Data communication over the telephone network : series V recommendations / International Telecommunication Union
Data communication networks : services and facilities, interfaces: recommendations X.1 - X.32 / 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 : open systems interconnection (OSI) protocol specifications, conformance testing: recommendations X.220 - X.290 /International Telecommunication Union