or headquarters crashes, all sites connected to it are also out of service. This is illustrated in Figure 5-9. Mesh design All points on the network nodes connect to one another in a flat or nonhierarchical manner. Peer-to-peer networks for music sharing are examples of mesh networks. Carriers often design their core, backbone networks in a mesh configured with multiple failover routes. Broadband and Wide Area Network Services Seattle Gigabit Internet Ethernet Los Angeles Centralized applications at headquarters T1 at smaller sites Houston Miami WAN = Wide Area Network Figure 5-9 A hub-and-spoke design for centralized applications such as e-mail. The disadvantage here is that if e-mail or another application at the central site crashes, all sites lose access to it. Integrated Services Digital Network Integrated Services Digital Network (ISDN) is a worldwide public standard for send- ing voice, video, data, or packets over the Public Switched Telephone Network (PSTN) in a digital format. There are two "flavors" of ISDN: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). However, manufacturers implemented some forms of ISDN differently from one another. This resulted in some incompatibilities between ISDN in Europe, North America, and Japan. PRI ISDN is used mainly in enterprise call centers and for other incoming calls to commercial organizations. The use of BRI ISDN is declining. When it is used, it's mainly deployed for videoconferencing. PRI and BRI ISDN use out-of-band signaling to carry dialed digits, caller identification, dial tone, and other signals. ISDN works over existing copper, fiber-optic, or satellite media. ISDN is gradually being replaced by IP services because of their greater efficiency, capacity, and flexibility. Private Lines, Network Topology, and Frame Relay The Similarities between PRI ISDN and T1 PRI has twenty-four 64-kilobit channels in the United States and Japan and 30 else- where in the world. PRI lines are similar to T1 because they both have 24 channels; however, PRI ISDN 