Basically, it represents a combined operation of relatively high-powered transmitters in broadcast configuration (large cells serving each a large area of the order of 50-70 km of diameter), and of relatively low-powered base-stations in a mobile network configuration (small cells serving each an area of up to a few kilometres). Programmes that attract a large audience are transmitted via the high-power broadcast network whilst programmes that are followed by a smaller audience make use of the small-cell configuration. The switch may be dynamic with time as audience levels change.