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.