pervasive use of mobile phones for voice- and data-communication.) In the United States, engineers from Bell Labs began work on a system to allow mobile users to place and receive telephone calls from automobiles, leading to the inauguration of mobile service on 17 June 1946 in St. Louis, Missouri. Shortly after, AT&T offered Mobile Telephone Service. A wide range of mostly incompatible mobile-telephone services offered limited coverage areas and only a few available channels in urban areas. As calls were transmitted as unencrypted analog signals, anyone with radio equipment that could receive those frequencies could eavesdrop. The commercial introduction (in Japan in 1979) of cellular technology, which allowed re-use of frequencies many times in small adjacent areas covered by relatively low-powered transmitters, made widespread adoption of mobile telephones economically feasible. In the USSR, Leonid Kupriyanovich, an engineer from Moscow, developed and presented a number of experimental pocket-sized communications radios in 1957–1961. The weight of one model, presented in 1961, was only 70 g and could fit in a palm. However, in the USSR the decision at first to develop the system of the automobile "Altai" phone was made. In 1965 the Bulgarian company "Radioelektronika" presented a mobile automatic phone combined with a base station at the Inforga-65 international exhibition in Moscow. Solutions of this phone were based on a system developed by Leonid Kupriyanovich. One base station, connected to one telephone wire line, could serve up to 15 customers. Advances in mobile telephony can be traced in successive generations from the early "0G" services like MTS and its successor Improved Mobile Telephone Service, to first-generation (1G) analog cellular networks (1979–), second-generation (2G) digital cellular networks (1991–), third-generation (3G) broadband data services (launched commercially in 2001) to the fourth-generation (4G) native-IP networks (launched in 2006 in South Korea). 5G began deployment in 2019. Early services MTS In 1949, AT&T commercialized Mobile Telephone Service. From its start in St. Louis, Missouri, in 1946, AT&T introduced Mobile Telephone Service to one hundred towns and highway corridors by 1948. Mobile Telephone Service was a rarity with only 5,000 customers placing about each week. Calls were set up manually by an operator and the user had to depress a button on the handset to speak and release the button to listen. The call subscriber equipment weighed about Subscriber growth and revenue generation were hampered by the constraints of the technology. Because only three radio channels