A network system includes a plurality of terminals classified into a plurality of groups, a network for selectively connecting the plurality of terminals to each other, and a control part for receiving a call indicating one of the groups from a source terminal which is one of the plurality of terminals...http://www.google.com.tw/patents/US5276687?utm_source=gb-gplus-share專利 US5276687 - Network system having different attributes of terminal equipment devices
