Lecture Notes (Week 1 and Weeh 2)
What
is a Computer?
A computer is a programmable machine. The two principal characteristics
of a computer are: it responds to a specific set of instructions in a well-defined manner
and it can execute a prerecorded list of instructions (a program).
Computer is an advanced
electronic device that takes raw data as input from the user and processes
these data under the control of set of instructions (called program) and gives
the result (output) and saves output for the future use. It can process both
numerical and non-numerical (arithmetic and logical) calculations. The basic
components of a modern digital computer are: Input Device, Output Device, and
Central Processor. A Typical modern computer uses LSI Chips.
Computer Classification,
By Size and Power
Computers can be generally classified by size and power as follows,
though there is considerable overlap:
Ø
Personal Computer: a small, single-user computer based on a
microprocessor. In addition to the microprocessor, a personal computer has a
keyboard for entering data, a monitor for displaying information, and a storage
device for saving data.
Ø
Workstation: a powerful, single-user computer. A workstation is
like a personal computer, but it has a more powerful microprocessor and a
higher-quality monitor.
Ø Minicomputer: a multi-user computer capable of supporting from 10
to hundreds of users simultaneously.
Ø Mainframe: a powerful multi-user computer capable of
supporting many hundreds or thousands of users simultaneously.
Ø Supercomputer: an extremely fast computer that can perform
hundreds of millions of instructions per second.
Computer
Hardware Definition
Hardware is a comprehensive term for all of the physical parts of
a computer, as distinguished from the data it contains or operates on, and the
software that provides instructions for the hardware to accomplish tasks. The
boundary between hardware and software is slightly blurry - firmware is
software that is "built-in" to the hardware, but such firmware is
usually the province of computer programmers and computer engineers in any case
and not an issue that computer users need to concern themselves with.
A typical computer (Personal Computer, PC) contains in a desktop
or tower case the following parts:
- Motherboard
which holds the CPU, main memory and other parts, and has slots for
expansion cards
- power supply - a
case that holds a transformer, voltage control and fan
- storage
controllers, of IDE, SCSI or other type, that control hard disk , floppy
disk, CD-ROM and other drives; the controllers sit directly on the
motherboard (on-board) or on expansion cards
- graphics
controller that produces the output for the monitor
- the hard disk,
floppy disk and other drives for mass storage
- interface
controllers (parallel, serial, USB, Firewire) to connect the computer to
external peripheral devices such as printers or scanners
Computer
Software Definition
Software is a generic term
for organized collections of computer data and instructions, often broken into
two major categories: system software that provides the basic non-task-specific
functions of the computer, and application software which is used by users to accomplish
specific tasks.
System software is
responsible for controlling, integrating, and managing the individual hardware
components of a computer system so that other software and the users of the
system see it as a functional unit without having to be concerned with the
low-level details such as transferring data from memory to disk, or rendering
text onto a display. Generally, system software consists of an operating system
and some fundamental utilities such as disk formatters, file managers, display
managers, text editors, user authentication (login) and management tools, and
networking and device control software.
Application software, on
the other hand, is used to accomplish specific tasks other than just running
the computer system. Application software may consist of a single program, such
as an image viewer; a small collection of programs (often called a software
package) that work closely together to accomplish a task, such as a spreadsheet
or text processing system; a larger collection (often called a software suite) of
related but independent programs and packages that have a common user interface
or shared data format, such as Microsoft Office, which consists of closely
integrated word processor, spreadsheet, database, etc.; or a software system,
such as a database management system, which is a collection of fundamental
programs that may provide some service to a variety of other independent
applications.
Software is created with
programming languages and related utilities, which may come in several of the
above forms: single programs like script interpreters, packages containing a
compiler, linker, and other tools; and large suites (often called Integrated
Development Environments) that include editors, debuggers, and other tools for
multiple languages.
The Internet
Internet is defined as a “connection of inter-connected
networks all freely exchanging information”. Another definition viewed internet
as “all the interconnected data sources and computers around the world”. Thus,
it is a world-wide broadcasting capability, a mechanism for information
dissemination and a medium for collaboration and interaction between
individuals and their computers without regards to geographic location. Just as
road networks link cities, and telephone networks link subscribers, the internet
as a network of computers links the computers for the purpose of communication.
The internet in this sense is a global infrastructure which enables any
computer connected to it to communicate with any other computer connected to
the internet at electronic speed, regardless of geographical location.
The Internet system comprises worldwide computer networks
that communicate directly through internet protocol (IP), a computer language
that allows any computer on any network to send data electronically to any
other computer on an equal basis. The
real national highway system and the new information highway have similar
military origin. During the 1950s,
American military and political leaders originally planned an inter-state
highway grid to make it easier to transport military vehicles across the
country in the event of a national emergency. The internet first started in
1960s as a defence communication system. It was initially called ARPAnet which
means Advanced Research Project, but later known as the “net”, allowed military
and academic researchers to conduct sensitive government probes that would
survive even in the event of nuclear disaster.
In developing the prototype for the military, Rand
Corporation, a cold war think-tank conceptualized communications networks with
no central authority. Each computer hub in the internet has similar status and
power so nobody can own the system outright and nobody has the power to kick
other off the network. There isn’t a master power switch, so authorities cannot
shut off the internet in an emergency.
At its inception stage, the internet was strictly for
military uses, where personnel from different locations interact with each
other via interconnected computers. During the development stage of internet
(1970s), the military computer network permitted different people in separate
locations to communicate with one another by simply leasing existing telephone
lines, they used the system to send letters and memos electronically (email)
and to post information on computer bulletin boards, which are sites that list
information about particular topics, such as health issues, computer programs
and employment services. At this stage, universities and government research
laboratories mainly used the internet: and later by corporations, especially
companies involved in computer software and other high-tech products, to
transmit and receive information.
By 1982, the Net had hit its entrepreneurial stage. The
National Science Foundations invested in a high-speed communication networks
designed to link computer centers around USA. This innovation permitted
dramatically increased use of the internet. After the end of cold war in late
1980s, the ARPAnet project officially ended; by this time however a growing
network of researchers, computer programmers, commercial interests and amateur
hackers had tapped into the Net, creating tens of thousands of decentralized
intersections. By 1993, the net developed multi-media capability allowing users
to travel with pictures, sound and video.
The internet today consists of more than fifty thousand
regional computer networks called servers, which are hosts run by universities,
corporations and government agencies, all interconnected by special high speed
phone lines.
The
Internet in Africa
The Internet in Africa
is limited by a lower penetration rate when compared to the rest of the world.
Measurable parameters such as the number of ISP (Internet Service Provider)
subscriptions, overall number of hosts, IXP (Internet Exchange Protocol) -traffic,
and overall available bandwidth all indicate that Africa is way behind the
"digital divide". Moreover, Africa itself exhibits an inner digital
divide, with most Internet activity and infrastructure concentrated in South
Africa, Morocco, Egypt as well as smaller economies like Mauritius and
Seychelles.
Obstacles to the
accessibility of Internet services in Africa include generally low
levels of computer
literacy in the population, poor infrastructures, and high
costs of Internet services. Power availability is also scarce, with vast rural
areas that are not connected to power grids as well as frequent black-outs
in major urban areas such as Dar es Salaam.
In 2000, Subsaharan
Africa as a whole had less fixed telephone lines than Manhattan,
and in 2006 Africa contributed to only 2% of the world's overall telephone
lines in the world. As a consequence of this general lack of connectivity, most
Africa-generated network traffic (something between 70% and 85%) is routed
through servers that are located elsewhere (mainly Europe).
Internet access
According to 2011
estimates, about 13.5% of African population has Internet access. While Africa
accounts for 15.0% of the world's population, only 6.2% of the World's Internet
subscribers are Africans. Africans who have access to broadband connections are
estimated to be in percentage of 1% or lower. In September 2007, African broadband
subscribers were 1,097,200, with a major part of these subscriptions from large
companies or institutions.
Internet access is also
irregularly distributed, with 2/3 of overall online activity in Africa being
generated in South Africa (which, on the other hand, only accounts for 5% of
the continent's population). Most of the remaining 1/3 is in Morocco and Egypt.
The largest percentage of Internet subscribers are found in small economies
such as Seychelles, where as much as 37% of the population has Internet access
(while in South Africa this value is 11% and in Egypt it is 8%).
It has been noted,
anyway, that data on Internet subscribers only partially reflect the actual
number of Internet users in Africa, and the impact of the network on African
daily life and culture. For example, cybercafes
and Internet
kiosks are common in the urban areas of many African countries.
There are also other informal means to "access" the Internet; for
example, couriers that print e-mail message and deliver
them by hand to recipients in remote locations, or radio stations
that broadcast information taken from the Internet.
Web Browsers
A web browser or
Internet browser is a software application for retrieving, presenting, and
traversing information resources on the World Wide Web. An information resource
is identified by a Uniform Resource Identifier (URI) and may be a web page,
image, video, or other piece of content. Hyperlinks present in resources enable
users to easily navigate their browsers to related resources.
Although
browsers are primarily intended to access the World Wide Web, they can also be
used to access information provided by Web servers in private networks or files
in file systems. Some browsers can also be used to save information resources
to file systems.
History of Web
Browsers.
The history of
the Web browser dates back in to the late 1980s, when a variety of technologies
laid the foundation for the first Web browser, WorldWideWeb, by Tim Berners-Lee
in 1991. That browser brought together a variety of existing and new software
and hardware technologies.
The introduction
of the NCSA Mosaic Web browser in 1993 – one of the first graphical Web
browsers – led to an explosion in Web use. Marc Andreessen, the leader of the Mosaic
team at NCSA, soon started his own company, named Netscape, and released the
Mosaic-influenced Netscape Navigator in 1994, which quickly became the world's
most popular browser, accounting for 90% of all Web use at its peak (see usage
share of web browsers).
Microsoft
responded with its browser Internet Explorer in 1995 (also heavily influenced
by Mosaic), initiating the industry's first browser war. By bundling Internet
Explorer with Windows, Microsoft was able to leverage its dominance in the
operating system market to take over the Web browser market; Internet Explorer
usage share peaked at over 95% by 2002. Internet Explorer has 60% browser usage
share as of September 2010 according to Net Applications, and it continues to
show a negative trend.
Opera first
appeared in 1996; although it has never achieved widespread use, with a browser
usage share that is stable around 2.4% as of September 2010,[4] it has a
substantial share of the fast-growing mobile phone Web browser market, being
preinstalled on over 40 million phones. It is also available on several other
embedded systems, including Nintendo's Wii video game console.
In 1998,
Netscape launched what was to become the Mozilla Foundation in an attempt to
produce a competitive browser using the open source software model. That
browser would eventually evolve into Firefox, which developed a respectable
following while still in the beta stage of development; shortly after the
release of Firefox 1.0 in late 2004, Firefox (all versions) accounted for 7.4%
of browser use.[3] As of September 2010, Firefox has a 23% usage share.
Apple's Safari
had its first beta release in January 2003; it has a dominant share of
Apple-based Web browsing, accounting for 5.3% of the entire browser market as
of September 2010 and is slowly gaining.[4] Its rendering engine, called
WebKit, is also running in the standard browsers of several mobile phone
platforms, including Apple iOS, Google Android, Nokia S60 and Palm webOS.
The most recent
major entrant to the browser market is Google's WebKit-based Chrome, first
released in September 2008. Its market share has quickly risen; as of September
2010, it has an 8% usage share and appears to be gaining further in the coming
months.
Function
The primary
purpose of a web browser is to bring information resources to the user. This
process begins when the user inputs a Uniform Resource Identifier (URI), for
example http://en.wikipedia.org/, into the browser. The prefix of the URI
determines how the URI will be interpreted. The most commonly used kind of URI
starts with http: and identifies a resource to be retrieved over the Hypertext
Transfer Protocol (HTTP). Many browsers also support a variety of other
prefixes, such as https: for HTTPS, ftp: for the File Transfer Protocol, and
file: for local files. Prefixes that the web browser cannot directly handle are
often handed off to another application entirely. For example, mailto: URIs are
usually passed to the user's default e-mail application and news: URIs are
passed to the user's default newsgroup reader.
In the case of
http, https, file, and others, once the resource has been retrieved the web
browser will display it. HTML is passed to the browser's layout engine to be
transformed from markup to an interactive document. Aside from HTML, web
browsers can generally display any kind of content that can be part of a web
page. Most browsers can display images, audio, video, and XML files, and often
have plug-ins to support Flash applications and Java applets. Upon encountering
a file of an unsupported type or a file that is set up to be downloaded rather
than displayed, the browser prompts the user to save the file to disk.
Interactivity in
a web page can also be supplied by JavaScript, which usually does not require a
plugin. JavaScript can be used along with other technologies to allow
"live" interaction with the web page's server via AJAX.
Information
resources may contain hyperlinks to other information resources. Each link
contains the URI of a resource to go to. When a link is clicked, the browser
navigates to the resource indicated by the link's target URI, and the process
of bringing content to the user begins again.
Features
Available web
browsers range in features from minimal, text-based user interfaces with
bare-bones support for HTML to rich user interfaces supporting a wide variety
of file formats and protocols. Browsers which include additional components to
support e-mail, Usenet news, and Internet Relay Chat (IRC), are sometimes
referred to as "Internet suites" rather than merely "web
browsers".
All major web
browsers allow the user to open multiple information resources at the same
time, either in different browser windows or in different tabs of the same
window. Major browsers also include pop-up blockers to prevent unwanted windows
from "popping up" without the user's consent.
Most web
browsers can display a list of web pages that the user has bookmarked so that
the user can quickly return to them. Bookmarks are also called
"Favorites" in Internet Explorer. In addition, all major web browsers
have some form of built-in web feed aggregator. In Mozilla Firefox, web feeds
are formatted as "live bookmarks" and behave like a folder of
bookmarks corresponding to recent entries in the feed. In Opera, a more
traditional feed reader is included which stores and displays the contents of
the feed.
Furthermore,
most browsers can be extended via plug-ins, downloadable components that
provide additional features.
