For economic and technical reasons, LANs
are not suitable for communications over long distances. On a LAN, the
computers must coordinate their use of the network and this
coordination takes time. Over long distances with greater delays in
communication, the computers would take more time coordinating the use
of the shared medium and less time sending data messages. In addition,
the costs of providing high-speed media over long distances are much
greater than in the case of local area networks. For these reasons, Wide
Area Network (WAN) technologies differ from LANs.
A WAN is a remote-access network that
connects sites across a broad geographic area. As shown in Figure
Wide area connections between computers use point-to-point,
serial communications lines. These lines are called point-to-point
because they connect only two computers, one on each side of the
line. They are called serial lines because the bits of information
are transmitted one after another in a series, like cars traveling
on a single lane highway.
Connections across WAN lines may be temporary or permanent. Telephone or dial-up lines, for example, might make a temporary connection to a remote network from a computer in a home or small office. In this case, the home or office computer makes a phone call to a computer on the boundary of the remote network. The telephone company provides the connection, or circuit, that is used for the duration of the call. After data is transmitted, the line is disconnected, just as it is for an ordinary voice call. If a company wants to transmit data at any time without having to connect and disconnect the line each time, the company can rent a permanent line or circuit from the common carrier. These leased lines are constantly available and operate at higher speeds than temporary dial-up connections. In both temporary and permanent cases, computers that connect over wide area circuits must use special devices called modems at each end of the connection. Modem devices are required because the electrical signals that carry digital computer data must be transformed, or modulated, before they can be transmitted on telephone lines. On the transmitting end of the connection, a modem (modulator-demodulator) transforms computer signals into phone signals. On the receiving end, the transformation is done from phone to computer signals. The modem is only one means of connecting computers or similar devices so they can communicate over long distances. Other much faster technologies include Integrated Services Digital Network (ISDN), Frame Relay, Asynchronous Transfer Mode (ATM), and so on. In general, WANs typically connect fewer computers than LANs and normally operate at lower speeds than LANs. WANs, however, provide the means for connecting single computers and many LANs over large distances. Thus, they enable networks to span whole countries, and even the entire globe.
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, the WAN
connects the central site, the branch office, the telecommuter site,
and the mobile user through a service provider. Connection
requirements
vary, depending on user requirements and cost. 
Typically, individuals and companies do not build their own WAN
connections. Government regulations only allow utility companies
to install lines on public property. Therefore, wide area
connections make use of the communications facilities put in place
by utility companies, called common carriers, such as the
telephone company.