7.1 X.25
7.1.1 X.25 standard
X.25 is a standard that defines the connection between a terminal and a packet-switching network. X.25 offers the closest approach to worldwide data communication available. Virtually every nation uses some X.25-addressable network.

X.25 originated in the early 1970s. The networking industry commonly uses the term X.25 to refer to the entire suite of X.25 protocols.

Engineers designed X.25 to transmit and receive data between alphanumeric “dumb” terminals through analog telephone lines. X.25 enabled dumb terminals to remotely access applications on mainframes or minicomputers.

Because modern desktop applications need LAN-to-WAN-to-LAN data communication, engineers designed newer forms of wide-area technology: Integrated Services Digital Network (ISDN) and Frame Relay. In many situations, these newer WANs complement or extend X.25, rather than replace it.

Many different network-layer protocols can be transmitted across X.25 virtual circuits (VCs), resulting in tunneling that has datagrams or other Layer 3 packets within the X.25 Layer 3 packets. This setup is shown in the Figure. Each Layer 3 packet keeps addressing legal for its respective protocol, whereas the X.25 VC transports the packet across the WAN.