as transport networks, or regional transport networks. They link hubs to headends. MANs typically operate over shorter distances than core, backbone networks. They carry a mix of voice, data, and video on demand, as well as television signals. Older MANs transmit traffic to headends over Synchronous Optical Network (SONET) rings. Cable operators and traditional telephone companies have transi- tioned some of their metropolitan networks to all-IP, Gigabit Ethernet networks or to a combination of SONET and 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. Unlike large core networks, they don't involve the immense upgrades as do core networks. MANs are equipped with either redundant fiber rings or the simpler Optical Transport Network (OTN) option as backup tech- nologies, 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. Mid- dle-mile networks can be the source of congestion on both wireless and wireline net- works because they transport increasing amounts of Internet, smartphone, and video traffic. This is straining existing infrastructure. 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 networks, the POP serves as the loca- tion where a provider's switches and connections for multiple carriers connect to the Carrier Networks Microwave middle-mile link between the last mile and the backbone (core) Rural carrier ABC Interconnection points with Ethernet or SONET and amplification gear Provider connection to backbone the Internet Fiber middle-mile link Rural carrier