ost instantaneously compensate for an outage. What they are referring to is survivability, which was one of the design goals of SONET and SDH. To accomplish this goal, both technologies support a self-healing feature that enables a fiber cut to be immediately recog- nized and compensated for by switching traffic onto a different path formed by a ring structure. The alternate path is built into one design Fiber in the WAN Figure 6.11 A hub or star net- work configuration. DACCS Legend: DACCS Digital Access and Cross-Connect System Path Terminating Terminal Multiplexer Repeater for SONET and SDH. However, there are two types of rings: a path- switched ring, which switches individual paths, and a line-switched ring, which switches the entire capacity of the optical line. The key differ- ences between the two ring topologies are the number of fibers used and the resulting cost of the ring. A path-switched ring uses two fibers and is less expensive to construct than a line-switched ring, which can use either two or four fibers. As we examine each type of ring, we will note that a line-switched ring can provide an additional level of switch- ing capability over a path-switched ring. PATH-SWITCHED RING A path-switched ring uses two fibers to form a concentric ring through the routing of point-to-point lines connecting individual nodes within a network. For each path between two nodes data are transmitted in both clockwise and counterclockwise directions, resulting in the term bidirectional ring used to refer to a path-switched ring configuration. Each receiving section in each node monitors both signals and selects the better one. Thus, any break in a signal between a pair of nodes allows the receiver to select the other fiber as a mecha- nism to continue operations. LINE-SWITCHED RING In comparison to path switching, a line-switching topology provides the capability to switch paths at any node within a ring if a fiber failure occurs. Thus, a line-switching ring enables switching at Chapter Six either the line or the pa