There exist a lot of wireless communication modules, all approved for use within the ISM radio bands (industrial/scientific/medical), for example, 433 MHz, 866 MHz and recently also 2.4 GHz. You get simple and cheap ones with low transmission data rates, and you can find excellent high-speed systems. “How much (will the thing set me back)?” is the most frequently asked question if you search for an RF module. Low-speed non-intelligent modules are cheap; high-speed intelligent ones, pretty expensive. That's easy but it doesn't help.
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