Message handling and directory services operations and definition of service : recommendations F.400 - F.422, F.500 / International Telecommunication Union
General characteristics of international telephone connections and circuits : recommendations G.100 - G.181 / International Telecommunication Union
Transmission media characteristics : recommendations G.601 - G.654 / International Telecommunication Union
General aspects of digital transmission systems : terminal equipments: recommendations G.700 - G.795 / International Telecommunication Union