Data communication networks : open systems interconnection (OSI) model and notation, service definition: recommendations X.200 - X.219 /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 : message handling systems: recommendations X.400 - X.420 /International Telecommunication Union
Data communication networks : directory: recommendations X.500 - X.521/International Telecommunication Union