When this is done, the networked routers function as a single entity with a single management interface. This simplifies the carrier's operations, upgrades, and remote diagnostics. Networking routers together is commonly done through virtualization. (See the section "Virtual Network Function [VNF]-Transforming Hardware Nodes into Software Functions" in Chapter 4, "Managing Broadband Networks" for information on virtualization.) Virtualization enables multiple functions including www.TechnicalPDF.com routers to appear as individual pieces of hardware on servers that can be networked together. In addition, traffic is balanced among all the networked routers. If one router fails, the others absorb its traffic. Ensuring Reliability in the Core and Edge Routers sold to carriers and to large web sites such as Google and Amazon are designed for "five 9s" of reliability. This means that they offer 99.999 percent uptime. They are sold with hot-swappable cards that connect to other services and networks. If a card fails, it can be replaced without taking the router out of service. They are also offered with options for duplicate processor cards and often come standard with dual power supplies. Power supplies connect to sources of electricity. It's also common for carriers to install duplicate routers that can be seamlessly brought online in the event of a failure. Duplicate routers are crucial at the edge, where if the router fails, all other networks and customers connected to it lose service. Enhancing Internet Performance by Using Content Delivery Networks Content Delivery Networks (CDNs) provide a number of services at the edge of the Internet that alleviate congestion on the Internet's backbone. In its simplest form, a CDN reduces congestion by decreasing the distance that the traffic must travel. One way they accomplish this is by storing frequently requested web pages at their servers, nearer to the end users, often in a service provider's location. This is referred to as caching. Many enterprises with large web sit