Download Lecture 02

Networks & Data Representation
Overview
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Computer networks
» Network connections
» LANs
» WANs
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The internet.
» Identifying computers on the internet
» The World Wide Web (WWW)
» Introduction to HTML
 Data Representation
» Analog and Binary signals
» Advantage Binary over Analog signal
» Bits and Bytes
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Computer Networks
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A network is two or more computers connected
together so that information and resources can be
shared.
Most computers are connected to some kind of
network.
Each computer has its own network address, which
uniquely identifies it among the others.
A file server is a network computer dedicated to
storing programs and data that are shared among
network users.
A file server often has a large amount of secondary
memory.
Client N
Client 1
Client 2
File Server
Client N-1
Client 3
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LANs and WANs
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A local-area network (LAN) is designed to cover
small distances and a small number of computers.
A LAN often connects the machines in a single
room or building.
A wide-are network (WAN) connects two or more
LANs, often over long distances.
Individual LANs are usually owned by a single
organization, but WANs often connect LANs from
many different groups in many different countries.
Long-distance
connection
(Wireline or Wireless)
LAN 1
LAN 2
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Internet
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The Internet is a WAN which spans the entire
planet.
The word Internet comes from the term
internetworking, which implies a network of
networks.
It started as a United States government project,
sponsored by the Advanced Research Projects
Agency (ARPA), and was originally called the
ARPANET.
The Internet grew quickly throughout the 1980s
and 90s.
Less than 600 computers were connected to the
Internet in 1983; now there are over 10 million.
The software which manages Internet
communication is called TCP/IP.
The programs in the Internet Protocol (IP) formats
the information for transfer.
The programs in the Transmission Control Protocol
(TCP) reassembles messages and handles lost
information.
Each computer on the Internet has a unique IP
address, such as: 196.1.64.8
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Internet
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Most computers also have a unique Internet name,
which is also referred to as an Internet address:
ccse.kfupm.edu.sa
aramco.com.sa
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The first part indicates a particular computer (ccse)
The rest is the domain name, indicating the
organization (kfupm.edu.sa)
The last section of each domain name usually
indicates the type of organization (Things will
change with Internet 2!!!) :
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edu: Educational institution.
com: Commercial business.
org: Non-profit organization.
Sometimes the suffix indicates the country:
New suffix categories are being considered.
A domain name can have several parts.
Unique domain names mean that multiple sites can
have individual computers with the same local
name.
When used, an Internet address is translated to an
IP address by software called the Domain Name
System (DNS).There is not a one-to-one
correspondence
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WWW
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The World-Wide Web (WWW)allows many
different types of information to be accessed using
a common interface.
A browser is a program which accesses and
presents information: text, graphics, sound, audio,
and programs.
A Web document usually contains links to other
Web documents, creating a hypermedia
environment.
The term Web comes from the fact that information
is not organized in a linear fashion.
Web documents are defined by the HyperText
Markup Language (HTML).
Information on the Web is found using a Uniform
Resource Locator (URL):
http://www.ccse.kfupm.edu.sa
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A URL may indicate an HTML document, or
some other kind of information.
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Analog & Binary Signals
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Everyone knows that computers are "digital" and
that they use "binary" numbers. We need to be
clear about their precise meaning and to be clear
about why computers are digital.
Binary means "two states." The two states are
sometimes called "1" and "0", "true" and "false", or
called "on" and "off“.
A bit is a single "on"/"off" value.
A good example is a toggle switch, such as a light
switch. You can turn it "on" or "off" but not
anything else. A light switch holds one bit of
information.
There are many advantages to binary. Here are
some (somewhat overlapping) important reasons
for using binary:
1.
2.
3.
4.
Simple; easy to build.
Unambiguous signals (hence noise immunity).
Flawless copies can be made.
Anything that can be represented with some sort of
pattern can be represented with patterns of bits.
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Analog Signal
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An analog signal is (usually) continuously
changing in value, its values can be anything within
a range of values, and its exact value at any time is
important.
If the signal shown below is an audio signal, the
exact value at each time is part of the information it
contains. For example, the value at time "T" must
be measured exactly.
It is obvious that any distortion on the signals could
lead to corruption of the information it is carrying.
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Digital Signal
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It is easy and fast (for electronics, as well as
humans) to tell if something is above a threshold or
below the threshold.
Here is the voltage signal we might send down our
wire to transmit the data "off" followed by "on".
(The wire is examined at times t1 and t2.)
The above signal is clear enough. However, the
wire is long, and someone nearby turns on a
vacuum cleaner and the signal get distorted as
shown in the next slide.
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Digital Signal –Contd.
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Even though the signal is noisy (at the analog
level), the binary values are transmitted perfectly.
You (and the electronics) can still tell that at time
"t1" the signal represents "off" and that at time "t2"
the signal represents "on" The receiving end just
needs to get the binary values.
The receiving end of the signal is only interested in
the binary values. All it has to do it check if the
signal is above or below the threshold. This can be
done perfectly (as long as the noise is not too
great.). Thus, binary signals are said to be flawless.
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Clock.
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Digital systems are built so that the "on" "off"
(binary) value is only tested at certain times, giving
the wire (or transistor, or...) a chance to change its
state between these times. This is why computer
systems have a "clock"---to keep all these times
synchronized.
So faster clocks mean wires can be tested more
times per second, and the whole system runs faster.
Processor chips (and the computers that contain
them) are often described in terms of their clock
speed
Clock speed is measured in Hertz, where one Hertz
is one clock tick per second. The symbol MHz
means mega Hertz, a million clock ticks per
second.
A 700 MHz Pentium processor checks binary
values 700 million times in each second. In
between these times values are allowed to change
and settle down.
The faster a processor chip is, the more times per
second values can be tested, and the more decisions
per second can be made.
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Bits & Bytes
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A bit is a single "on"/"off" value. Only these two
values are possible.
One bit of information is so little that usually
computer memory is organized into groups of eight
bits. Each eight bit group is called a byte.
When more than eight bits are required for some
data, a whole number of bytes are used. One byte is
about enough memory to hold a single character.
Often very much more than eight bits are required
for data, and thousands, millions, or even billions
of bytes are needed. These amounts have names, as
seen in the table:
Name
Number of Bytes
Power 2
Byte
1
20
kilobyte
1024
210
megabyte 1,048,576
220
gigabyte
230
1,073,741,824
Any system of symbols can be translated into bit
patterns. An example is how English characters
have been translated into eight-bit patterns called
ASCII. (You will learn more about this later.) 12