d architecture. A key advantage in this topology is that there is no single point of failure. The ring architecture has the same disadvantages as the meshed architecture. Ring topology is shown in Figure 9-5. After you've seen the four types of network topology in an optical wireless system, it is useful to see an example of how carriers apply them in a densely Figure 9-4 Meshed FSO system configuration. Free space optical link Network Chapter 9 Figure 9-5 Ring FSO topology. Network populated high-rise office building environment, or more specifically, in a MAN environment. In Figure 9-6,3 the tall light grey building represents a carrier's point of presence (POP) that serves as a gateway between a WAN and MAN. The MAN is established via an optical wireless (FSO) system. The carrier's POP exchanges information with its nearby hubs (dark grey build- ings) via back-haul FSO links. In this example, these back-haul FSO links are configured in a combined ring and meshed topology. Communication between the carrier's hub and its serving customer premise buildings (white buildings connected by thin lines to the hub buildings) is set up in a hub- and-spoke or point-to-multipoint topology. Figure 9-6 is an example of how these network topologies can be applied to the MAN environment. 9.1.2 Key Network Elements An FSO system consists of two major network elements: free-space optical telescopic transceivers and network management software and hardware. The free-space optical transceiver acts as a transmitting and receiving ter- minal for transporting optical signals to another transceiver. For the trans- ceivers to interface properly with an area network such as a LAN and MAN, network management software and hardware are necessary. The hardware would be a switch that is usually bundled with the transceiver from the same manufacturer. The network management software controls how the optical signals are transmitted and received from the transceiver and how the signals are routed to an area network. If high availability is a cr