Wide Area Networks

Local Area Networks (LANs), discussed in Chapter 6, typically cover small geographical areas. They are designed around relatively simple bus or ring topologies. Some networks such as Wide Area Networks (WANs), however, cover much larger areas, sometimes spanning several continents. In such cases the LAN protocols are inappropriate and new ones must be defined.
LAN uses include file transfer, electronic mail and file servers just as for WAN. WAN can be used for remote log-ins also (An application in which a user in one location logs into a computer at another). WAN protocols must distinguish between various applications.
There is a difference in routing between LAN and WAN. Routing strategies are more complex in WAN than in LAN. The fact that there are many ways to go from one point to another by itself makes the situation more complex.
To add to the complexity, sometimes a link in a chosen route experiences a failure. What does the network protocol do with all the data travelling in that route? In some cases, a route may prove to be so popular that too much data travels over it. The result is congestion and sometimes failures. Can network protocols avoid such situations? If they can not, what can they do to minimize their effects? When data is delayed due to failures and congestion, it must be stored somewhere while WAN protocols decide what to do with it. Network nodes must be equipped with software and buffers to do this.
LANs are controlled and managed by a single organization or department. If a problem occurs, users know whom to call. Some WANs such as the Internet have evolved mainly due to voluntary efforts of universities and government agencies. Consequently, there is no central authority responsible for fixing problems or updating protocols so that problems do not recur. The success of such network operations depends on the cooperation of the organizations that use them.
  WAN TRANSMISSION METHODS
WAN transmission methods use different switching techniques. Switching techniques are used to create one or more data paths called channels for transmitting data. The channels May be created using one communication cable or using several cables that offer a range of paths along which data can be transmitted. Switching can enable multiple nodes to simultaneously transmit and receive data or it can enable data to be transmitted over different routes to achieve maximum efficiency in terms of speed and cost. The following are the common switching techniques used in WANs:
•   Time-Division Multiple Access (TDMA)
•      Frequency-Division Multiple Access (FDNIA)
•      Statistical Multiple Access
•      Circuit Switching
•      Message Switching
•      Packet Switching.

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