A two-way paging system utilizes four local frequencies for transmissions between pager units (22) and a central control station (20). A first local frequency (f.sub.1) carries a local clock; a second local frequency (f.sub.2) carries communications packets from the central control station to paging...http://www.google.fr/patents/US5613212?utm_source=gb-gplus-shareBrevet US5613212 - Paging method and apparatus
