wn among many industry players as the provider network. An ISP's network is a typical example of an access network where the boundaries among a local telephone company, a long-distance company, and an ISP become more blurred. Everyone is stepping on everyone else's toes in a competitive stampede that is bound to reshape the industry landscape while optimizing the communications services and cost. Access networks consolidate (aggregate) customer traffic from the humble home-based PC modem users to the more sophisticated broadband cable clients. These networks prepare to feed the traffic through a larger pipe into the WAN. This could be done over the Plain Old Telephone Service (POTS), the Internet, or something else. Cable-based broadband access clients are multiplexed through the local cable-TV company's head-end equipment. The client might also use some sort of Digital Subscriber Line (DSL) modem or the latest wireless broadband last-mile access technology. It does not take a rocket scientist to realize that the provider networks also comprise two types of routers: provider network core routers and provider network edge routers. These routers are illustrated in Figure 3.1, which provides a macroscopic view of reality. Once again, the terms edge and core are used loosely here SO we will not follow this example. The typical speeds encountered inside an access network currently range between OC-3 and OC-48. The top layer in this hierarchy is the WAN, which interconnects provider or carrier networks and is often referred to as the backbone (for example, of the Internet). The WAN also contains edge and core routers. The transmission technologies most often used at the WAN level are optical. The typi- cal speeds achieved on a WAN currently range between OC-48 and OC-192. In some metropolitan areas, a trend is evolving to adopt the new emerging 10 Gigabit Ethernet as well. Historically, the metropolitan network was largely used as a transport-layer medium (such as Synchronous Optical Network/Synchronous Digital Hierarc