e sending end and from digital to analog at the receiving end. All digital cellular services improve privacy. Snoopers with scanners can easily listen in on analog cellular signals. Eavesdropping on digital transmissions is more dif- ficult because the digital bits are scrambled before they are transmitted over the open air between handsets and an operator's equipment. Because 2G service is designed for voice traffic, it is not efficient for transmitting data. Therefore, some carriers have started using spectrum on which 2G service oper- ates for 4G service. This is referred to as refarming. They have decommissioned some 2G services and use the spectrum for 3G or 4G services. Refarming refers to changing the type of air interface deployed on particular spectrum bands. The two most prevalent 2G air interfaces in the world, in order of worldwide sub- scriber use, are Global System for Mobile Communications (GSM) and Code-Division Multiple Access (CDMA). Mobile and Wi-Fi Networks GSM Service GSM is the most widely used 2G air interface worldwide. Its strength lays in its wide- spread global penetration, which enables consumers to use their telephones when they travel. GSM phones have clip-on Subscriber Identity Module (SIM) cards. These cards contain a microchip that stores user identity and other information such as speed-dial lists. The SIM card encrypts voice and data before they are transmitted. GSM is the digital cellular standard that was originally decided on by European governments and was first deployed in 1992. GSM divides channels of 200KHz spec- trum into eight time slots. Seven of the time slots carry traffic and the eighth carries control signals. The control channel also carries Short Message Service (SMS), which is otherwise known as text messaging. GSM uses a form of Time-Division Multiplex- ing (TDM). T-Mobile USA and AT&T Mobility operate GSM networks in the United States. Roaming Using Mobile Devices in Other Networks Roaming is the capability to use voice or mobile data devices in another carrie