Linear Drive Optics: The Future of High-Speed Optical Connectivity

Introduction

One of the hottest topics at the recent OFC 2023 conference was linear drive optics, a new technology that promises to revolutionize high-speed optical connectivity in data centers. Linear drive optics, also known as Linear Pluggable Optics (LPO), is an emerging solution that addresses the ever-increasing demand for higher bandwidth and lower power consumption in data center networks.

What is Linear Drive Optics?

Linear drive optics is a concept that aims to reduce power consumption by eliminating one of the two digital signal processors (DSPs) typically found in optical transceivers. The idea is illustrated in Figure 1, which compares linear drive pluggable transceivers with co-packaged optics (CPO).

Figure 1: Concept of Linear Drive Optics

In both cases, one of the two DSPs is removed, resulting in reduced power consumption. However, while CPO moves the optics closer to the ASIC, linear drive pluggable optics remain on the front panel of the switch, separated by up to 12 inches of copper traces on a printed circuit board (PCB).

The key enabler for linear drive optics is the development of advanced 100G SerDes (Serializer/Deserializer) by companies like Broadcom and Nvidia. These SerDes are designed to preserve signal integrity across 12 inches of copper traces and directly drive the pluggable optics without any additional re-timers or DSPs.

Power Consumption Advantages

The driving force behind the development of linear drive optics is the need to reduce power consumption in data center networks. As shown in Figure 2, both CPO and linear drive pluggable transceivers offer substantial reductions in power consumption compared to standard retimed pluggable transceivers.

Figure 2: Improvements in power consumption of CPO and linear drive pluggables

Since both CPO and linear drive technology use the same approach to reduce power consumption, the results are very similar. Linear drive pluggables require a bit more electronics than CPO, but both designs eliminate power-hungry DSP re-timers.

Market Adoption Challenges

Despite the promising advantages of linear drive optics, market adoption faces several challenges. Incumbent solutions, such as retimed pluggable transceivers, have the advantage of established standards and a supporting ecosystem, which are critical for large customers to ensure scalability of supply and continuing cost reductions.

Table 1 summarizes the pros and cons of linear drive pluggables and CPO, as presented by Janet Chen of Meta.

	Retimed Pluggable	Linear-Drive Pluggable	Linear-Drive CPO
Power	●	●	●
cost	●	●	●
Latency		●	●
Product Maturity	●	●	●
Serviceability	●	●	●
Late Binding Commitment	●	●	●
Link Performance		●	●
Link Accountability	●	●	●
Interoperable Ecosystem	●	●	●

Link performance and accountability are among the most challenging parameters for linear drive pluggables. If a link is not performing properly, it can be difficult to pinpoint the cause – whether it's a problem with one of the SerDes lanes, one of the PCB traces, or the transceiver itself. In contrast, retimed pluggables decouple performance on an optical link from parameters of the electrical channel, making it easier to separate and validate the optical and electrical specs.

First Deployments and Potential Applications

Developing standards for linear drive pluggable optics is expected to take at least a year, followed by another year for large customers to qualify suppliers. This means that the first deployments of any new standard linear drive pluggables are likely to occur at least two years from now.

Figure 3: Scaling network bandwidth of AI clusters with current optical technologies

However, trials and limited deployments of proprietary solutions are anticipated by the end of 2024. The urgency to reduce power consumption is particularly high in AI clusters, where Nvidia plans to use linear drive optics by the end of 2024. Nvidia's Ethernet adapter cards CX7, shown in Figure 4, may be a good starting point for adoption of linear drive pluggable optics, as the transceivers are placed within an inch from the ASIC chip, ensuring minimal signal degradation.

Figure 4: Nvidia's CX7 Ethernet Adapter card

Other suppliers of Ethernet adapters or Network Interface Cards (NICs) are also expected to experiment with linear drive optics in 2024, as long as they can control both sides of the link to ensure adequate performance.

Deployments of linear drive pluggable transceivers on Ethernet, InfiniBand, and NVlink switches may prove to be more challenging. If linear drive pluggables fail to deliver at higher data rates (e.g., 200G per lane), CPO may become the longer-term solution.

Conclusion

Linear drive optics is an emerging technology that holds great promise for addressing the power consumption and bandwidth challenges faced by data center networks. While market adoption faces hurdles, such as developing standards and ensuring link performance, the potential benefits have generated significant interest from industry leaders like Nvidia and Meta.

As the demand for high-speed optical connectivity continues to grow, linear drive optics and CPO may play complementary roles, with the former offering a pluggable solution and the latter serving as a longer-term option, particularly for higher data rates. The coming years will be crucial in determining the success and extent of adoption for these innovative technologies.

Reference

[1] V. Kozlov, "Emerging market for Linear Drive Optics," LightCounting, April 26, 2023. [Online]. Available: www.lightcounting.com