X25

X.25 is a family of ITU-T standards for packet-switched networks, originally published in 1976 by CCITT (now ITU-T). It defines the interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) and specifies the protocols used for transmitting data over wide-area networks. X.25 was designed to provide reliable, error-controlled communication across heterogeneous networks and long distances, compatible with a variety of physical media.

The X.25 architecture includes a three-layer model: the data link layer using an HDLC-like LAPB protocol for reliable point-to-point links, the packet layer that provides virtual circuits and call control, and the interface between DTE and DCE. In the packet layer, communication proceeds along virtual circuits, which may be switched (SVC) or permanent (PVC). Virtual circuits are identified within the network by a virtual circuit identifier (VCI), and each data transfer is associated with addressing that specifies both the source and destination endpoints. The maximum packet size (MPS) is negotiable and can be configured up to several thousand bytes, with typical values used in practice often around 4096 bytes. X.25 supports call setup, data transfer, and disconnect procedures, along with flow control and error handling primarily at the link and packet layers.

X.25 networks were widely deployed for remote terminal access, banking networks, and early distributed systems, enabling interoperable communication across diverse equipment. Over time, they were largely supplanted by more modern protocols and technologies such as Frame Relay and IP-based networks. Today, X.25 is viewed as a legacy technology, with occasional remaining implementations in specialized or legacy systems.