DSL. However, their largest investments are in mobile networks and networking and cloud services to enterprise customers. Mobile Operators AT&T Mobility and Verizon Wireless are the largest mobile operators in the United States; they are also the only ones with nationwide coverage. Each of their networks covers areas that encompass approximately 300 million of the 312 million people in the United States, as of June 2011. Verizon Wireless is 55 percent owned by Verizon Communications; the United Kingdom's Vodafone owns the remaining 45 percent. Cellular service, which was first available in 1984, was primarily used for voice until the late 1990s when text messaging became available on GSM mobile services. By 2001, Vodafone reported that it derives more revenue from text messaging than voice services. Advancement in mobile technologies has resulted in subscribers increasingly using their handsets for Internet access and e-mail as well as text messaging. Subscrib- ers have substituted text messaging and e-mail for voice calling and are purchasing multipurpose smartphones that handle Internet access and apps in addition to voice. As mentioned earlier in the chapter, the term "app" is short for an "application" designed The State of the Industry specifically for particular smartphones and tablet computers (as opposed to compre- hensive and full-featured applications designed for use on more powerful computers, such as Microsoft Word or Adobe Photoshop). In the face of declining voice revenue, carriers are seeking to increase revenue and profits by promoting data plans to customers and upgrading networks for additional capacity. These network upgrades require sizable capital investments in equipment and spectrum. Spectrum is made up of the airwaves that carry wireless signals. See Chapter 7 for more information on spectrum and mobile technologies. Carriers now also "cap" the amount of data that subscribers can transmit monthly and offer tiered options, with higher rates applied to customers who transmit higher amounts 