QSFP-DD form factor packaging. But those in OSFP and QSFP-DD form factor also support 8-channel electrical signal interface.
In comparison, the QSFP-DD package size is smaller (similar to the QSFP28 package of the conventional 100G optical module), making it a better 400G Ethernet connection alternative for data center applications; the 400G OSFP is slightly larger in size, because it requires more power consumption and it is more suitable for telecommunications application.
4. 400G-FR8/LR8
400G FR8/LR8 optical transceivers transmit PAM4 signal over 8 channels of 53Gbps on the electrical port. There are some differences in the optical interface. As for 400G SR8/FR8/LR8 modules, their electrical and optical port function similarly; both are 8 channels of 53Gbps PAM4 signals by the CDR technology designed inside the module to achieve the electrical to optical or optical to electrical signal conversion. The following diagram depicts how a 400G-FR8/LR8 optical transceiver is working.
? Diagram of 400G FR8/LR8 Optical Module Based on PAM4 Modulation
5. 400G-FR4
400G DR4/FR4/LR4 modules are designed with built-in Gearbox chips that multiplex the two lines’ electrical inputs signal into one-line outputs signal and modulates it to the light. Therefore, the data rate on the optical port is twice that of the electrical port side, that is, 4 channels of 106Gbps PAM4 signals. Below is a diagram of 400G FR4 with duplex fiber.
? Diagram of 400G FR4, Duplex Fiber Optical Transceiver
6. A Brief Overview of 400G Interface Standard
400GE interfaces including 400G Base DR4, 400G Base FR8 and 400G Base LR8 is standardized in IEEE 802.3bs. In addition, due to the cost advantage of multimode fiber in short-distance transmission, IEEE established the 802.3cm working group in March 2018. The technology for multimode fiber to achieve 400G transmission by using 8 (SR8) or even 4 (SR4.2) channels previously discussed in the industry is included in the specification discussion. At present, multimode 400GE interface including 400GBASE-SR8 and 400GBASE-SR4.2 is specified in IEEE 802.3cm standard.
7. Developing Trend of 400G Transceiver Application
The technology of 400G optical modules is attracting the attention in the industry. It is expected that the way to achieve 400G data transmission with 4 optical signal channels may become the mainstream solution for 400G Ethernet in the future. Meanwhile, the electrical interface of the 400G optical module may also be gradually upgraded to the form of 4-channel 106Gbps PAM4 in order to save the Gearbox chip to save power consumption and cost. For ultra-long-distance transmission (>80km), 400G transceivers will apply coherent communication + DWDM method to implement the 400G data connection. 400G-ZR being developed by the OIF organization is a typical example for such interconnecting optical networks.
Conclusion
400G optical transceivers based on NRZ in the past and currently PAM4 modulation technique are the key devices for 400G optical network interconnection in data center. 400G-FR8/LR8 and 400G FR4 exemplify the PAM4 technology in 400G application. Continuous increase in bandwidth density and maturity of key technologies for volume deployment is driving 400G Ethernet to take shape in the cloud data center.
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