Unit 3

Information Technology Infrastructure

d. Types of Networks , Topologies

e. Network Goals
 Network goals

• What is computer network ?

i. Computer network means a collection of autonomous computers
interconnected by a single technology.
ii. Two computers are said to be interconnected if they are able to exchange
information.
iii. Network consists of two or more computers linked together with data
communication devices for the purpose of communicating data & sharing
resources.
iv. The connection between computers need not be via copper wires only.
Fiber Optics, microwaves, infrared and communication satellites can also
be used.
Components of n / w include :-
a) Node / client
b) Server for centralized mgmt.
c) Cabling system etc.
 Uses of Computer Network

• Business applications:-
a) Resource Sharing :- The goal here is to make all programs,
equipments and especially data available to anyone on the network
without regards to location of the resource and the user. E.g.
Common Printer shared in a network by many office workers.
b) Sharing Information:- For smaller companies all the computers are
likely to be in a single office or perhaps a single building but for
larger ones , the computers may be scattered over dozens of
offices and plants in many countries. If a sales person in New York
might sometimes need to access to a product inventory database in
Singapore. In other words, the mere fact that a user happens to be
15,000 km away from the data should not prevent him from using
the data stored at a distance.
Contd…
c) Communication Medium :- Every company that has two or more
computers now has e-mail (electronic mail) which employees
generally use for a great deal of daily communication. It is easy for
two or more workers who work far apart to write a report together.
When one worker makes change to online document, the other can
see the a change immediately, instead of waiting several days for a
letter. Such a speedup makes co-operation among far-flung groups
of people easy where it previously had been impossible.
d) Video-Conferencing:- Using this technology, employees at distant
locations can hold a meeting, seeing and hearing each other and
even writing on a shared virtual blackboard. This is a powerful tool
for eliminating cost and time previously devoted to travel.
e) Do Business Electronically :- Many companies do business
electronically with other companies, especially with suppliers and
customers. Being able to place orders in real time (i.e. as needed)
reduces the need for large inventories and enhances efficiency.
 f) E-commerce:- (electronic- commerce)
Doing business with consumers over internet. Airlines, bookstores
etc. booking/shopping from home (on-line). On- Line auctions of
second hand goods have become massive industry.

Tag Full Name Example

B2C Business-to- Consumer Ordering books online
B2B Business-to- Business Car manufacturer ordering tires
from supplier.
G2C Government-to- Consumer Government distributing tax
forms electronically.
C2C Consumer-to- Consumer Auctioning second Hand goods
online.
P2P Peer-to-Peer File sharing

g) Other communication-oriented applications include using the internet

to carry telephone calls, video calls ,Internet radio and also in mobiles.
h) Social Interaction :- For forming groups within community or exchanging
views over social matters.
 Types of Network
• Computer networks can be divided into Local, Metropolitan
and Wide- Area networks.
• Distance is important as a classification metric because different
techniques are used at different scales i. e types decided upon no. of
computers, geographical area, size in terms of no. of devices, size in
terms of distance.

Inter-processor Processors located

distance in same
1m Square meter Personal Area network
10 m Room
100 m Building Local area network
1 km Campus
10 km City Metropolitan area network
100 km Country
Wide area network
1000 km Continent
10,000 km Planet Internet
 LAN (Local Area Network)

• A LAN is digital communication system capable of interconnecting a

large number of computers, terminals and other peripherals devices
within a limited geographical area under 1 km.
• It normally operates within a compact area such as office building or
campus.
• Note:- LAN is owned by the organization.
• Transmission channels generally used are co- axial or fiber optic
cables.
• Transmission speed is very high.
 MAN (Metropolitan Area Network)
• It represents a LAN technology optimized for longer distances.
• It is an extension of LAN. Public/private n /w used for connecting
various locations in suburbs in a metro city.
• It interconnects various buildings on the entire campus or other
facilities within a city (wide area).
• It provides high speed switched connectivity across distances
typically for a city.
• Major advantages of Metropolitan Area Network are
i. High Bandwidth
ii. Low Delay
ii. High transmission Quality
• Main features:-
a) LAN interconnection
b) Bulk Data Transfer
c) Digitized Voice transfer
d) Compressed video traffic
e) Conventional terminal traffic
E . Cable T V network.
 WAN (Wide Area Network)

• It is digital communication system which interconnects different

sites, computer installations, user terminals & may also unable
LANs to communicate with each other.

• Public/private n /w used for connecting various cities & towns.

• This type of network works nation/ world wide.

• Transmission media generally used are public systems such as

telephone lines or microwaves and satellite link.

• WAN is also known as long-haul-network.

• Owned by multiple organizations.

• E . G internet.
 Types of n / w based on roles of computer

1. Server based /- server-Client

2. Peer / peer-peer

1. Server based n /w & domains :-

• Defined by presence of server on the n /w.

• Divides processing tasks between clients & server.
• Domains are collection of n /w’ s & clients that share security
trust information.
• Domain security & logon permissions are controlled by special
servers & domain controllers.
 Advantages of server based n / w

• Strong Central Security

• Central file storage
• Ability to servers to pool available h /w & s /w resources, lowering
overall costs.
• Sharing costly resources like Laser printers, plotters etc.
• Optimizes dedicated servers. (Give entirely to specific person ,
activity or cause.)
• Easy manageability of large no. of users
• Central organization.
 Disadvantages of server based n /w

• Expensive dedicated h /w
• Expensive n /w operating system software & client licenses.
• A dedicated n / w administrator.

2. Peer Networks:-

• Computers are connected to each other

• There are no servers
• Organized into work groups
• Not optimized to share resources
 Advantages of Peer Network

• No extra investment in server h /w & s /w is required

• Easy setup

• No n /w administrator required.

• Ability of users to control resources sharing.

• No reliance on other computers for their operation.

• Lower cost for small n /w.

 Disadvantages of Peer Network

• Additional load on computers because of resource sharing.

• Inability of peers to handle as many n /w connections as servers.
• Lack of central organization which can make data hard to find.
• No central point of storage for file archiving.
• Requirement:- that users administer their own computers.
• Weak & intrusive security.
• Lack of central organization which makes large peer n /w’ s hard to
work within.
UNIT 1 , section d

1. What is topology ?

2. What is BUS topology ?

3. What is STAR topology ?

4. What is RING topology ?

5. What is Completely connected topology ?

6. What is HYBRID topology ?

 Topology
• The way in which the connections are made is called Topology.
• It especially refers to physical layout of computers and how the
cable is run between them. i.e. it refers to the way in which the
networks nodes (computers or other devices that need to
communicate) are linked.
• Choice of topology for computer network depends on combination
of factors such as:
1. Desired performance of the system.
2. Desired reliability of the system.
3. Size (number of nodes and their geographical distance) of the
system.
4. Expandability of the system.
5. Cost of components
6. Availability of communication lines.
7. Delays involved in routing information from one node to another.
•Although number of possible network topologies can be limitless they are
categorized into MULTI ACCESS BUS, STAR, RING, MESH (Completely
connected n /w), HYBRID n /w.

Multi Access BUS

Diagram

Single communication line shared by ay all nodes

Computer (nodes)
• All nodes share a single communication line.

•When a node wants to send a message it appends destination address

to the message and checks whether communication line is free.
• As soon as line becomes free it broadcasts (places ) the message
on the line.
• As the message travels on the line it is checked by each node
to whom it is addressed.
• The message is picked by the addressee node which sends an
acknowledgement to source node and frees the line.
• Co-axial cables are used in BUS n /w.
• Also called as ‘multipoint’, ‘multi drop ‘ or ’broadcast ‘ network.
•Convenient to use in small area LAN with high speed communication
channel and also where satellite communication is used as many
computers at different geographical locations share one satellite channel.
 Advantages

• Helps in reducing the number of physical lines.

• Failure of a node does not affect communication between other
nodes.
• Easy to extend.
• Simple
• Easy to understand.
• Less expensive.
Disadvantages
• All nodes in n /w should have good communication and decision
making capability.
• If shared communication line fails then entire n / w fails.
• Heavy traffic can slow down the transmission as only one
computer can send a message at a time & thus affects the speed.
• A computer must wait until the shared line is free.
• Termination problem has to be dealt with.
 Termination

• It is an important issue in bus topology. Being passive topology

( does not use amplifiers) the electric signal travels the entire length
of the cable & when it reaches at the end of the cable it gets
bounced back. When echoes back & forth along an un terminated
BUS it is called RINGING .To overcome this problem of ringing
terminators should be used.
• Terminators absorb the electrical energy &stop the reflection .
• Cables cannot be left un-terminated in bus n /w .
 Star Network

• All computers are connected through central location called host node & can
communicate via HOST.
• The routing function is performed by the host node that controls
communication between any two other nodes by establishing a logical path
between them.

Diagram of a star configuration of computer n /w

Computer (nodes)

HOST NODE
 Advantages
• Easy to add & modify new computer.
• Transmission delays reduced.
• If any node other than the host node fails , remaining nodes are
unaffected.
• Easy to troubleshoot .(Host node can be easily diagnosed to trace
faults.)
• Several cable types can be used.
Disadvantages
• If the central node fails whole n/ w fails.
• It is expensive since all cables need to pull to one central points
which require more number of cables.
 Ring Topology

Diagram of a Ring configuration of computer n /w

• It shows ring/ circular n /w in which each node has two adjacent

nodes for direct communication.
• Message flows along a ring in one direction.
• A node receives data from one of its adjacent computers & the only
decision a node has to take is whether the data is for its own use, if
it is addressed to itself it utilizes it else passes it to next node.
 Advantages
• It works well where no central node making routing decisions is
needed.
• It is more reliable than star as communication is not dependent
on single central node. If one link between two nodes fails alternate
routing is possible.

Disadvantages
• Addition of new nodes increases communication delays as delay is
directly proportional to number of nodes.
• It requires more complicated software than star n /w .
• Adding or removing computer disrupts the n / w.
 Completely Connected Topology

• It has a separate physical link for connecting each node to any other
node.
• Each node has point-to-point link with all other nodes in the n /w.
• The control is distributed with each node deciding its communication
priorities.
 Advantages
• It is very reliable as any link failure will affect only direct
communication between the nodes connected by that link.
• Each node of the network need not have individual routing
capability.
• Communication is very fast between two nodes.

Disadvantages

• It is the most expensive network from the point of view of link cost.
For n number of nodes n(n-1)/2 links are required & hence linking
cost increases.
 Hybrid Topology
• Considering the advantages & disadvantages of each type of n /w,
in reality a pure star, ring or completely connected n /w is rarely
used.
• Usually Hybrid i.e. combination of networks is used.
• Exact configuration of a n /w depends on needs and structure of the
organization.

Diagram of hybrid n/ w having star, ring & Completely connected

Ring
Star Completely connected
 Internet
• The internet is a network of computers linking many different types of
computers all over the world.
• It is a communication service that provides universal service which
allows arbitrary pair of computers to communicate.
• It is a network of networks sharing a common mechanism for
addressing (identifying) computers and a common set of
communication protocols between two computers on the network.

HISTORY
• It has its root in the APRANET system of Advanced Research Project
Agency of the U. S. Dept. of defense.
• APRANET was the first WAN and had only 4 sites in 1969.
• The internet evolved from basic idea of APRANET for interconnecting
computers for exchanging information.
• The U.S. government lifted the restrictions in 1989 on use of internet
and allowed it to be used for commercial purposes as well.
• Thereafter internet has grown rapidly to become world’s largest
network.
 Terms related to INTERNET

• Electronic mail service :- (e- mail in short ) Enables an internet user

to send a mail (message) to another internet user in any part of the
world.
• An e-mail message takes a few seconds to several minutes to reach
its destination because it travels from one network to another.
• It is similar to postal mail service.
• Postal address – similar to – e-mail address
• Each internet user has logical mailbox-similar to –-mail box in
house.
• Sender specifies the e-mail address of the receiver mailbox-similar
to –-postal address of receiver.
• E- mail service delivers the message into receiver’s mailbox and the
receiver can extract it as per his/ her convenience just as postal mail
system
• The receiver can save, delete, forward the message or respond to it.
• E-mail not only contains text documents but also image, audio and
video data wit the only restriction that the data must be digitized
(in computer-readable format.)
• Advantages:-

1. It is faster than paper mail.

2. Unlike telephone, the persons communicating need not be available
at same time.
3. Unlike fax documents, e-mail documents can be stored in a computer
and can be edited easily using editing programs.
 File Transfer Protocol

• Known as FTP service enables an internet user to move file from

one computer to another on internet.
• A file may have digital information- text document , image ,movie,
software etc.
• Downloading- moving one file from a remote computer to ones
own computer.
• Uploading- moving one file from ones own computer to a remote
computer .
• File transfer takes place in following manner:-
1. A user executes the ftp command from his/her computer
specifying address of the remote computer as parameter.
2. An FTP client process establishes a connection with FTP process
running on remote computer called (FTP server process )
3. The system then asks the user to enter his/her login name and
password on the remote computer to ensure that the user is
authorized.
4. After successful login desired files are downloaded or uploaded
using get (for downloading) and put (for uploading) commands.

To overcome the problem of access rights to vast number of users

Anonymous FTP site is used.
• Anonymous FTP site is a computer allowing the user to log in
with username as anonymous and password i.e. user’s e-mail id.
• These are publicly accessible sites as any user on internet can
access & download files that are stored for sharing.
• In organizations FTP is used for more secure file transfer
operations (authentication.)
 TELNET

• It enables an internet user to log in to another computer from his/

her local computer.
• When a telnet command is executed on local computer by a user a
login session on remote computer is started.
• This is called remote login.
• To start a remote login session a user types the command telnet
and address of the remote computer on hi/her local computer. The
system then asks the user to enter a login name (User ID) and
password.
• After authorization is done a login session is established & the telnet
enters input mode & communication between local & remote
computer is processed.
Uses:-
• Remote computer- super computer.
Local computer-ordinary computer. Use computing powers of
Remote computer.
2. Using a s /w from remote computer.
3. Accessing a database / archive from remote computer.
4. To access his/ her personal computer via telnet if that user
is attending a conference in another city.

USENET NEWS
Usenet service enables a group of internet users to exchange their
views/ideas/information on some common topic of interest.
• Moderated newsgroup
• Non - Moderated newsgroup.
• WWW (World Wide Web)
• WWW Browsers
• Uses of Internet.