s packet switches, design fundamentals dictate the specifics of the train which have no notion of circuits, paths, flows, or any (such as the wheel gauge and carriage shape), toward the other end-to-end connection. equivalent of a road system, where any type of vehicle There's an economic and fault-tolerance factor you ranging from a motorcycle to a truck can travel on should keep in mind when considering the router-using a single, simple infrastructure. Like the road system (and flavor of packet switching. The economic issue revolves unlike the rail), traffic is granted its own autonomy. around the relative costs of computer cycles and Instead of working to a strict, centrally controlled time- communications circuits. If processing power is cheap, table and signaling system, the packets simply set out and it's not unreasonable to figure out the routes every packet steer themselves toward their destinations. should take in order to make the best use of expensive The aptly-named "superhighway" therefore has circuits. If circuits are cheap, you may prefer to set up a series of advantages. It will not only be able to run a mesh of connections using relatively dumb circuit different sorts of traffic at high speed, but it will be switches, rather than dedicate a bunch of high-powered, designed in such a way that it can accommodate further special-purpose computers to routing packets. change. Unlike the real-world highway, the telecom The fault-tolerance issue (often expressed as the highway can mandate faster and faster speeds and a vast overheated claim that the Internet was designed to sur- increase in the number of lanes running between any two vive a nuclear war) is that connectionless networks are points. Like the real-world highway, its strength is its more resilient than connection-oriented ones. If a link or simplicity. The self-addressed packet standard is the a router goes down, the overall system is designed to find equivalent of tarmac. The telecom equivalent of new routes around the problem. A broken conn