Mar 11
22
What is a wide area network
WAN – Wide Area Network spans a large geographical area, often a country or continent. It contains a collection of machines intended for running user programs or applications. The hosts are connected by a communication subnet. The job of the subnet is to carry message from host to host, just as the telephone system carries words from speaker to the listener.
By separating the pure 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 distinct components. They are transmission lines and switching elements. Transmission lines also called as circuit, channels or trunks move bits between machines. The switching elements are specialized computers used to connect two or more transmission lines. When data arrives on an incoming line, the switching element will choose an outgoing line to forward them on. The collection of communication lines and switching elements (routers) (but not the host computers) is called subnet.
In most WANs the network contains numerous telephone lines and cables. Each one of these lines connecting a pair of routers. If two routers that do not share a cable nevertheless wish to communicate each other, they must do this indirectly, via other routers.
When a packet sent from one router to another via one or more intermediate routers, the packet is received at each intermediate router in its entirety, stored there until the required output line is free, and then forwarded. A subnet using this principle is called a point-to-point, store-and-forward, or packet switched subnet. Nearly all wide area networks except those using satellites have store-and-forward subnets. When the packets are 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. 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 hear the upward transmissions of their fellow routers to the satellite as well. Sometimes 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.
