“At the Optical Communications Summit held in San Diego this year, applications in the field of data center interconnection (DCI) have become a hot topic. Data center interconnection is becoming an important part of the rapid development in the field of networking, and recent exciting developments in the field of optical fiber cabling have focused on this. This article will explore the reasons for the continuous development of this field, focusing on how several new cabling technologies make the interconnection part of the data center more friendly to installers.
At the Optical Communications Summit held in San Diego this year, applications in the field of data center interconnection (DCI) have become a hot topic. Data center interconnection is becoming an important part of the rapid development in the field of networking, and recent exciting developments in the field of optical fiber cabling have focused on this. This article will explore the reasons for the continuous development of this field, focusing on how several new cabling technologies make the interconnection part of the data center more friendly to installers.
What are the best practices for designing and deploying extremely high-density data center interconnections?
A quick search on the Internet for spending announcements for hyperscale or multi-tenant data centers will reveal multiple expansion plans, with a total scale of billions of dollars. What can you gain from this kind of investment? Usually, it is a data center campus, which consists of several data room modules located in different buildings. These data room modules are usually larger than a football field, and the traffic between data room rooms usually exceeds 100 Tbps (Figure 1).
As shown in Figure 1.Sample data center campus layout
There are many detailed reasons for why these data centers have grown so large, but we can simplify them into two trends. The first is the exponential east-west traffic growth brought about by communication between machines; the second trend is the application of flatter network architectures, such as spine and leaf structures and Clos architecture networks. Its goal is to establish a large-scale network structure in the park, which also enables data transmission between data centers to reach or exceed 100Tbps.
It is conceivable that a network construction of this scale will encounter a number of special challenges in the entire network, from power and cooling to the connection of equipment. In the interconnection of network devices, a variety of methods have been evaluated to provide a transmission rate of 100 Tbps (or even higher), but the common model is to transmit at a lower rate through multi-core single-mode fiber. It should be noted that the length of these connections is usually 2-3 kilometers or less. Through our modeling analysis, at least for the next few years, using more optical fiber to transmit at low data rates is still the most cost-effective method. This cost model reveals why the industry invests so much money to develop high-core count fiber optic cables and related hardware.
Now that we understand the demand for high-core count fiber optic cables, we can turn our attention to alternative solutions in the data center interconnect market. The industry agrees that ribbon optical cable is the only feasible solution for this application. The installation time of traditional loose tube optical cable and single-core optical fiber termination is too long, and the optical fiber connector fusion splicing hardware is too large to be practical. For example, it takes more than 200 hours for a 3456-core optical cable designed with a loose tube structure to be spliced, assuming that each splicing takes 4 minutes. If you use a ribbon fiber configuration, the splicing time drops to less than 40 hours. In addition to saving these time, the capacity of ribbon splicing equipment is usually four to five times the density of single-core fiber splicing under the same hardware space occupation.
Once the industry believes that the ribbon cable is the best choice, it will quickly realize that the required fiber density cannot be achieved in the existing duct space through the traditional ribbon cable design. Therefore, the industry has set out to double the optical fiber density inside the traditional ribbon optical cable.
Two design methods have emerged for the structure of optical cables. The first method uses a standard matrix ribbon with tighter packaging subunits, while the other method uses a standard fiber optic cable structure design with a center or slot design, and a loosely bonded ribbon fiber design that can be stacked on top of each other (see figure 2).
figure 2. Design of different ribbon cables for very high-density applications.
Now that we understand these new ribbon cable designs, we must also explore ways to terminate them and the challenges they face. According to the National Electrical Code (NEC), because the optical cable is only suitable for outdoor fire protection, it must be converted to an indoor fire rated optical cable within 50 feet of entering the building, usually by changing the MTP?／MPO or LC ribbon pigtail (optical cable with pre-installed connector at one end) or integrated hardware with coupler and pigtail (the hardware is pre-installed with coupler and pigtail) spliced in a high-density splicing cabinet To achieve. Therefore, in this application environment, users no longer only consider the design of outdoor optical cables, but seek a complete end-to-end solution for these expensive and labor-intensive link deployments (Figure 3).
image 3.The outdoor fiber optic cable with very high core count is connected to the indoor fiber optic cable through the fiber splicing cabinet
When deciding on the best point-to-point solution, several factors must be considered. Time studies have shown that the most time-consuming process is the identification of ribbon fiber ribbons and fiber optic cable branch routing of splice trays. “Branch” refers to the process when the ribbon fiber enters the hardware to the splice tray after the cable is stripped. In order to protect the ribbon fiber, it will be protected by a corrugated tube or mesh sleeve. As the number of fiber cores in an optical cable increases, this step will become more time-consuming and labor-intensive.
Generally, hundreds of feet of corrugated pipe or mesh sleeve are required to install and splice a single 3456 fiber link. The same time-consuming process will also be applied to indoor optical cables, whether they are direct fusion or fusion to the hardware provided with pigtails and couplers. At present, the branch operation time of different optical cable products on the market can vary greatly.
Some optical cables integrate branchable and routed optical cable sub-unit harnesses in both indoor and outdoor optical cables, and do not need to branch when connected to the splice tray, while some products require multiple accessories to branch and protect the optical cable. This kind of fiber optic cable is usually installed on a special splice cabinet, and the splice tray design has also been optimized to match the number of fibers in the routing subunit.
Figure 4. Optical cable with ribbon fiber bundle subunit.
Figure 5: A sample of a branch assembly of a very high-density optical cable.
Another time-consuming task is ribbon identification and correct ordering to ensure correct splicing. Because a 3456 optical cable contains 288 12-core optical fiber ribbons, a clear identification is required for sorting after the outer sheath of the optical cable is removed. Standard matrix ribbons can be printed with inkjet printers to identify characters, and many network designs rely on connection numbers of different lengths and numbers to help identify ribbons. This step is critical, because a large number of fibers and routes must be identified. When the fiber optic cable is damaged or cut after the initial installation, this kind of ribbon marking also becomes crucial in network repair.
Optical cables with 3456-core optical fibers seem to be just a starting point, because the industry has begun to discuss optical cables with more than 5,000-core optical fibers. As the pipe size has not become larger, another emerging trend is that the size of the optical fiber coating used has been reduced from the industry standard 250 microns to 200 microns. The size of the core and cladding remains unchanged, so the optical performance is not affected. This reduced optical fiber coating size can be in the same size pipe as before, allowing hundreds or thousands of additional optical fibers to be laid.
Another trend is the increasing demand from customers for point-to-point solutions. Optical cables containing thousands of fibers solve the problem of duct density, but they also bring many challenges in terms of risk and network deployment speed. Innovative solutions that help eliminate these risks and reduce the speed of deployment will continue to mature and evolve.
The demand for extremely high-density fiber optic cables seems to be accelerating. Artificial intelligence, 5G, and larger data center campuses are all driving the need for interconnection of these data centers in some way. These deployments will continue to challenge the industry to develop end-to-end solutions that can be effectively scaled to maximize the use of pipeline resources instead of making problems more and more troublesome.
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