the signaling network to central- ized Network Operations Centers (NOCs) where technicians see visual indications of alarms on wall-mounted, computerized displays. Moreover, sections of carrier net- works can be quickly reconfigured from commands sent by centralized network con- trol centers. In addition, when carriers transmit traffic to other providers, signaling messages are used to identify each carrier's traffic. This is the method by which carriers bill one another for services that they provide. No carrier has facilities everywhere, SO they must rely on other providers' networks outside of their coverage area. For example, a provider that sells toll-free service to customers nationwide pays a fee for each call that terminates in another carrier's coverage area. Termination refers to the traffic destina- tion. In the preceding example, a call terminates at the subscriber's location where it is answered. Signaling in Mobile Wireless Networks Second-generation (2G) and third-generation (3G) cellular networks use Signal- ing System 7 (SS7) technology to support roaming for voice traffic. Every cellular Signaling provider has a database called the Home Location Register (HLR) where complete information is kept about each subscriber. They also have a Visitor Location Register (VLR), which keeps temporary records for callers who are visiting from other areas. When a cellular subscriber roams, each visited system exchanges SS7 messages with the "home" system. The home system also marks its HLR SO that it knows where to send calls for roaming customers. As mobile networks transition to fourth-generation services, they will use differ- ent signaling protocols. Long-Term Evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMAX) are fourth-generation (4G) upgrades that provide higher capacity for data and voice. In the United States, AT&T and Verizon are rolling out LTE. Sprint is converting its WiMAX network to LTE. 3G mobile protocols specify circuit switching for voice and IP for data traffic, adh