Even though it predates the OSI model, the protocols and physical standards over which X.25 works are considered to map to the model’s lowest three layers. These protocols and standards are described following this reference figure:
X.25 protocols and standards and their relationship to the OSI model.
Network Layer. At the Network layer, X.25 implements the Packet-Layer Protocol (PLP). PLP is responsible for call setup and teardown functions, data transfer between DTE devices, and the fragmentation and reassembly of data. A PLP header is added to higher-layer data during the encapsulation process, and identifies the type of payload (control information or data), the PLP packet type, and the virtual circuit that the packet is associated with.
Data Link Layer. At the Data Link Layer, X.25 implements a protocol known as Link Access Procedure, Balanced (LAPB). LAPB is a variation of the HDLC protocol that handles framing, error and flow control mechanisms, as well as acknowledgements for frames as they travel between nodes on an X.25 network.
Physical Layer. X.25 is capable of using a variety of different physical and electrical interfaces to connect DTE to DCE devices. X.25 has traditionally used the X.21bis standard to provide full-duplex connectivity at speeds up to 19.2 Kbps. Other physical interfaces like EIA/TIA-232 are also commonly used with X.25.
