ireless networks must meet government specifications to receive certifications. That is why even a Bluetooth mouse is certified and assigned an FCC ID number. There are no assurances that congestion from many networks in the same unlicensed spectrum will not interfere with one another or overwhelm capacity. Second-, Third-, and Fourth-Generation Digital Networks SECOND-, THIRD-, AND FOURTH-GENERATION DIGITAL NETWORKS Each successive generation of digital networks uses spectrum more efficiently and provides capabilities for additional features. Third-generation (3G) networks have improved capabilities to carry data. Fourth-generation (4G) networks support broad- band services, and in the future they will replace fixed-line Internet access services such as Digital Subscriber Line (DSL). Second-Generation Digital Service Although second-generation (2G) digital cellular service was first deployed in the 1990s, it is still the most prevalent form of cellular technology, worldwide. 2G cellular service provides more capacity than analog mobile service and enables features such as caller ID, speed dialing, and voicemail message notification in the form of stutter dial tone. These features were not available on analog cellular service. Digital wireless services use multiplexing access techniques to carry more voice traffic on the same amount of spectrum than the analog cellular service they sup- planted. Mobile carriers introduced disparate, incompatible multiplexing techniques, referred to as air interfaces. The air interface in the customer's device must match the one used in the carrier's cell site. Thus, although all digital services have more capac- ity than analog cellular, the air interfaces added for additional capacity are not always compatible with one another. Digital handsets include Digital Signal Processors (DSPs) to convert analog voice signals into a digital format. DSPs are specialized, very-high-speed computer chips. The DSPs code and decode the voice signals, converting them from analog to digital at th