With the changing application scenarios, the demands for data center networks from users are constantly evolving. In long-distance wavelength division applications, users emphasize performance and aim to achieve longer transmission distances and higher spectral efficiency. In short-distance applications within data centers, users are more concerned about cost, taking into account the given distance, size, and power consumption.
Metropolitan Integrated Service Network Architecture Diagram
Gordon Moore, the co-founder of Intel, proposed Moore's Law, which predicts that the number of transistors on an integrated circuit doubles every 18 months, supporting the semiconductor industry for over half a century. In the field of optoelectronics, there is also the "Optical Moore's Law," where short-distance optical transceivers evolve approximately every four years, resulting in a halving of the cost per bit and power consumption.
"Optical Moore's Law" in Data Center Networking
To achieve higher data rates, optical transceivers generally employ three approaches: increasing the speed of optical devices (higher baud rates), increasing the number of channels (multi-lane), and adopting advanced modulation techniques to reduce the transmission cost per bit, meeting the requirements of the "Optical Moore's Law."
Technological Path for Increasing Optical Transceiver Speed
Adoption of PAM4 Advanced Modulation Technique
PAM4 (Pulse Amplitude Modulation 4) technology is an inherently more efficient modulation technique that effectively improves bandwidth utilization. PAM signals are a popular signal transmission technology following NRZ (Non-Return-to-Zero) encoding.
NRZ signals use two signal levels, high and low, to represent digital logic signals 1 and 0, respectively. Each clock cycle can transmit 1 bit of logical information. PAM4 signals, on the other hand, use four different signal levels for signal transmission, allowing 2 bits of logical information to be transmitted per clock cycle, namely 00, 01, 10, and 11. Therefore, under the same baud rate conditions, PAM4 signals have twice the bit rate of NRZ signals, doubling the transmission efficiency and effectively reducing costs.
The 400G transceivers will fully adopt PAM4 technology to enhance transmission efficiency and reduce transceiver costs.
Increasing the Number of Channels (Multi-lane)
Historical data has shown that solutions with more than 8 lanes (such as x10, x16) can bring about challenges in channel yield and reliability, making it difficult for them to become mainstream solutions. A cost- and power-efficient multi-lane architecture is typically based on x4 or x8 configurations.
Among them, the 100G CWDM4 and 100G SR4 transceivers based on the 4x25G architecture have become the mainstream solutions for the previous generation of DC optical interconnects.
Higher Baud Rate Optoelectronic Chips
DC 100G optical transceivers, based on the 25Gbaud optoelectronic chip industry chain (DML, VCSEL), employ NRZ signaling and have achieved commercial success based on a 4-channel architecture. Various 25Gbaud optoelectronic chips (DML, EML, VCSEL) are evolving towards higher baud rates of 56Gbaud. The 56Gbaud EML industry chain is already available, while the 56Gbaud DML and VCSEL are still in the research stage.
Chip Requirements in the Data Center
TOR to Leaf(100m)
4CH: 25Gbaud VCSEL+NRZ
8CH: 25Gbaud VCSEL+PAM4
Leaf to Spine (500m)
4CH: 25Gbaud SiP/DML+NRZ
4CH: 56Gbaud EML/SiP
Leaf to Spine(2km)
4CH: 25Gbaud DML+NRZ
4CH: 56Gbaud EML/SiP+
As the scale and complexity of data centers continue to grow, finding low-cost and low-power optical interconnect solutions becomes crucial. The ongoing evolution of the Optical Moore's Law provides data center networks with higher bandwidth and improved efficiency. By adopting PAM4 modulation, increasing the number of channels, and utilizing higher baud rate optoelectronic chips, data centers can achieve faster and more reliable data transmission, meeting the ever-growing demands of data processing.
In the future, with continuous technological innovation and development, we can expect the emergence of more innovative optical interconnect solutions, bringing higher performance and lower total ownership costs to data center networks.