General characteristics of international telephone connections and circuits : recommendations G.100 - G.181 / International Telecommunication Union
International analogue carrier systems : recommendations G.211 - G.544 / International Telecommunication Union
General aspects of digital transmission systems : terminal equipments: recommendations G.700 - G.795 / International Telecommunication Union
Digital networks, digital sections and digital line systems : recommendations G.801 - G.961 / International Telecommunication Union