Wide Area Network (WANs)

Wide Area Network:- 

A Wide Area Network or WAN, spans a large geographical area, often a country or continent. It contains a collection of machines intended for running user (i.e. application) programs. we will follow traditional usage and call these machines hosts. the term and system is sometimes also used in the literature. The host are connected by a communication subnet, or just subnet for short. The job of the subnet is to carry message from host to host. just as the telephone system carries words from speaker to listener. by separating the poor communication aspects of the network (the subnet) from the application aspects (the hosts), the complete network design is greatly simplified. 

    In most Wide Area Networks, the subnet consists of two distinet components: transmission lines and switching elements, Transmission lines (also called circuits, channel or trunks) move bits between machines. 

   The switching elements are specialized computers used to connect two or more transmission lines. when data arrive on an incomeing line, the switching element must choose an outgoing line to forward them on. unfortunately, there is no standard terminology used to name these computers. They are variously called packet switching nodes, intermediate systems, and data switching exchanges, among other things. as a generic term for the switching computers, we will use the word router, but the reader should we aware that no consensus on terminology exists here. In this model shown in figure second, each host is generally connected to a LAN on which a router is present, although in some cases a host can be connected directly to a router. The collection of communication lines and routers (but not the hosts) from the subnet.

   An aside about the "subnet" is worth making. originally, its only meaning was the collection of routers and communication lines that moved packets from the source host to the destination host. however, some years later, it also acquired a second meaning in conjuction with network addressing (which we will discus in chap). Hence the term has a certain ambiguity about it. unfortunately, no widely-used alternative existe for its initial meaning, so with some hesitation we will use it in both senses. From the context, It will always be clear which is meant.

  In most WANs the network contains numerous cables or telephone lines, each one connecting a pair of routers. If two routers that do not share a cable nevertheless wise to communicate, they must do this indirectly, via other routers. when a packet is sent from one router to another via one or more intermediate routers, the packets is received at each intermediate routers in its entirety, stored there until the required output line is free and then forwarded. A subnet using this principle is called a point-t-point, store and forward, or packet switched subnet. Nearly all wide area Networks (aspect those using sattelites) have store- and forward subnets. When the packets ae small and all the same size, they are often called cells.

 When a point-to-point subnet is used, an important design issue is what the router interconnection topology should look like. Figure Third Shows several possible topologies. Local networks that were designed as such usually have a symmetric topology. In Contrast, wide Area networks typically have irregular topologies.

     A second possibility for a wan is a satellite or ground radio system. Each router has an antenna through which it can send and receive. All routers can hear the output from the satellite, and in some cases they can also here the upward transmission of there follow 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....