Telephony

Mobile phones transmit to a local cell site (transmitter/receiver) that ultimately connects to the public switched telephone network (PSTN) through an optic fiber or microwave radio and other network elements. When the mobile phone nears the edge of the cell site's radio coverage area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital modulation schemes. Recent developments in Sweden (such as DROPme) allow for the instant downloading of digitial material from a radio broadcast (such as a song) to a mobile phone.Satellite phones use satellites rather than cell towers to communicate. They come in two types: INMARSAT and Iridium. Both types provide world-wide coverage. INMARSAT uses geosynchronous satellites, with aimed high-gain antennas on the vehicles. Iridium uses 66 Low Earth Orbit satellites as the cells.