 to Packet Switching Sometimes it helps to think about telecom networks in the context of something familiar, such as the transportation system. First, consider the physical roadways. A standard plain old telephone service (POTS) line or traditional twisted-pair alternative is equivalent to a gravelly, potholed alley-allowing only one lane of traffic in each direction that can travel at low speed due to various physical impairments. That alley, when widened and repaved, Establishing Connections: Networking Modes and Switching Modes can allow vehicles to travel more quickly, improving performance, much as DSL does for twisted-pair. These alleyways may feed into a main street, with a couple lanes of traffic each direction; we can think of this as the coaxial network. The main street, in turn, may feed into a highway with multiple lanes of traffic in each direction, and this would be our fiber-optic highway. As the air serves airplanes, the air also serves our wireless systems. The next step in this comparison is to consider the various points at which roadways need to intersect and merge. This is the province of switches and routers. A switch is anal- ogous to a four-way intersection: It connects separate roadways, much as a switch con- nects two individual transmission lines (indeed, rail yards have numerous switches that direct trains among different tracks). A router is more like an access road off the highway, directing you one step closer to your destination (e.g., go right to points east; go left to points west). The final element is the vehicles, which are the packets in the telecom realm. Some vehicles are large and can carry a lot of passengers with only one driver, such as a bus; in the case of telecom, this is analogous to IP. Others offer high performance and advanced navigational controls but can carry only one passenger along with the driver, such as a Ferrari; in the communications context, this is ATM. The bus has the benefits of carrying more passengers with less overhead, but it is larger, and the