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100G DWDM Solution: PAM4 or Coherent?

May 27, 2019

With the increasing demand for high-speed network applications such as 5G, cloud computing, and storage networks, optical transmission networks are also evolving rapidly. And 100G DWDM solutions have gradually improved and matured. Direct modulation and coherent technology can easily extend the transmission distance up to 80km or even 1000km for metro and DWDM networks. So, what is the difference between these two technologies? Which one is more suitable for 100G DWDM transmission? I believe you will find the answer after reading this article.

What's the 100G DWDM Solution?

100G DWDM solution provides the most straightforward method to connect 100G traffic over long-haul applications. And PAM4 and coherent technologies offer cost-optimized solutions in WDM and ultra-long distances transmission over 80km. So what's the difference between these two technologies? Which one is better for the 100G DWDM solution? Find the answers in the following descriptions.

100G DWDM Solution: What Are 100G PAM4 and Coherent Technology?

QSFP28 PAM4 is a technology encoding two bits of data with four distinct signal levels, and it doubles the signal bandwidth of a connection with each signal level representing 2 bits of logic information. It plays a key role in multi-order modulation and makes PAM4 the most efficient and cost-effective enabler of 100G DWDM solutions. 100G PAM4 QSFP28 transceivers adopt PAM4 modulation technology to achieve high-speed signal transmission and the construction of 100G in large data centers, campuses, and enterprise networks.
100G PAM4 Signaling Technology

Figure 1: 100G PAM4 Signaling Technology

Coherent technology uses amplitude, phase, and polarization of light to focus more data on the wave that is being transmitted in a 100G DWDM solution. Thus, much higher bitrates can be achieved on a single wavelength by the adoption of DSP (Digital Signal Processor). CFP DCO (Digital Coherent Optic), CFP2 coherent ACO (Analog Coherent Optic), CFP2 coherent DCO are the common types of transceivers that use coherent technology to achieve 100G transmission, making them ideal for DWDM data center interconnection and metro network.
Coherent Optics Working Principle in DWDM Solution

Figure 2: Coherent Optics Working Principle in DWDM Solution

100G DWDM Solution: Advantages & Disadvantages of 100G PAM4 and Coherent Optics

The following chart describes the advantages and disadvantages of QSFP28 PAM4 and coherent modules in 100G DWDM solution in detail.

Solution Advantages Disadvantages
100G PAM4 QSFP28 Simple and cost-efficient solution with higher bandwidth and lower consumption. Additional amplifiers, DWDM multiplexers, and dispersion compensators are required for transmission over 5km.
Can be used directly in the switch of an embedded DWDM network. A higher Signal-to-noise ratio (SNR) will be produced.
Coherent Optics Built-in DSP chip and electronic dispersion compensation (EDC) to improve amplification and receiving sensitivity. Higher power consumption and more expensive for the project.
Ideal for long-haul distance transmission with a reach of 1000km. Two DSPs are required from the same vendor at both ends of the link. In some cases, the line cards must be identical.

Which Is Better for 100G DWDM Solution: 100G PAM4 or Coherent Optics?

100G PAM4 are ideal for distances between 5-80km in large data centers, 100G DWDM networks, campus, and enterprise networks (although amplifier and dispersion compensation modules are required). A 10G/100G DWDM 80km hybrid transmission for data center interconnection has been stated in the picture below with the adoption of FS QSFP28 PAM4 modules.
100G PAM4 Transceiver for Data Center Interconnection

Figure 1: 100G PAM4 Transceiver for Data Center Interconnection

100G coherent optics are ideal for metro carriers and regional/long-haul transmission applications (80km-1000km). The figure below shows the application of FS 100G CFP2-DCO transceivers for 200G DWDM network, with a fiber link loss of 25dB, achieving the interconnection of lineside backbone DWDM data centers between 100km.
100G Coherent CFP2-DCO Transceiver for 200G DWDM network

Figure 2: 100G Coherent CFP2-DCO Transceiver for 200G DWDM network

Conclusion

After reading this article, do you have a better understanding of the 100G DWDM solution? 100G PAM4 is more economical and convenient, but the transmission distance is shorter. The coherent module supports long transmission distance but has limitations on the equipment. The specific choice needs to refer to your actual application requirements.

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