In a push-to-talk communication system, a domain name server transfers a registration server role, call server role, and media server role. A first server receives the registration server role and responsively registers push-to-talk users and maintains a first time of day. A second server receives the...http://www.google.com.hk/patents/US7983705?utm_source=gb-gplus-share專利 US7983705 - Push-to-talk communication management system
