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X.25 is a standard for wide area network (WAN) communication administered by the International Telecommunication Union-Telecommunication Standardization Sector. Most commonly, it is used for dial-up Internet service. It defines the way that private users connect to the network and how those connections are maintained. Users are generally charged for the connection service based on how much they use it, just like an electric or water bill.
The X.25 standard was adopted in the mid 1970s. It is most often used by common carriers, such as telephone companies that use packet-switched networks. When information moves through a packet-switched network, it is automatically divided up into packets of a particular size. The information is moved as a packet even if the content and format of the various pieces are unrelated. A limited number of packets can be moved at any one time, leading to slow processing times when there is high demand on the network.
A network following the X.25 standard is made up of three basic parts. Data terminal equipment are the network hosts and personal computers that use the network. Data circuit terminating equipment are the modems and packet switches that allow communication between computers and the network. Packet switching exchanges are the switches that actually transfer data through the network.
When a user wants to download something, the user's computer sends a signal through the phone line and the modem into the X.25 network. A Packet-Layer Protocol sets up a virtual circuit, which is basically a temporary connection between the user's computer and the server or computer from which the user is downloading. The request for a connection can be either accepted or rejected. An X.25 network uses the X.121 address format to make sure the virtual circuits it creates are between the correct components of the network.
The data requested by the user is then segmented into packets. These packets are sent through the X.25 network to the user's computer at the standard rate. The data is then "unpacked" for use on the user's computer.
In order to process the packets that are created during data transfer, many computers rely on a packet assembler and disassembler. This device operates between the computer and the modem. In addition to assembling and disassembling the packets, it also stores packets until the device is able to process them. This is called buffering.
The X.25 standard was widely used in the 1980s, mostly by phone companies and for financial transactions. A few telephone companies still use the system. Most telecommunications companies have switched to the Internet protocol since it is simpler and does not suffer from the same traffic constraints.