Minggu, 10 Juli 2011


2.1          What is a Network?
You may have come across other types of network such as a bussines network or a friendship network. An example of such a friendship network is facebook.com where people from all over the word can echange information with each other.
A network is anny collection of idependent computer or device that communicate with one another over a shared network medium.
2.2          Component of a Network
A network is simply a group of device that are inter-connected. Some examples of these device are:
-          Computers
-          Printers
-          Scanners
-          Modems
The function of a network is to pass information between device that are connected. When you use a computer to communicate with another computer that someone else is using, you must use a network.
The network may be formed by using:
-          Copper cables
-          Fibre-optic cables
-          Radio waves
-          Infra-red rays
2.2.1      Network Interface Cards
Network interface cards, commonly referred to as network cards or NICs, are use to connect a PC to a network. The network card provides a connection between the network cable and the computer.
Your Computer can use a network card in order for it to connect to a network. These are also other types of devices that your computer uses to connect to a network.

2.2.2      Hub
The hub performs a very important role in connecting different computers together.  All members of the network share the same media to transmit data onto a single network. This means that each member will only get a percentage of the available network bandwith.
2.2.3      Cables
Network cables are used to connect the computer and device together. These cables carry the data that is sent in the network.

2.3          Types of Networks
There are several types of networks:
2.3.1      LANs (Local Area Networks)
A LAN is network ussualy confined to a small geographical area. This can be a building or a college campus.
A LAN can be small, linking as few as 3 computer. A large LAN can also link hundred of computers.
LANs are commonly found in bussines and educational organizations.

2.3.2      MANs (Metropolitan Area Network
 A MAN covers an area larger than a LAN and smaller than a WAN. It Connects 2 or more LANs, and usually covers an entire metropolitan area, such as a large city and its suburbs. Most universities use MANs.

2.3.3      WANs (Wide Are Network)
WANs connect multiple LANs and MANs tha are located id different places. This often done because computers may be located in different plaaces. A WAN ma be located throughout a country and even around the world.

2.4          Network Topology
What is a network topology? In communication networks, a topology is a usually schematic description of the arrangement of a network, including its nodes and connecting lines. There are two ways of defining network geometry: the physical topology and the logical (or signal) topology.
Network Topology is the schematic description of a network arrangement, connecting various nodes(sender and receiver) through lines of connection.
BUS Topology
Bus topology is a network type in which every computer and network device is connected to single cable. When it has exactly two endpoints, then it is called Linear Bus topology.

Features of Bus Topology
  1. It transmits data only in one direction.
  2. Every device is connected to a single cable
Advantages of Bus Topology
  1. It is cost effective.
  2. Cable required is least compared to other network topology.
  3. Used in small networks.
  4. It is easy to understand.
  5. Easy to expand joining two cables together.
Disadvantages of Bus Topology
  1. Cables fails then whole network fails.
  2. If network traffic is heavy or nodes are more the performance of the network decreases.
  3. Cable has a limited length.
  4. It is slower than the ring topology.

RING Topology
It is called ring topology because it forms a ring as each computer is connected to another computer, with the last one connected to the first. Exactly two neighbours for each device.

Features of Ring Topology
  1. A number of repeaters are used for Ring topology with large number of nodes, because if someone wants to send some data to the last node in the ring topology with 100 nodes, then the data will have to pass through 99 nodes to reach the 100th node. Hence to prevent data loss repeaters are used in the network.
  2. The transmission is unidirectional, but it can be made bidirectional by having 2 connections between each Network Node, it is called Dual Ring Topology.
Advantages of Ring Topology
  1. Transmitting network is not affected by high traffic or by adding more nodes, as only the nodes having tokens can transmit data.
  2. Cheap to install and expand
Disadvantages of Ring Topology
  1. Troubleshooting is difficult in ring topology.
  2. Adding or deleting the computers disturbs the network activity.
  3. Failure of one computer disturbs the whole network.

STAR Topology
In this type of topology all the computers are connected to a single hub through a cable. This hub is the central node and all others nodes are connected to the central node.

Features of Star Topology
  1. Every node has its own dedicated connection to the hub.
  2. Hub acts as a repeater for data flow.
  3. Can be used with twisted pair, Optical Fibre or coaxial cable.
Advantages of Star Topology
  1. Fast performance with few nodes and low network traffic.
  2. Hub can be upgraded easily.
  3. Easy to troubleshoot.
  4. Easy to setup and modify.
  5. Only that node is affected which has failed, rest of the nodes can work smoothly.
Disadvantages of Star Topology
  1. Cost of installation is high.
  2. Expensive to use.
  3. If the hub fails then the whole network is stopped because all the nodes depend on the hub.
  4. Performance is based on the hub that is it depends on its capacity

MESH Topology
It is a point-to-point connection to other nodes or devices. All the network nodes are connected to each other. Mesh has n(n-2)/2 physical channels to link n devices.
There are two techniques to transmit data over the Mesh topology, they are :
  1. Routing
  2. Flooding

In routing, the nodes have a routing logic, as per the network requirements. Like routing logic to direct the data to reach the destination using the shortest distance. Or, routing logic which has information about the broken links, and it avoids those node etc. We can even have routing logic, to re-configure the failed nodes.
In flooding, the same data is transmitted to all the network nodes, hence no routing logic is required. The network is robust, and the its very unlikely to lose the data. But it leads to unwanted load over the network.

Types of Mesh Topology
  1. Partial Mesh Topology : In this topology some of the systems are connected in the same fashion as mesh topology but some devices are only connected to two or three devices.
  2. Full Mesh Topology : Each and every nodes or devices are connected to each other.
Features of Mesh Topology
  1. Fully connected.
  2. Robust.
  3. Not flexible.
Advantages of Mesh Topology
  1. Each connection can carry its own data load.
  2. It is robust.
  3. Fault is diagnosed easily.
  4. Provides security and privacy.
Disadvantages of Mesh Topology
  1. Installation and configuration is difficult.
  2. Cabling cost is more.
  3. Bulk wiring is required.

TREE Topology
It has a root node and all other nodes are connected to it forming a hierarchy. It is also called hierarchical topology. It should at least have three levels to the hierarchy.

Features of Tree Topology
  1. Ideal if workstations are located in groups.
  2. Used in Wide Area Network.
Advantages of Tree Topology
  1. Extension of bus and star topologies.
  2. Expansion of nodes is possible and easy.
  3. Easily managed and maintained.
Disadvantages of Tree Topology
  1. Heavily cabled.
  2. Costly.
  3. If more nodes are added maintenance is difficult.
  4. Central hub fails, network fails.

HYBRID Topology
It is two different types of topologies which is a mixture of two or more topologies. For example if in an office in one department ring topology is used and in another star topology is used, connecting these topologies will result in Hybrid Topology (ring topology and star topology).

Features of Hybrid Topology
  1. It is a combination of two or topologies
  2. Inherits the advantages and disadvantages of the topologies included
Advantages of Hybrid Topology
  1. Reliable as Error detecting and trouble shooting is easy.
  2. Effective.
  3. Scalable as size can be increased easily.
  4. Flexible.
Disadvantages of Hybrid Topology
  1. Complex in design.
  2. Costly.

 2.5          The Internets

The Internet is a system of networks that are linked worldwide and facilities data communication services.
Some examples of data communication services are remote login, file transfer, electronic mail, the World Wide Web and newsgroups.
Internet websites provide personal, educational, political an economic resources to the entire planet.