LPO Transceiver

In recent years, the optical communication industry has developed rapidly, thanks to the promotion of 5G and artificial intelligence. The fiber optic infrastructure has grown rapidly. The network speed of data centers is gradually developing from 400G to 800G and 1.6T, and may even reach 3.2T in the near future. The rapid development of data center networks has promoted the continuous improvement of the Transceiver rate. However, in order to meet the needs of modern high-performance networks, Transceivers need to be improved in multiple aspects beyond the rate, including power consumption and packaging. This has led to the emergence of various new technologies.

Challenges in Transceiver Development

Technological iteration is not just a doubling of numbers. After reaching the 400G stage, while addressing the issue of increasing data transmission rates, we must also face the challenges brought by increased power consumption and costs.

In the early stages, the power consumption of a 10G Transceiver was approximately 1W. However, as we reached the 400G and 800G stages, the power consumption of Transceivers increased sharply, reaching 30W, accounting for 40% or more of the total power consumption of the entire device. Compared to 2010, the total power consumption in 2022 increased by 22 times. The increase in energy consumption of optical communication equipment has brought a huge burden to the energy utilization and cost of the entire data center. To address these issues, the industry has explored two main solutions: CPO (Conventional Pluggable Transceiver) and LPO (Linearly Driven Pluggable Transceiver).

Overview of LPO Transceiver  

What is an LPO Transceiver

The LPO Transceiver, fully known as Linear-drive Pluggable Optics, is an Transceiver packaging technology that utilizes a linear drive strategy. It replaces DSP with transimpedance amplifiers (TIA) and driver chips (DRIVER) that possess excellent linearity and equalization capabilities. In data communication, the LPO Transceiver employs linear analog components, eliminating the need for complex CDR or DSP systems. Compared to DSP solutions, LPO Transceivers consume less power and exhibit lower latency, making them well-suited for short-distance, high-bandwidth, low-power, and low-latency data communication needs in AI computing centers.

LPO Transceiver: An Innovative Module Utilizing Linear Drive Technology

The LPO Transceiver employs linear analog components in its data communication, eliminating the need for complex CDR or DSP systems. Compared to DSP solutions, the LPO Transceiver offers lower power consumption and latency, making it suitable for short-distance, high-bandwidth, low-power, and low-latency data communication needs in AI computing centers.

What Are DSP and CDR

DSP is an algorithm that runs on a chip. While not all traditional Transceivers employ digital signal processing (DSP) technology, it is crucial for meeting the signal processing requirements of high-speed Transceivers. It has functions such as digital clock recovery and dispersion compensation, which can reduce the impact on the system's bit error rate. However, DSP also has high power consumption and costs. For instance, in a 400G Transceiver, using a 7nm DSP consumes approximately 4W of power, accounting for about 50% of the entire module's power consumption.

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

Understanding Linear Drive Technology

The linear drive technology of the LPO solution removes the DSP/CDR chip from the Transceiver and integrates the related functions into the switching chip at the device end. In the Transceiver, only the driver chip (DRIVER) and transimpedance amplifier (TIA) with high linearity are retained, while continuous-time linear equalization (CTLE) and equalization (EQ) functions are integrated separately to compensate for high-speed signals to a certain extent.

Advantages of LPO Transceivers

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

Low Power Consumption

Removing the DSP will undoubtedly significantly reduce power consumption. As Macom's data shows, the power consumption of an 800G multi-mode Transceiver with integrated DSP can exceed 13W. In contrast, an 800G LPO Transceiver using linear drive technology obviously has much lower power consumption, only 4W.

Low Cost

As previously mentioned, the bill of materials (BOM) cost of DSP accounts for a large portion, approximately 20-40%. Removing the DSP can effectively reduce this cost. Integrating the EQ function into the driver and TIA will introduce some additional costs, but the overall expenditure will decrease. According to industry analysis, when using 800G Transceivers, the estimated BOM cost is approximately 4316-5036 yuan, with the DSP chip costing between 360-504 yuan. Incorporating the EQ function into the driver and TIA will result in a slight cost increase, ranging from 22-36 yuan. Through this calculation method, the overall system cost will be reduced by approximately 8%, equivalent to saving approximately 356-432 yuan.

Low Latency

Removing the DSP will reduce a processing step, thereby reducing the latency of data transmission. This advantage is particularly important in artificial intelligence computing scenarios, as minimizing latency is a crucial factor in achieving excellent performance.

Easy Maintenance

Within the CPO framework, if a system device malfunctions, it requires power shutdown and replacement of the entire board, which is quite inconvenient for maintenance tasks. In contrast, the pluggability of LPO Transceivers enables efficient replacement without shutting down the entire system, further enhancing the overall convenience of the LPO solution and simplifying fiber optic cabling and equipment maintenance processes.

Challenges Currently Facing LPO Transceivers

There are currently two major issues that need to be addressed for LPO Transceivers.

Short Transmission Distance

Removing the DSP function incurs certain costs. The TIA and driver chips cannot fully replace the DSP, resulting in an increase in the system's bit error rate. An increased bit error rate inevitably leads to a shortened transmission distance. The industry generally believes that LPO Transceivers are only suitable for specific short-distance application scenarios, such as connections between servers and switches within data center cabinets. LPO Transceivers can connect distances ranging from several meters to tens of meters. In the future, they may be extended to distances within 500m.

Standardization of LPO Technology Has Not Been Achieved

The standardization of LPO technology is still in its early stages, and therefore, there may be some interoperability issues. As a result, LPO technology is currently more suitable for relatively closed systems with a single source. Implementing LPO technology requires enterprises to possess certain technical capabilities, including the ability to design technical specifications and solutions, investigate the boundary conditions of equipment and Transceivers, and conduct extensive integration and interoperability testing.

In addition, some experts have pointed out that LPO technology poses significant design issues for the telecommunication channels on the system end. Currently, the mainstream specification for SerDes is 112G, and it will soon be upgraded to 224G. Experts believe that LPO technology cannot meet the requirements of 224G SerDes.

Conclusion

The development of LPO technology marks an important breakthrough in Transceiver technology. Its main difference lies in the adoption of linear drive technology, replacing DSP with standard Transceivers, and bringing advantages such as reduced power consumption and price. Especially when key issues such as low power consumption, economy, low latency, and ease of maintenance need to be addressed, LPO Transceivers are an ideal choice for improving the efficiency and performance of optical communication systems.