Data communication networks : services and facilities, interfaces: recommendations X.1 - X.32 / International Telecommunication Union
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) protocol specifications, conformance testing: recommendations X.220 - X.290 /International Telecommunication Union
Data communication networks : interworking between networks, mobile data transmission systems, internetwork management: recommendations X.300 - X.370 /International Telecommunication Union