X.25 is a CCITT recommendation for a synchronous interface between a data terminal equipment (DTE) operating in a packet mode and the data circuit terminating equipment (DCE) on the public data network. The elements of the interface is defined independently as:
Physical - the mechanical, electrical, functional, and procedural characteristics to activate, maintain, and deactivate the physical connection between the DTE and DCE.
Link - the link access procedure for data interchange across the link between the DTE and the DCE.
Packet - the packet format and control procedures for the exchange of packets containing control information and user data between the DTE and the DCE.
These three levels are associated with the first three levels in the International Standards Organization (ISO) Open System Interconnect (OSI) Reference Model.
Physical
The interface characteristics for a DTE connected to a packet switched transmission service by a dedicated circuit or by a circuit switched network is defined in Recommendation X.21 and X.21bis (V.24). Recommendation X.21 is only for connections via a circuit-switched data network while X.21bis can be used for connections via a circuit-switched data network and a telephone network.
Link
The Link Access Procedure, LAP and LAPB, is the protocol and the frame format is High-level Data Link Control (HDLC). LAPB defines procedures for link set-up, error control, flow control, and link disconnect. The frames used within the LAPB ensures error-free packets on the access link.
Packet
The packet level procedures pertain to virtual calls on the X.25 access link, and is independent of the internal protocol of the packet switched network. Procedures are defined for the setting up and clearing of switched virtual circuits, data transmission, and restart of switched and permanent circuits.
The packet format includes a user facility field which allow users to subscribe to various features such as Closed User Group, Reverse charging, and fast select.