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Kibabii University College (KIBUCO)
School of Computing and Informatics (SCAI)
Department of Computer science
Course Description
Year II Semester I (2014/2015)
CSC 223
Course Lecturer
Details
Lecture Hours
Pre-requisite
Purpose/Aim
Course Objective
(Indicative Learning
Outcomes)
Course Content

Data Communication
Samuel W
...
com or wbarasa@mmust
...
ke
Friday, 7-9am - ABB 014, 9-11am - ABB 012
N/A
This course introduces the concepts of data communication and
computer networking since networks exist so that data can be sent
from one point to another
...

Week One: Introduction
Data communication;
Components; data representation; direction of data flow
...

Protocols and standards;
Week Two: Network Models
Layered tasks;
Internet model;
OSI model;
TCP/IP Suite
Week Three: Data and Signals
Data Transmission;
Introduction to Analog and Digital transmission; Signal types; Analog
and Digital; Analog signals;
Periodic Analog Signals;
Sine wave; phase; time and frequency domains; composite signals
bandwidth;
Digital signals;
Analog versus digital;
Transmission impairment;

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Attenuation; Distortion; Noise
Data rate limits;
Noiseless Channel: Nyquist bit rate;
Noisy Channel: Shannon capacity; using both limits
Performance;
Bandwidth, Throughput, propagation speed, propagation time,
wavelength
...

Week Six: Bandwidth Utilization
Multiplexing; FDM; WDM; TDM
...

Week Eleven: Channelization
FDMA; TDMA; CDMA
Lectures, Tutorials, Group Discussions, and Demonstrations

Learning and
Teaching
Methodologies
Instructional
Classroom with audio and visual aids
Materials/Equipment
Course Assessment
Type
Examination
Continuous Assessment
CAT One - Date: (Week 7)
CAT Two - Date: (Week 10)
Total
Recommended
Title
Reading
Computer Networks

Weight (%)
70
15
15

30
100
Publisher
Prentice-Hall
India (1996)
Addison Wesley;
(1993)

Author
Tanenbaum
A
...
,
Quarterman
J
...

Behrouz A
...
Shisoka
Chairperson, Computer Science Department

Signature: …………
...
Business decisions have to be made ever more quickly, and the decision makers require
immediate access to accurate information
...

Data communication and networking have found their way not only through business and
personal communication, they have found many applications in political and social issues
...
Communities in the world are not isolated anymore
...

Data Communications
When we communicate, we are sharing information
...

Between individuals, local communication usually occurs face to face, while remote
communication takes place over distance
...

The word data refers to information presented in whatever form is agreed upon by the parties
creating and using the data
...
For data communications to occur, the
communicating devices must be part of a communication system made up of a combination of
hardware (physical equipment) and software (programs)
...

 Delivery: The system must deliver data to the correct destination
...

 Accuracy: The system must deliver the data accurately
...

 Timeliness: The system must deliver data in a timely manner
...
In the case of video and audio, timely delivery means delivering data as they are
produced, in the same order that they are produced, and without significant delay: This
kind of delivery is called real-time transmission
...
It is the uneven delay in the
delivery of audio or video packets
...
If some of the packets arrive with 3D-ms delay and others with 4D-ms
delay, an uneven quality in the video is the result
...
The message is the information (data) to be communicated
...

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




Sender
...
It can be a computer,
workstation, telephone handset, video camera, and so on
...
The receiver is the device that receives the message
...

Transmission medium
...
Some examples of transmission media include
twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves
...
A protocol is a set of rules that govern data communications
...
Without a protocol, two devices may be
connected but not communicating, just as a person speaking French cannot be understood
by a person who speaks only Japanese
...

Text
In data communications, text is represented as a bit pattern, a sequence of bits (Os or Is)
...
Each set is called a
code, and the process of representing symbols is called coding
...
The American Standard Code for Information Interchange (ASCII),
developed some decades ago in the United States, now constitutes the first 127 characters in
Unicode and is also referred to as Basic Latin
...
However, a code such as ASCII is not used to
represent numbers; the number is directly converted to a binary number to simplify mathematical
operations
...
In its simplest form, an image is composed of a
matrix of pixels (picture elements), where each pixel is a small dot
...
For example, an image can be divided into 1000 pixels or 10,000 pixels
...

After an image is divided into pixels, each pixel is assigned a bit pattern
...
For an image made of only black and-white dots (e
...
, a
chessboard), a I-bit pattern is enough to represent a pixel
...
For example, to show four levels of gray scale, you can use 2-bit
patterns
...

There are several methods to represent color images
...
The
intensity of each color is measured, and a bit pattern is assigned to it
...

Audio
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Audio refers to the recording or broadcasting of sound or music
...
It is continuous, not discrete
...
In Chapters 4 and 5,
we learn how to change sound or music to a digital or an analog signal
...
Video can either be produced
as a continuous entity (e
...
, by a TV camera), or it can be a combination of images, each a
discrete entity, arranged to convey the idea of motion
...

Data Flow
Simplex
In simplex mode, the communication is unidirectional, as on a one-way street
...

Keyboards and traditional monitors are examples of simplex devices
...
The simplex mode can use the entire
capacity of the channel to send data in one direction
...
: When
one device is sending, the other can only receive, and vice versa
...
When cars
are traveling in one direction, cars going the other way must wait
...
Walkie-talkies and CB (citizens band) radios are both half-duplex systems
...

Full-Duplex
In full-duplex both stations can transmit and receive
...
In
full-duplex mode, going in one direction share the capacity of the link: with signals going in the
other This sharing can occur in two ways: Either the link must contain two physically separate
paths, one for sending and the other for receiving; or the capacity of the is divided between
signals traveling in both directions
...
When two people
are communicating by a telephone line, both can talk and listen at the same time
...

The capacity of the channel, however, must be divided between the two directions
...
A
node can be a computer, printer, or any other device capable of sending and/or receiving data
generated by other nodes on the network
...

Instead of one single large machine being responsible for all aspects of a process, separate
computers (usually a personal computer or workstation) handle a subset
...
The most important of these are
performance, reliability, and security
...
Transit
time is the amount of time required for a message to travel from one device to another
...
The performance of a network
depends on a number of factors, including the number of users, the type of transmission medium,
the capabilities of the connected hardware, and the efficiency of the software
...
We often need
more throughput and less delay
...
If we try to
send more data to the network, we may increase throughput but we increase the delay because of
traffic congestion in the network
...

Security
Network security issues include protecting data from unauthorized access, protecting data from
damage and development, and implementing policies and procedures for recovery from breaches
and data losses
...

Physical Structures
Before discussing networks, we need to define some network attributes
...
A link is a communications pathway
that transfers data from one device to another
...
For communication to occur, two devices
must be connected in some way to the same link at the same time
...

Point-to-Point: A point-to-point connection provides a dedicated link between two devices
...
Most point-topoint connections use an actual length of wire or cable to connect the two ends, but other
options, such as microwave or satellite links, are also possible
...

Multipoint: A multipoint (also called multidrop) connection is one in which more than two
specific devices share a single link
...

If several devices can use the link simultaneously, it is a spatially shared connection
...

Physical Topology
The term physical topology refers to the way in which a network is laid out physically i
...
one or
more devices connect to a link; two or more links form a topology
...
There are four basic topologies possible: mesh, star, bus, and ring
...
The
term dedicated means that the link carries traffic only between the two devices it connects
...

 A mesh topology is robust
...

 There is the advantage of privacy or security
...
Physical boundaries prevent other users
from gaining access to messages
...
Traffic can be
routed to avoid links with suspected problems
...

Disadvantage
The main disadvantages of a mesh are related to the amount of cabling and the number of I/O
ports required
...

 Second, the sheer bulk of the wiring can be greater than the available space (in walls,
ceilings, or floors) can accommodate
...

For these reasons a mesh topology is usually implemented in a limited fashion, for example, as a
backbone connecting the main computers of a hybrid network that can include several other
topologies
...


Star Topology
In a star topology, each device has a dedicated point-to-point link only to a central controller,
usually called a hub
...
Unlike a mesh topology,
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a star topology does not allow direct traffic between devices
...

Advantages
 A star topology is less expensive than a mesh topology
...
This factor also makes it
easy to install and reconfigure
...

 Other advantages include robustness
...
All other
links remain active
...
As long as the hub is working, it can be used to monitor link problems and
bypass defective links
...
If the hub goes down, the whole system is dead
...

For this reason, often more cabling is required in a star than in some other topologies (such as
ring or bus)
...
High-speed LANs often use a star
topology with a central hub
...
A bus topology, on the other
hand, is multipoint
...
Nodes
are connected to the bus cable by drop lines and taps
...
A tap is a connector that either splices into the main
cable or punctures the sheathing of a cable to create a contact with the metallic core
...
Therefore, it becomes
weaker and weaker as it travels farther and farther
...

Advantages
Advantages of a bus topology include ease of installation
...
In this way, a
bus uses less cabling than mesh or star topologies
...
In a bus, this
redundancy is eliminated
...
Each
drop line has to reach only as far as the nearest point on the backbone
...
A bus is usually designed to be
optimally efficient at installation
...
Signal
reflection at the taps can cause degradation in quality
...

Adding new devices may therefore require modification or replacement of the backbone
...
The damaged area reflects signals back in the direction of origin,
creating noise in both directions
...

Ethernet LANs can use a bus topology, but they are less popular now
...
A signal is passed in one direction, from device to device, until it
reaches its destination
...
When a device receives a
signal intended for another device, its repeater regenerates the bits and passes them along
...
Each device is linked to only its
immediate neighbors (either physically or logically)
...
The only constraints are media and traffic considerations
(maximum ring length and number of devices)
...
Generally in a ring, a signal is circulating at all times
...
The
alarm alerts the network operator to the problem and its location
...
In a simple ring, a break in the ring (such as a
disabled station) can disable the entire network
...

Hybrid Topology
A network can be hybrid
...


Network Types
Today when we speak of networks, we are generally referring to two primary categories: localarea networks and wide-area networks
...

Local Area Network
A local area network (LAN) is usually privately owned and links the devices in a single
office, building, or campus
...
Currently, LAN
size is limited to a few kilometers
...
The resources to be shared can include hardware (e
...
, a printer), software (e
...
, an
application program), or data
...
One of the computers may be given a large capacity disk drive
and may become a server to clients
...
In this example, the size of the LAN may be determined by licensing
restrictions on the number of users per copy of software, or by restrictions on the number of
users licensed to access the operating system
...
In general, a given LAN will use only one type of transmission medium
...

Wireless LANs are the newest evolution in LAN technology
...
A WAN can be as complex as the backbones that connect the Internet or as
simple as a dial-up line that connects a home computer to the Internet
...
This type of WAN is often used to provide Internet access
...
A switched WAN, is
used in the backbone of global communication today
...

Internetwork: Today, it is very rare to see a LAN or a WAN in isolation; they are connected to
one another
...

As an example, assume that an organization has two offices, one on the east coast and the other
on the west coast
...
To make the communication between employees at different offices possible,
the management leases a point-to-point dedicated WAN from a service provider, such as a
telephone company, and connects the two LANs
...

Metropolitan Area Networks
A metropolitan area network (MAN) is a network with a size between a LAN and a WAN
...
It is designed for customers who need a highspeed connectivity, normally to the Internet, and have endpoints spread over a city or part of city
...

Protocols and Standards
Protocols
In computer networks, communication occurs between entities in different systems
...
However, two entities cannot simply send
bit streams to each other and expect to be understood
...
A protocol is a set of rules that govern data communications
...

The key elements of a protocol are syntax, semantics, and timing
...
For example, a simple protocol might expect the first 8 bits of data to be the
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address of the sender, the second 8 bits to be the address of the receiver, and the rest of the
stream to be the message itself
...
How is a particular
pattern to be interpreted, and what action is to be taken based on that interpretation? For
example, does an address identify the route to be taken or the final destination of the message?
Timing: The term timing refers to two characteristics: when data should be sent and how fast
they can be sent
...

Standards
Standards are essential in creating and maintaining an open and competitive market for
equipment manufacturers and in guaranteeing national and international interoperability of data
and telecommunications technology and processes
...

Data communication standards fall into two categories: de facto (meaning "by fact" or "by
convention") and de jure (meaning "by law" or "by regulation")
...
De facto standards are often established
originally by manufacturers who seek to define the functionality of a new product or technology
...

Standards Organizations
Standards are developed through the cooperation of standards creation committees, forums, and
government regulatory agencies
...
The ISO is active in developing cooperation in the realms of
scientific, technological, and economic activity
...
The United Nations
responded by forming, as part of its International Telecommunication Union (ITU), a committee,
the Consultative Committee for International Telegraphy and Telephony (CCITT)
...
On March 1, 1993, the name of this
committee was changed to the International Telecommunication Union Telecommunication
Standards Sector (ITU-T)
...
S
...
However, all ANSI activities are undertaken with the welfare of the
United States and its citizens occupying primary importance
...

International in scope, it aims to advance theory, creativity, and product quality in the fields of
electrical engineering, electronics, and radio as well as in all related branches of engineering
...

Electronic Industries Association (EIA): Aligned with ANSI, the Electronic
Industries Association is a nonprofit organization devoted to the promotion of electronics
manufacturing concerns
...
In the field of information technology, the EIA has made
significant contributions by defining physical connection interfaces and electronic signaling
specifications for data communication
...
Standards committees are procedural bodies and by nature slowmoving
...
The forums work with universities and users to test,
evaluate, and standardize new technologies
...
The forums present their conclusions to the standards bodies
...
The purpose of these
agencies is to protect the public interest by regulating radio, television, and wire/cable
communications
...

Internet Standards
An Internet standard is a thoroughly tested specification that is useful to and adhered to by
those who work with the Internet
...

There is a strict procedure by which a specification attains Internet standard status
...
An Internet draft is a working document (a work in
progress) with no official status and a 6-month lifetime
...
Each RFC is edited,
assigned a number, and made available to all interested parties
...

ISOC
The Internet Society (ISOC) is an international, nonprofit organization formed in
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1992 to provide support for the Internet standards process
...
ISOC also promotes research and other scholarly activities
relating to the Internet
...
The main
purposes of the IAB are to oversee the continuing development of the TCP/IP Protocol Suite and
to serve in a technical advisory capacity to research members of the Internet community
...

Another responsibility of the IAB is the editorial management of the RFCs, described earlier
...

IETF
The Internet Engineering Task Force (IETF) is a forum of working groups managed by the
Internet Engineering Steering Group (IESG)
...
IETF also develops and reviews
specifications intended as Internet standards
...
Currently nine areas have been defined
...

IRTF
The Internet Research Task Force (IRTF) is a forum of working groups managed by the
Internet Research Steering Group (IRSG)
...

Network Models
Protocol Layering
In data communication and networking, a protocol defines the rules that both the sender and
receiver and all intermediate devices need to follow to be able to communicate effectively
...

Protocol layering enables us to divide a complex task into several smaller and simpler tasks
...
A layer needs to be able to receive a set of services from the
lower layer and to give the services to the upper layer; we don’t care about how the layer is
implemented
...
If we did not use protocol layering, we would have to make each intermediate
system as complex as the end systems, which makes the whole system more expensive
...
There is no need
for each layer to provide a service to the upper layer and give service to the lower layer
...

First Principle
The first principle dictates that if we want bidirectional communication, we need to make each
layer so that it is able to perform two opposite tasks, one in each direction
...
The second layer needs
to be able to encrypt and decrypt
...

Second Principle
The second principle that we need to follow in protocol layering is that the two objects under
each layer at both sites should be identical
...
The object under layer 2 at both sites should be a ciphertext letter
...
Data and Signals
One of the major functions of the physical layer is to move data in the form of electromagnetic
signals across a transmission medium
...


Generally, the data usable to a person or application are not in a form that can be transmitted
over a network
...
Transmission media work by conducting energy along a physical path
...


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ANALOG AND DIGITAL
Both data and the signals that represent them can be either analog or digital in form
...
The term analog data refers to information that is continuous;
digital data refers to information that has discrete states
...
On the other hand, a digital clock that reports the hours and the minutes
will change suddenly from 8:05 to 8:06
...
When
someone speaks, an analog wave is created in the air
...

Digital data take on discrete values
...
They can be converted to a digital signal or modulated into an analog signal
for transmission across a medium
...
Analog data are continuous and take continuous values
...

Analog and Digital Signals
Like the data they represent, signals can be either analog or digital
...
As the wave moves from value A to
value B, it passes through and includes an infinite number of values along its path
...
Although each value
can be any number, it is often as simple as 1 and O
...
The
vertical axis represents the value or strength of a signal
...

Signals can be analog or digital
...

Periodic and Nonperiodic Signals
Both analog and digital signals can take one of two forms: periodic or nonperiodic (sometimes
refer to as aperiodic, because the prefix a in Greek means "non")
...
The completion of one full pattern is
called a cycle
...

Both analog and digital signals can be periodic or nonperiodic
...

PERIODIC ANALOG SIGNALS
Periodic analog signals can be classified as simple or composite
...
A composite periodic analog
signal is composed of multiple sine waves
...
When we visualize it as
a simple oscillating curve, its change over the course of a cycle is smooth and consistent, a
continuous, rolling flow
...
For electric signals, peak amplitude is normally measured in volts
...

Frequency refers to the number of periods in I s
...
Period is the inverse of frequency, and frequency is the
inverse of period, as the following formulas show
...

Period is formally expressed in seconds
...
Units of period and frequency are shown in
Table Units of period and frequency
Unit
Seconds (s)
Milliseconds (ms)
Microseconds (µs)
Nanoseconds (ns)
Picoseconds (ps)

Wavelength

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Equivalent
1s
10-3 s
10-6 s
10-9 s
10-12 s

Unit
Hertz (Hz)
Kilohertz (kHz)
Megahertz
(MHz)
Gigahertz (GHz)
Terahertz (THz)

Equivalent
1 Hz
103 Hz
106 Hz
109 Hz
1012 Hz

Wavelength is another characteristic of a signal traveling through a transmission medium
...

While the frequency of a signal is independent of the medium, the wavelength depends on both
the frequency and the medium
...
In data
communications, we often use wavelength to describe the transmission of light in an optical
fiber
...

Wavelength can be calculated if one is given the propagation speed (the speed of light) and the
period of the signal
...
For example, in a vacuum, light is propagated with a speed of 3 x 108 mls
...

The wavelength is normally measured in micrometers (microns) instead of meters
...
5µm) because the
propagation speed in the cable is decreased
...
We have been
showing a sine wave by using what is called a time-domain plot
...
Phase is
not explicitly shown on a time-domain plot
...
A frequency-domain plot is concerned with only the peak value and the
frequency
...

Composite Signals
So far, we have focused on simple sine waves
...
We can send a single sine wave to carry electric energy from one place to another
...
As another example, we can use a single sine wave to
send an alarm to a security center when a burglar opens a door or window in the house
...

If we had only one single sine wave to convey a conversation over the phone, it would make no
sense and carry no information
...
We need to send a composite signal
to communicate data
...


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According to Fourier analysis, any composite signal is a combination of simple sine waves with
different frequencies, amplitudes, and phases
...

Bandwidth
The range of frequencies contained in a composite signal is its bandwidth
...
For example, if a composite signal contains
frequencies between 1000 and 5000, its bandwidth is 5000 - 1000, or 4000
...

DIGITAL SIGNALS
In addition to being represented by an analog signal, information can also be represented by a
digital signal
...
A
digital signal can have more than two levels
...

Bit Rate
Most digital signals are nonperiodic, and thus period and frequency are not appropriate
characteristics
...

The bit rate is the number of bits sent in 1s, expressed in bits per second (bps)
...
We can define something similar for a digital signal: the bit length
...

Bit length =propagation speed x bit duration
Digital Signal as a Composite Analog Signal
Based on Fourier analysis, a digital signal is a composite analog signal
...
We can intuitively corne up with this concept when we
consider a digital signal
...
A vertical line in the time domain means a frequency of infinity
(sudden change in time); a horizontal line in the time domain means a frequency of zero (no
change in time)
...

Fourier analysis can be used to decompose a digital signal
...
If the digital signal is nonperiodic, the
decomposed signal still has an infinite bandwidth, but the frequencies are continuous
...
For the remainder of the discussion, let
us consider the case of a nonperiodic digital signal, similar to the ones we encounter in data
communications
...

Baseband Transmission
Baseband transmission means sending a digital signal over a channel without changing the
digital signal to an analog signal
...

Baseband transmission requires that we have a low-pass channel, a channel with a bandwidth
that starts from zero
...
For example, the entire bandwidth of a cable connecting two
computers is one single channel
...
Again we have a low-pass
channel, and we can use it for baseband communication
...
The imperfection causes signal
impairment
...
What is sent is not what is received
...

Attenuation
Attenuation means a loss of energy
...
That is why a
wire carrying electric signals gets warm, if not hot, after a while
...
To compensate for this loss, amplifiers are used to amplify the
signal
...

The decibel (dB) measures the relative strengths of two signals or one signal at two different
points
...

dB = 10log10 P2 / P1
Variables PI and P2 are the powers of a signal at points 1 and 2, respectively
...
In this case, because
20 | P a g e

power is proportional to the square of the voltage, the formula is dB = 20 log 10 (V2IV1)
...

Distortion
Distortion means that the signal changes its form or shape
...
Each signal component has its own propagation speed
through a medium and, therefore, its own delay in arriving at the final destination
...

In other words, signal components at the receiver have phases different from what they had at the
sender
...

Noise
Noise is another cause of impairment
...
Thermal noise is the random motion
of electrons in a wire which creates an extra signal not originally sent by the transmitter
...

These devices act as a sending antenna, and the transmission medium acts as the receiving
antenna
...
One wire acts as a sending antenna and
the other as the receiving antenna
...


DATA RATE LIMITS
A very important consideration in data communications is how fast we can send data, in bits per
second
...
Data rate depends on three factors:
1
...
The level of the signals we use
3
...

Noiseless Channel: Nyquist Bit Rate
For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate
BitRate = 2 x bandwidth x 10g2 L
In this formula, bandwidth is the bandwidth of the channel, L is the number of signal levels used
to represent data, and BitRate is the bit rate in bits per second
...
Although the idea is theoretically
correct, practically there is a limit
...
If the number of levels in a signal is just 2, the receiver can easily
distinguish between a 0 and a 1
...
In other words, increasing the levels of a
signal reduces the reliability of the system
...


Example
Does the Nyquist theorem bit rate agree with the intuitive bit rate described in baseband
transmission?
Solution
They match when we have only two levels
...
However, the Nyquist
formula is more general than what we derived intuitively; it can be applied to baseband
transmission and modulation
...

Example
Consider a noiseless channel with a bandwidth of 3000 Hz transmitting a signal with two signal
levels
...
The maximum bit rate can be calculated as
BitRate =2 x 3000 X log2 4 = 12,000 bps
Example
We need to send 265 kbps over a noiseless channel with a bandwidth of 20 kHz
...
625 L =26
...
7 levels
Since this result is not a power of 2, we need to either increase the number of levels or reduce the
bit rate
...
If we have 64 levels, the bit rate is 240
kbps
...
In 1944, Claude
Shannon introduced a formula, called the Shannon capacity, to determine the theoretical highest
data rate for a noisy channel:
Capacity =bandwidth X log2 (1 +SNR)
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In this formula, bandwidth is the bandwidth of the channel, SNR is the signal-to-noise ratio, and
capacity is the capacity of the channel in bits per second
...
In other words, the formula
defines a characteristic of the channel, not the method of transmission
...
In other words, the noise is so strong that the signal is faint
...
In other words,
we cannot receive any data through this channel
...
A telephone line
normally has a bandwidth of 3000 Hz (300 to 3300 Hz) assigned for data communications
...
For this channel the capacity is calculated as
C =B log2 (1 + SNR) =3000 log2 (l + 3162) =3000 log2 3163
=3000 x 11
...
860 kbps
...


PERFORMANCE
Up to now, we have discussed the tools of transmitting data (signals) over a network and how the
data behave
...

Bandwidth
One characteristic that measures network performance is bandwidth
...

Bandwidth in Hertz
We have discussed this concept
...
For example, we can say the
bandwidth of a subscriber telephone line is 4 kHz
...
For example, one can say the bandwidth of a Fast Ethernet network (or
the links in this network) is a maximum of 100 Mbps
...

Relationship
There is an explicit relationship between the bandwidth in hertz and bandwidth in bits per
seconds
...
The relationship depends on whether we have baseband transmission or transmission
with modulation
...

 The first, bandwidth in hertz, refers to the range of frequencies in a composite signal or
the range of frequencies that a channel can pass
...

Example
The bandwidth of a subscriber line is 4 kHz for voice or data
...

Example
If the telephone company improves the quality of the line and increases the bandwidth to 8 kHz,
we can send 112,000 bps by using the same technology

Throughput
The throughput is a measure of how fast we can actually send data through a network
...
A link may have a bandwidth of B bps, but we can only send T bps through this link
with T always less than B
...
For example, we may have a
link with a bandwidth of 1 Mbps, but the devices connected to the end of the link may handle
only 200 kbps
...

Imagine a highway designed to transmit 1000 cars per minute from one point to another
...

The bandwidth is 1000 cars per minute; the throughput is 100 cars per minute
...
What is the throughput of this network?
Solution
We can calculate the throughput as
24 | P a g e

Throughput= (12,000 x 10,000)/60 =2 Mbps
The throughput is almost one-fifth of the bandwidth in this case
...
We can say that latency is made
of four components: propagation time, transmission time, queuing time and processing delay
...

The propagation time is calculated by dividing the distance by the propagation speed
...
For example, in a vacuum, light is propagated with a speed of 3 x 108 mfs
...

Example
What is the propagation time if the distance between the two points is 12,000 km? Assume the
propagation speed to be 2
...

Solution
We can calculate the propagation time as
Propagation time = (12000 x 1000)/(2
...

Transmission Time
In data communications we don't send just 1 bit, we send a message
...
However, there is a time between the first bit leaving the sender and the last bit arriving
at the receiver
...
The time required for transmission of a message depends on the size of the message and the
bandwidth of the channel
...
5-kbyte message (an e-mail) if
the bandwidth of the network is 1 Gbps? Assume that the distance between the sender and the
receiver is 12,000 km and that light travels at 2
...

Solution
We can calculate the propagation and transmission time as
Propagation Time = (12000 x 1000)/(2
...
020 ms
Note that in this case, because the message is short and the bandwidth is high, the dominant
factor is the propagation time, not the transmission time
...

Example
What are the propagation time and the transmission time for a 5-MB (megabyte) message (an
image) if the bandwidth of the network is 1 Mbps? Assume that the distance between the sender
and the receiver is 12,000 km and that light travels at 2
...

Solution
We can calculate the propagation and transmission times as
Propagation time = (12,000 x 1000) / (2
...
The propagation time can be
ignored
...
The queuing time is not a fixed factor;
it changes with the load imposed on the network
...
An intermediate device, such as a router, queues the arrived messages
and processes them one by one
...

Bandwidth-Delay Product
Bandwidth and delay are two performance metrics of a link
...
To use the
maximum capability of the link, we need to make the size of our burst 2 times the product of
bandwidth and delay; we need to fill up the full-duplex channel (two directions)
...
The sender then waits for receiver
acknowledgment for part of the burst before sending another burst
...

Example
26 | P a g e

We can think about the link between two points as a pipe
...
We can say the volume
of the pipe defines the bandwidth-delay product
...
We can roughly say that jitter is a
problem if different packets of data encounter different delays and the application using the data
at the receiver site is time-sensitive (audio and video data, for example)
...

Digital Transmission
A computer network is designed to send information from one point to another
...

In this topic, we show the schemes and techniques that we use to transmit data digitally
...
Second, we discuss analog-to-digital conversion techniques, methods which change an
analog signal to a digital signal
...

Summary
Digital-to-digital conversion: Line coding is used to convert digital data to a digital signal
...
The section also describes block coding, which is used
to create redundancy in the digital data before they are encoded as a digital signal
...
The last topic in this section discusses scrambling, a
technique used for digital-to-digital conversion in long-distance transmission
...
Delta modulation is used to improve the efficiency of the pulse code
modulation
...
In parallel transmission, we send multiple bits at a time; in serial
transmission, we send one bit at a time
...
We said that data can be either digital or analog
...
In this topic, we see
how we can represent digital data by using digital signals
...

Line coding is always needed; block coding and scrambling may or may not be needed
...
We assume that data, in
the form of text, numbers, graphical images, audio, or video, are stored in computer memory as
sequences of bits
...
At the sender,
digital data are encoded into a digital signal; at the receiver, the digital data are recreated by
decoding the digital signal
...
In data communications, our
goal is to send data elements
...
In digital data communications, a signal element carries data
elements
...
In other words, data
elements are what we need to send; signal elements are what we can send
...

Data Rate Versus Signal Rate
The data rate defines the number of data elements (bits) sent in 1s
...
The signal rate is the number of signal elements sent in 1s
...
There are
several common terminologies used in the literature
...

One goal in data communications is to increase the data rate while decreasing the signal rate
...
In our vehicle-people analogy, we need to carry more people in
fewer vehicles to prevent traffic jams
...

Bandwidth
We discussed earlier that a digital signal that carries information is nonperiodic
...
However, most
digital signals we encounter in real life have a bandwidth with finite values
...
The effective bandwidth is finite
...

Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite
...
This average is called the baseline
...
A long string of 0s or 1s can cause a drift in
the baseline (baseline wandering) and make it difficult for the receiver to decode correctly
...

DC Components
When the voltage level in a digital signal is constant for a while, the spectrum creates very low
frequencies (results of Fourier analysis)
...
We can say that DC component means 0/1 parity
28 | P a g e

that can cause base-line wondering
...

Also a long-distance link may use one or more transformers to isolate different parts of the line
electrically
...

Self-synchronization
To correctly interpret the signals received from the sender, the receiver’s bit intervals must
correspond exactly to the sender’s bit intervals
...

A self-synchronizing digital signal includes timing information in the data being transmitted
...
If the receiver’s clock is out of synchronization, these points can
reset the clock
...
Some encoding schemes that we will discuss
have this capability to some extent
...

Some encoding schemes that we will discuss have this capability
...
For example, a scheme that
uses four signal levels is more difficult to interpret than one that uses only two levels
...
NRZ, RZ, and biphase (Manchester, and differential Manchester)
 Bipolar (AMI and pseudotermary)
 Multilevel (2B/IQ, 8B/6T, and 4D-PAM5
 Multitransition (MLT-3)
There are several schemes in each category
...

NRZ (Non-Return-to-Zero)
29 | P a g e

Traditionally, a unipolar scheme was designed as a non-return-to-zero (NRZ) scheme in which
the positive voltage defines bit 1 and the zero voltage defines bit 0
...

Compared with its polar counterpart, this scheme is very costly
...
For this reason,
this scheme is normally not used in data communications today
...
For example, the voltage level
for 0 can be positive and the voltage level for 1 can be negative
...
We can have two versions of
polar NRZ: NRZ-L and NRZ-I, as shown in Figure 4
...
The figure also shows the value of r, the
average baud rate, and the bandwidth
...
In the second variation, NRZ-I (NRZ-Invert), the
change or lack of change in the level of the voltage determines the value of the bit
...

TRANSMISSION MODES
Of primary concern when we are considering the transmission of data from one device to another
is the wiring, and of primary concern when we are considering the wiring is the data stream
...
In parallel
mode, multiple bits are sent with each clock tick
...
While there is only one way to send parallel
data, there are three subclasses of serial transmission: asynchronous, synchronous, and
isochronous
...

30 | P a g e

Computers produce and consume data in groups of bits much as we conceive of and use spoken
language in the form of words rather than letters
...
This is called parallel transmission
...
That way each bit has its own wire, and all n bits of one group can be transmitted
with each clock tick from one device to another
...
All else being equal, parallel transmission can
increase the transfer speed by a factor of n over serial transmission
...
Parallel transmission requires n communication
lines (wires in the example) just to transmit the data stream
...


Serial Transmission
In serial transmission one bit follows another, so we need only one communication channel
rather than n to transmit data between two communicating devices
...

Since communication within devices is parallel, conversion devices are required at the interface
between the sender and the line (parallel-to-serial) and between the line and the receiver (serialto-parallel)
...


Asynchronous Transmission
Asynchronous transmission is so named because the timing of a signal is unimportant
...
As long as those patterns
are followed, the receiving device can retrieve the information without regard to the rhythm in
which it is sent
...
Each group, usually 8
bits, is sent along the link as a unit
...

Without synchronization, the receiver cannot use timing to predict when the next group will
arrive
...
This bit, usually a 0, is called the start bit
...
These bits, usually 1s, are called stop bits
...
In addition, the transmission of each byte may then be followed by a gap of
31 | P a g e

varying duration
...

In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits
(1s) at the end of each byte
...

The start and stop bits and the gap alert the receiver to the beginning and end of each byte and
allow it to synchronize with the data stream
...
But within each byte, the
receiver must still be synchronized with the incoming bit stream
...
The receiving device resynchronizes at the
onset of each new byte
...

After n bits, the receiver looks for a stop bit
...

Asynchronous here means “asynchronous at the byte level,” but the bits are still
synchronized; their durations are the same
...
But it is cheap and effective, two advantages that make it an
attractive choice for situations such as low-speed communication
...
A user types only one character at a time, types extremely slowly in
data processing terms, and leaves unpredictable gaps of time between characters
...
Each byte, however, is introduced onto the transmission link without a
gap between it and the next one
...
In other words, data are transmitted as an unbroken string of 1s and 0s, and
the receiver separates that string into the bytes, or characters, it needs to reconstruct the
information
...

It is the responsibility of the receiver to group the bits
In reality, those divisions do not exist; the sender puts its data onto the line as one long string
...
The receiver counts the bits as they arrive and
groups them in 8-bit units
...
Timing becomes very important, therefore, because the
accuracy of the received information is completely dependent on the ability of the receiving
device to keep an accurate count of the bits as they come in
...
With no extra bits or gaps to introduce at
the sending end and remove at the receiving end, and, by extension, with fewer bits to move
across the link, synchronous transmission is faster than asynchronous transmission
...
Byte synchronization is accomplished in the data-link layer
...
Although there is no gap between characters in
synchronous serial transmission, there may be uneven gaps between frames
...
For example, TV images are broadcast at the rate of 30 images
per second; they must be viewed at the same rate
...
For this type of application, synchronization
between characters is not enough; the entire stream of bits must be synchronized
...

MULTIPLEXING
Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of
the devices, the link can be shared
...
As data and
telecommunications use increases, so does traffic
...
today’s technology includes highbandwidth media such as optical fiber and terrestrial and satellite microwaves
...
If the bandwidth of a
link is greater than the bandwidth needs of the devices connected to it, the bandwidth is wasted
...

In a multiplexed system, n lines share the bandwidth of one link
...
1 shows the basic
format of a multiplexed system
...
At the receiving
end, that stream is fed into a demultiplexer (DEMUX), which separates the stream back into its
component transmissions (one-to-many) and directs them to their corresponding lines
...
The word channel refers to the portion of a link
that carries a transmission between a given pair of lines
...

There are three basic multiplexing techniques: frequency-division multiplexing (Analog),
wavelength-division multiplexing (Analog), and time-division multiplexing (Digital)
...

Frequency-Division Multiplexing
Frequency-division multiplexing (FDM) is an analog technique that can be applied when the
bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be
transmitted
...
These modulated signals are then combined into a single composite signal that can
be transported by the link
...
These bandwidth ranges are the channels through which the
various signals travel
...

In addition, carrier frequencies must not interfere with the original data frequencies
...
A digital signal can be
converted to an analog signal before FDM is used to multiplex them
...
Inside the multiplexer, these similar
signals modulate different carrier frequencies (f1, f2, and f3)
...

Demultiplexing Process
The demultiplexer uses a series of filters to decompose the multiplexed signal into its constituent
component signals
...

SomeApplications of FDM
A very common application of FDM is AM and FM radio broadcasting
...
A special band from 530 to 1700 kHz is assigned to AM radio
...
Each AM station needs 10 kHz of bandwidth
...
The signal
that goes to the air is a combination of signals
...
Without multiplexing, only one AM station could

34 | P a g e

broadcast to the common link, the air
...
Multiplexing is done at the data-link layer
...
However, FM has a wider band of 88 to 108 MHz
because each station needs a bandwidth of 200 kHz
...
Each TV channel has its own bandwidth of 6 MHz
...
Each user is assigned two 30-kHz
channels, one for sending voice and the other for receiving
...
Remember that an FM
signal has a bandwidth 10 times that of the modulating signal, which means each channel has 30
kHz (10 × 3) of bandwidth
...


Wavelength-Division Multiplexing
Wavelength-division multiplexing (WDM) is designed to use the high-data-rate capability of
fiber-optic cable
...

Multiplexing allows us to combine several lines into one
...
The idea is the same: We are
combining different signals of different frequencies
...

Very narrow bands of light from different sources are combined to make a wider band of light
...

Although WDM technology is very complex, the basic idea is very simple
...
The combining and splitting of light sources are easily handled by a prism
...
Using this technique, a multiplexer can be made to combine several input beams of
light, each containing a narrow band of frequencies, into one output beam of a wider band of
frequencies
...

One application of WDM is the SONET network, in which multiple optical fiber lines are
multiplexed and demultiplexed
...
It achieves even greater efficiency
...
Instead of sharing a portion of the bandwidth as in FDM, time is
shared
...

Note that the same link is used as in FDM; here, however, the link is shown sectioned by time
rather than by frequency
...

We are concerned with only multiplexing, not switching
...
The delivery is
fixed and unvarying, unlike switching
...
Digital
data from different sources are combined into one timeshared link
...

TDM is a digital multiplexing technique for combining several low-rate channels into one
high-rate one
...
We first discuss
synchronous TDM and then show how statistical TDM differs
...

Time Slots and Frames
In synchronous TDM, the data flow of each input connection is divided into units, where each
input occupies one input time slot
...
Each
input unit becomes one output unit and occupies one output time slot
...
If an input time slot
is T s, the output time slot is T/n s, where n is the number of connections
...

In synchronous TDM, a round of data units from each input connection is collected into a frame
(we will see the reason for this shortly)
...
If the duration of the input
unit is T, the duration of each slot is T/n and the duration of each frame is T (unless a frame
carries some other information, as we will see shortly)
...

Digital Signal Service
Telephone companies implement TDM through a hierarchy of digital signals, called digital
signal (DS) service or digital hierarchy
...
To implement those services, the telephone
companies use T lines (T-1 to T-4)
...
So far only T-1 and T-3 lines are commercially available
...
The two systems are conceptually identical,
but their capacities differ
...
For example, the
digital version of cellular telephony divides the available bandwidth into 30-kHz bands
...
This means that each 30-kHz band is
now made of six time slots, and the digitized voice signals of the users are inserted in the slots
...


Switching
An internet is a switched network in which a switch connects at least two links together
...
The two most
common types of switched networks are circuit-switched and packet-switched networks
...

Circuit-Switched Network
In a circuit-switched network, a dedicated connection, called a circuit, is always available
between the two end systems; the switch can only make it active or inactive
...

The switches used in this mode have forwarding tasks but no storing capability
...
In other words, instead of the continuous communication we see between two telephone
sets when they are being used, we see the exchange of individual data packets between the two
computers
...

A router in a packet-switched network has a queue that can store and forward the packet
...
If only two computers (one at each site) need to communicate with
each other, there is no waiting for the packets
...
Packet-switched network is
more efficient than a circuit switched network, but the packets may encounter some delays
...
Whenever we have multiple devices, we have the
problem of how to connect them to make one-to-one communication possible
...
These methods, however, are impractical
and wasteful when applied to very large networks
...
Other topologies employing multipoint
connections, such as a bus, are ruled out because the distances between devices and the total
number of devices increase beyond the capacities of the media and equipment
...
A switched network consists of a series of interlinked nodes,
called switches
...
In a switched network, some of these nodes are connected to
the end systems (computers or telephones, for example)
...

Three Methods of Switching
Traditionally, three methods of switching have been discussed: circuit switching, packet
switching, and message switching
...

The third has been phased out in general communications but still has networking applications
...
We discuss only circuit switching and packet switching; message switching
is more conceptual than practical
...

Switching at Physical Layer
At the physical layer, we can have only circuit switching
...
The switches at the physical layer allow signals to travel in one path or another
...
However, the term packet in this case
means frames or cells
...

38 | P a g e

Switching at Network Layer
At the network layer, we can have packet switching
...
Currently the Internet uses a datagram approach, but the
tendency is to move to a virtual-circuit approach
...
The communication at the
application layer occurs by exchanging messages
...

CIRCUIT-SWITCHED NETWORKS
A circuit-switched network consists of a set of switches connected by physical links
...
However, each
connection uses only one dedicated channel on each link
...

A circuit-switched network is made of a set of switches connected by physical links, in which
each link is divided into n channels
...
We have
shown only two end systems for simplicity
...
This is called the setup phase; a circuit (channel) is reserved on each link,
and the combination of circuits or channels defines the dedicated path
...

After all data have been transferred, the circuits are torn down
...

 Before starting communication, the stations must make a reservation for the resources to
be used during the communication
...

 Data transferred between the two stations are not packetized (physical layer transfer of
the signal)
...

 There is no addressing involved during data transfer
...
Of course, there is end-to-end
addressing used during the setup phase, as we will see shortly
...

39 | P a g e

Setup Phase
Before the two parties (or multiple parties in a conference call) can communicate, a dedicated
circuit (combination of channels in links) needs to be established
...

In the next step to making a connection, an acknowledgment from system M needs to be sent in
the opposite direction to system A
...

Note that end-to-end addressing is required for creating a connection between the two end
systems
...

Data-Transfer Phase
After the establishment of the dedicated circuit (channels), the two parties can transfer data
...


Efficiency
It can be argued that circuit-switched networks are not as efficient as the other two types of
networks because resources are allocated during the entire duration of the connection
...
In a telephone network, people normally
terminate the communication when they have finished their conversation
...
In this case, allowing resources to be dedicated means that other
connections are deprived
...
During data transfer the data are not delayed at each switch; the resources
are allocated for the duration of the connection
...
If the
message is going to pass through a packet-switched network, it needs to be divided into packets
of fixed or variable size
...

In packet switching, there is no resource allocation for a packet
...

Resources are allocated on demand
...

When a switch receives a packet, no matter what the source or destination is, the packet must
40 | P a g e

wait if there are other packets being processed
...
For example, if we do not have a reservation at a restaurant, we
might have to wait
...

Datagram Networks
In a datagram network, each packet is treated independently of all others
...
Packets in this
approach are referred to as datagrams
...
We briefly discuss datagram
networks here as a comparison with circuit-switched and virtual-circuit switched networks
...

The switches in a datagram network are traditionally referred to as routers
...

The datagram networks are sometimes referred to as connectionless networks
...
There are no setup or teardown phases
...

Routing Table
If there are no setup or teardown phases, how are the packets routed to their destinations in a
datagram network? In this type of network, each switch (or packet switch) has a routing table
which is based on the destination address
...
The destination addresses and the corresponding forwarding output ports are
recorded in the tables
...
Figure 8
...

A switch in a datagram network uses a routing table that is based on the destination address
...
When the switch receives the packet, this destination address is
examined; the routing table is consulted to find the corresponding port through which the packet
should be forwarded
...

The destination address in the header of a packet in a datagram network remains the same
during the entire journey of the packet
...
If a source sends a packet and there is
a delay of a few minutes before another packet can be sent, the resources can be reallocated
during these minutes for other packets from other sources
...

Although there are no setup and teardown phases, each packet may experience a wait at a switch
before it is forwarded
...

CELLULAR TELEPHONY
Cellular telephony is designed to provide communications between two moving units, called
mobile stations (MSs), or between one mobile unit and one stationary unit, often called a land
unit
...

To make this tracking possible, each cellular service area is divided into small regions called
cells
...
Each base station, in turn, is controlled by a switching office, called
a mobile switching center (MSC)
...
It is a computerized center that is responsible for
connecting calls, recording call information, and billing
...
A node in the network can be a satellite,
an Earth station, or an end-user terminal or telephone
...
Another restriction on using natural satellites is their distances from the
Earth, which create a long delay in communication
...
Satellites can
provide transmission capability to and from any location on Earth, no matter how remote
...

Channelization (or channel partition, as it is sometimes called) is a multiple-access method in
which the available bandwidth of a link is shared in time, frequency, or through code, among
different stations
...

FDMA
In frequency-division multiple access (FDMA), the available bandwidth is divided into
frequency bands
...
In other words, each band is
reserved for a specific station, and it belongs to the station all the time
...
To prevent station interferences, the
allocated bands are separated from one another by small guard bands
...

FDMA specifies a predetermined frequency band for the entire period of communication
...

We need to emphasize that although FDMA and frequency-division multiplexing (FDM)
conceptually seem similar, there are differences between them
...
The channels that are combined are low-pass
...
The bandwidth of each
channel is shifted by the multiplexer
...
The datalink layer in each
station tells its physical layer to make a bandpass signal from the data passed to it
...
There is no physical multiplexer at the physical layer
...
They are mixed when they are
sent to the common channel
...
Each station is allocated a time slot during which it can send data
...

The main problem with TDMA lies in achieving synchronization between the different stations
...
This may be
difficult because of propagation delays introduced in the system if the stations are spread over a
large area
...
Synchronization is normally
accomplished by having some synchronization bits (normally referred to as preamble bits) at the
beginning of each slot
...

We also need to emphasize that although TDMA and time-division multiplexing (TDM)
conceptually seem the same, there are differences between them
...
The process uses a physical multiplexer that interleaves data units from each channel
...
The data-link layer in each
station tells its physical layer to use the allocated time slot
...


CDMA
Code-division multiple access (CDMA) was conceived several decades ago
...
CDMA differs from
FDMA in that only one channel occupies the entire bandwidth of the link
...

In CDMA, one channel carries all transmissions simultaneously
...
CDMA simply means communication with different codes
...
Another two people can talk in Chinese if they are the only ones who
understand Chinese, and so on
...


44 | P a g e



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