ing 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 car- riers to install duplicate routers that can be seamlessly brought on line 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, 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 serv- ers, nearer to the end users, often in a service provider's location. This is referred to as caching. Many enterprises with large web sites use CDNs to replicate their web content at many edge locations to prevent delays at their web or e-commerce sites. For traffic that traverses the Internet backbone, CDNs use mathematical algorithms to map out the best paths for traffic to take. CDNs are also key enablers of quality video on the Internet. Specialized servers are used to optimize video. In addition, some CDN providers, including Cambridge, Massachusetts-based Akamai Technologies, partner with companies such as Bright- cove, Inc., which is a provider of cloud content services that transmit video in formats compatible with the many devices (including mobile handsets) that use different meth- ods of displaying video. Brightcove equipment is located with Akamai servers. The Structure of the Internet As the amount of data on mobile networks increases, CDNs will place a growing number of their servers close to cell towers to enable mobile providers to handle their data and video traffic. CDNs provide other services, as well, including security, cloud computing, and wide area n