A single international standard integrated services digital network (ISDN), basic rate interface (BRI) is arranged so that either a data terminal and two voice terminals, three voice terminals, or four voice terminals can be operated simultaneously through the interface. Code converters, associated with...http://www.google.de/patents/US4970723?utm_source=gb-gplus-sharePatent US4970723 - ISDN, basic rate interface arranged for quad voice
