ion Availability Approximate Downtime/Year 53 min 5 min Based on 525,600 min/year. component downtimes, and then converting the sum to time-based availability. The ranges shown in the figure are typical for the different layers of infrastructure. The physical cloud infrastructure capabilities include physical properties of the plant such as generators, battery, heating, and cooling. Traditional Tier 4 data centers (DCs) supporting critical functions are designed with complete redundancy in the physical plant in order to achieve very high availability (ADC, 2006). In contrast, commercial clouds achieve a lower cost profile by using less redundancy. This is illustrated in the wide availability range shown in the figure. As carriers virtu- alize their networks, decisions need to be made about how reliably this layer should be engineered. On the one hand, improving the physical infrastructure layer to 5-9s or 6-9s could have little overall benefit and, in fact, may be quite costly. On the other hand, low physical layer availability may dominate the availability profile at a site. Though engineering to low availability may be less costly, the need to engineer greater capacity at neighboring sites to absorb frequent failures may offset these savings. Furthermore, low availability also has negative consequences to transaction reliability and latency. Given the availability of the commercial hardware and software that resides in the site, a middle-ground approach to physical infrastructure availability may provide a good cost/performance tradeoff. Next, in the fabric layer, WAN access will generally have high availability due to transport layer diversity, a principle of carrier networks (Wu, 1992; Krishnan, 1994; Klincewicz, 2013). Similarly, Architecting the Network Cloud for High Availability is standard and cost effective to design fully redundant LAN fabrics within a site, leading to high availability. Therefore, the fabric layer is generally not a significant contributor to downtime. The compute/storage layer of the