Kerala Plus One Computer Science Notes Chapter 11 Computer Networks
A network consists of two or more computers that are linked in order to share resources.
Two or more computers connected through a communication media that allows ex-change of information between computers is called a computer network, eg., internet.
Need for network
The advantages of networks are:
- Resource sharing: All the computers in a network can share software.
- Reliability: If one computer fails, the other computer can perform the work without any delay.
- Price Vs Performance: Instead of a main frame 10 personal computers are used with less cost and same performance.
- Communication medium: It is a powerful communication medium. We can exchange information between computers in a network.
- Scalable: System performance can be increased by adding computers to a network.
- Bandwidth: The maximum amount of data that can be transmitted by the medium measured in Hertz.
- Noise: It is the unwanted electrical or electromagnetic interference that adversely affect the transmitted data signals.
- Node: It is a computer or an I/O device connected to a network.
Data communication system
Data communication is the exchange of information between two computer (devices).
- Message: It is the data/information to be transmitted from one computer to another.
- Sender: It is a computer or a device that sends data. It is also called source or transmitter.
- Receiver: It is a computer or a device that receives data.
- Protocol: The rules and conventions for transmitting data.
It is the path through which message transmitted from the sender to the receiver. There are two I types guided and unguided media.
1. Guided Media: These are divided into:
- Twisted pair cable: These are two types : unshielded twisted pair and shielded twisted pair. Two copper wires individually insulated and twisted around each other ; and put in a plastic cover.
- Coaxial cable: A sturdy copper wire is I insulated by plastic, it is covered just like a I mesh by a conductor which is enclosed in an protective plastic coating. It is
expensive, less flexible and more difficult to install. But it is more reliable and carry for higher data rates.
- Optical fibre: These are made of glass fibres that are enclosed in a plastic jacket. It uses light instead of electrical signals. The light sources are LED or ILD.
2. Unguided Media: These are divided into:
- Radio waves: It transmits data at different frequencies ranging from 3 KHz to 300 GHz.
- Microwaves: Microwave signals can travel in straight line if there is any obstacle in its path, it can’t bend. So it uses tall towers instead of short one.
- Infrared wave: These waves are used for transmitting data in short distance and its frequency range is 300 GHz to 400 GHz.
Wireless communication technologies using radio waves
- Bluetooth: It uses radio waves in the frequency range of 2.402 GHz to 2.480 GHz. It transmit data in short distance.
- WiFi (Wireless Fidelity): It uses radio waves to transmit information across a net-work in a range 204 GHz to 5 GHz in short distance.
- Wi MAX (Wireless Microwave Access): It uses microwaves to transmit in-formation across a network in a range 2 GHz to 11 GHz over long distance.
- Satellite: The ground stations are connected via the satellite. The data signals transmitted from earth to satellite (uplink) and from the satellite to the earth (down- link).
Data communication device
It acts as an interface between computer and the communication channel. They are:
1. Network Interface Card (NIC)
This device enables a computer to connect to a network and transmit information.
It is a small, simple and inexpensive device j used to connect computers (devices) to a network. If a computer wants to transmit data to another computer, first it sends to the hub, the hub retransmits this data to all other computers. Each and every computer gets the data and check whether it is for them or not. It increases the network traffic and hence the transmission speed is low.
It is an expensive device used to connect computers (device) to a network. Unlike hub, switch transmit data not to all computers, it retransmits data only to the intended computer. So the traffic is less and speed is high.
It is device used to strengthen weak signals on the network.
It is a device used to link same type of net-works.
It is similar to a bridge, but it can connect two networks with different protocols.
It is used to connect two different networks with different protocols.
Data terminal equipments
This device are used to control data flow to and from a computer.
- Modem: It is a device used to connect the computer to the internet. It converts digital signal into analogue signal (modulation) and vice versa (Demodulation).
- Multiplexer: It combines the inputs from different channels of a medium and produces one output.
Physical or logical arrangement of computers on a network is called structure or topology. It is the geometrical arrangement of computers in a network. Different topologies are:
1. Star topology:
It has a server all other computers are connected to it. Advantages are add or remove workstations to a star network is easy and the failure of a work-station will not effect the other. The disadvantages is that if the server fails the entire network will fail.
2. Bus topology:
All the computers are attached to a single cable called bus. Here one computer transmits all other computers listen. Therefore it is called broadcast bus. The transmission from any station will travel in both the direction. The connected computers can hear the message and check whether it is for them or not. Advantages are add or remove computer is very easy. It requires less cable length and the installation cost is less. Disadvantage is fault detection is very difficult because of no central computer.
3. Ring topology:
All the computers are connected in the shape of a ring and it is a closed loop. Here also there is no central computer. Here a computer transmits a message, which is tagged along with its destination computer’s address. The message travels in one direction and each node check whether the message is for them. If not, it passes to the next node. It requires only ; short cable length.
4. Hybrid topology:
It is a combination of any two or more network topologies. It can be two types:
- Tree topology: The structure of a tree topology is the shape of an inverted tree : with a central node and branches as node.
[ It is a variation of bus topology. The data transmission takes place in the way as in bus topology. The disadvantage is that if one node fails, the entire portion will fail.
- Mesh Topology: Each node is connected
to more than one node. It is just like a mesh (net). There are multiple paths between computers. If one path fails, we can transmit data through another path.
Type of networks
The network are classified based upon the amount of geographical area that covers.
- Personal Area Network (PAN): It is used to connect devices situated in a small radius by using guided media or unguided media.
- Local Are Network (LAN): This is used to connect computers in a single room or buildings of one location by using twisted pair wire or coaxial cable. Data transfer rate is high and error rate is less.
- Metropolitan Area Network (MAN):
It is a network spread over a city. MAN have lesser speed than LAN and the error rate is less. Here optical fiber cable is used, eg., Cable TV network.
- Wide Area Network (WAN): The used to connect computers over a large geographical area. It is a network of networks. Here the computers are connected using telephone lines or Microwave station or satellites. The error rate in data transmission is high, eg., internet.
Logical Classification of Networks
1. Peer to Peer:
In this configuration, all the computers have equal priority. That means each computer can function as both a client and server. There is no dedicated server.
2. Client Server:
In this configuration, a computer is powerful which acts as a dedicated server and all others are clients (work stations). A server fulfills the needs of the clients. They are:
- File server: A computer that store and manages files for other devices on a net?work.
- Web server: A computer that handles the requests for web pages.
- Print server: A computer that handles the print jobs from other computers on a network.
- Database server: A computer that manages the database.
A protocol is a collection of rules and regulations to transfer data from one location to another.
Different types of protocols are:
- Transmission Control Protocol (TCP): It uses a set of rules to exchange mes?sages with other internet points at the in?formation packet level.
- Internet Protocol (IP): It uses a set of rules to send and receive messages at the internet address level.
- File Transfer Protocol (FTP): It is used for transferring files between computers connected to local network or internet.
- Hyper Text Transfer Protocol (HTTP): It is a protocol used for WWW for enabling the web browser to access web server and request HTML documents.
Identification of Computers over Network:
A computer gets a data packet on a network, it can identity the sender’s address easily.
- Media Access Control (MAC) address:
It is a unique 12 digit hexadecimal number (IMEI for mobile phones, it is a 15 digit decimal number) assigned to each NIC by its manufacture. This address is known as MAC address and its permanent.
- Internet Protocol (IP) address: An IP address has 4 parts numeric address. Each parts contains 8 bits. By using 8 bits we can represent a decimal number between
O to 255. Each part is separated by dot.
Uniform Resource Locator (URL)
Every resource on the internet has a unique URL. It has three parts.
- http: It means hyper text transfer protocol. It is a protocol used to transfer hyper text.
- www: Means World Wide Web. With an email address we can open our mail box from anywhere in the world.
- Third part is a unique name. It is the official website name of signals Window System.