The X.25 protocol describes the interface between data terminal equipment (DTE) and data transmission equipment (DCE) for terminal equipment that operates with data packet switching in public data networks and is brought in via permanently switched lines.
The performance characteristics of the X.25 protocol are described in Recommendation X.2. This describes the service; X.25, on the other hand, describes how the service is implemented in the protocol. The features can be fixed for an agreed period of time or requested for a connection. The X.25 protocol was one of the first protocols to be structured according to the OSI reference model. It supports the lower three layers and thus belongs to the transport-oriented protocols.X.21, which was designed specifically for access to public networks. However, since the majority of the terminal devices to be connected have not implemented this interface, recommendation X.21bis, which is compatible with the V.24 / RS-232-C standard used worldwide, also applies. However, this only covers transmission speeds of up to 9,600 bit/s (19.2 kbit/s); for higher transmission speeds, the X.21 interface is absolutely necessary.
On the data link layer, the X.25 protocol uses the LAP and LAP-B protocols. These protocols, with their cyclic-redundant block backup procedures and flow control, form a secured system connection that is offered to the network layer above.fixed connections. This provides the user with high network availability and the network operator with optimally controllable network utilization. The network layer turns the secured system connection into an end system connection. The upper access point of the X.25 protocol is the NSAP, which forms the interface between layer 3 and the transport layer.