CHSE INFORMATION TECHNOLOGY NOTES (IT)

 Chse information technology|chse it notes |+2 2nd year it |chse computer networks|

Computer networks 

What is a network?

A computer network is a collection of interconnected computers and otherhardware devices which are linked together through communication channels

Or A computer network is a system in which computers are connected to share information and resources.

What is the need of network?

There are various reasons which favour need of network.

-To share data at different locations of the organization.

-To share information with other people on the network.

-To make quick link with the other persons using network.

-Provides fast and effective communication from one computer to another computer

-To communicate in the form of mail messages and chatting.

-To gain knowledge from the worldwide resources using Internet

Types of Network
On the basis of coverage or geographical spread, a network can be divided into following types:

1. Local Area Network (LAN) When the network of computer is confined to a small or localised area such as school, office or building, it is known as Local Area Network (LAN). Computers or users in a local area network can share data, information, software and common hardware devices such as printer, modem, hard disk, etc.

2. Metropolitan Area Network (MAN) A metropolitan area network is the network which is spread over a city. The functions of a MAN is similar to LAN. Like a LAN, computers in a MAN are also capable of sharing data, information, software and hardware. The difference is only on the basis of geographical area coverage.

3. Wide Area Network (WAN) The network of computers which is spread across the countries probably over the entire world is known as a Wide Area Network (WAN).

-A WAN can be a group of several LANs interconnected together.

-The largest WAN is the Internet.

4. Personal Area Network (PAN) The PAN refers to a computer network, which is used for communication among computer devices spread over a few meters. PAN may be wired  Bluetooth personal area network(PAN) . It can be composed of upto eight devices.

 Some of the different networks based on their purpose 

-Storage Area Network or SAN

-Enterprise Private Network or EPN

-Virtual Private Network or VPN

Elements of a network

Host: the main or controlling computer connected to others computer or terminal to which it provides data for computing service via a network.

Node: in a network, a node is a connection point, either a redistribution point or an endpoint for data transmissions. In general, a node has capability to recognize and process or forward transmissions to other nodes. A node can be a computer or some other device, such as a printer. Every node has a unique network address.

Servers:

Servers are computers that hold shared programs, files, and the network operating system. Servers allow access to network resources to all the users of the network.

Clients:

Clients are computer devices which access and uses the network as well as shares network resources. They are also users of the network, as they can send and receive requests from the server.

Network devices

Network devices are the components used to connect computer and other electronic devices together, so that they can share files or resources like printers or fax machines. The most common type of network devices used by the public to set-up a Local Area Network (LAN) are hub, switch, repeater and if online access is desired, a high-speed modem.

Network Interface Card (NIC)

A Network Interface Card is a hardware device which connects a Network Interface Card computer with the network. They are installed on the motherboard of computers and are responsible for developing physical contact with network devices and other computers(nodes).  Thus every node, which is to be connected to a network, must have an NIC installed on it.  

Repeater

When the data is transmitted over a network for long distances, the data signal gets weak after certain distance.

Therefore, some device is required which can re-strengthen the data signal before it gets too weak. Repeater is such a device. A repeater regenerates the received signal and re-transmits it to its destination

Hub

Hub is a device that splits network connection into multiple computers. It acts a distribution center so whenever a computer requests any information from a computer or from the network it sends the request to the hub through a cable. The hub will receive the request and transmit it to the entire network.

-These are made up of twisted pair cables

Whenever any data packet is received by any port in a workstation which may be an e-mail, graphics or any document, is forwarded to all remaining ports. Thus, the required data packet is transported to desired user or device.

There are two types of hub:

Active Hub It amplifies the signals and passes from one connected device to another. These hubs are working as repeaters to boost up the signals.

Passive Hub It simply passes signals from one connected device to another without making any changes.

Switch

A switch is an intelligent hub. Just as in hub, devices in switches are connected to them through twisted pair cabling. It has the same function as that of a hub: to connect multiple computers/devices in a network. But the difference between the two is in the way they re-transmit the received information. Unlike a hub, instead of sending to each device attached to it the received information, a switch sends the information selectively only to those computers for which it is intended. 

Routers

Routers are network devices which shared data on the basic of IP address. It identify the IP addresses and send data only to the computers whose IP address is provided to the routers . Thus it restricts the outer computers whose IP address is not assigned to the router. Hence can be used where the requirements of security and private data is high .

Routers are more expensive than other networking devices like hubs,bridges and switches.

Gateway The gateway is a network device which is used to connect dissimilar networks. The gateway establishes an intelligent connection between a local network and external networks, which are completely different in structure.
-The gateway also acts as a proxy server and a firewall, which prevents the unauthorised access.

Modem (Modulator/Demodulator) Modem is a device that converts digital signal to analog signal (modulator) at the sender’s site and converts back analog signal to digital signal (demodulator) at the receiver’s end, in order to make communication possible via telephone lines.

Functions of Modem The essential function of a modem is to create an easily transmitted and decode signal. Modem divides the information into packets called frames. They minimise the error that occur while the transmission of signals. 

Modems are of two types:

Internal Modem It refers to the modem that is fixed within a computer, e.g. dial-up and wireless.

External Modem A modem that resides in a self-contained box outside the computer system, e.g. USB modem, Cable modem.

Transmission mode

Simplex mode: In this mode data travels in one direction i.e.communication is unidirectional. We cannot send a message back to the sender.Unidirectional communication is done in implex systems, like: loudspeaker,television broadcasting, television and remote, keyboard and monitor etc.

Half-Duplex mode: In this mode data travels in both directions but one at a time, that is when the sender is sending the data then at that time we can't send the sender our message. The data is sent in one direction. Example of half duplex is a walkie-talkie in which message is sent are sent in both the directions,but one at a time.

Full-Duplex mode: In this mode data can be sent in both directions simultaneously. Telephone Network in which there is communication between two persons by a telephone line, through which both can talk and listen at the same time is an example of Full Duplex transmission..

Transmission media:

In a computer network, computere have to exchange information (data) among themselves. The information being in form of electromagnetic signals can travel through vacuum, air or other transmission media from one point to another (from source to receiver) in a network. A transmission medium can be wired (guided) or wireless (unguided)

 Guided Media: Depending upon their construction type, wired mediacan be classified in to three categories:

(1) Twisted Pair cables

(2) Co-axial cables

(3)Optic Fiber cables

Twisted Pair Cables / Ethernet Cables:

It consists of 2 separately insulated conductor wires wound about each other. Generally, several such pairs are bundled together in a protective shield. They are the most widely used Transmission Media. Twisted Pair is of two types: 

I)Unshielded Twisted Pair (UTP): 

This type of cable has the ability to block interference and does not depend on a physical shield for this purpose. It is used for telephonic applications. 

Advantages: 

Least expensive

Easy to install

High speed capacity

Susceptible to external interference

Disadvantages

Lower capacity and performance in comparison to STP

Short distance transmission due to attenuation

II)Shielded Twisted Pair (STP): 

This type of cable consists of a special jacket to block external interference. It is used in fast-data-rate Ethernet and in voice and data channels of telephone lines. 

Advantages: 

Better performance at a higher data rate in comparison to UTP

Eliminates crosstalk

Comparitively faster

Disadvantages

Comparitively difficult to install and manufacture

More expensive

Bulky 

Coaxial Cable 

It has an outer plastic covering containing 2 parallel conductors each having a separate insulated protection cover. Coaxial cable transmits information in two modes: Baseband mode(dedicated cable bandwidth) and Broadband mode(cable bandwidth is split into separate ranges). Cable TVs and analog television networks widely use Coaxial cables. 

Advantages: 

 High Bandwidth

Better noise Immunity

Easy to install and expand

Inexpensive

Disadvantages: 

Single cable failure can disrupt the entire network

 Optical Fibre Cable 

It uses the concept of reflection of light through a core made up of glass or plastic. The core is surrounded by a less dense glass or plastic covering called the cladding. It is used for transmission of large volumes of data. 

The cable can be unidirectional or bidirectional. The WDM (Wavelength Division Multiplexer) supports two modes, namely unidirectional and bidirectional mode.

Advantages: 

 Increased capacity and bandwidth

Light weight

Less signal attenuation

Immunity to electromagnetic interference

Resistance to corrosive materials

Disadvantages: 

 Difficult to install and maintain

High cost

 Unguided Media: 

It is also referred to as Wireless or Unbounded transmission media.No physical medium is required for the transmission of electromagnetic signals. 

There are 3 major types of Unguided Media: 

(i) Radiowaves 

These are easy to generate and can penetrate through buildings. The sending and receiving antennas need not be aligned. radios and cordless phones use Radiowaves for transmission. 

Further Categorized as (i) Terrestrial and (ii) Satellite. 

(ii) Microwaves 

It is a line of sight transmission i.e. the sending and receiving antennas need to be properly aligned with each other. The distance covered by the signal is directly proportional to the height of the antenna. Frequency Range:1GHz – 300GHz. These are majorly used for mobile phone communication and television distribution. 

(iii) Infrared 

Infrared waves are used for very short distance communication. They cannot penetrate through obstacles. This prevents interference between systems. Frequency Range:300GHz – 400THz. It is used in TV remotes, wireless mouse, keyboard, printer, etc.

  

Network Topologies

The network topology refers to the arrangement or pattern of computers (i.e. nodes or workstations), which are interconnected in a network.

Commonly used network topologies are as follows:

1. Bus Topology Bus topology is also referred as linear topology. Under this arrangement, all nodes in the network are connected by a single length of transmission medium, which is normally a co-axial cable. Both ends of the cable are terminated by terminators. Data transmission from any workstation can travel in both directions.

Advantages of Bus Topology

-In bus topology, all nodes are connected through a single length of cable, so very short cable length is used.

-Extension is very easy ,new node can be connected in a line 

-cheap than any other topology

Disadvantages of Bus Topology

When there is any problem in data communication with any node, whole network stops functioning.

In case of any fault in data transmission, fault isolation is very difficult. We have to check the entire network to find the fault.

2. Ring Topology In ring or circular topology, the nodes are connected in a circular way. In such arrangement, each node is connected to exactly with its two neighbouring nodes. In other words, when the last node in a bus network is connected with first node and the entire network becomes circular network is known as a network using ring topology.

Advantages of Ring Topology

Short cable length is required for connecting the nodes together.

The architecture of network is simple and easy to maintain.

Disadvantages of Ring Topology

In ring topology, each node is connected in a circular way with its two neighbouring nodes, so when there is transmission problem anywhere in the network, entire network stops functioning.

Fault diagnosis is very difficult in a network formed using ring topology.

3. Star Topology In star topology, there is a direct and dedicated connection of every . node of the network with the central node or server. In star topology, the server is directly connected with each and every node in the network via a hub.

Advantages of Star Topology

The star topology is the most reliable as there is a direct connection of every node in the network with the central node or server.

If there is any problem in connection with any node, other nodes in the network are not affected.

Disadvantages of Star Topology

Since, every node is directly connected with the central node or the server, so a large quantity of the cable is required.

There is dependency on the central node. If there are some problems with the central node, entire network stops functioning.

4. Mesh Topology A network set-up where each computer and network device is interconnected with one another to provide an alternative route in case of the host is down or busy.

Advantages of Mesh Topology

Excellent for long distance networking.

Communication is also possible through the alternate route, if one path is busy.

Disadvantages of Mesh Topology

Long wire/cable length, hence increase in the cost of installation and maintenance

There are high chances of redundancy in many of the network connections.

Set-up and maintenance of this topology is very difficult. Even administration of the network is tough.

5. Tree Topology The tree topology is an extension and variation in bus topology. Here, the shape of a network is like an inverted tree, where the central node is at the root which is branching out as the network extends. Tree topology is suitable for the applications which have a hierarchical flow of data and control. Since, it is a modified form of bus topology, hence it is a hybrid topology.

Advantages of Tree Topology

The tree topology simulates hierarchical flow of data. So, it is suitable for applications, where hierarchical flow of data and control is required.

We can easily extend the network whenever required.

Disadvantages of Tree Topology

There is a requirement of long cable length.

In tree topology, there is dependency on the root node.

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