Issue link: https://www.e-digitaleditions.com/i/1066130
40 I ICT TODAY TRENDS IN DATA CENTER ARCHITECTURES Leaf-spine architectures are popular because they reduce the number of "hops" from server to server, which minimizes latency. One of the consequences of this approach is that it requires more physical connections. The physical layer becomes more important, because it needs to accommodate a simplex, duplex or parallel networking scheme. This means that sometimes the design, implementation, testing and validation can get fairly complex. Further complicating this scenario is that as the rate of change in network speeds has accelerated, optical loss budgets are shrinking, and supported channel lengths are shortening. As a result, there is less budget to work with in the physical layer, so the connectivity used needs to be much better. The good news is there are now more choices in connectivity and media types; the bad news is there are MANY more choices. STRATEGIES FOR MOVING TO 100 GB/S The Data Center Roadmap (Figure 1) shows that customers are currently running anywhere from 10 Gb/s to 100 Gb/s. While 40 Gb/s and 100 Gb/s have been on the market for a couple of years now, most data centers do not have a need to utilize the full 40/100 Gb/s as a single connection. With one MPO connector plugged into a QSFP transceiver, a switch could be provisioned to send a full 40 Gb/s or 100 Gb/s between devices. It can also break down that 40 Gb/s speed into four 10 Gb/s lanes or a 100 Gb/s down into ten 10 Gb/s lanes while only utilizing one port on the switch. This allows for more efficient switch port utilization, but it also creates a more complex physical layer architecture. Pertaining to the roadmap in Figure 1, 100G BiDi is proprietary right now, but when the 802.3cm standard is completed in 2019, there will likely be a standards-based version available. There will also likely be MSAs around 200G. Currently the technology is available for 50G/λ over SWDM, and OEMs are waiting for the demand to increase. Likely, this will be an interim step before 400G interfaces are cost effective. 100G/λ work is underway and is expected somewhere beyond 2022. The trend in the industry is to simplify cabling as soon as the technology permits. This is driven by the overall applied link cost (cost per Gb). For example, 100GBASE-SR10 has been replaced by SR4 and may soon be displaced by SWDM4 or BiDi technology. To support higher speeds, different techniques like increased lane rates and more efficient modulation schemes are being used. For example, above 25Gb/s lane rate, PAM-4 modulation will replace NRZ. This allows for doubling the number of bits per symbol. FIGURE 1: The data center multimode speed roadmap shows the current standards (in black). Blocks with question marks indicate areas where there are multiple possible outcomes that should be considered for the future. 100GBASE-SR10 40GBASE-SR4 40GBASE-SWDM4 400G-SWDM4? 400GBASE-SR16 100G-SWDM4 800G-SWDM4? 400GB-SWDM4? 200G-SWDM4? 1 Tb/s? 400 Gb/s 800 Gb/s 200 Gb/s 100 Gb/s 10 Gb/s Lane rate > 1 2 4 8 10 16 # lanes 25 Gb/s 50 Gb/s 100 Gb/s NRZ Encoding > PAM-4