Explain about Wide Area Network (WAN).

Explain about Wide Area Network (WAN). 

Wide Area Network:   A wide area network, or WAN, spans a large geographical area, often a country orcontinent. It contains a collection of machines intended for running user (i.e., application)programs. These machines are called as hosts. The hosts are connected by a communication subnet, or just subnet for short. The hosts are owned by the customers (e.g., people's personal computers), whereas the communication subnet is typically owned and operated by a telephone company or Internet service provider. The job of the subnet is to carry messages from host to host, just as the telephone system carries words from speaker to listener. Separation of the pure communication aspects of the network (the subnet) from the application aspects (the hosts), greatly simplifies the complete network design. In most wide area networks, the subnet consists of two distinct components: transmission lines and switching elements. Transmission lines move bits between machines. They can be made of copper wire, optical fiber, or even radio links.  

  In most WANs, the network contains numerous transmission lines, each one connecting a pair of routers. If two routers that do not share a transmission line wish tocommunicate, they must do this indirectly, via other routers. When a packet is sent from onerouter to another via one or more intermediate routers, the packet is received at eachintermediate router in its entirety, stored there until the required output line is free, and thenforwarded. A subnet organized according to this principle is called a store-and-forward orpacket-switched subnet. Nearly all wide area networks (except those using satellites) havestore-and-forward subnets. When the packets are small and all the same size, they are often called cells.

The principle of a packet-switched WAN is so important. Generally, when a processon some host has a message to be sent to a process on some other host, the sending host first cuts the message into packets, each one bearing its number in the sequence. These packets are then injected into the network one at a time in quick succession. The packets are transportedindividually over the network and deposited at the receiving host, where they are reassembledinto the original message and delivered to the receiving process. A stream of packetsresulting from some initial message is illustrated in Fig.3.1.  In this figure, all the packets follow the route ACE, rather than ABDE or ACDE. Insome networks all packets from a given message must follow the same route; in others eachpacked is routed separately. Of course, if ACE is the best route, all packets may be sent alongit, even if each packet is individually routed.  


Not all WANs are packet switched. A second possibility for a WAN is a satellitesystem. Each router has an antenna through which it can send and receive. All routers canhear the output from the satellite, and in some cases they can also hear the upwardtransmissions of their fellow routers to the satellite as well. Sometimes the routers are connected to a substantial point-to-point subnet, with only some of them having a satellite antenna. Satellite networks are inherently broadcast and are most useful when the broadcast property is important