services are not limited to low-priced devices and low monthly minutes of usage. Carriers are also starting to introduce prepaid as an adjunct to postpaid services. For example, a subscriber might have an unlimited plan for voice and text messaging but pay-as-you-go plans for data. Some subscribers who have Wi-Fi-enabled tablet computers might only need to purchase data service during months when they travel or plan to be in locations without free Wi-Fi. In addition, parents might want to purchase prepaid voice or data services for teenagers or even themselves as a way to control monthly mobile expenses. The advantages for carriers are mainly the elimination of the costs associated with issuing bills and collecting overdue payments. For some carriers, it's a way to attract lower-income or immigrant customers who don't have credit. In countries where mobile penetration is high and most people already have existing postpaid service, prepaid is another way to add customers. Prepaid services are offered both on a retail and wholesale basis. Sprint Nextel has four different prepaid product lines that it offers in addition to its postpaid offer- ings. Other prepaid companies such as America Móvil's Straight Talk sign intercon- nect agreements with carriers such as AT&T who it pays to carry prepaid traffic in the United States. America Móvil is headquartered in Mexico and offers service through- out Latin America. Straight Talk is its mobile virtual network operator. It resells pre- paid on other carriers' networks outside of its own coverage area. Whether purchasing prepaid directly through a carrier or through a reseller, cus- tomers are connected to a prepaid platform, which authenticates their device and tracks usage to determine either whether to charge a credit card or if the subscriber's amount of time or usage has been used up. The prepaid platform is connected to the carrier's core mobile network. Wal-Mart sells prepaid services using Straight Talk's prepaid platform. WI-FI STANDARDS, ARCHITECTURE, AND USE IN C