1. Recommendation X.2, International Data Transmission Services and Optional User Facilities in Public Data Networks.
2. X.25: The Universal Packet Network Interface;Drukarch,1980
3. Recommendation 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 by Dedicated Circuit.
4. Progress on X.25;Jamet,1984
5. Recommendation 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 or an Integrated Services Digital Network or a Circuit Switched Public Data Network.