The rise of AI requires new computing models and enhanced methods of data transfer, creating an ever-increasing need for innovative, high-performance, low-latency connectivity solutions. PCIe over Optical is poised to play a key role in enabling the growth of AI. Here we explore some of the intricacies of PCIe over Optical and explore its implementation, challenges, and possibilities.
Driving force
The data center landscape is in the midst of a transformation. AI models like ChatGPT are rapidly scaling from 1.5 billion parameters (ChatGPT 2.0) to 175 billion parameters (ChatGPT 4.0). The next generation of generative AI (genAI) platforms is expected to use around 100 trillion parameters, which (to add context) is roughly the number of synapses in the human brain. And this requires massive data processing power.
The relentless rise in data consumption is creating exponential demand on data center networks, with global data generation expected to exceed 180 zettabytes by next year, a 50% increase from 2023 and a 200% increase from 2020.
Figure 1: Projected growth of AI parameters and global data creation.
This rapid increase in data volumes highlights the need for robust connectivity solutions that can handle high-speed data transfers with minimal latency.
Distributed computing is becoming a key model for achieving this, where memory and storage are shared in a centralized pool, improving efficiency and capacity. This model relies on low-latency connectivity solutions. As distances increase and speeds increase, Linear Pluggable Optics (LPO) becomes a potential solution for distribution. Where direct attached cables cannot meet reach demands, LPO supports longer reach with minimal impact on latency and power.
And unlike traditional fully retimed optical transceivers, LPOs eliminate clock data recovery (CDR) and digital signal processing (DSP) components, resulting in lower power consumption and latency compared to typical optical pluggable transceivers.
However, this simplification requires highly sophisticated SerDes technology to ensure interoperability and efficient data transfer.
Interoperability and Implementation
No technology is successful in isolation, and one of the major obstacles to implementing LPO is interoperability. The ecosystem of support is growing.
The Optical Internetworking Forum (OIF) has been working on CEI-112G-Linear since 2020, and a 2024 demonstration at OFC 2024 showed that multi-vendor LPO configurations are possible and that this work can be used with PCIe to increase bandwidth while reducing power, latency, and cost.
The optical budget for CEI-112G-Linear has already been established in the IEEE 802.3 specification, which defines the electrical channels and interfaces that allow the link to be closed without the use of a DSP, demonstrating that interoperability is possible. In fact, at the OFC 2024 event, Alphawave participated with Broadcom and 12 module vendors in LPO’s largest interoperability event, demonstrating compatibility.
Separately, Alphawave has worked with multiple optical vendors to consistently achieve BERs below 1×10 using the PCIe 6.0 data to drive optical systems using Alphawave PipeCORE PCIe 6.0 Subsystem IP for both PCIe and CXL running on evaluation boards.-9That’s at least a triple-digit performance margin.
And momentum is building with additional optical demonstrations driven by Alphawave and its ecosystem partners, including InnoLight, Tektronix, TE Connectivity, Ayar Labs, Amphenole, Nubis, and others. These demonstrations took place at DesignCon, OFC, CXL DevCon, and PCIe DevCon, raising excitement for upcoming applications using PCIe and CXL over Optical.
Figure 2: The Alphawave evaluation board drives 64Gbps into the Nubis optical engine, demonstrating less than 15dB total electrical loss at Nyquist and a BER of less than 1×10.-9.
However, several complex issues remain: nonlinearities in optical components complicate transmitter construction, as well as part-to-part variations and the ability to deploy these solutions at scale.
Conclusion
By leveraging technologies such as LPO and advanced SerDes, PCIe over Optical offers a path to higher bandwidth, lower power, and lower latency data transfer. Although there are challenges to address, such as achieving interoperability and managing signal integrity, the potential benefits far outweigh these hurdles.
As the data environment continues to evolve, the role of PCIe over Optical to enable efficient, high-speed data transfer will become increasingly important. At PCIe 7.0 and above speeds, standard copper connections are unlikely to work rack-to-rack, making re-timers or LPOs the only realistic options to support the industry. The ecosystem is building. With continued advancements and real-world demonstrations validating these hybrid capabilities, PCIe and CXL operating over optical channels is poised to revolutionize the data center of the future.
