Last year, Mexican police raided the drug cartel Los Zetas three times. They weren't looking for cocaine or marijuana; they were targeting the homegrown radio network that stretches across the northeastern chunk of the country, where the Zetas control drug and migrant smuggling routes. Los Zetas—comprised of former members of Mexican special forces—have been building the system of radio towers and receivers since 2006. The cartels divide up their territory into plazas, each with a boss responsible for buying and maintaining equipment and distributing handheld radios. By controlling their own mobile network, the Zetas can rapidly inform its members about an imminent raid by police or a rival cartel in a controlled and encrypted fashion.
Transmission technology is cheap. The Zetas can easily upgrade and maintain infrastructure, despite the raids. No sooner than masts, transmitters, and receivers are confiscated, they are replaced again.