To help prevent call bridging and increase the reliability of channel de-assignment in a trunked radio repeater system, the working channel sends a drop channel "pre-alert" message to on-channel mobile units just before sending the drop channel signal. This "pre-alert" message instructs the mobile units...http://www.google.de/patents/US5086507?utm_source=gb-gplus-sharePatent US5086507 - Drop channel pre-alert for trunked radio communication system
