in which an STS-N signal builds up. For example, an STM-1 frame would be represented as 9 rows times 270 columns, similar to an STS-3 frame, but using different technology. Now that we have a general idea of the SDH hierarchy, let's conclude this section by discussing some of the topologies supported by SONET and SDH. Networking Topologies Both SONET and SDH support a number of networking topologies. Those topologies range in scope from a simple point-to-point line struc- ture to a self-healing ring architecture. Fiber in the WAN POINT-TO-POINT CONFIGURATION The most common and least costly network architecture supported by SONET and SDH is point-to-point network configuration. This network configuration is commonly used to access a ring and also functions as a basic block for the construction of a ring. While a point-to-point configuration is more economical than a ring, many subscribers are now electing to access a carrier's communications infrastructure via a local ring since it provides a local loop fault-tolerance capability. Figure 6.10 illustrates two versions of SONET and SDH point-to-point topologies. A basic point-to-point network configuration is shown at the top portion of the figure. Note that the path terminating terminal mul- tiplexer (PTM) serves as a device which multiplexes multiple DS1s, E1s, and other signals onto an OC-1 signal in a SONET environment or onto the signal for an STM-0 frame in an SDH environment. As illustrated at the top of Figure 6.10, a basic point-to-point network configuration includes two PTMs connected via fiber, which may or may not require one or more optical repeaters or amplifiers. POINT-TO-MULTIPOINT CONFIGURATION The basic point-to- point network structure shown at the top of Figure 6.10 can be easily modified into a point-to-multipoint structure. This modification is a. Point-to-point Figure 6.10 Point-to-point and point-to-multipoint configurations. b. Point-to-multipoint Legend: Path Terminating Terminal Multiplexer Add/Drop Multiplexer Repeater Chapter Six accom