For the first 100 years of telephony, each telephone required a wire from the home or office to a central location in the telephone network (the central office). After radio was invented, car telephones became available, but only since the early 1980s could large numbers of people have car telephones, now called cellular phones. In the 1990s, there has been an explosion in wireless communications. Wireless technology has expanded from high-tier systems for vehicles and individuals to low-tier systems for the home and office. The design of high-tier systems differs from that of low-tier systems.