Local area networks (LANs).

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LAN

Local area networks (LANs) are used in local places for short communication. They are located in buildings such as schools and collages. The main purpose of LAN is to link a number of PCs to each other so they can share resources. LAN can consist of anything from two small PCs to a large number of computers all housed in several buildings linked together with cables. The only problem with this is the physical problem of laying direct cable. There are many ways in which LAN can be linked logically, each different network layout is known as a topology. The main advantages with LAN compared with several stand alone PCs are that the network can share resources of disk drives and printers, share software, access files that are stored on the network from any workstation, communicate with other PCs on the network without the need of having to pass disks to each other which is useful when the computers are located in different rooms. There are many different functions of LAN but the main ones are to allow communication from one PC to another by e-mail for example and to send and receive messages between the PC and file server or between the PC and the printer server. In some cases LAN may include a mini or mainframe computer that has number of ‘dumb’ terminals connected to it. In many cases they are not needed because the PCs are already connected to a powerful computer which is known as the File Server. This has large hard drive to store data, which all the other computers in the network can access. A typical network will also include a printer server, which is a PC that is connected to the printer, and the file server so that every one in the network can use it to print out their work.

The topology of the network will have implications for the protocol used to manage the connections as well as the type of cable chosen. There are three basic types of topologies:  

  • BUS
  • RING
  • STAR

BUS

All the computers and other devices in a bus network share one cable. All the messages can be transmitted in both directions from a point along the single bus from any PC to another device. The cable runs from device to device by using tee connectors that plug into the network adapter cards. Each end device has a terminator on one end of the tee and a cable going out to the next device on the other end, while all devices in the middle have one cable coming in and one going out. The terminators on each end device simply stop the network signal from reflecting back into the cable and colliding with other transmissions. Problems arise when several users want to send information all at the same time. To stop this happening the station checks if the bus is busy. If it is busy then the station that wants to send the message will have to wait. A bus network is extremely cheap and easy to install. If other PCs and other devices wanted be linked up. The whole network will have to be disturbed. It is also easy to locate a fault in the cable because there is only the one. But this depends how long the cable is. There are also problems with bus networks as well

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for example if there is a fault in the cable then the whole network will be shutdown.

Advantages:

  • Less expensive than a star topology due to less footage of cabling and no network hubs
  • Simple for smaller networks not requiring higher speeds

Disadvantages:

  • Limited in size and speed
  • One bad connector can take down entire network
  • Terminators are required at both ends of the backbone cable

RING

In a ring topology, the network signal is passed through each network card of each device and passed ...

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