sive Optical Networks (PONs), discussed later in this chapter. PONs are a lower-cost method of extending fiber to premises and neighborhoods. Using Ring Topology for Greater Reliability SONET and Ethernet can operate as a straight point-to-point line between sites, or in the more fail-safe ring topology. When fiber in a point-to-point arrangement is cut, ser- vice is lost. However, reliability on fiber is critical because each failure affects poten- tially hundreds or thousands of customers, particularly if a failure occurs in long-haul networks. Topology refers to the design of how devices are physically connected. It is the "view from the top." When a medium such as copper carries a conversation from one telephone sub- scriber to another, a failure only impacts one customer. Because of the large volumes transmitted by fiber, failures in these networks can put hundreds of businesses, police stations, or hospitals out of service. For this reason, the majority of carriers deploy bidirectional ring topology in long-haul and MANs, where each fiber ring, multiplexer, and power supply is duplicated. Carrier Networks Redundant Fiber Rings in Core and Metropolitan Networks Redundant connections to local central office Fiber rings Chicago Los Angeles Washington Dallas Figure 4-2 Fiber rings between Points of Presence (POPs), and a connection to a local provider in each city. Each POP has a switch and redundant SONET or Ether- net gear connected to the fiber rings. Ring topology is costly because the fiber and the multiplexers are all duplicated, even though this combined capacity is not used on a day-to-day basis. In the bidirec- tional SONET/SDH or Ethernet ring, one set of fiber strands is used for sending and receiving; the other is the protect ring (spare ring). If one set of fiber strands is broken or the electronics on it fail, the protect ring reroutes traffic in the other direction, as is illustrated in Figure 4-2. Read the section "Mesh Configuration Backups," later in this chapter, to learn about new, lower-cos