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Shining DAC Copper Cables in Cloud DC

Today, many data centers are deploying higher-speed server cables, which are being installed in various private, public, and hybrid cloud access layer positions around the world.

These server and storage connections are used to support a variety of emerging services and applications across multiple industries. Data centers with the fastest throughput and processing times have a digital advantage, and network equipment and connections that meet industry standards are leading the trend, helping to drive transformative innovations in the market. New mobile cloud application tools continue to emerge, making it convenient for users to connect to more digital services from anywhere and unlock the best experiences, all thanks to high-speed cabling.

Some industries are already leveraging the advantages of fast response times and large processing capabilities, such as finance, software services, healthcare, manufacturing, etc. New 400G network switches are being deployed, connecting to the network core, and ultimately achieving faster end-server rates. Currently, the industry is very enthusiastic about the discussion of next-generation 400G and 800G port rates, which are being adopted by hyperscale data centers, cloud providers, and some large enterprises. Although the 400G rate will eventually enter more large enterprises for switch-level uplinks to handle more and more data, the bandwidth and latency of server connections need to keep up with the development of e-commerce and emerging technologies such as advanced data analysis, machine learning (ML), artificial intelligence (AI), telemedicine, online banking, high-resolution video content, and other real-time applications. Fortunately, high-speed direct attach cables (DAC) are keeping up with these growing demands, ensuring that the network interface connections from switches to servers do not become the weakest link.

Direct attach copper cables are known for their lowest latency, which is the lowest latency among all short-distance connection options. This is because optical connections produce measurable latency when converting electrical signals to optical signals and then back to electrical signals. When deploying 25G and 100G server connections, active optical cables (AOC) are not needed for high-speed connections within 3 meters. In addition, direct attach copper cables do not require forward error correction (FEC), which is a method used in high-speed connections to detect and correct errors, adding additional latency. The gauge (AWG) of DAC cables can help overcome the need for FEC, which is important because large public clouds are not suitable for every business. For example, financial and healthcare institutions that are subject to strict regulations often find it difficult to comply due to strict security requirements for customer private information; this alone can make small-scale private cloud systems the best choice.

Public clouds are popular because they are well-suited to provide unlimited computing power when typical application usage is unknown. However, having a private or hybrid cloud makes more economic sense when there is a continuous demand for a large amount of workflow processing. For large enterprise users with a scale of 10 to 100+ cabinets, it is usually found that operating a private cloud is more economical in the long run. In today's data centers, systems with 100G downlink and 400G uplink are leading the way. 100G QSFP28 switches and server network interface connections are readily available on the market and provide options to branch from 100G ports to high-density 25G server and storage connections. Larger enterprise data centers are already considering the need for 200G downlink port rates, which are expected to require faster connection rates in the future.

Agility is also a key consideration when designing cloud systems, and choosing the right cabling can help you reduce uncertainty in your IT budget. Many local and public cloud data centers utilize top-of-rack (ToR) server cabinet modules to deploy more computing power when needed. The repeatable design makes it possible to quickly deploy server cabinet modules and allows users to easily expand step by step. Combined with direct attach copper cables, ToR systems can also be more economical because cloud server systems are well-known for being very power-consuming and generating a lot of heat.

In addition to the low cost of direct attach copper cables, power consumption is another key consideration for cost. Many short-distance connections in ToR systems are ideal for using DAC. Passive DAC ports have the lowest power consumption, which helps contribute to reducing power consumption and cooling costs for data centers. Direct attach copper cables are a popular choice for short-distance connections; however, if you need longer distances, active optical cables (AOC) with embedded transceivers can transmit over long distances but consume more power than using DAC. Structured cabling with transceiver solutions require more power consumption, typically 1.2w per port for 25G and 3.5w for 100G.

The ability to support faster transmission rates is closely related to the binary encoding scheme used to convert data into digital signals. Non-return-to-zero (NRZ) is the most commonly used encoding scheme and has been used in data transmission for a long time. NRZ uses two different voltage levels to represent binary numbers, where a positive voltage represents "1" and a negative voltage represents "0". This encoding has significantly evolved over the past few decades and is mainly used to support bit rates of 1G, 10G, and 25G per channel in data center links.

PAM4 encoding provides twice the bit rate within the same signal period as NRZ encoding by using four levels instead of two and supports single-channel signal rates of 50G and 100G without increasing channel loss. For small form-factor pluggable technology, PAM4 now provides single-channel SFP56 direct attach 50G and four-channel QSFP56 direct attach 200G, which is the next speed step for short-distance server connections. PAM4 is also the reason for enabling 400G applications, with dual-density 8-channel QSFP-DD interfaces relying on the same PAM4 50G bit rate to achieve 400 gigabits (i.e., 50GGx 8 channels), which is very suitable for switch-to-switch deployments.

However, the increased throughput provided by PAM4 comes at a cost. PAM4 requires FEC, which adds latency, typically between 100 to 500 microseconds. Due to the increased latency of PAM4's FEC, the current best combination of rate and latency is the 4-channel QSFP28 standard DAC, which supports 100G using NRZ and uses a 25G bit rate that does not require FEC, up to 3 meters.

Although most enterprise data centers are just starting to use single-channel SFP28 DAC for 25G server connections, 4-channel QSFP28 DAC and breakout cables make it possible to migrate to high-speed, low-latency server connections to support emerging real-time applications. For applications such as high-frequency financial trading, edge computing, interactive gaming, video conferencing, artificial intelligence, real-time monitoring, and data analysis, a delay of more than 100ms can significantly affect performance. For example, in gaming, a delay of more than 100ms means players will feel noticeable lag. For data centers that want to support these applications, the latency design of these switch-to-server connections should be a key focus, making QSFP28 DAC the best choice.

ToR switch-to-server deployments of DAC will continue to exist and will take you where you need to go. When planning your next server and storage deployment, remember that not all cables are equally reliable, and the chosen cables should be electrically tested to industry standards to ensure trouble-free high-speed connections. Using industry-standard cabling ensures interoperability, which means that the solution you choose should work with switches from any vendor in the same ecosystem, such as Ethernet and InfiniBand. The key takeaway is that the introduction of PAM4 encoding technology enables DAC and AOC to support 10 to 400G direct attach, including the lowest latency 100G option using NRZ encoding technology, to support emerging real-time applications and help keep budget overruns from becoming too outrageous.

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