ities have nodes with wiring center (transmission) equipment. Older MANs transmit traffic to headends' SONET rings. Cable operators and traditional telephone companies are transitioning parts of their metropolitan networks to all-IP, Gigabit Ethernet. It is complex and costly to upgrade all of a carrier's MANs simultaneously. However, carriers have the option of gradually upgrading a few networks at a time. These upgrades don't involve the immense upgrades as those in core networks. MANs are equipped with either redundant fiber rings or the simpler Optical Transport Network (OTN) option as backup technologies in the event of a fiber cut or equipment failure. MIDDLE-MILE NETWORKS Middle-mile networks are the sections of networks between the access provider and the connections that carry traffic to the Internet and to national and international core networks. Figure 4-8 shows an example of connections on middle-mile networks. Middle-mile networks can be the source of congestion on both wireless and wireline networks because they transport increasing amounts of Internet, smartphone, and video traffic. This is straining existing infrastructure. backbone (core) Rural carrier ABC www.TechnicalPDF.com Ethemetor SONET A amplification gear Provider backbone connection to the Internet Fiber middle-mile link Rural carrier XYZ POP=Pointof Presence Figure 4-8 The middle-mile connection between rural last-mile providers and a core network interconnection point for transport to the Internet. Middle-mile networks include the fiber or wireless connections between switches in small cities and the carrier's equipment that transmits traffic to a major carrier's Point of Presence (POP). Within middle-mile www.TechnicalPDF.com networks, the POP serves as the location where a provider's switches and connections for multiple carriers connect to the Internet or to a national or undersea cable. The core switches of large providers are also located at POPs. Independently operated data centers where data is managed for multiple enterprises 