Efficient 100G and 400G optical transmission solution for very large-scale data centers


Global network data center traffic has maintained rapid growth for a decade and has not shown any signs of abating for the foreseeable future. Smartphones and other mobile devices, social media and applications, streaming videos, augmented and virtual reality are gaining popularity, and the rapid growth is attracting more and more new users. The number of devices held by each user continues to grow, The amount of data used by each device is also rising, so the traffic in the data center has increased significantly. According to analyst forecasts, by 2020, the number of devices connected to the Internet worldwide will reach 200 billion. Recent evidence suggests that the maturing consumer electronics market may be just the tip of the iceberg. The growth rate of data bandwidth in cloud computing and machine-to-machine deployments is surpassing consumer data communication rates, driving huge demand for high-capacity data center infrastructure.

Over the past decade, top Internet site companies such as AWS, Microsoft, Google, and Facebook have been busy deploying larger and larger data centers to meet customer needs. For some of them, in every building The number of computer servers in use now exceeds 100,000. According to Cisco estimates, by 2020, nearly half of the installed servers in all data centers will be located in hyperscale data centers. These servers will account for 68% of processing power and more than half (53%) of total data center traffic.

400G optical transmission

Inside each ultra-large-scale data center building, there may be tens of thousands or even hundreds of thousands of computer servers connected to each other through Ethernet switches at different levels to form a collective computing capability. Network company's own services (such as Google or Facebook), or leased to enterprise customers (such as Amazon's AWS or Microsoft's Azur). Although there are many ways to achieve the interconnection of calculator servers, in 2018, the typical ultra-large-scale data center network connection is characterized by the use of DACs (direct-attached copper cables), at 25 or 2x25 Gbps, in several counts. Connect the server to the top-of-rack (ToR) server within a distance of meters, and then use a large number of 100 Gbps optical links to interconnect the ToR switches through a large-scale switching structure (commonly referred to as a leaf-ridge architecture). Depending on the number of such data centers, typical optical interconnects can range up to 500 meters (DR), but larger data centers require distances of up to 2 kilometers (FR).

The current generation of 100G optical transmission modules are based on 4-channel optical transmitters and optical receivers, which run in parallel at 25Gbps speeds, respectively, to achieve an aggregate bandwidth of 100 Gbps. There are currently two types of 100G optical transmission systems: For users who want to deploy more optical fibers and reduce the cost of each optical transmission system, PSM-4 (parallel single-mode-4) optical transmission systems are suitable.

100G / 400G transition to 100G PAM-4 technology is coming

Current hyperscale data centers are characterized by faster interconnect speed transitions, which often occur every three years. The highly innovative 100G interconnect system is becoming mainstream, has been widely deployed in the past two years, and the next speed transition is approaching. Although 200 Gbps is currently being considered, the industry consensus is that 400 Gbps will be the next natural choice.

The current packaging process based on 4x25G 100G technology is too complicated and cannot be expanded to 400G. In order to reduce the cost of 100G and support 400G optical components in an economical way, the industry is turning to a new technology that uses PAM-4 (4-level pulse amplitude modulation) encoded optical components at 50 GBaud, thereby Achieve 100G speed per channel, and then achieve 4G speed through 4x100G aggregation. With advances in digital signal processing and high-speed optoelectronic devices such as high-speed silicon photonics, we expect the industry to quickly adopt and implement such technologies.

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