Operating System

an operating system (OS) is a collection of software that manages computer hardware resources
and provides common services for computer programs. The operating system is a vital
component of the system software in a computer system. This tutorial will take you through
step by step approach while learning Operating System concepts.

Definition
An Operating System (OS) is an interface between a computer user and computer hardware. An
operating system is a software which performs all the basic tasks like file management, memory
management, process management, handling input and output, and controlling peripheral
devices such as disk drives and printers.
Some popular Operating Systems include Linux Operating System, Windows Operating
System, VMS, OS/400, AIX, z/OS, etc.
An operating system is a program that acts as an interface between the user and the computer
hardware and controls the execution of all kinds of programs.

Following are some of important functions of an operating System.

 Memory Management
 Processor Management
 Device Management
  File Management
 Security
 Control over system performance
 Job accounting
 Error detecting aids
 Coordination between other software and users

Memory Management
Memory management refers to management of Primary Memory or Main Memory. Main
memory is a large array of words or bytes where each word or byte has its own address.
Main memory provides a fast storage that can be accessed directly by the CPU. For a program
to be executed, it must in the main memory. An Operating System does the following activities
for memory management −
 Keeps tracks of primary memory, i.e., what part of it are in use by whom, what part are
not in use.
 In multiprogramming, the OS decides which process will get memory when and how
much.
 Allocates the memory when a process requests it to do so.
 De-allocates the memory when a process no longer needs it or has been terminated.

Processor Management
In multiprogramming environment, the OS decides which process gets the processor when and
for how much time. This function is called process scheduling. An Operating System does the
following activities for processor management −
 Keeps tracks of processor and status of process. The program responsible for this task is
known as traffic controller.
 Allocates the processor (CPU) to a process.
 De-allocates processor when a process is no longer required.

Device Management
An Operating System manages device communication via their respective drivers. It does the
following activities for device management −
 Keeps tracks of all devices. Program responsible for this task is known as the I/O
controller.
 Decides which process gets the device when and for how much time.
  Allocates the device in the efficient way.
 De-allocates devices.

File Management
A file system is normally organized into directories for easy navigation and usage. These
directories may contain files and other directions.
An Operating System does the following activities for file management −
 Keeps track of information, location, uses, status etc. The collective facilities are often
known as file system.
 Decides who gets the resources.
 Allocates the resources.
 De-allocates the resources.

Other Important Activities

Following are some of the important activities that an Operating System performs −
 Security − By means of password and similar other techniques, it prevents unauthorized
access to programs and data.
 Control over system performance − Recording delays between request for a service
and response from the system.
 Job accounting − Keeping track of time and resources used by various jobs and users.
 Error detecting aids − Production of dumps, traces, error messages, and other
debugging and error detecting aids.
 Coordination between other softwares and users − Coordination and assignment of
compilers, interpreters, assemblers and other software to the various users of the
computer systems.
Services
An Operating System provides services to both the users and to the programs.

 It provides programs an environment to execute.

 It provides users the services to execute the programs in a convenient manner.
Following are a few common services provided by an operating system −

 Program execution
 I/O operations
 File System manipulation
  Communication
 Error Detection
 Resource Allocation
 Protection

Program execution
Operating systems handle many kinds of activities from user programs to system programs like
printer spooler, name servers, file server, etc. Each of these activities is encapsulated as a
process.
A process includes the complete execution context (code to execute, data to manipulate,
registers, OS resources in use). Following are the major activities of an operating system with
respect to program management −

 Loads a program into memory.

 Executes the program.
 Handles program's execution.
 Provides a mechanism for process synchronization.
 Provides a mechanism for process communication.
 Provides a mechanism for deadlock handling.

I/O Operation
An I/O subsystem comprises of I/O devices and their corresponding driver software. Drivers
hide the peculiarities of specific hardware devices from the users.
An Operating System manages the communication between user and device drivers.

 I/O operation means read or write operation with any file or any specific I/O device.
 Operating system provides the access to the required I/O device when required.

File system manipulation

A file represents a collection of related information. Computers can store files on the disk
(secondary storage), for long-term storage purpose. Examples of storage media include
magnetic tape, magnetic disk and optical disk drives like CD, DVD. Each of these media has its
own properties like speed, capacity, data transfer rate and data access methods.
A file system is normally organized into directories for easy navigation and usage. These
directories may contain files and other directions. Following are the major activities of an
operating system with respect to file management −

 Program needs to read a file or write a file.

  The operating system gives the permission to the program for operation on file.
 Permission varies from read-only, read-write, denied and so on.
 Operating System provides an interface to the user to create/delete files.
 Operating System provides an interface to the user to create/delete directories.
 Operating System provides an interface to create the backup of file system.

Communication
In case of distributed systems which are a collection of processors that do not share memory,
peripheral devices, or a clock, the operating system manages communications between all the
processes. Multiple processes communicate with one another through communication lines in
the network.
The OS handles routing and connection strategies, and the problems of contention and security.
Following are the major activities of an operating system with respect to communication −

 Two processes often require data to be transferred between them

 Both the processes can be on one computer or on different computers, but are connected
through a computer network.
 Communication may be implemented by two methods, either by Shared Memory or by
Message Passing.

Error handling
Errors can occur anytime and anywhere. An error may occur in CPU, in I/O devices or in the
memory hardware. Following are the major activities of an operating system with respect to
error handling −

 The OS constantly checks for possible errors.

 The OS takes an appropriate action to ensure correct and consistent computing.

Resource Management
In case of multi-user or multi-tasking environment, resources such as main memory, CPU
cycles and files storage are to be allocated to each user or job. Following are the major activities
of an operating system with respect to resource management −

 The OS manages all kinds of resources using schedulers.

 CPU scheduling algorithms are used for better utilization of CPU.

Protection
Considering a computer system having multiple users and concurrent execution of multiple
processes, the various processes must be protected from each other's activities.
Protection refers to a mechanism or a way to control the access of programs, processes, or users
to the resources defined by a computer system. Following are the major activities of an
operating system with respect to protection −

 The OS ensures that all access to system resources is controlled.

 The OS ensures that external I/O devices are protected from invalid access attempts.
 The OS provides authentication features for each user by means of passwords

Applications of Operating System

Following are some of the important activities that an Operating System performs −
 Security − By means of password and similar other techniques, it prevents unauthorized
access to programs and data.
 Control over system performance − Recording delays between request for a service
and response from the system.
 Job accounting − Keeping track of time and resources used by various jobs and users.
 Error detecting aids − Production of dumps, traces, error messages, and other
debugging and error detecting aids.
 Coordination between other softwares and users − Coordination and assignment of
compilers, interpreters, assemblers and other software to the various users of the
computer systems.

Type of Operating System

Operating systems are there from the very first computer generation and they keep evolving
with time. In this chapter, we will discuss some of the important types of operating systems
which are most commonly used.

Batch operating system

The users of a batch operating system do not interact with the computer directly. Each user
prepares his job on an off-line device like punch cards and submits it to the computer operator.
To speed up processing, jobs with similar needs are batched together and run as a group. The
programmers leave their programs with the operator and the operator then sorts the programs
with similar requirements into batches.
The problems with Batch Systems are as follows −
  Lack of interaction between the user and the job.
 CPU is often idle, because the speed of the mechanical I/O devices is slower than the CPU.
 Difficult to provide the desired priority.

Time-sharing operating systems

Time-sharing is a technique which enables many people, located at various terminals, to use a
particular computer system at the same time. Time-sharing or multitasking is a logical extension
of multiprogramming. Processor's time which is shared among multiple users simultaneously is
termed as time-sharing.
The main difference between Multiprogrammed Batch Systems and Time-Sharing Systems is
that in case of Multiprogrammed batch systems, the objective is to maximize processor use,
whereas in Time-Sharing Systems, the objective is to minimize response time.
Multiple jobs are executed by the CPU by switching between them, but the switches occur so
frequently. Thus, the user can receive an immediate response. For example, in a transaction
processing, the processor executes each user program in a short burst or quantum of
computation. That is, if n users are present, then each user can get a time quantum. When the
user submits the command, the response time is in few seconds at most.
The operating system uses CPU scheduling and multiprogramming to provide each user with a
small portion of a time. Computer systems that were designed primarily as batch systems have
been modified to time-sharing systems.
Advantages of Timesharing operating systems are as follows −

 Provides the advantage of quick response.

 Avoids duplication of software.
 Reduces CPU idle time.
Disadvantages of Time-sharing operating systems are as follows −

 Problem of reliability.
 Question of security and integrity of user programs and data.
 Problem of data communication.

Distributed operating System

Distributed systems use multiple central processors to serve multiple real-time applications and
multiple users. Data processing jobs are distributed among the processors accordingly.
The processors communicate with one another through various communication lines (such as
high-speed buses or telephone lines). These are referred as loosely coupled systems or
distributed systems. Processors in a distributed system may vary in size and function. These
processors are referred as sites, nodes, computers, and so on.
The advantages of distributed systems are as follows −
  With resource sharing facility, a user at one site may be able to use the resources available at
another.
 Speedup the exchange of data with one another via electronic mail.
 If one site fails in a distributed system, the remaining sites can potentially continue operating.
 Better service to the customers.
 Reduction of the load on the host computer.
 Reduction of delays in data processing.

Network operating System

A Network Operating System runs on a server and provides the server the capability to manage
data, users, groups, security, applications, and other networking functions. The primary purpose
of the network operating system is to allow shared file and printer access among multiple
computers in a network, typically a local area network (LAN), a private network or to other
networks.
Examples of network operating systems include Microsoft Windows Server 2003, Microsoft
Windows Server 2008, UNIX, Linux, Mac OS X, Novell NetWare, and BSD.
The advantages of network operating systems are as follows −

 Centralized servers are highly stable.

 Security is server managed.
 Upgrades to new technologies and hardware can be easily integrated into the system.
 Remote access to servers is possible from different locations and types of systems.
The disadvantages of network operating systems are as follows −

 High cost of buying and running a server.

 Dependency on a central location for most operations.
 Regular maintenance and updates are required.

Real Time operating System

A real-time system is defined as a data processing system in which the time interval required to
process and respond to inputs is so small that it controls the environment. The time taken by the
system to respond to an input and display of required updated information is termed as
the response time. So in this method, the response time is very less as compared to online
processing.
Real-time systems are used when there are rigid time requirements on the operation of a
processor or the flow of data and real-time systems can be used as a control device in a
dedicated application. A real-time operating system must have well-defined, fixed time
constraints, otherwise the system will fail. For example, Scientific experiments, medical
imaging systems, industrial control systems, weapon systems, robots, air traffic control systems,
etc.
There are two types of real-time operating systems.
Hard real-time systems
Hard real-time systems guarantee that critical tasks complete on time. In hard real-time systems,
secondary storage is limited or missing and the data is stored in ROM. In these systems, virtual
memory is almost never found.
Soft real-time systems
Soft real-time systems are less restrictive. A critical real-time task gets priority over other tasks
and retains the priority until it completes. Soft real-time systems have limited utility than hard
real-time systems. For example, multimedia, virtual reality, Advanced Scientific Projects like
undersea exploration and planetary rovers, etc.

LAN - Local Area Network

A Local Area Network (LAN) is a private network that connects computers and devices within a
limited area like a residence, an office, a building or a campus. On a small scale, LANs are used
to connect personal computers to printers. However, LANs can also extend to a few kilometers
when used by companies, where a large number of computers share a variety of resources like
hardware (e.g. printers, scanners, audiovisual devices etc), software (e.g. application programs)
and data.

WAN - Wide Area Network

A Wide Area Network (WAN) is a much larger network than LAN and MAN. It often covers
multiple contries or contenants. It is quiet expensive and a single organization may not have own
it. Satellite is used to manage WAN.

MAN - Metropolitan Area Network

A Metropolitan Area Network (MAN) is a larger network than LAN. It often covers multiple
cities or towns. It is quiet expensive and a single organization may not have own it.

Following are the important differences between LAN and WAN.

Sr. Key LAN WAN

No.

1 Definition LAN stands for Local Area Network. WAN stands for Wide Area Network.

2 Ownership LAN is often owned by private WAN ownership can be private or public.
 Sr. Key LAN WAN
No.

organizations.

3 Speed LAN speed is quiet high. WAN speed is lower than that of LAN.

Delay Network Propagation Delay is short in Network Propagation Delay is longer in

4
LAN. WAN.

Congestion LAN has low congestion as compared WAN has higher congestion than LAN.
5
to WAN.

Fault Fault Tolerance of LAN is higher than Fault Tolerance of WAN is lower than
6
Tolerance WAN. LAN.

Maintenance Designing and maintaining LAN is easy Designing and maintaining WAN is
7
and less costly than WAN. complex and more costly than LAN

Following are the important differences between LAN and MAN.

Sr. Key LAN MAN

No.

Definition LAN stands for Local Area Network. MAN stands for Metropolitan Area
1
Network.

Ownership LAN is often owned by private MAN ownership can be private or public.
2
organizations.

3 Speed LAN speed is quiet high. MAN speed is lower than that of LAN.
 Sr. Key LAN MAN
No.

Delay Network Propagation Delay is short in Network Propagation Delay is average in

4
LAN. MAN.

Congestion LAN has low congestion as compared MAN has moderate congestion than
5
to MAN. LAN.

Fault Fault Tolerance of LAN is higher than Fault Tolerance of MAN is lower than
6
Tolerance MAN. LAN.

Maintenance Designing and maintaining LAN is Designing and maintaining MAN is

7
easy and less costly than MAN. complex and more costly than LAN.

Following are the important differences between MAN and WAN.

Sr. Key MAN WAN

No.

Definition MAN stands for Metropolitan Area WAN stands for Wide Area Network.
1
Network.

Ownership MAN ownership can be private or WAN ownership also can be private or
2
public. public.
 Sr. Key MAN WAN
No.

Speed MAN speed is higher than that of WAN speed is lower than that of MAN.
3
WAN.

Delay Network Propagation Delay is Network Propagation Delay is high in WAN.

4
average in MAN.

Congestion MAN has moderate congestion as WAN has high congestion than LAN.
5
compared to MAN.

Fault Fault Tolerance of MAN is higher Fault Tolerance of WAN is lower than
6
Tolerance than WAN. MAN.

Maintenance Designing and maintaining MAN is Designing and maintaining WAN is more
7
complex and costly. complex and costly than MAN

Data Communication & Computer Network

History Of Networking
ARPANET - the First Network
ARPANET − Advanced Research Projects Agency Network − the granddad of
Internet was a network established by the US Department of Defense (DOD). The work
for establishing the network started in the early 1960s and DOD sponsored major
research work, which resulted in development on initial protocols, languages and
frameworks for network communication.
It had four nodes at University of California at Los Angeles (UCLA), Stanford Research
Institute (SRI), University of California at Santa Barbara (UCSB) and University of
Utah. On October 29, 1969, the first message was exchanged between UCLA and SRI.
E-mail was created by Roy Tomlinson in 1972 at Bolt Beranek and Newman, Inc.
(BBN) after UCLA was connected to BBN.
Internet
ARPANET expanded to connect DOD with those universities of the US that were
carrying out defense-related research. It covered most of the major universities across
the country. The concept of networking got a boost when University College of London
(UK) and Royal Radar Network (Norway) connected to the ARPANET and a network of
networks was formed.
The term Internet was coined by Vinton Cerf, Yogen Dalal and Carl Sunshine of
Stanford University to describe this network of networks. Together they also developed
protocols to facilitate information exchange over the Internet. Transmission Control
Protocol (TCP) still forms the backbone of networking.
Telenet
Telenet was the first commercial adaptation of ARPANET introduced in 1974. With this
the concept of Internet Service Provider (ISP) was also introduced. The main function
of an ISP is to provide uninterrupted Internet connection to its customers at affordable
rates.

World Wide Web

With commercialization of internet, more and more networks were developed in
different part of the world. Each network used different protocols for communicating
over the network. This prevented different networks from connecting together
seamlessly. In the 1980s, Tim Berners-Lee led a group of Computer scientists at
CERN, Switzerland, to create a seamless network of varied networks, called the World
Wide Web (WWW).

World Wide Web is a complex web of websites and web pages connected together
through hypertexts. Hypertext is a word or group of words linking to another web page
of the same or different website. When the hypertext is clicked, another web page
opens.
The evolution from ARPANET to WWW was possible due to many new achievements
by researchers and computer scientists all over the world. Here are some of those
developments −

Year Milestone
 1957
Advanced Research Project Agency formed by US

1969
ARPANET became functional

1970
ARPANET connected to BBNs

1972 Roy Tomlinson develops network messaging or E-mail. Symbol @ comes

to mean "at"

1973
APRANET connected to Royal Radar Network of Norway

1974 Term Internet coined

First commercial use of ARPANET, Telenet, is approved

1982
TCP/IP introduced as standard protocol on ARPANET

1983
Domain Name System introduced

1986 National Science Foundation brings connectivity to more people with its
NSFNET program

ARPANET decommissioned
1990
First web browser Nexus developed
HTML developed

2002-2004
Web 2.0 is born

Data communications refers to the transmission of this digital data between two or more
computers and a computer network or data network is a telecommunications network that
allows computers to exchange data. The physical connection between networked computing
devices is established using either cable media or wireless media. The best-known computer
network is the Internet.
This tutorial should teach you basics of Data Communication and Computer Network (DCN)
and will also take you through various advance concepts related to Data Communication and
Computer Network.

Why to Learn Data Communication & Computer Network?

Network Basic Understanding
A system of interconnected computers and computerized peripherals such as printers is called
computer network. This interconnection among computers facilitates information sharing
among them. Computers may connect to each other by either wired or wireless media.
Network Engineering
Networking engineering is a complicated task, which involves software, firmware, chip level
engineering, hardware, and electric pulses. To ease network engineering, the whole networking
concept is divided into multiple layers. Each layer is involved in some particular task and is
independent of all other layers. But as a whole, almost all networking tasks depend on all of
these layers. Layers share data between them and they depend on each other only to take input
and send output.
Internet
A network of networks is called an internetwork, or simply the internet. It is the largest network
in existence on this planet.The internet hugely connects all WANs and it can have connection to
LANs and Home networks. Internet uses TCP/IP protocol suite and uses IP as its addressing
protocol. Present day, Internet is widely implemented using IPv4. Because of shortage of
address spaces, it is gradually migrating from IPv4 to IPv6.
Internet enables its users to share and access enormous amount of information worldwide. It
uses WWW, FTP, email services, audio and video streaming etc. At huge level, internet works
on Client-Server model.
Internet uses very high speed backbone of fiber optics. To inter-connect various continents,
fibers are laid under sea known to us as submarine communication cable.

Applications of Communication & Computer Network

Computer systems and peripherals are connected to form a network.They provide numerous
advantages:

 Resource sharing such as printers and storage devices

 Exchange of information by means of e-Mails and FTP
 Information sharing by using Web or Internet
 Interaction with other users using dynamic web pages
  IP phones
 Video conferences
 Parallel computing
 Instant messaging

Personal Area Network

A Personal Area Network (PAN) is smallest network which is very personal to a user. This may
include Bluetooth enabled devices or infra-red enabled devices. PAN has connectivity range up
to 10 meters. PAN may include wireless computer keyboard and mouse, Bluetooth enabled
headphones, wireless printers and TV remotes.

For example, Piconet is Bluetooth-enabled Personal Area Network which may contain up to 8
devices connected together in a master-slave fashion.

Local Area Network

A computer network spanned inside a building and operated under single administrative system
is generally termed as Local Area Network (LAN). Usually,LAN covers an organization’
offices, schools, colleges or universities. Number of systems connected in LAN may vary from
as least as two to as much as 16 million.
LAN provides a useful way of sharing the resources between end users.The resources such as
printers, file servers, scanners, and internet are easily sharable among computers.

LANs are composed of inexpensive networking and routing equipment. It may contains local
servers serving file storage and other locally shared applications. It mostly operates on private
IP addresses and does not involve heavy routing. LAN works under its own local domain and
controlled centrally.
LAN uses either Ethernet or Token-ring technology. Ethernet is most widely employed LAN
technology and uses Star topology, while Token-ring is rarely seen.
LAN can be wired,wireless, or in both forms at once.

Metropolitan Area Network

The Metropolitan Area Network (MAN) generally expands throughout a city such as cable TV
network. It can be in the form of Ethernet,Token-ring, ATM, or Fiber Distributed Data Interface
(FDDI).
Metro Ethernet is a service which is provided by ISPs. This service enables its users to expand
their Local Area Networks. For example, MAN can help an organization to connect all of its
offices in a city.
Backbone of MAN is high-capacity and high-speed fiber optics. MAN works in between Local
Area Network and Wide Area Network. MAN provides uplink for LANs to WANs or internet.

Wide Area Network

As the name suggests,the Wide Area Network (WAN) covers a wide area which may span
across provinces and even a whole country. Generally, telecommunication networks are Wide
Area Network. These networks provide connectivity to MANs and LANs. Since they are
equipped with very high speed backbone, WANs use very expensive network equipment.
WAN may use advanced technologies such as Asynchronous Transfer Mode (ATM), Frame
Relay, and Synchronous Optical Network (SONET). WAN may be managed by multiple
administration.

Internetwork
A network of networks is called an internetwork, or simply the internet. It is the largest network
in existence on this planet.The internet hugely connects all WANs and it can have connection to
LANs and Home networks. Internet uses TCP/IP protocol suite and uses IP as its addressing
protocol. Present day, Internet is widely implemented using IPv4. Because of shortage of
address spaces, it is gradually migrating from IPv4 to IPv6.
Internet enables its users to share and access enormous amount of information worldwide. It
uses WWW, FTP, email services, audio and video streaming etc. At huge level, internet works
on Client-Server model.
Internet uses very high speed backbone of fiber optics. To inter-connect various continents,
fibers are laid under sea known to us as submarine communication cable.
Internet is widely deployed on World Wide Web services using HTML linked pages and is
accessible by client software known as Web Browsers. When a user requests a page using some
web browser located on some Web Server anywhere in the world, the Web Server responds
with the proper HTML page. The communication delay is very low.
Internet is serving many proposes and is involved in many aspects of life. Some of them are:

 Web sites
 E-mail
 Instant Messaging
 Blogging
 Social Media
 Marketing
 Networking
 Resource Sharing
 Audio and Video Streaming

Network Topology
A Network Topology is the arrangement with which computer systems or network devices are
connected to each other. Topologies may define both physical and logical aspect of the network.
Both logical and physical topologies could be same or different in a same network.
The way in which devices are interconnected to form a network is called network
topology. Some of the factors that affect choice of topology for a network are −
 Cost − Installation cost is a very important factor in overall cost of setting up an
infrastructure. So cable lengths, distance between nodes, location of servers,
etc. have to be considered when designing a network.
 Flexibility − Topology of a network should be flexible enough to allow
reconfiguration of office set up, addition of new nodes and relocation of existing
nodes.
 Reliability − Network should be designed in such a way that it has minimum
down time. Failure of one node or a segment of cabling should not render the
whole network useless.
 Scalability − Network topology should be scalable, i.e. it can accommodate load
of new devices and nodes without perceptible drop in performance.
 Ease of installation − Network should be easy to install in terms of hardware,
software and technical personnel requirements.
 Ease of maintenance − Troubleshooting and maintenance of network should be
easy.

Bus Topology
Data network with bus topology has a linear transmission cable, usually coaxial, to
which many network devices and workstations are attached along the
length. Server is at one end of the bus. When a workstation has to send data, it
transmits packets with destination address in its header along the bus.
The data travels in both the directions along the bus. When the destination terminal
sees the data, it copies it to the local disk.
Advantages of Bus Topology
These are the advantages of using bus topology −

 Easy to install and maintain

 Can be extended easily
 Very reliable because of single transmission line

Disadvantages of Bus Topology

These are some disadvantages of using bus topology −

 Troubleshooting is difficult as there is no single point of control

 One faulty node can bring the whole network down
 Dumb terminals cannot be connected to the bus

Ring Topology
In ring topology each terminal is connected to exactly two nodes, giving the network
a circular shape. Data travels in only one pre-determined direction.
When a terminal has to send data, it transmits it to the neighboring node which
transmits it to the next one. Before further transmission data may be amplified. In this
way, data raverses the network and reaches the destination node, which removes it
from the network. If the data reaches the sender, it removes the data and resends it
later.
Advantages of Ring Topology
These are the advantages of using ring topology −

 Small cable segments are needed to connect two nodes

 Ideal for optical fibres as data travels in only one direction
 Very high transmission speeds possible

Disadvantages of Ring Topology

These are some the disadvantages of using ring topology −
 Failure of single node brings down the whole network
 Troubleshooting is difficult as many nodes may have to be inspected before
faulty one is identified
 Difficult to remove one or more nodes while keeping the rest of the network
intact

Star Topology
In star topology, server is connected to each node individually. Server is also called the
central node. Any exchange of data between two nodes must take place through the
server. It is the most popular topology for information and voice networks as central
node can process data received from source node before sending it to the destination
node.
Advantages of Star Topology
These are the advantages of using star topology −
 Failure of one node does not affect the network
 Troubleshooting is easy as faulty node can be detected from central node
immediately
 Simple access protocols required as one of the communicating nodes is always
the central node
Disadvantages of Star Topology
These are the disadvantages of using star topology −
 Long cables may be required to connect each node to the server
 Failure of central node brings down the whole network

Tree Topology
Tree topology has a group of star networks connected to a linear bus backbone cable.
It incorporates features of both star and bus topologies. Tree topology is also called
hierarchical topology.
Advantages of Tree Topology
These are some of the advantages of using tree topology −
 Existing network can be easily expanded
 Point-to-point wiring for individual segments means easier installation and
maintenance
 Well suited for temporary networks

Disadvantages of Tree Topology

These are some of the disadvantages of using tree topology −
 Technical expertise required to configure and wire tree topology
 Failure of backbone cable brings down entire network
 Insecure network
 Maintenance difficult for large networks

Transmission Media
For any networking to be effective, raw stream of data is to be transported from one
device to other over some medium. Various transmission media can be used for
transfer of data. These transmission media may be of two types −
 Guided − In guided media, transmitted data travels through cabling system that
has a fixed path. For example, copper wires, fibre optic wires, etc.
 Unguided − In unguided media, transmitted data travels through free space in
form of electromagnetic signal. For example, radio waves, lasers, etc.
Each transmission media has its own advantages and disadvantages in terms of
bandwidth, speed, delay, cost per bit, ease of installation and maintenance, etc. Let’s
discuss some of the most commonly used media in detail.

Twisted Pair Cable

Copper wires are the most common wires used for transmitting signals because of
good performance at low costs. They are most commonly used in telephone lines.
However, if two or more wires are lying together, they can interfere with each other’s
signals. To reduce this electromagnetic interference, pair of copper wires are twisted
together in helical shape like a DNA molecule. Such twisted copper wires are
called twisted pair. To reduce interference between nearby twisted pairs, the twist
rates are different for each pair.

Up to 25 twisted pair are put together in a protective covering to form twisted pair
cables that are the backbone of telephone systems and Ethernet networks.
Advantages of twisted pair cable
Twisted pair cable are the oldest and most popular cables all over the world. This is
due to the many advantages that they offer −

 Trained personnel easily available due to shallow learning curve

 Can be used for both analog and digital transmissions
 Least expensive for short distances
 Entire network does not go down if a part of network is damaged
Disadvantages of twisted pair cable
With its many advantages, twisted pair cables offer some disadvantages too −

 Signal cannot travel long distances without repeaters

 High error rate for distances greater than 100m
 Very thin and hence breaks easily
 Not suitable for broadband connections

Shielding twisted pair cable

To counter the tendency of twisted pair cables to pick up noise signals, wires are
shielded in the following three ways −

 Each twisted pair is shielded.

 Set of multiple twisted pairs in the cable is shielded.
 Each twisted pair and then all the pairs are shielded.
Such twisted pairs are called shielded twisted pair (STP) cables. The wires that are
not shielded but simply bundled together in a protective sheath are called unshielded
twisted pair (UTP) cables. These cables can have maximum length of 100 metres.
Shielding makes the cable bulky, so UTP are more popular than STP. UTP cables are
used as the last mile network connection in homes and offices.

Coaxial Cable
Coaxial cables are copper cables with better shielding than twisted pair cables, so
that transmitted signals may travel longer distances at higher speeds. A coaxial cable
consists of these layers, starting from the innermost −
 Stiff copper wire as core
 Insulating material surrounding the core
 Closely woven braided mesh of conducting material surrounding the insulator
 Protective plastic sheath encasing the wire
Coaxial cables are widely used for cable TV connections and LANs.
Advantages of Coaxial Cables
These are the advantages of coaxial cables −
 Excellent noise immunity
 Signals can travel longer distances at higher speeds, e.g. 1 to 2 Gbps for 1 Km
cable
 Can be used for both analog and digital signals
 Inexpensive as compared to fibre optic cables
 Easy to install and maintain

Disadvantages of Coaxial Cables

These are some of the disadvantages of coaxial cables −

 Expensive as compared to twisted pair cables

 Not compatible with twisted pair cables

Optical Fibre
Thin glass or plastic threads used to transmit data using light waves are called optical
fibre. Light Emitting Diodes (LEDs) or Laser Diodes (LDs) emit light waves at
the source, which is read by a detector at the other end. Optical fibre cable has a
bundle of such threads or fibres bundled together in a protective covering. Each fibre is
made up of these three layers, starting with the innermost layer −
 Core made of high quality silica glass or plastic
 Cladding made of high quality silica glass or plastic, with a lower refractive
index than the core
 Protective outer covering called buffer

Note that both core and cladding are made of similar material. However, as refractive
index of the cladding is lower, any stray light wave trying to escape the core is
reflected back due to total internal reflection.

Optical fibre is rapidly replacing copper wires in telephone lines, internet

communication and even cable TV connections because transmitted data can travel
very long distances without weakening. Single node fibre optic cable can have
maximum segment length of 2 kms and bandwidth of up to 100 Mbps. Multi-node fibre
optic cable can have maximum segment length of 100 kms and bandwidth up to 2
Gbps.
Advantages of Optical Fibre
Optical fibre is fast replacing copper wires because of these advantages that it offers −

 High bandwidth
 Immune to electromagnetic interference
 Suitable for industrial and noisy areas
 Signals carrying data can travel long distances without weakening

Disadvantages of Optical Fibre

Despite long segment lengths and high bandwidth, using optical fibre may not be a
viable option for every one due to these disadvantages −

 Optical fibre cables are expensive

 Sophisticated technology required for manufacturing, installing and maintaining optical
fibre cables
 Light waves are unidirectional, so two frequencies are required for full duplex
transmission

Infrared
Low frequency infrared waves are used for very short distance communication like TV
remote, wireless speakers, automatic doors, hand held devices etc. Infrared signals
can propagate within a room but cannot penetrate walls. However, due to such short
range, it is considered to be one of the most secure transmission modes.

Radio Wave
Transmission of data using radio frequencies is called radio-wave transmission. We
all are familiar with radio channels that broadcast entertainment programs. Radio
stations transmit radio waves using transmitters, which are received by the receiver
installed in our devices.
Both transmitters and receivers use antennas to radiate or capture radio signals. These
radio frequencies can also be used for direct voice communication within
the allocated range. This range is usually 10 miles.

Advantages of Radio Wave

These are some of the advantages of radio wave transmissions −

 Inexpensive mode of information exchange

 No land needs to be acquired for laying cables
 Installation and maintenance of devices is cheap

Disadvantages of Radio Wave

These are some of the disadvantages of radio wave transmissions −

 Insecure communication medium

 Prone to weather changes like rain, thunderstorms, etc.