PAM4 and Coherent Technology in 100G DWDM Optical Modules

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100G transmission in data centers is possible by using standard optical modules such as CFP and QSFP28. Although they are very suitable for transmitting 100G traffic within racks and data centers, it becomes a problem when 100G traffic needs to be transmitted over long distances, such as long-distance connections between different data centers (e.g., more than 40km), which requires 100G DWDM optical modules.

DWDM technology is not new in the industry, as DWDM SFP transceivers are widely used to expand 10G network capacity. In terms of functionality, 100G DWDM is very similar to its predecessor. However, in 100G networks it is usually used to achieve longer distance transmission.

PAM4 and coherent are two industry-leading solutions that provide greater bandwidth and transmission distance. When comparing 100G DWDM PAM4 vs. coherent optical modules, it depends on which features the network requires and benefits from. In this article, we will analyze these two options to help enterprises make an informed decision.

PAM4 optical module

Pulse Amplitude Modulation (or PAM4 optics) was created in response to the need for optical modules for short-reach links. It uses four different pulse amplitudes to convey information. Each amplitude has two bits, which doubles the data rate and makes PAM4 twice as bandwidth efficient as the traditional binary model.

Advantages of PAM4

Prior to PAM4, the binary NRZ modulation format was used for 100G long-haul and 40G data transmission networks. PAM4 comes with four different code patterns for encoding two data bits, so with this modulation format, the bandwidth of the connection can be doubled. Today, single-wavelength PAM4 modulation technology is being used because it is considered the most efficient and cost-effective enabler for 100G data transmission.

PAM4 optical modules can be directly used in switches of embedded DWDM networks. This is a cost-effective and simple solution for customers willing to build embedded DWDM data networks.

Disadvantages of PAM4

One of its disadvantages is that PAM4 optical modules require amplifiers for transmission over 5km. In this case, a separate DWDM multiplexer equipped with dispersion compensation and an amplifier is required to connect the data center. In addition, if the PAM4 optical module is used with an existing DWDM network, DCM and amplifiers must be prepared in advance to avoid problems in the future.

Another disadvantage is that PAM4 is susceptible to noise interference. Its additional voltage levels require reduced level spacing, resulting in the need for a higher signal-to-noise ratio, which is why PAM4 works best in short-distance optical systems.

Case 1: Applications below 5 km

Case 2: Applications over 5 km

Coherent optical module

Coherent optical communication is optical transmission that uses local oscillator light for coherent detection, and it uses a digital signal processor (DSP). It has the ability to reach or even exceed a transmission rate of 100 Gigabits, requiring only a pair of optical fibers to transmit terabits of data, and is usually used in long-distance applications.

Related advantages

The main advantage of coherent optical modules is the built-in DSP chip and electronic dispersion compensation (EDC), which PAM4 does not have. This chip does not require separate DSP and dispersion compensation modules, but uses EDC to increase the amplification rate.

Coherent optics can transmit up to 1000km, without the need for third-party systems for long-distance support. Using techniques from radio communications, coherent optics can improve receiver sensitivity while maintaining selective tuning, which allows channels to be spaced close but still separated.

Related Disadvantages

Although CFP and CFP2 digital coherent optics work quickly and efficiently, they use more power and cost more than other forms factors. These drawbacks can affect total operating costs, creating challenges for enterprises that need quality 100G DWDM pluggable optical modules within an economical budget.

Another disadvantage of coherent optical modules is that at both ends of the link, two DSPs from the same vendor will be required. Different DSPs cannot work together, and in some cases, the line cards must also be identical.

Summarize

In summary, 100G PAM4 optical modules can be selected within 80km, and 100G coherent optical modules can be selected for more than 80km. Data centers can also use optical modules in any combination according to their specific requirements. The popular DWDM technology has been extended to 200G rate, which helps to keep the complexity of data communication networks and reduce costs. Compared with the 10G services below that passive multiplexers enable multi-channel CWDM/DWDM networks, DWDM will become the main multiplexer for 100G systems.