X.25 is a standard suite of protocols used for packet switching across computer networks. The X.25 protocols works at the physical, data link, and network layers of the OSI model.
Each X.25 packets contains up to 128 bytes of data. The X.25 network handles packet assembly at the source device, delivery, and then dis-assembly at the destination. X.25 packet delivery technology includes not only switching and network-layer routing, but also error checking and re-transmission logic should delivery failures occur. X.25 supports multiple simultaneous conversations by multiplexing packets and using virtual communication channels.
Function of X.25::
The role of X.25 is to transport data both ways between a pair of “DTE”s (data terminal equipment) by X.25. One DTE may belong to many pairs at once.
The X.25 protocol uses layering technology that is similar to the first three layers of the OSI reference model. The layers provide a way to seperate the services of the protocol into distinct levels. The three layer are called the Physical Layer, the Data Link Layer and the Network Layer.
- The Physical Layer: Describes the interface with the physical environment. It is concerned with connections between a DTE and a DCE and does not do any switching work.
- The Data Link Layer (also called the Frame level): Conerned with the communications between a DTE and a DCE. Provides framing, error detection and recovery, and flow control.
- The Network Layer (also called the Packet Level): The network layer does not concern routing algorithms but does create x25 packets, assign logical channels, manage packet flow control and connection with the network.