sábado, 17 de julio de 2010

LANs and topology

Ever since the invention of the telephone enabled us to speak to someone in the next town and the television spawned a remote control, we have been unhappy about the thought of getting out of our chair to communicate with anyone or anything.
It is a natural progression to apply the same thoughts to our computers and other forms of data communications.
As computers have became more compatible, it seemed a good idea to exchange information by a cable connecting two or more systems. This made it possible to have shared facilities, whether it be massive storage areas, printers or software. It also allows staff to work on the same project and share the corporate facilities.
Sharing data is called networking and is categorized by the physical area that is interconnected. The smallest subdivision is called a 'Local Area Network' or LAN.

A LAN can be as small as you like, as in Figure 19.1 or, more realistically, it could interconnect a whole building or a collection of buildings or a large manufacturing site or a university with several thousand connections.

Characteristics of a LAN

  • A LAN uses a privately owned communication system rather than the normal telephone system.
  • They can operate at high data rates.
  • All computers can initiate the transfer of data to any other one.
  • They generally save costs by sharing costly equipment, software or data libraries.
  • They improve productivity and allow the exchange of data to be monitored for security purposes.
A whole city can be interconnected or possibly several LANs can be joined to provide a larger system and, in this case, the network is referred to as a MAN, 'Metropolitan Area Network'. Any larger network is called a 'Wide Area Network' or WAN.

What bits of hardware are we likely to meet in a LAN?

Communication route

Cable, almost certainly. The cables used are chosen to meet the requirements of the LAN, principally a matter of distance and the data rate. The cables used are optic fiber, copper twisted pair and copper coaxial cable. There are a few LANs that use a wireless communication system but this is unusual.


A server supplies a facility to the network. The name comes from the idea of the device 'serving' the needs of the network.
A typical example may be a 'print server'. Instead of supplying maybe a hundred mediocre quality printers to all the staff, we may decide to have a single super high quality printer than can handle all the printing requirements with better quality, higher speed and less expense.
All LANs have a 'file server'. This device controls access to all the shared files to store the files when not in use and to prevent two people modifying a file simultaneously. Having the files held centrally means that we can wander off to another part of the building and can use any computer to access the previously used files. There are two more advantages. If the software needs upgrading, we have only a single copy to be loaded. Backing up the work is easy since all the
shared files are held on the file server. Once all the staff have gone home at night, the server can carry out a backup of the day's work before going to sleep.
If we want to access an external network, we can make contact via the normal telephone communication system by using a modem like the one we use with the Internet. As with the print server, it is more economic for a 'communication server' to control a series of modems than to provide each staff member with their own modem.

Connecting our computer to a network

To connect our computer to a network we need to install network interface cards (NICs). The power is switched off, the case of the PC is taken off, and the card inserted into one of the expansion slots. The software is run to make sure the computer will be recognized by the network and the case is put back on again. Done. We are now part of the club.

Topology at last

We have choices when it comes to how we connect several computers and other devices. These connection patterns are what we call topology, an impressive name for something quite simple.
There are only three basic designs from which all other layouts are derived. Just before we start, we should mention the term 'node'. This is the name given to any piece of equipment that can be connected to a network such as computers, terminals or printers.

Bus topology

Here is the first one and probably the most obvious. We have a length of optic fiber or copper cable and connect all the nodes, one after another. This layout is shown in Figure 19.2 but in reality, the bus does not need to be straight, it simply wanders around connecting to each of the nodes.

Bus topology is very simple to construct and can be expanded simply by joining new devices to the bus cable. If the bus cable is damaged, the whole network may fail and your whole workplace goes for a coffee break until it is fixed and this may take some time. We can get over this reliance on a single cable by using star topology but this brings its own problem, as we will see.

Star topology

In this design, we have a central connection called a hub. This is a central computer or server that is connected to each node as in Figure 19.3.

The central hub asks each node in turn whether it has a message to send. If a node, say node 2, wishes to send data to node 5, it says 'yes' and sends it to the hub which then reroutes it to node 5. The hub goes on to check with node 3, then 4 and so on. Providing the hub is fast enough it can handle all routing requirements. It can do more than this; it can provide the management with details of who is sending what data to whom. This can monitor data holdups so that the system can be upgraded if necessary, but it can also check to see who is playing games during work time.
As each node can be easily disconnected without interfering with the whole system, faults can be isolated more easily. The snag is that it the hub fails, the whole system goes down.

Ring topology

This is a modified version of the bus topology. The two ends of the bus are simply joined together to form a ring as shown in Figure 19.4.

If node 2 wishes to send a message to node 5, a system of tokens is used. A token is a short electronic code that is passed from node to node round the ring. Node 2 waits until it receives the token then attaches its message and an address. The token, message and address are passed from to mode 3, then 4, then 5. Node 5 recognizes the address and removes the attached message and the token continues around the ring until someone else wants to send some data. There are a couple of problems with this system in that a failure of any node will stop the rotation of the token and the whole network will fail immediately. Expanding the network by adding another node is only possible if the ring is broken and the network is shut down.

Hybrid topologies

We can use the three basic topologies in combination with each other to create a large number of possibilities as required in the network being installed.

Three popular hybrids are illustrated in Figure 19.5.
The first is called the clustered star topology. To do this, we start with a bus and instead of separate nodes, we can connect star clusters.
Tree topology also starts with the bus system but reduces the length of cable used by combining some of the routes.
Star-wired ring is, as the name suggests, a combination of the ring and the star.
The nodes pass through the hub before going onto the ring to pass the data its destination. The hub can also disconnect a faulty node and allow the system to remain operative. As can be imagined, there are many amazing possibilities but if we are aware of the basic patterns we will be able to find our way around the others.

Kevin M Contreras H
CI 18.255.631



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