COMPUTER NETWORKING Introduction The advent of computer and communication technology have changed our daily life. Observe the following daily life examples-

Watching Cable TV

Cash Withdrawal from ATM

Sending and receiving E-mails

Booking Railway or Air-lines Tickets.

Sending and receiving SMS through Mobile.

What is a Network – A computer network is a collection of interconnected

computers and other devices which are able to communicate with each other

and share hardware and software resources.

Advantages/Why we build a Network?

1. Sharing Resources:

To share data and peripherals among users irrespective of their physical

location. Ex. Sharing database, program files, printer and scanners etc.

2. Improved Communication:

Enables reliable, secure and faster communication between users. It saves our time.

3 Reduced Communication cost:

Using today’s public network we can send a large quantity of data at very low

cost. Internet and Mobile network playing a very important role in sending and

receiving text, image, audio and video data.

4. Offers easy communication methods: Example - E-mail, SMS etc. Components of a Network

Sender: A device or a computer that sends the data.

Receiver: A device or a computer that receives the data.

Message: Information to be communicated. It may be text, images, sound

or video.

Medium: A transmission medium is a physical path through which the data

flows from sender to receiver. A cable, fiber-optics or radio waves can be the

medium.

Protocol: A set of rules that manage data communication. It represents an

agreement between the communicating devices.

Networking Hardware- To form a computer network a lot of hardware

devices are required. Some of these devices along with their functionalities are

mentioned below :

Transmission Media-

Def- A transmission medium is a medium of data transfer over a network. It

can be wired or wireless.

Wired Media – A number of various types of cables are used to transfer data

over computer networks. These are twisted pair cable, co-axial cable, and

optical fiber cable.

1. Twisted pair cable – Most widely used cable for creating small computer

networks. It contains four twisted pairs covered in an outer shield. An

RJ-45 connecter is used to connect this cable to a computer. It is of two

types:

1. UTP (Unshielded (unprotected) Twisted Pair): As the name suggest

in UTP cables individual pairs are not shielded (protected).

UTP Cable UTP Cable with

RJ-45 Connector

(Registered Jack)

Characteristics –

1. Low-cost cable available for setting up small networks.

2. Thin and flexible cable and therefore it offers ease of installation.

3. Carry data upto a length of 100m.

2. STP (Shielded Twisted Pair) –

It is the same cable as the UTP, but

with each pair shielded individually.

An outer shield then covers all the

pairs like in UTP.

Characteristics –

1. As compared to UTP, STP offers better immunity (resistance) against

internal and external electromagnetic interferences.

2. Expensive than UTP cable.

3. As compared to UTP cable, STP cable is difficult to install.

2. Coaxial Cable ( or coax) – Coaxial cables are group of specially wrapped &

insulated wire lines. They consist of a central copper wire surrounded by a PVC

insulation over which there is a sleeve (cover) mesh. The copper wire sleeve is

shielded again by an outer shield. Signal is transmitted by inner copper wire. It

is most common in cable TV network.

Characteristics :

1. It can carry data for a large distance ( 185m to

500m) .

2. Bulkier and less flexible than twisted pair.

3.Due to its thickness and less flexibility, it is

difficult to install as compared to twisted pair

cable.

4. Data transmission is better than twisted pair.

5. Much higher noise immunity.

3. Optical Fiber Cable – Optical fibers are hair-thin threads of glass or plastic.

Optical fiber can transmit data at much higher speed than copper wires or

coaxial cables with no significant loss over long distances.

An optical fiber has following parts:

Core - It is the thin glass rod at the center through which the light travels

Cladding (covering) - It is the outer optical material surrounding the core that

reflects the light back into the core

Buffer coating - It is the plastic coating that protects the cable from damage and moisture

Characteristics :

1. Can carry data for a very large distance.

2. Most efficient cable available for network.

3. Especially skilled people are required to install optical fiber cable.

4. Most expensive.

5. less susceptible to noise

Wireless Media (Unguided media):

1.No physical link established between two or more devices, communicating

wirelessly. Wireless signals are spread over in the air and are received and

interpreted by appropriate antennas.

2.Electromagnetic waves are used for wireless communication over computer

networks.

3.No physical medium is necessary for wireless signals, making them a

versatile way to build a network.

Radio wave – Radio waves are used for communication over distances

ranging from a few meters upto covering an entire city and can penetrate

(go through) buildings easily.

Examples of Radio Waves transmission: - Cordless phones, AM AND

FM radio broadcast, Garage door openers.

Characteristics :

1. These waves are Omni-directional, so the

transmitting and receiving antennas need not be

aligned.

2. Ease of communication over difficult terrain

(Area of land).

3. Relatively inexpensive than wired media.

4. Permission from concerned authorities is

required for use of radio wave transmission.

5. Less secure mode of transmission

Microwave – Microwave signals travel in straight lines and cannot penetrate

any solid object. Therefore for long distance microwave communication, high

towers are built and microwave antennas are put on their tops.

An example of usage of microwaves for communication is as follows:

In the big cities where land is very costly and a lot of formalities have to be

completed to get permission to dig land for cabling, microwave antennas can

be put on top of high rise buildings and communication can be started in a

short time.

Characteristics :

1. Faster than radio communication.

2. Ease of communication over difficult

terrain.

3. The transmission is in straight line so

the transmitting and receiving antennas

need to be properly aligned.

4. Relatively inexpensive than wired

media.

5. Free from land acquisition rights.

1. Satellite Link - Satellite links are used for very long distance wireless

communication which may range from intercity to worldwide.

Transmission from the earth to a satellite is known as uplink.

Transmission from a satellite to the earth is known as downlink.

Characteristics :

1. Area coverage very large.

2. Fast communication.

3. Very expensive.

4. Requires legal permission.

5. Normally owned by governments or by government approved

organizations of various countries

4. Bluetooth – Bluetooth technology uses radio waves. This technology is

used for short range wireless communication (approx. 10m).

Characteristics:

1. Slow data transfer rate.

2. Used for short range communication.

3. Line of sight between communicating devices is not required.

5. Infrared Waves – These waves are used for short range communication

(approx. 5m). It is inexpensive compared to other mechanisms.

Examples – Home-entertainment remote-control devices, Cordless mouse.

Characteristics:

1. No government permission is required.

2. The waves do not cross any solid object in between.

3. Does not require an antenna.

4. Transmission is safe.

Node – A node is a device which is directly connected to a computer network.

It can be a computer or any other device like scanner, printer etc.

Network Devices –

Other than transmission media many other devices are required to form

computer networks. Some of these devices are :

1. NIC - Any computer which has to be part of a computer network must have

an NIC (Network Interface Card) installed in it. A computer communicates with

other computer on a network with the help of NIC only.

2. Hub – A hub is an electronic device that connects several nodes to build a

network and redirects the received information to all the connected nodes in

broadcast mode. Hubs can be passive or active.

Active hubs – Electrically amplify the signals as it

moves from one connected device to another.

Passive hub – Allow the signals to pass from one

computer to another without any change.

http://www.youtube.com/watch?v=NxEyelY8R0s&list=PLC57B37876E

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3. Switch – A switch is an intelligent device that connects several nodes to

build a network and redirect the received information only to the intended

node. Switch can be considered advance from of hub.

4. Repeater – A repeater is a device that is used to regenerate a signal which

is on the way through a communication channel. A repeater regenerates the

received signal and re-transmits it to its destination.

5. Gateway – A gateway is a device, which is used to connect different types

of network and perform the necessary translation so that the connected

networks can communicate properly.

Network Topologies – The way in which the computers or other devices are

physically interconnected to build a network is called a Topology.

The most popular topologies are:

1. Bus topology – In this topology all nodes are connected to a main cable

called backbone. If any node has to send some information to any other

node, it sends the signal to the backbone. The signals travels through the

entire length of the backbone and is received by the node for which it is

intended.

Characteristics:

1. It is easy to install.

2. It requires less cable length and hence it is cost effective.

3. In case of cable fault, the entire network breaks down.

4. Easy to add additional node.

5. Failure of a node does not affect the network.

2. Star Topology – In this topology each node is directly connected to a

switch/hub. Each node requires a single cable. Any communication between the

node must pass through the central node.

Star topology generally requires more cable than bus topology.

Characteristics :

1. It is easy to install.

2. Failure of hub/switch leads to failure of entire

network.

3. It requires more cable than bus topology.

4. It is easy to add and remove additional node.

5. If any node other than host node fails, remaining

nodes are unaffected.

3. Tree topology - Tree topology is a combination of bus and star topology. It

is used to combine multiple star topology networks. All these stars are

connected together like a bus.

Characteristics:

1. It offers easy way of network expansion.

2. Even if one network (star) fails, the other networks remain connected and

working.

Types of Networks

A computer network may span any amount of geographical area. It can be on a

table, in a room, in a building, in a city, in a country, across continents or

around the world.

On the basis of area covered computer network are classified as :

1. Personal Area Network (PAN) - A PAN is network of communicating

devices ( Computer, Phone, Camera, MP3/MP4 Players) in the

proximity(nearness) of an individual. This range is typically upto 10

meters.

Example – Transfer songs from one cell phone to another, when files are

transferred from PC to an MP3 player.

A PAN can be set up using guided media (USB cable) or unguided media (Bluetooth, Infrared).

2. Local Area Network (LAN) – Small computer networks that are limited

to a localized area (e.g. an office, a building or a factory, home, school) are

known as Local Area Network. A LAN is managed by a single person or

organization.

It can cover an area of a few meters to a few kilometers.

A LAN can be set up using wired media (UTP cables, Co-axial cables etc.)

or wireless media (Infrared, radio waves).

3. Metropolitan Area Network (MAN) – Metropolitan Area Networks are

the networks spread over a city. For example , cable TV networks,

interconnected offices of a state government. A MAN is typically owned and

operated by a single entity such as a govt. body or a large corporation.

4. Wide Area Network (WAN) – These are the networks spread over large

distances says across countries or even continents. WANs usually contain a

number of interconnected individual computers, LANs, MANs, and may be

other WANS. Example – Internet.

Area of coverage – No upper limit.

Identification of computers and users over a network

MAC (Media Access Control) address – A MAC address is a unique 12 digit

( 6 digits for manufacturer code and 6 digits for serial number ) hexadecimal

number assigned to each NIC. MAC address of an NIC never changes.

example 00:A0:C9 : 14:C8:35

first six digit 00:A0:C9 indicates manufacturer code and the last digit

14:C8:35 are Serial number .

IP Address – An IP (Internet Protocol ) address is a unique number assigned

to each node on a network. IP address are expressed in decimal format. For

example following is an IP address :

202.27.94.137

IP address setting of a node can be changed by the user.

An IP address is a group of four bytes (or 32 bits).

IP address Vs MAC Address – The IP address is assigned by the network

administrator or the internet service provider while the MAC address is

assigned by the manufacturer. Thus if a computer is transferred from one

network to another, its IP address gets changed where as the MAC address

remains the same.

Domain Name – The system which assigns names to computers (web server)

and maintain a database of these names and corresponding IP address.

A domain generally contains following parts:

1. Top level domain name or primary domain name such as .com, .net, .org,

.edu, .in etc.

2. Sub-domain name(s)

example – cbse.nic.in

in – is the primary domain name

nic – is the sub-domain of in

cbase - is the sub-domain of nic.

Generic Domain Names :

.com - commercial business

.edu - educational institutions

.gov - gevt. Agencies

.mil - Military

.net - Network organizations

.org - Organizations (nonprofit)

Domain Name Resolution – Domain Name Resolution is the process of

getting the corresponding IP address from a domain name.

Attack on Network Security:

Some kinds of attack on network security are as follows:

Denial of service attacks: Denial of service attacks are those attacks that

prevent the legitimate (valid) users of the system, from accessing or using the

resources, information and capabilities of the system.

Examples of such attacks are:

1. Denial of Access to information: Corrupting, Encrypting or changing

the status of information so that it is not accessible (available) to its valid

user.

2. Denial of access to application: Forced shutting of an application as

soon as the users open it.

3. Denial of access to Resources: Blocking a resource, may be a printer or

scanner or USB port, of a computer from proper working.

4. Denial of access to a website: Continuously sending bulk request to a

website so that it is not available to any other user.

Intrusion Problems: An Intrusion problem is an attempt to mischievously

steal some information from someone's computer.

Examples of Intrusion are:

Snooping (to find out private things about somebody) – refers to gaining

unauthorised access to another person's or organization's data.

This may be done in a number of ways:

a. By getting someone’s login information.

b. Reading the files in an unauthorized manner.

c. Using some software which keeps track of the activities and data

being sent or received on someone’s computer.

Eavesdropping (To listen secretly to the private conversation of others) –

Refers to gaining unauthorized access to another person’s or organization’s

data while the data is on its way on the network.

this may be done in a number of ways:

1. By setting up parallel telephone lines.

2. By installing some software in the target computer.

3. By installing some receiver which capture the data while on its

way.

Network Security: refers to all activities undertaken to protect a computer network from attacks to its security.

Various techniques used for network security are given below:

1. Login-password: By assigning login names and strong password to the

users of a system, it can be ensured that only authorized people access a

computer.

2. Firewall: A firewall is a hardware device or software that controls the

incoming and outgoing network traffic. A firewall permits only the data to

enter or leave a computer/Network for which permission have been

granted by the computer/network administrator.

3. Anti Virus Software: Anti-virus software’s work against not only the

virus but also against almost all kind kinds of malware.

4. File Permissions: by giving appropriate rights to respective users,

computer security as well as network security can be increased.

ANSWER THE FOLLOWING QUESTIONS

1. Tara Nathani wants to upload and download files from/to a remote

internet server, write the name of the relevant communication protocol,

which will let her do the same.

Ans – FTP (File Transfer Protocol).

2. Define any two threats of Network Security.

3. Which protocol is used for the transfer of hyper text documents on the

internet?

Ans – HTTP (Hyper Text Transfer Protocol)

4. Which transmission media should be used to transfer data across two

continents at very high speed?

5. Give two examples of guided media and two examples of unguided

media

Ans – Guided media - Twisted pair cable, Coaxial cable

Unguided media – Microwave, radio wave.

6. Why a switch is called an intelligent hub?

7. When is a repeater used in a computer network?

8. Two engineers in the same room have connected their Palm-top using

Bluetooth for working on a group presentation. Out of the following,

what kind of network have they formed?

LAN, MAN, PAN, WAN

9. Two neighbourhood schools, at a distance of 120 meters from each

other, decide to join their LANs using UTP cable so that they can share

their e-learning resources. But after joining their LANs they are not able

to share the resources due to loss of signal in-between. Which device

should they use so that the signal is amplified in-between ?

10. How would you extract manufacturer-id and card no. from a MAC

address ?

Ans – The first three bytes of a MAC address represent manufacturer-id

and the last three bytes of a MAC address represent card-no.

11. Sujata says that the following numbers indicate an address :

208.77.188.166

What is the above address called ? To which object/device is it

assigned?

Ans : The above address is called an IP address or Internet Protocol

Address. It is assigned to devices participating in a network.

12. Jai Khanna is confused between the terms Domain Name and URL.

Explain the difference with the help of appropriate examples of each

Ans- A URL (Uniform Resource Locator) is the complete address of a

document on the web, whereas a domain name specifies the location of

document’s web server. A domain name is a component of the URL

used to access web sites.

For example the web address

http://www.example.net/index.html is a URL.

in this URL www.example.net is the domain name.

13. Ms. Taufiq Ahmed wants to prevent unauthorized access to/form his

company’s local area network. Write the name of a system

(software/hardware) , which he should install to do the same.

14. Beauty Lines Fashion Inc. is a fashion company, with design unit and

market unit 135 meters away from each other. The company recently

connected their LANs using Ethernet cable to share the stock related

information. But after joining their LANs, they are not able to share the

resources due to loss of signal in-between. Which device out of the following

should you suggest to be installed for a smooth communication ?

(i) UPS (ii) Modem (iii) Repeater

15. Which of the following is not a feature of Networking ?

(i) Resource sharing (ii) Uninterrupted Power Supply

(iii) Reduced cost (iv) Reliability

16. Identify the type of Topology from the following:

(i) If each node is connected with the help of independent cable with the

help of a central switching (communication controller )

(ii) If each node is connected with the help of a single co-axial cabel.

17. Define following with reference to Threats to Network Security :

(i) Trojan Horse (ii) Worm

18. Give one advantage and one disadvantage of using Star topology over

bus topology.

Ans : Advantage - As compared to Bus topology it gives far much better

performance, signals don’t necessarily get transmitted to all the

workstations.

Disadvantage - Longer cable length.

19. Write the name of the most suitable wireless communication channels for each of the following situations. (i) (i) Communication between two offices in different countries.

(ii) To transfer the data from one mobile phone to another. 20. Expand the following term: (i) HTTP (ii) FTP 21. What do you understand by Network Security? Name two common threats to it. Ans : Network security is needed to protect data during their transmission and to guarantee that data transmissions are authentic.

1. Trojan horse programs

2. Worms 22.

17

14. ABC Pvt. Ltd. Has set up its new center at Manglore for its offices and web based activities. It

has four buildings as shown in the diagram below :

Center to center distances between various buildings Number of computers

(a) Suggest a cable layout of connections between the buildings.

(b) Suggest the most suitable place (i.e., building) to house the server of this organization with

a suitable reason.

(c) Suggest the placement of the following devices with justification : (i) Repeater (ii)

Hub/Switch

Ans :

(a) Two layouts are being suggested any of which may be considered :

Layout Option 1 :

Layout Option 2 :

A to B 50 m

B to G 150 m

G to L 25 m

A to L 170 m

B to L 125 m

A to G 90 m

A 25

B 50

G 125

L 10

A

B

G

L

A

B

G

L

A

B

G

L

(b) The most suitable place/building to house the server of this organization would be building G, as

this building contains the maximum number of computers, thus decreasing the cabling cost for most

of the computers as well as increasing the efficiency of the maximum computers in the network. Also

housing the server here will make most of the traffic local.

(c) (i) For Layout 1, since the cabling distance between A to G, and that between B to G are quite

large, so a repeater each, would ideally be needed along their path to avoid loss of signals during the

course of data flow in these routes.

For layout 2, since the distance between building A to G is large so a repeater would ideally be placed

in between this path

(ii) in both the layouts, a hub/switch each would be needed in all the buildings, to interconnect the

group of cables from the different computers in each building.

Layout 1

Repeater

Repeater

A

B

G

L

Repeater A

B

G

L

Repeater

Repeater

A

B

G

L

Hub

Hub

Hub

Hub

Bus Topology

A bus topology consists of a main run of cable with a terminator at each end. All nodes like workstations, printers, laptops servers etc., are connected to the linear cable. The terminator is used to absorb the signal when the signal reaches the end, preventing signal bounce. When using bus topology,

when a computer sends out a signal, the signal travels the cable length in both

directions from the sending computer. When the signal reaches the end of the

cable length, it bounces back and returns in the direction it came from. This is

known as signal bounce. Signal bounce will create problem in network, because

if another signal is sent on the cable length at the same time, the two signals

will collide.

Definition

Noise

Noise is unwanted electrical or electromagnetic energy that degrades the

quality of signals and data. Noise occurs in digital and analog systems, and

can affect files and communications of all types, including text, programs,

images, and audio.

In a hard-wired circuit such as a telephone-line-based Internet hookup,

external noise is picked up from appliances in the vicinity (neighborhood), from

electrical transformers, from the atmosphere, and even from outer

space. Normally this noise is of little or no consequence. However, during

severe thunderstorms, or in locations were many electrical appliances are in

use, external noise can affect communications. In an Internet hookup it slows

down the data transfer rate, because the system must adjust its speed to

match conditions on the line.

Polyvinyl Chloride

Amplitude Modulation

Frequency Modulation

What is the difference between HTTP and FTP?

- Both HTTP and FTP are file transfer protocols based on TCP, and they are published in RFC (Request

for Comments).

- HTTP is used to transfer the content of a web page from web server to a client’s web browser, while FTP

is used to both upload and download files between FTP server and FTP client. So, HTTP is referred as a

one-way system and FTP is grouped under two-way system.

- When using an URL including http, it means that the user is connecting to a web server, and when using

an URL containing ftp, it says that the user is working with a file server.

- HTTP transfer only the web page content to the web browser in order to view it, and the transferred file is

not copied to the memory, but FTP transfers the entire file to the other device, and also it is saved in the

memory space.

- FTP generally requires a user login to the server in order to exchange files, but HTTP does not need

authentication for that.

- FTP is more efficient in transferring larger files, whereas HTTP is more efficient for transferring smaller

files such as web pages.