A communications system for carrying messages via a radio channel between one central site of a plurality of central sites and a plurality of two-way remote data units is disclosed. Each central site has a radio coverage area and each remote unit has a unique address and association with one of the central...http://www.google.com.au/patents/US4644351?utm_source=gb-gplus-sharePatent US4644351 - Two way personal message system with extended coverage
