or routing traffic, and the media server plays announce- ments and detects tones. The session border controller converts signals between networks to compatible types of signals. Employing Redundancy to Back Up Centralized IP Servers Redundancy is particularly critical for centralized VoIP servers because they support large numbers of subscribers. Newer ones are capable of supporting up to 600,000 VoIP lines. If any one of these servers fails, entire VoIP networks can be incapacitated. The most common causes of failure are power losses and fiber-optic cabling cuts. To protect networks from these outages, providers often build backup Network Opera- tions Centers (NOCs) in different geographic regions. The main centers have the abil- ity to automatically fail over to the backup center in the event of a malfunction. Once the main center is operational again, service automatically reverts to it, as depicted in Figure 4-5. Carrier Networks Managed IP core NOC 1 (West) NOC 2 (East) Failover/back Router Router Network Operations Centers (NOCs) with VolP servers IP access network CM = Cable modem = Routes for failover to NOC 2 and service resumption at NOC 1 Figure 4-5 A Network Operations Center (NOC) with paths (routes) to a backup NOC. (Courtesy BroadSoft) IP Multimedia Subsystem for Mobile and Wired Access to an Application IP Multimedia Subsystem (IMS) is an open-standards architectural framework made up of multiple protocols and standards by which customers can access the same appli- cation from both wireline and mobile devices. These applications, including unified messaging and multiplayer games, are installed in a provider's network. In essence, IMS separates the access signals from the application. It is a form of Fixed Mobile Convergence (FMC) with which both wireline and wireless network operators can install and maintain a single application for their customers. Subscribers can then use these core applications on an interoperable basis. IMS provides methods to convert signals from mobile switches to those com