CDMA) and began upgrading to pre-4G Long-Term Evolution (LTE) service in 2011. Meanwhile, European standards bodies adopted GSM as a uniform technology for all of Europe. GSM was first used for commercial service in 1992. Prior to standard- izing on GSM, Europe had a mix of incompatible analog technologies. The presence of a uniform air interface made using the same mobile handset across the continent convenient. Other countries around the globe adopted GSM because of roaming compatibility. Mobile and Wi-Fi Networks Connections to Customers and Mobile Networks via the Cell Site cell is the physical area in which a set of frequencies is used. Each cell site has a Base Transceiver Station (BTS) and antenna. (The BTS has different names in later- generation services.) With the increase in pedestrian use of mobile telephones, more cell sites are located in shopping malls and downtown areas. Carriers often share space at cell sites to save costs. For example, a large operator might lease space on a tall water tower for its antenna and on the ground for ancillary equipment. Companies such as Crown Castle and American Tower in the United States, spe- cialize in building towers and leasing space for cell sites. Instead of a single mobile carrier owning a tower or the land around the tower, a third-party company builds the tower and leases space on it as well as space for the rest of the equipment at base sta- tions. This spares carriers from investing capital in towers and real estate. In Europe, carriers often share towers. In these arrangements, one of the carriers leases space at the tower to other carriers. Mobile handsets and data devices are connected to cellular networks by the BTS. The BTS is connected to the antennas by coaxial cabling. It transmits and receives mobile calls from the cell site's antenna and amplifies (strengthens) signals. It also performs the conversion of signals between over-the-air radio frequency formats such as GSM and CDMA to those compatible with landline networks, and vise versa. The Ba