What is the LPO Transceiver?

The optical communications industry has achieved significant advances in recent years, fueled by 5G and AI, resulting in a rapid growth of optical infrastructure. The data center network speed is gradually developing from 400G to 800G and 1.6T, and will even reach 3.2T in the near future. The rapid development of data center networks has spurred continuous advancements in the speed of optical modules. However, to meet the demands of modern high-performance networks, optical transceivers need to evolve in various aspects beyond just speed, including power consumption, packaging, and more. This has led to the emergence of various new technologies.

Table of Contents

Challenges in Transceiver Development

The Introduction of LPO Transceiver

The Advantages of LPO Transceiver

The Current Challenges of LPO Transceiver

Conclusion

Challenges in Transceiver Development

Technology iteration involves more than just doubling numbers. After getting to the 400G stage, it becomes imperative to address not only the enhancement of data transmission speed but also the challenges associated with increased power consumption and costs.

In the early days, a 10G transceiver used just about 1W of power. Until 400G and 800G, transceiver power usage surged, reaching 30W, accounting for 40% or more of the machine's total power consumption. Compared to 2010, total power in 2022 has increased by 22 times. The increase in energy consumption of optical communication devices puts a major burden on the overall use of energy and costs for the whole data center. To address these challenges, the industry explored two main solutions, Conventional Pluggable Optics (CPO) and Linear-Drive Pluggable Optics (LPO).

The Introduction of LPO Transceiver

What is LPO Technology?

LPO (Linear-drive Pluggable Optics) is a transceiver packaging technology. It uses a linear drive strategy to replace DSPs with a Transimpedance Amplifier (TIA) and Driver Chip (DRIVER) with excellent linearity and EQ capabilities. It utilizes specialized components, including ASIC substrates, ASIC (Retimer), and ASIC die, to optimize signal processing and enable efficient plug-and-play functionality in optical communication systems.

LPO Transceiver: Innovative Modules with Linear-Drive Technology

The LPO transceiver uses linear analog components in its data communication, eliminating the need for complex CDR or DSP systems. Compared to DSP solutions, LPO transceiver exhibits major savings in power consumption and latency, making them suitable for the needs of short-range, high-bandwidth, low-power, and low-latency data communication in AI computing centers.

What are DSP and CDR?

Not all traditional transceivers incorporate Digital Signal Processing (DSP). However, in high-speed transceivers where stringent signal requirements prevail, DSP becomes essential. DSP is a chip-running algorithm that proves crucial in meeting the demanding signal processing needs of high-speed transceivers. It has a digital clock recovery function, and dispersion compensation function, which can reduce distortion on the system BER impact. However, it also has high power consumption and costs. For example, in the 400G transceiver, the 7nm DSP used, the power consumption is about 4W, accounting for about 50% of the power consumption of the entire module.

Clock Data Recovery (CDR) is also used for data restoration. It extracts the data sequence from the received signal and recovers the clock timing signal corresponding to the data sequence, thus restoring the specific information received.

Understanding Linear-Drive Technology

The linear-drive technology of LPO solution is to remove the DSP/CDR chip in the transceiver and integrate the related functions into the switching chip on the device side. In the transceiver, only the Driver Chip (DRIVER) and Trans-Impedance Amplifier (TIA) with high linearity are left, while Continuous Time Linear Equalization (CTLE) and Equalization (EQ) functions are integrated, respectively, for compensating high-speed signals to a certain extent.

The Advantages of LPO Transceiver

Compared with traditional transceivers, the advantages of LPO transceiver are mainly reflected in the four aspects of power consumption, cost, latency, and maintenance.

Low Power Consumption

The absence of DSP undoubtedly results in a significant reduction in power consumption. As indicated by Macom's data, an 800G multimode transceiver incorporating DSP functionality can surpass 13W in power consumption. In contrast, an 800G LPO transceiver leveraging linear-drive technology exhibits remarkably lower power consumption, measuring less than 4W.

Low Cost

As previously highlighted, the Bill of Materials (BOM) cost attributed to DSP constitutes a significant portion, ranging from 20-40%, and this cost is effectively eliminated with the removal of DSP. The integration of EQ functionality in both the driver and TIA does introduce a slight incremental cost. However, the overall expenditure experiences a net reduction. According to industry analyses, in the context of an 800G transceiver, the BOM cost is estimated at approximately 600-700 dollars, with the DSP chip alone accounting for a cost range of 50-70 dollars. The inclusion of EQ functionality in the driver and TIA results in a marginal cost increase, ranging from 3-5 dollars. Through this calculated approach, the overall system cost realizes a reduction of approximately 8%, translating to a savings of around 50-60 dollars.

Low Latency

The elimination of DSP results in a reduction of one processing step, consequently decreasing data transmission latency. This advantage holds particular significance in AI computing scenarios, where minimized latency is a crucial factor for optimal performance.

Easy Maintenance

Within the CPO framework, the need to power off and replace the entire board in the event of a system device malfunction presents a considerable inconvenience for maintenance tasks. In contrast, the pluggable nature of LPO transceivers allows for efficient replacement without the need to power down the entire system, further enhancing the overall convenience of the LPO solution. This not only simplifies fiber cabling but also streamlines equipment maintenance.

The Current Challenges of LPO Transceiver

For LPO transceivers, there are currently two main challenges that need to be addressed.

Short Transmission Distance

There is a cost associated with deleting DSP. TIA and driver chips cannot fully replace DSP, hence the system's bit error rate will rise. With a larger bit error rate, the transmission distance is inevitably shorter. The industry usually considers that the LPO transceiver is only appropriate for specific short-distance application scenarios, such as the connection between servers and switches inside data center cabinets. The LPO transceiver can connect distances ranging from a few to tens of meters. In the future, it may be extended to within 500 meters.

No Standardization of LPO Technology

LPO standardization is still in its early stages, therefore there may be some interoperability issues. So, LPO technology is currently better suited to systems that are reasonably closed and have only one source. Enterprises that implement LPO technology must have certain technical capabilities, including the capacity to design technical specifications and solutions, investigate the boundary conditions of devices and transceivers, and conduct a significant number of integration and interoperability tests.

Furthermore, several experts have noted that LPO technology presents significant design issues for electrical channels on the system side. SerDes' current mainstream specification is 112G, which will soon be updated to 224G. Experts feel that LPO technology cannot meet the criteria of 224G SerDes.

Conclusion

The development of LPO technology marks an important leap in optical transceivers. The primary differentiator is the use of linear-drive technology, which replaces DSP with standard transceivers and provides benefits such as decreased power consumption and prices. So, the LPO transceiver is a promising alternative for improving the efficiency and performance of optical communication systems, especially in contexts where low power consumption, cost-effectiveness, low latency, and ease of maintenance are essential concerns.