First, understand the technical explanation of 400G-ZR/ZR+

When 400G-ZR/ZR+ first came out, several the Company jointly announced the results of their verification of how to achieve 400G transmission at "JANOG50" held in Hakodate. We provided a technical explanation of the specifications and block diagrams of 400G-ZR/ZR+. The goal of this seminar was to provide a deeper understanding of the content announced in Hakodate.

  

Electrical and optical data rates

Before introducing digital coherent optics, we have used the block diagram of 400G-ZR/ZR+ to explain why the data rate of electrical signals has increased to 425Gbps while still using 400Gbps communication, and why the data rate of optical signals has increased to 478.75Gbps.

Modulation method (transmitter)

The most basic modulation method for transmitting digital data on an optical signal is called NRZ (Non Return to Zero), which puts the digital signal directly on the optical amplitude. This method is used in a variety of products. The more complex PAM4 (Pulse Amplitude Modulation, 4-level) also puts the digital signal on the optical amplitude, but in a format where two bits are mapped to one signal level. This means that twice the data capacity can be sent even with the same signal repetition period.

The modulation method used in 400G-ZR/ZR+ is called 16QAM (16 Quadrature Amplitude Modulation), which superimposes the digital signal not only on the amplitude of the light but also on the phase. This modulation method is suitable for large-volume communications that send large amounts of data, as it can increase the bit rate while keeping the baud rate low.

Using multiple diagrams, we explained how modulated signals are created and what I and Q signals are. We also explained "polarized multiplexing technology," a mechanism that allows communication at twice the capacity at one time.

Modulation method (receiving side)

In the explanation on the receiving side, we provided a detailed explanation of how the received optical signal is processed and converted into an electrical signal, as well as how the receiving sensitivity is improved by interfering with local light (coherent detection).

Where do I place the FEC?

In communications over 400G, RS-FEC is always applied on the host side. Therefore, the electrical signal transmitted by the optical transceiver from the host side is also subjected to RS-FEC. However, in the case of digital coherent optics such as 400G-ZR/ZR+, the host's RS-FEC is temporarily terminated and a separate optical-specific FEC (C-FEC for 400G ZR, O-FEC for 400G ZR+) is applied to apply a stronger FEC. This is thought to contribute to longer distance transmission.