A voltage controlled crystal oscillator 25, for example, as used in a mobile communications terminal 23, has its output frequency Fref stabilised against temperature drift using frequency correction information received, for example, in a downlink signal 35 from a base station 33. The feed unit comprises a plurality of filling channels. The classroom barrier is simple to operate, bullet resistant, and impenetrable for some predetermined length of time. Contemplated messages include digitized video or other movies, books, music, or any other type of information.