Data communication networks : transmission, signalling and switching, network aspects, maintenance and administrative arrangements: recommendations X.40 - X.181 / International Telecommunication Union
Data communication networks : open systems interconnection (OSI) model and notation, service definition: recommendations X.200 - X.219 /International Telecommunication Union
Data communication networks : interworking between networks, mobile data transmission systems, internetwork management: recommendations X.300 - X.370 /International Telecommunication Union
Data communication networks : message handling systems: recommendations X.400 - X.420 /International Telecommunication Union