LANs and WANs
Introduction
Broadly, networks can be split into two categories, LANs and WANs. Local Area Networks are networks that are made up of computers connected up to each other and are geographically close to each other, for example, in a room, or in a building, or in a number of close buildings. Wide Area Networks, on the other hand, are made up of computers connected up to each other over a wide geographical area. The Internet is a good example of a WAN. Companies who have offices scattered over the country may have a Wide Area Network.
Network topologies
Computers that are going to be connected together can be connected in different ways. The way that the computers on a network are connected together is known as the topology of the network.
Bus networks
A bus network is one where the stations are connected to a main communications cable, called a bus. The bus has terminators at the end of the cable. This set-up is very simple to implement compared to alternative topologies because very little cable is needed. If the main cable breaks, however, then the whole network stops working and it can be difficult to pinpoint where exactly the fault is. When a message needs to be sent, the workstation checks to see if the line is busy with another message. The cable can only carry one message at a time. If the cable isn't busy then a station can put its message on the cable. If there is a message on the cable then it waits until it is free. This simple method avoids most ‘collisions’. This is when two messages are put on the network cable at exactly the same time, which results in corrupted messages. Collisions are possible because two stations can see at exactly the same time that the cable is free and at exactly that same time, can put their own messages on the cable! Fortunately, stations can also detect when these collisions have occurred. When they do detect a collision, the stations involved are asked to re-send their messages.
Ring networks
A ring network is like a bus network with the ends connected together! This gives a better performance than a bus network, but is not as fast as a star network. There is a security problem with both bus and ring networks. When a message is sent, it is put on the cable. All stations can read all the messages on the cable and this means that someone who has access to the network with the right software can 'grab' packets of information that they are not entitled to!
Star networks
A star network is one in which the server is in the middle, and cables run from the server to each workstation. This kind of network is very secure because all of the communications go from one station to the target station via the server and other stations cannot 'grab' the communication. Security can be managed from one central point - the server. This kind of network takes up a lot of cable compared to a bus network, for example, and this can be expensive. In this kind of network, the server polls each station to see if it has anything to send. Polling is when the server goes from one station to the next in turn, checking if it needs to be serviced. If the server finds a station that needs to send a message, for example, then it will deal with it.
