The International Telegraph and Telephone Consultative Committee (CCID) is the most prominent organization for international telecommunication standards. CCITT is organized into study groups, which are further subdivided into committees, to study and make recommendations regarding international telecommunications issues and concerns. The most important series of ISDN are
• I-series -which contains the overall ISDN standards
• X-series- which includes data-communication standards
• Q-series- which provides signaling standards