Shulou(Shulou.com)11/24 Report--

Hello, everyone. I'm Mr. Jujube.

In today's article, we will talk about a very popular concept of optical communication-LPO.

In recent years, the development momentum of optical communication industry is very strong.

Under the continuous stimulation of national strategies such as 5G, broadband China, East and West calculation, the domestic optical communication technology has made great breakthroughs, and the optical infrastructure has also made a qualitative leap.

Especially this year, the popularity of AIGC models and the rise of smart computing and supercomputing have led to a new upsurge in the development of optical communications. The backbone network 400G is about to be fully landed, and the data centers 800G and 1.6T are also eager to try.

Evolution of OSPF (Source: Arista Network) the challenge of the evolution of █ optical communication actually, the technical iteration of optical communication is not a simple digital doubling.

After entering the 400G phase, the problem we have to solve is not only the improvement of the rate, but also the power consumption and cost caused by the high rate.

Speed up is like moving goods by car. When the cargo is getting heavier and heavier, the engine needs to be upgraded. The greater the emission of the generating power, the greater the fuel consumption, and the higher the engine price and fuel cost.

Let's take the optical module as an example.

As the key device of the optical network, but also the most widely used device, the optical module has always been the focus of the industry. Its power consumption and price are closely related to users' willingness to purchase.

Optical module as early as 2007, a 10 gigabyte (10Gbps) optical module, the power is only about 1W.

With the iteration of 40G, 100G, 400G and 800G, the power consumption of optical module soars all the way to 30W.

You know, a switch can have more than one optical module. When fully loaded, there are often dozens of optical modules (if there are 48, that is, 48 × 30 = 1440W).

Generally speaking, the power consumption of optical modules accounts for more than 40% of the power consumption of the whole machine. This means that the power consumption of the whole machine is very likely to exceed 3000W.

The sharp increase in energy consumption of optical communication equipment also brings great pressure on the energy consumption and cost of the whole data center, which is not conducive to the dual-carbon goal of the communication network.

Compared with 2010, the energy consumption of optical devices will be increased by 26 times. (picture from Cisco) in order to solve the problem of energy consumption caused by the rising rate of optical communication, the industry has carried out a lot of technical exploration.

CPO, which was very popular last year, is one of the solutions.

I've specifically introduced CPO before (link: what exactly is NPO / CPO? ), I won't go into details here.

This year, in addition to CPO, the industry has put forward a new scheme, which is-- LPO.

█ what is LPOLPO, the English full name is Linear-drive Pluggable Optics, that is, linear driver pluggable optical module.

As can be seen from the name, it is a kind of optical module packaging technology.

The so-called "Pluggable", we usually see the optical module, are pluggable.

As shown in the following figure, there is an optical module port on the switch, and the corresponding optical module can be plugged into the optical fiber. If it's broken, you can change it.

LPO emphasizes "pluggable" to distinguish it from the CPO scheme. In the CPO scheme, the optical module cannot be plugged or unplugged. The optical module (optical engine) is moved closer to the switching chip and directly "tied" together.

Then, the key difference between LPO and traditional optical modules lies in linear drive (Linear-drive).

The so-called "linear drive" means that the LPO uses linear direct drive technology, and the DSP (digital signal processing) / CDR (clock data recovery) chip is removed from the optical module.

Here comes the question-what is linear direct drive? What role does DSP play? Why can it be cancelled? What will be the impact after cancellation?

Here, let's start with the basic architecture of the optical module.

In the previous introduction of coherent optical technology (link), Xiaozao Jun mentioned that optical module transmission is the process of turning an electrical signal into an optical signal and an optical signal into an electrical signal.

At the sending end, the signal changes from digital signal to analog signal through digital-to-analog conversion (DAC). At the receiving end, the analog signal is converted into a digital signal after analog-to-digital conversion (ADC).

After a meal of operation, the digital signal is a little messy and distorted. At this time, DSP is needed to "repair" the digital signal.

DSP is a chip for running algorithms. It has the function of digital clock recovery and dispersion compensation (removing the influence of noise, nonlinear interference and other factors), which can resist and compensate the distortion and reduce the influence of distortion on the bit error rate of the system.

Various compensation and estimation of DSP

The function of each module of DSP (Note: DSP is not available in all traditional optical modules. However, in the high-speed optical module, the signal requirements are high, so the basic need for DSP. )

In addition to DSP, the main electrical chips in the optical module include laser driver (LDD), trans-resistance amplifier (TIA), limiting amplifier (LA), clock and data recovery chip (CDR,Clock and Data Recovery) and so on.

CDR is also used for data restore. It extracts the data sequence from the received signal and recovers the clock timing signal corresponding to the data sequence, thus restoring the received specific information.

DSP is very powerful. However, its power consumption and cost are also very high.

For example, in the 400G optical module, the power consumption of the 7nm DSP is about 4W, accounting for about 50% of the power consumption of the whole module.

The composition of the power consumption of the optical module from the point of view of cost, the BOM (Bill of Materials) cost of DSP accounts for about 20-40% of the 400G optical module.

The LPO scheme is to kill the DSP / CDR chip in the optical module and integrate the related functions into the switch chip on the device side.

In the optical module, only Driver (driver chip) and TIA (Trans-Impedance Amplifier) with high linearity are left, and CTLE (Continuous Time Linear Equalization, continuous-time linear equalization) and EQ (Equalization) functions are integrated to compensate the high-speed signal to a certain extent.

As shown in the following figure:

The advantages of █ LPO the advantages of LPO are: low power consumption, low cost, low latency, easy maintenance.

Low power consumption without DSP, power consumption must be reduced.

According to the data of Macom, the power consumption of 800G multimode optical module with DSP function can exceed 13W, while the power consumption of 800G multimode optical module using MACOM PURE DRIVE technology is less than 4W.

Needless to say, low cost. As mentioned earlier, the BOM cost of DSP accounts for about 20-40%, but this is gone.

Driver and TIA integrate EQ, with a slight increase in cost, but an overall decline.

Industry analysis: in 800G optical module, the cost of BOM is about $600,700, and the cost of DSP chip is about $50,70,000,000. The EQ function is integrated into Driver and TIA, which will increase the cost by $3 to $5. As a result, the total cost of the system can be reduced by about 8%, or about $50 to $60.

It is worth mentioning that DSP is also a technology mastered by a small number of manufacturers, such as Broadcom and Inphi. The elimination of DSP, to some extent, also reduces the dependence on a small number of vendors.

Low delay without DSP, reducing a processing process, data transmission delay is also reduced.

This advantage is particularly important for AI computing and supercomputing scenarios.

Easy to maintain this is relative to the CPO solution.

In the CPO scheme, if any of the devices in the system is broken, the power will be turned on and the whole board will be replaced, which is very inconvenient to maintain.

There is no significant change in the package of LPO, which supports hot plug, simplifies optical fiber cabling and equipment maintenance, and is more convenient to use.

█ LPO's current challenge-short communication distance-removes DSP, of course, comes at a price. TIA and Driver chips can not completely replace DSP, so it will increase the bit error rate of the system. If the bit error rate is high, the transmission distance will naturally be shorter.

It is generally believed that LPO is only suitable for specific short-range application scenarios. For example, the connection from the server to the switch in the data center cabinet and the connection between the data center cabinets.

The development of primary LPO, the connection distance from a few meters to dozens of meters. In the future, it may expand to less than 500 meters.

Standardization has just started, and the standardization of LPO is still in its early stage, so there may be some challenges in interconnection.

For enterprises, if they adopt LPO, they need to have certain technical capabilities, to be able to formulate technical specifications and programs, to explore the boundary conditions of equipment and modules, and to carry out a large number of integration and interconnection tests.

In other words, LPO is currently more suitable for systems that are more closed and single-vendor. If you adopt multiple suppliers and do not have the strength to control them, then there may be the problem of "the problem is more difficult to define and wrangle with each other", it is better to use the traditional DSP scheme.

In addition, some experts pointed out that LPO brings some challenges to the design of the electrical channel on the system side. At present, the mainstream specification of SerDes is 112G, which will be upgraded to 224G soon. Experts believe that LPO cannot keep up with the requirements of 224G SerDes.

The industrialization progress of █ LPO the LPO plan has actually been put forward by some enterprises before, but it has not achieved any results because of technical limitations.

At this year's OFC conference, LPO was proposed again and soon became the focus of the industry.

Major customers in the international market, such as AWS, Meta, Microsoft and Google, have expressed interest in LPO. Many optical communication giants have also invested resources in research and development.

At present, Zhongji Xuchuang, Xinyi Sheng, Cambridge Technology and other companies have launched 800G LPO solutions. Recently, some enterprises should have achieved small-scale shipments.

The key to the LPO solution is the chip. The main suppliers of TIA&Driver with high linearity are Macom, Semtech, American letter and so on.

According to the forecast, LPO will be commercialized on a large scale in 2024. The more optimistic institutions in the industry believe that LPO can occupy half of the market share in the future. More conservative institutions believe that the share of CPO / LPO will reach about 30 per cent in 2026.

█ conclusion, the above is about the introduction of LPO.

The logical essence of LPO is balance and trade-off. Based on a specific application scenario (short distance), it abandons DSP / CDR at the expense of a little bit performance (bit error rate), but in exchange for lower power consumption, cost and delay.

It and CPO have their own advantages. Although they were born later than CPO, they will land faster than CPO.

Comparison of advantages and disadvantages according to the current trend, LPO will be the most potential technology route in the 800G era.

With the development of AIGC wave, data center optical network will accelerate to 800G evolution. The golden age of LPO is coming.

-- The End--

References:

1. "Application and industrialization of Silicon Optical in New Generation PON, 800G Interconnection and coherent sinking", Pan Dong

2. "Optical Communication opportunities and challenges in the AI era", Li Junjie

3. "who will dominate the future data center optical interconnection in LPO vs CPO", Xue Zhenfeng

4. "800g Optical Module, AI Computing Base", Societe Generale Securities

5. "the evolution path of new optical module technology in the era of AI computing", Great Wall Securities

6. "LPO technology is the most potential route in the 800G era and is expected to have mass production by the end of 2024." Guosheng Securities

7. "revealing the New weapon of Network Construction of Intelligence Center: the emergence of LPO Technology", Ruijie Network

8. "Summary of expert exchange meeting on linear driven optical module".

This article comes from the official account of Wechat: fresh Jujube classroom (ID:xzclasscom), author: Xiaozaojun

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