The series of ITU-T Recommendations specifying protocols relating to data networks, open systems communications, and security. See Table X-2 for selected X-series Recommendations by 100s group, and Table X-3 for specific recommendations of interest. For a full listing of ITU-T Recommendations, see the contact information in Appendix A.

Table X-2: ITU-T X Series Recommendation Groups

Recommendation Group	Description
100	Data communications and networks
200	Open systems interconnection
300	Data transmission services between public networks, and between public networks and other networks
400	Message Handling Services and Systems (MHS)
500	Open Systems Interconnection (OSI): Directory services
600	Open Systems Interconnection (OSI): Multi-peer communications
700	Open Systems Interconnection (OSI): Systems Management
800	Open Systems Interconnection (OSI): Security
900	Open Distributed Processing

Table X-3: Select X Series Recommendations

Recommendation	Description
X.3	Packet assembly/disassembly facility (PAD) in a public data network (PDN)
X.20bis	Use on public data networks (PDNs) of Data Terminal Equipment (DTE) designed for interfacing to asynchronous duplex V-Series modems
X.21	Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for synchronous operation on public data networks
X.21bis	Use on public data networks of Data Terminal Equipment (DTE) which is designed for interfacing to synchronous V-Series modems
X.25	Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and connected to public data networks (PDNs) by dedicated circuit
X.28	DTE/DCE interface for a start-stop mode Data Terminal Equipment accessing the Packet Assembly/Disassembly (PAD) in a public data network (PDN) situated in the same country
X.29	Procedures for the exchange of control information and user data between a Packet Assembly/Disassembly (PAD) and a packet mode DTE or another PAD
X.31	Support of packet mode terminal equipment by an ISDN
X.32	Interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and accessing a packet-switched public data network through a public switched telephone network (PSTN) or an integrated services digital network (ISDN) or a circuit-switched public data network (PDN)
X.75	Packet-switched signaling system between public networks providing data transmission services
X.121	International numbering plan for public data networks (PDNs)
X.122	Numbering plan interworking for the E.164 and X.121 numbering plans
X.140	General quality of service parameters for communication via public data networks (PDNs)
X.141	General principles for the detection and correction of errors in public data networks (PDNs)
X.142	Quality of service metrics for characterizing frame relay/ATM service interworking performance
X.144	User information transfer performance parameters for public frame relay data networks
X.145	Connection establishment and dis-engagement performance parameters for public frame relay data networks providing SVC services
X.146	Performance objectives and quality of service classes applicable to frame relay
X.147	Frame relay network availability
X.148	Procedures for the measurement of the performance of public data networks (PDNs) providing international frame relay service
X.149	Performance of IP networks when supported by public frame relay data networks
X.151	Frame relay operations and maintenance -- Principles and functions
X.200	Information technology -- Open Systems Interconnection (OSI) - Basic Reference Model: The basic model
X.300	General principles for interworking between public networks and between public networks and other networks for the provision of data transmission services
X.400	Message handling services: Message handling system and service overview
X.500	Information technology -- Open Systems Interconnection (OSI) - The Directory: Overview of concepts, models and services
X.601	Multi-peer communications framework
X.700	Management framework for Open Systems Interconnection (OSI) for CCITT applications
X.800	Security architecture for Open Systems Interconnection for CCITT applications
X.901	Information technology - Open Distributed Processing - Reference Model: Overview