The Global Dialing Scheme (GDS) is numbering plan for H.323 audio-visual communication networks (often used for videoconferencing). Based on the numerology provided by the United Nations International Telecommunications Union, GDS numerology resembles the international telephone system numbering plan, with some exceptions. The Global Dialing Scheme uses a hierarchy of gatekeepers to route call set-up information nationally and internationally. National gatekeepers have knowledge of all zones within a country, World gatekeepers have knowledge of all National gatekeepers.