module -1 fuill notes.pdf

MOBILE COMPUTING
Course Code: 20CSE813A
Dr.Santhosh Krishna B V
Associate Professor
Department of Computer Science and Engineering
New Horizon College of Engineering
Module-1
prepared by DR.BVS- NHCE

Module-1
Introduction: Introduction to Mobile Computing – Challenges in mobile computing,
coping with uncertainties, resource poorness, bandwidth, etc. Applications of Mobile
Computing- Generations of Mobile Communication Technologies- Multiplexing –
Spread spectrum -MAC Protocols – SDMA- TDMA- FDMA- CDMA

INTRODUCTION

Mobile computing
Definition
Mobile computing (sometimes called ubiquitous computing and also at times called nomadic computing) is
widely described as the ability to compute remotely while on the move.
This is a new and fast emerging discipline that has made it possible for people to access information from
anywhere and at anytime.
We can also view mobile computing as encompassing two separate and distinct concepts: mobility and
computing.
Computing denotes the capability to automatically carry out certain processing related to service invocations
on a remote computer. Mobility, on the other hand, provides the capability to change location while
communicating to invoke computing services at some remote computers.

Mobile Computing vs. Wireless Networking
Wireless Networks Fixed Networks
There is no requirement of any physical
configuration in the wireless network.
In Fixed Networks, a physical configuration is
required in any condition.
The data loss rate is high in Wireless
Networks.
In Fixed Networks, a perfect link is established
between the devices, so; the data loss rate is very
low.
In Wireless Networks, the data transmission
rate is comparatively low, so it provides less
speed.
In Fixed Networks, the rate of data transmission
is high, so it provides high speed.
Latency is high in Wireless Networks, which
finally results in more delay.
There is no issue of latency in Fixed Networks
because there is a perfect connection established
between the devices that provide less delay.
The Wireless Networks may be hacked; that's
why the security is always low in this type of
network.
Fixed Networks connections are highly secured.

Mobile Computing Applications
1.Business:
1. Managers can use mobile computers say, critical presentations to major customers. They
can access the latest market share information. To enable the company to keep track of
all activities of their travelling employees, to keep databases consistent etc. With wireless
access, the laptop can be turned into a true mobile office, but efficient and powerful
synchronization mechanisms are needed to ensure data consistency.
2.Infotainment:
1. Now a days there is a huge market of Entertainment for humans while they are on move.
Watching movies, listening music and playing a game is become a part of life for
entertainment. In this case mobile computing performs a major role to provide
uninterrupted internet connection to digital devices.
3.Replacement of Wired Networks:
1. wireless networks can also be used to replace wired networks. e.g., remote sensors, for
tradeshows, or in historic buildings. Due to economic reasons, it is often impossible to
wire remote sensors for weather forecasts, earthquake detection, or to provide
environmental information.

Mobile Computing Applications
4. Credit Card Verification
At Point of Sale (POS) terminals in shops and supermarkets, when customers use credit
cards for transactions, the intercommunication required between the bank central
computer and the POS terminal, in order to effect verification of the card usage, can take
place quickly and securely over cellular channels using a mobile computer unit. This can
speed up the transaction process and relieve congestion at the POS terminals.
5.Emergencies:
An ambulance with a high-quality wireless connection to a hospital can carry vital
information about injured persons to the hospital from the scene of the accident. All the
necessary steps for this particular type of accident can be prepared and specialists can be
consulted for an early diagnosis.
6.Tourism:
Tourism is the largest industry for all the countries. Mostly tourism places are at remote
location from the developed cities. In this case wireless communication performs a vital
role in connectivity for people who are enjoying their tour. They are continuously in
contact with the family and friends and searching for travel services, hotel services food
services etc.

7.E-Governance:
Governments are using many communication services to update their rural areas. To provide
health, education, safety , farming , weather forecast and many other related information to
governance, governments are connecting rural areas with head quarter offices for monitoring.
8.Education:
2020 COVID Pandemic teach us a great lesson to provide distance education to all who can not
reach to the college or schools. Wireless communication and digital devices are the backbone
for such distance learning concept. Now all private and public industries are moving towards
the online education with help of good wireless connectivity.
Manage Personal Records
Social Media and Group Message
Transaction

Characteristics of Mobile Computing
A computing environment is said to be “mobile”, when either the sender or the receiver of information can be on the
move while transmitting or receiving information. The following are some of the important characteristics of a mobile
computing environment
Ubiquity: The dictionary meaning of ubiquity is present everywhere. In the context of mobile computing, ubiquity
means the ability of a user to perform computations from anywhere and at anytime. For example, a business
executive can receive business notifications and issue business transactions as long he is in the wireless coverage
area.
Location awareness: A hand-held device equipped with global positioning system (GPS) can transparently provide
information about the current location of a user to a tracking station.
Adaptation: Adaptation in the context of mobile computing implies the ability of a system to adjust to bandwidth
fluctuation without inconveniencing the user. In a mobile computing environment, adaptation is crucial because of
intermittent disconnections and bandwidth fluctuations that can arise due to a number of factors such as handoff,
obstacles, environmental noise, etc.

Characteristics of Mobile Computing
Broadcast: Due to the broadcast nature of the underlying communication network of a mobile computing
environment, efficient delivery of data can be made simultaneously to hundreds of mobile users. For example, all
users at a specific location, such as those near a railway station, may be sent advertising information by a taxi
service operator
Personalization: Services in a mobile environment can be easily personalized according to a user’s profile. This is
required to let the users easily avail information with their hand-held devices. For example, a mobile user may
need only a certain type of information from specific sources. This can be easily done through personalization.

Challenges
1.Constrained Resources
The most obvious limitation of a wireless sensor is the fact that the resources available to the sensor are
severely constrained relative to a desktop computer or even a personal digital assistant (PDA). Although these
limitations are obvious, the various ways these limitations influence the design across distinct layers of the
protocol stack are not immediately apparent.
No centralized authority
Limited power
Wireless communication
Limited computation and storage
Storage constraints
Limited input and output options

2.Security
Sensor networks are deployed for a wide range of purposes. Some of these sensors require absolutely no
security. A noncritical application, such as a sensor that simply recognizes that someone has picked up an object
and responds, may not need any security. On the other hand, sensors deployed for military applications require
stringent security mechanisms.
Small keys
Limited computation
Changing network membership
Arbitrary topology

3.Mobility
Ad hoc networks are not necessarily mobile. The ad hoc nature of the network arises simply from the lack of
fixed infrastructure. Of course, the absence of a fixed network infrastructure leads directly to the conclusion
that mobility is a reasonable feature for these devices.
Mobility of ad hoc nodes presents a number of additional challenges to deploying and sustaining energy-
efficient ad hoc networks. Although some energy may be used in mobility if the node is providing the
movement on its own, as opposed to being transported by a human or an animal, the overhead is
unavoidable because of the effects that mobility has on the wireless communication protocols. In other
words, the overhead occurs because mobility is useful to the user
Mobility requirements
Loss of connectivity
Data loss

Generations of Mobile Computing

 Pre-cell phone mobile telephony technology, such as radio
telephones that some had in cars before the arrival of cell phones.
 Communication was possible through voice only.
 These mobile telephones were usually mounted in cars or trucks.
 Technologies :
PTT(Push to Talk)
MTS (Mobile Telephone System)
IMTS (Improved MTS)

First Mobile Radio Telephone-1924

First Generation Cellular Systems
 First generation (1G) of cellular systems introduced in the late
1970s and early 1980s
 Evolved out of the growing number of mobile communication
users
 The use of semiconductor technology and microprocessors made
mobile devices smaller and lighter
 1G systems were based on analogue communication in the
900MHz frequency range
 Voice transmission only – easy to tap
 The most prominent 1G systems are
 Advanced Mobile Phone Systems (AMPS) - America
 Nordic Mobile Telephone (NMT) - France
 Total Access Communications System (TACS) – UK
 Jan 1985 Vodafone introduced the TACS system

Frequency Division Multiple Access (FDMA)
 Splits allocated spectrum into 30 channels, each channel is 30kHz
 Allocates a single channel to each established phone call
 The channel is agreed with the serving base-station before
transmission takes place on agreed and reserved channel
 Channel used by device to transmit and receive on this channel
 Ineffective methods since each analogue channel can only be used
by one user at a time
 FDMA does not take full advantage of available spectrum

Frequency Division Multiple Access (FDMA)
Frequency

DRAWBACKS OF 1G
Poor Voice Quality
Poor Battery Life
Large Phone Size
No Security
Limited Capacity
Poor Handoff Reliability
1G Wireless System
Drawbacks of 1G

Region America Europe Japan
Parameter AMPS ETACS NTT
Multiple Access FDMA FDMA FDMA
Duplexing FDD FDD FDD
Forward Channel 869-894 MHZ 935-960 MHZ 870-885 MHZ
Reverse Channel 824-849 MHZ 890-915 MHZ 925-940 MHZ
Channel Spacing 30 KHZ 25 KHZ 25 KHZ
Data Rate 10 Kbps 8 Kbps 0.3 Kbps
Capacity 832 Channels 1000 Channels 600 Channels
AMPS: Advanced mobile phone system
ETACS: European Total Access Comm. system
NTT: Nippon Telephone and Telegraph

Second Generation Cellular Systems
 Development driven by the need to improve speech quality,
system capacity, coverage and security
 First system that used digital transmission
 Examples of Second Generation (2G) cellular systems ...
 Digital AMPS (D-AMPS) in the US,
 Personal Digital Communication (PDC) in Japan,
 Intrim Standard `94 (IS-94) in Korea and the US
 Global System for Mobile Communication (GSM)
 The GSM standard was defined by ETSI in 1989
 Originally called “ Groupe Spéciale Mobile which later changed
to the English version
 A majority of countries over the world have adopted GSM900
and the GSM1800 which are all based on the same original
GSM specification.
 The US uses an additional GSM 1900

2G TECHNOLOGY
 2G technology refers to the 2nd generation which is
based on GSM.
 It was launched in Finland in the year 1991.
 2G network use digital signals.
 It’s data speed was upto 64kbps.
Features Includes:
 It enables services such as text messages,
picture messages and MMS (multi media message).
 It provides better quality and capacity .

2G
 2G requires strong digital signals
to help mobile phones work. If there
is no network coverage in any specific
area , digital signals would weak.
 These systems are unable to
handle complex data such as Videos.
2G Wireless System
DRAWBACKS OF 3G

2.5G Technology
 2.5G is a technology between the second (2G) and
third (3G) generation of mobile telephony.
 2.5G is sometimes described as 2G Cellular
Technology combined with GPRS.
Features Includes:
 Phone Calls
 Send/Receive E-mail Messages
 Web Browsing
 Speed : 64-144 kbps
 Camera Phones
 Take a time of 6-9 mins. to download a 3 mins. Mp3 song

& 2.5 Digital Cellular System
Generation Technology Voice Data
Rate
2G GSM 10 Kbps
2G CDMA 10Kbps
2.5G GPRS 50Kbps
2.75 EDGE 200kbps
GPRS : General packet radio service.
EDGE : Enhanced Data For GSM Evolution

3rd GENERATION
2G networks were built mainly for voice data and slow
transmission. Due to rapid changes in user expectation, they
do not meet today's wireless needs.
3G networks provide the ability to transfer voice data and
non-voice data over the same network simultaneously.
Applications :
Internet, e-mail, fax, e-commerce, music,
video clips, and videoconferencing
The aim of the 3G is to allow for more coverage and growth
with minimum investment.

3G TECHNOLOGY
3G technology refer to third generation which was
introduced in year 2000s.
Data Transmission speed increased from
144kbps- 2Mbps.
Typically called Smart Phones and
features increased its bandwidth
and data transfer rates to accommodate
web-based applications and audio
and video files.

Feature of 3G Technology OF 3G
TECHNOLOGY
 Providing Faster Communication
 Send/Receive Large Email Messages
 High Speed Web / More Security
Video Conferencing / 3D Gaming
 TV Streaming/ Mobile TV/ Phone Calls
 Large Capacities and Broadband Capabilities
 11 sec – 1.5 min. time to download a 3 min Mp3 song.

DRAWBACKS OF 3G TECHNOLOGY
 Expensive fees for 3G Licenses Services
 It was challenge to build the infrastructure
for 3G
 High Bandwidth Requirement
 Expensive 3G Phones.
 Large Cell Phones

3G Cellular System Standards.
Generation Technology Data Rate
3G WCDMA/U
MTS
384 Kbps 10Kbps-
voice & 50
Kbps for
data
3G CDMA 384 Kbps
3G HSDPA/HSU
PA
5-30 Mbps
3.5G 1X EVDO
A,B,C
5-30 Mbps
WCDMA: wide band CDMA
UMTS: Universal mobile telecomm. Standards
HSDPA/HSUPA: High speed down link or uplink packet
access
1x EVDO: Evaluation data Optimized.

2G
1G
3G 1.High bandwidth requirement.
2. High spectrum licensing fees.
3.Huge capital.
The GSM is a circuit switched, connection oriented
technology, where the end systems are dedicated for the
entire call session. This causes inefficiency in usage of
bandwidth and resources. They are unable to handle
complex data such as video.
Poor voice quality, Poor battery life, Large phone size, No
security, frequent call drops, Limited capacity and poor
handoff reliability.

4th GENERATION
• 4G development focuses around achieving ultra-
broadband speeds, competing with and in some cases
outstripping the speeds provided by your home internet
connection.
• 4G average speeds are targeted to be in the 100Mbps to
1Gbps range, roughly 10 to 100 times (dependent on
location) faster than 3G networks. At that rate, that 4-
minute MP3 download would take you mere seconds.
• A 4G phone can run on a 3G network just fine, and it’ll be
ready for the 4G revolution when the time comes.

• There are two major systems in U.S, which are using the 4G
mobile technology – WiMax, backed by Clearwire and Long
Term Evolution or LTE.
• WiMax’s majority owner is Sprint Nextel. Sprint currently has
two mobile phones, the HTC Evo and the Samsung Epic,
which achieve speeds 10 times faster than 3G; coverage is
still limited to major metropolitan cities. Outside of these
areas, data speeds revert to 3G.
• Long Term Evolution is backed by Verizon. According to
cnet, Verizon has completed initial 4G wireless test, but not
available for widespread use until end of 2012.

Features include:
- A spectrally efficient system
- High network capacity
- Huge data rate
- Perfect connectivity & global roaming
- High quality of service
- Security & Privacy
Speed:
The data transfer is 100 Mbps for outdoor and 1Gbps
for indoor.
The word “MAGIC” also refers to 4G wireless
technology which stands for
Mobile multimedia, Any-where, Global mobility
solutions over, Integrated wireless and Customized
services.

The design is that 4G will be based on OFDM (Orthogonal
Frequency Division Multiplexing), which is the key enabler
of 4G technology. Other technological aspects of 4G are
adaptive processing and smart antennas, both of which will
be used in 3G networks and enhance rates when used in
with OFDM

High-speed data access
High quality streaming video
Combination of wi- fi and wi-max
Capable of providing 100Mbps – 1Gbps speed.
One of the basic term used to describe 4G is MAGIC.
MAGIC:
Mobile Multimedia
Anytime Anywhere
Global Mobility Support
Integrated Wireless Solution
Customized Personal Services
Also known as Mobile Broadband Everywhere.
4G TECHNOLOGY

4G (Anytime, Anywhere)
The next generations of wireless technology that promises
higher data rates and expanded multimedia services.
Capable to provide speed 100Mbps-1Gbps.
High QOS and High Security
Provide any kind of service at any time as per user requirements,
anywhere.
Features Include:
 More Security
 High Speed
 High Capacity
 Low Cost Per-bit etc.

DRAWBACKS OF 4G
Battery uses is more
Hard to implement
Need complicated hardware
Expensive equipment required
to implement next generation
network.

Generation Technology Data rate
4G LTE
(Long term
evaluation)
100-200Mbps
4G WiMax
(world wide
Interapability
For micro wave
access )
100-200Mbps

COMPARISON BETWEEN 3G Vs 4G
Technology 3G 4G
Data Transfer Rate 3.1 MB/sec 100 MB/sec
Internet Services Broadband Ultra Broadband
Mobile - TV Resolution Low High
Bandwidth 5-20 MHz 100MHz
Frequency 1.6-2 GHz 2-8 GHz
Download and upload 5.8 Mbps 14 Mbps
The basic difference between 3G and 4G is in data transfer and signal
quality.

Figure: Evolution of Mobile system from 2G
to 3G

 LTE is a standard for wireless communication of
high-speed data for mobile phones and data
terminals.
 Supports at least 200 active data clients in every
5 MHz cell.
 It is based on the GSM/EDGE and UMTS/HSPA
network technologies, increasing the capacity and
speed using new modulation techniques.
 Ability to manage fast moving mobiles and
supports MBSFN (Multicast Broadcast Single
Frequency Network).
 It can deliver services such as Mobile TV

 Formally submitted as a candidate 4G system to
ITU-T in late 2009, was approved into ITU,
International Telecommunications Union, IMT-
Advanced and was finalized by 3GPP in March
2011.
 Data rate is 2Gbps.
 It targets faster switching between power states
and improved performance at the cell edge.
 Improves the capacity and coverage, and ensures
user fairness.
 Also introduces multicarrier to be able to use
ultra wide bandwidth, up to 100 MHz of spectrum
supporting very high data rates.

5G TECHNOLOGY
5G technology refer to short name of fifth Generation
which was started from late 2010s.
Complete wireless communication
with almost no limitations.
It is highly supportable to WWWW
(Wireless World Wide Web).

BENEFITS OF 5G TECHNOLOGY
High Speed, High Capacity
5G technology providing large broadcasting of data in Gbps .
Multi - Media Newspapers, watch T.V programs with the
clarity
as to that of an HD Quality.
Faster data transmission that of the
previous generations.
Large Phone Memory, Dialing Speed,
clarity in Audio/Video.
Support interactive multimedia , voice,
streaming video, Internet and other
5G is More Effective and More Attractive.

COMPARISON BETWEEN 4G Vs 5G
The following basic differences between 4G and 5G are:

EVOLUTION OF 1G TO 5G TECHNOLOGY

FEATURE OF WIRELESS TECHNOLOGY
5G
4G

Technology 1G 2G/2.5G 3G 4G 5G
Deployment 1970/198
4
1980/1999 1990/2002 2000/2010 2014/2015
Bandwidth 2kbps 14-64kbps 2mbps 200mbps >1gbps
Technology Analog
cellular
Digital
cellular
Broadbandwidt
h/cdma/ip
technology
Unified ip
&seamless combo
of
LAN/WAN/WLAN/PA
N
4G+WWWW
Service Mobile
telephony
Digital
voice,short
messaging
Integrated high
quality audio,
video & data
Dynamic
information
access, variable
devices
Dynamic information
access, variable
devices
with AI capabilities
Multiplexing FDMA TDMA/CDMA CDMA CDMA CDMA
Switching Circuit Circuit/circuit for
access network&air
interface
Packet except
for air interface
All packet All packet
Core
network
PSTN PSTN Packet
network
Internet Internet
Handoff Horizontal Horizontal Horizontal Horizontal& Horizontal&
COMPARISON OF 1G TO 5G TECHNOLOGIES

CONCLUSION
 All totally the best way to help all users is to use 5G as the
next wireless system and in totally it is safety and secure for
public, this the need that demands the solution.
 Today’s wired society is going wireless and if it has problem,
5G is answer.
 5G technology is going to give tough competition to
Computers and Laptops.
 It will be available in the market 2020 at affordable cost with
more reliability than previous mobiles.
CONCLUSION

Multiplexing Techniques

Multiplexing Techniques
• Multiplexing techniques are used to allow many users to share a common
transmission resource. In our case the users are mobile and the
transmission resource is the radio spectrum. Sharing a common resource
requires an access mechanism that will control the multiplexing
mechanism.
• As in wireline systems, it is desirable to allow the simultaneous
transmission of information between two users engaged in a connection.
This is called duplexing.
• Two types of duplexing exist:
• Frequency division duplexing (FDD), whereby two frequency channels are assigned
to a connection, one channel for each direction of transmission.
• Time division duplexing (TDD), whereby two time slots (closely placed in time for
duplex effect) are assigned to a connection, one slot for each direction of
transmission.

• Multiplexing in 3 dimensions
• time (t) (TDM)
• frequency (f) (FDM)
• code (c) (CDM)
• Goal: multiple use
of a shared medium
s2
s3
s1
Multiplexing
f
t
c
k2 k3 k4 k5 k6
k1
f
t
c
f
t
c
channels ki

Narrowband versus Wideband
• These multiple access schemes can be grouped into two categories:
• Narrowband systems - the total spectrum is divided into a large number of
narrow radio bands that are shared.
• Wideband systems - the total spectrum is used by each mobile unit for both
directions of transmission. Only applicable for TDM and CDM.

Frequency Division Multiplexing (FDM)
• Separation of the whole spectrum into smaller frequency bands
• A channel gets a certain band of the spectrum for the whole time – orthogonal
system
• Advantages:
• no dynamic coordination
necessary, i.e., sync. and
framing
• works also for analog signals
• low bit rates – cheaper,
delay spread
• Disadvantages:
• waste of bandwidth
if the traffic is
distributed unevenly
• inflexible
• guard bands
• narrow filters
k2 k3 k4 k5 k6
k1
f
t
c

f
t
c
k2 k3 k4 k5 k6
k1
Time Division Multiplexing (TDM)
• A channel gets the whole spectrum for a certain amount
of time – orthogonal system
• Advantages:
• only one carrier in the
medium at any time
• throughput high - supports bursts
• flexible – multiple slots
• no guard bands ?!
• Disadvantages:
• Framing and precise
synchronization
necessary
• high bit rates
at each
Tx/Rx

f
Hybrid TDM/FDM
• Combination of both methods
• A channel gets a certain frequency band for a certain amount of time (slot).
• Example: GSM, hops from one band to another each time slot
• Advantages:
• better protection against
tapping (hopping among
frequencies)
• protection against frequency
selective interference
• Disadvantages:
• Framing and
sync. required
t
c
k2 k3 k4 k5 k6
k1

Code Division Multiplexing (CDM)
• Each channel has a unique code
(not necessarily orthogonal)
• All channels use the same
spectrum at the same time
• Advantages:
• bandwidth efficient
• no coordination and
synchronization necessary
• good protection against
interference and tapping
• Disadvantages:
• lower user data rates due to high
gains required to reduce
interference
• more complex signal regeneration
2.19.1
k2 k3 k4 k5 k6
k1
f
t
c

Issues with CDM
• CDM has a soft capacity. The more users the more codes that are used. However as more codes are used the
signal to interference (S/I) ratio will drop and the bit error rate (BER) will go up for all users.
• CDM requires tight power control as it suffers from far-near effect. In other words, a user close to the base
station transmitting with the same power as a user farther away will drown the latter’s signal. All signals
must have more or less equal power at the receiver.
• Rake receivers can be used to improve signal reception. Time delayed versions (a chip or more delayed) of
the signal (multipath signals) can be collected and used to make bit level decisions.
• Soft handoffs can be used. Mobiles can switch base stations without switching carriers. Two base stations
receive the mobile signal and the mobile is receiving from two base stations (one of the rake receivers is
used to listen to other signals).
• Burst transmission - reduces interference

Types of CDM I
• Two types exist:
• Direct Sequence CDM (DS-CDM)
• spreads the narrowband user signal (Rbps) over the full spectrum by multiplying it by a
very wide bandwidth signal (W). This is done by taking every bit in the user stream and
replacing it with a pseudonoise (PN) code (a long bit sequence called the chip rate). The
codes are orthogonal (or approx.. orthogonal).
• This results in a processing gain G = W/R (chips/bit). The higher G the better the system
performance as the lower the interference. G2 indicates the number of possible codes.
Not all of the codes are orthogonal.
F req u en cy
T im e
C o d e
C D M A

Types of CDM II
• Frequency hopping CDM (FH-CDM)
• FH-CDM is based on a narrowband FDM system in which an individual user’s
transmission is spread out over a number of channels over time (the channel choice is
varied in a pseudorandom fashion). If the carrier is changed every symbol then it is
referred to as a fast FH system, if it is changed every few symbols it is a slow FH system.
A
A
A
A
A
A
A
A
A
B
B
B
B
B
B
B B
B

Spread Spectrum in Mobile Computing

Spread Spectrum in Mobile Computing
• Spread spectrum is a technique used for wireless communications in
telecommunication and radio communication. In this technique, the
frequency of the transmitted signal, i.e., an electrical signal,
electromagnetic signal, or acoustic signal, is deliberately varied and
generates a much greater bandwidth than the signal would have if its
frequency were not varied.
• In other words, "Spread Spectrum is a technique in which the transmitted
signals of specific frequencies are varied slightly to obtain greater
bandwidth as compared to initial bandwidth."
• Now, spread spectrum technology is widely used in radio signals
transmission because it can easily reduce noise and other signal issues.

Example of Spread Spectrum
• We know that a conventional wireless signal frequency is usually specified in megahertz (MHz) or
gigahertz (GHz). It does not change with time (Sometimes it is exceptionally changed in the form
of small, rapid fluctuations that generally occur due to modulation).
• Suppose you want to listen to FM stereo at frequency 104.8 MHz on your radio, and then once
you set the frequency, the signal stays at 104.8 MHz. It does not go up to 105.1 MHz or down to
101.1 MHz. You see that your set digits on the radio's frequency dial stay the same at all times.
• The frequency of a conventional wireless signal is kept as constant to keep bandwidth within
certain limits, and the signal can be easily located by someone who wants to retrieve the
information.

• In this conventional wireless communication model, you can face at
least two problems:
1.A signal whose frequency is constant is subject to catastrophic interference.
This interference occurs when another signal is transmitted on or near the
frequency of a specified signal.
2.A constant-frequency signal can easily be intercepted. So, it is not suitable for
the applications in which information must be kept confidential between the
source (transmitting party) and the receiver.
• The spread spectrum model is used to overcome with this conventional
communication model. Here, the transmitted signal frequency is deliberately
varied over a comparatively large segment of the electromagnetic radiation
spectrum. This variation is done according to a specific but complicated
mathematical function. If the receiver wants to intercept the signal, it must be
tuned to frequencies that vary precisely according to this function.

Types of Spread Spectrum
Spread Spectrum can be categorized into two types:
• Frequency Hopping Spread Spectrum (FHSS)
• Direct Sequence Spread Spectrum(DSSS)

Frequency Hopping Spread Spectrum (FHSS)
• The Frequency Hopping Spread Spectrum or FHSS allows us to utilize
bandwidth properly and maximum. In this technique, the whole
available bandwidth is divided into many channels and spread
between channels, arranged continuously.
• The frequency slots are selected randomly, and frequency signals are
transmitted according to their occupancy.
• The transmitters and receivers keep on hopping on channels available
for a particular amount of time in milliseconds.
• So, you can see that it implements the frequency division multiplexing
and time-division multiplexing simultaneously in FHSS.

Frequency Hopping Spread Spectrum –Types
• The Frequency Hopping Spread Spectrum or FHSS can also be
classified into two types:
• Slow Hopping: In slow hopping, multiple bits are transmitted on a
specific frequency or same frequency.
• Fast Hopping: In fast hopping, individual bits are split and then
transmitted on different frequencies.

Block diagram

Advantages of Frequency Hopping Spread
Spectrum (FHSS)
• The following are some advantages of frequency hopping spread spectrum (FHSS):
• The biggest advantage of Frequency Hopping Spread Spectrum or FHSS is its high
efficiency.
• The Frequency Hopping Spread Spectrum or FHSS signals are highly resistant to
narrowband interference because the signal hops to a different frequency band.
• It requires a shorter time for acquisition.
• It is highly secure. Its signals are very difficult to intercept if the frequency-hopping
pattern is not known; that's why it is preferred to use in Military services.
• We can easily program it to avoid some portions of the spectrum.
• Frequency Hopping Spread Spectrum or FHSS transmissions can share a frequency band
with many types of conventional transmissions with minimal mutual interference. FHSS
signals add minimal interference to narrowband communications, and vice versa.
• It provides a very large bandwidth.

Disadvantages of Frequency Hopping Spread
Spectrum (FHSS)
• The following are some disadvantages of Frequency Hopping Spread
Spectrum (FHSS):
• FHSS is less Robust, so sometimes it requires error correction.
• FHSS needs complex frequency synthesizers.
• FHSS supports a lower data rate of 3 Mbps as compared to the 11 Mbps
data rate supported by DSSS.
• It is not very useful for range and range rate measurements.
• It supports the lower coverage range due to the high SNR requirement at
the receiver.
• Nowadays, it is not very popular due to the emerging of new wireless
technologies in wireless products.

Applications of Frequency Hopping Spread
Spectrum (FHSS)
• Following is the list of most used applications of Frequency Hopping
Spread Spectrum or FHSS:
• The Frequency Hopping Spread Spectrum or FHSS is used in wireless
local area networks (WLAN) standard for Wi-Fi.
• FHSS is also used in the wireless personal area networks (WPAN)
standard for Bluetooth.

Direct Sequence Spread Spectrum (DSSS)
• The Direct Sequence Spread Spectrum (DSSS) is a spread-spectrum
modulation technique primarily used to reduce overall signal
interference in telecommunication.
• The Direct Sequence Spread Spectrum modulation makes the
transmitted signal wider in bandwidth than the information
bandwidth. In DSSS, the message bits are modulated by a bit
sequencing process known as a spreading sequence.
• This spreading-sequence bit is known as a chip. It has a much shorter
duration (larger bandwidth) than the original message bits. Following
are the features of Direct Sequence Spread Spectrum or DSSS.

DSSS
• In Direct Sequence Spread Spectrum or DSSS technique, the data that
needs to be transmitted is split into smaller blocks.
• After that, each data block is attached with a high data rate bit sequence
and is transmitted from the sender end to the receiver end.
• Data blocks are recombined again to generate the original data at the
receiver's end, which was sent by the sender, with the help of the data rate
bit sequence.
• If somehow data is lost, then data blocks can also be recovered with those
data rate bits.
• The main advantage of splitting the data into smaller blocks is that it
reduces the noise and unintentional inference.

Direct Sequence Spread Spectrum or DSSS -
Types
• Wide Band Spread Spectrum
• Narrow Band Spread Spectrum

Advantages of Direct Sequence Spread Spectrum
(DSSS)
• The following are some advantages of Direct Sequence Spread Spectrum or
DSSS:
• Direct Sequence Spread Spectrum or DSSS is less reluctant to noise; that's
why the DSSS system's performance in the presence of noise is better than
the FHSS system.
• In Direct Sequence Spread Spectrum or DSSS, signals are challenging to
detect.
• It provides the best discrimination against multipath signals.
• In Direct Sequence Spread Spectrum, there are very few chances of
jamming because it avoids intentional interference such as jamming
effectively.

Disadvantages of Direct Sequence Spread
Spectrum (DSSS)
• The following are some disadvantages of Direct Sequence Spread
Spectrum or DSSS:
• The Direct Sequence Spread Spectrum or DSSS system takes large
acquisition time; that's why its performance is slow.
• It requires wide-band channels with small phase distortion.
• In DSSS, the pseudo-noise generator generates a sequence at high
rates.

Applications of Direct Sequence Spread Spectrum
(DSSS)
• Following is the list of most used applications of Direct Sequence
Spread Spectrum or DSSS:
• Direct Sequence Spread Spectrum or DSSS is used in LAN technology.
• Direct Sequence Spread Spectrum or DSSS is also used in Satellite
communication technology.
• DSSS is used in the military and many other commercial applications.
• It is used in the low probability of the intercept signal.
• It supports Code division multiple access.

MAC Protocols

MAC Protocols
• MAC stands for Media Access Control. A MAC layer protocol is the
protocol that controls access to the physical transmission medium on
a LAN.
• It tries to ensure that no two nodes are interfering with each other’s
transmissions, and deals with the situation when they do.

Objectives of MAC Protocols
• Collision avoidance
• Energy efficiency
• Scalability
• Latency
• Fairness
• Throughput
• Bandwidth utilization

CSMA/CD MAC
• CSMA/CD architecture used in Ethernet is a common MAC layer
standard.
• It acts as an interface between the Logical Link Control sublayer and
the network's Physical layer.

Interference / Collisions
Interference on node b
(“Hidden terminal problem”)
a
b
c
a b
a
b
c d
Interference on node b
a and b interfere and hear noise only
Packets which suffered
collisions should be re-sent.
Ideally, we would want all
packets to be sent collision-
free, only once…

Hidden node and Exposed node
problem
• Suppose that there are three mobile A, B, and C. The transmission
range of A reaches B, but not C. The transmission range of C
reaches B, but not A. Finally the transmission range of B reaches A
and C.

Hidden Terminal Problem
A starts sending signal to B, C does not receive this transmission.
C also wants to send data to B and senses the medium. The
medium appears to be free, the carrier sense fails. C also starts
sending data. It will create a collision at B. But A cannot detect this
collision at B and continues with its transmission. A is hidden for C
and vice versa.
Hidden node problem has been solved by using Request
to send(RTS) and Clear to send frames also called as
handshake frames.

Exposed Terminal Problem
The sender mistakenly thinks the medium is in use, so
that it unnecessarily delays the transmission.
Device A is transmitting the data to B. Device B wants to
send to C and listens to the channel.B hears A’s
transmission, and B mistakenly thinks that it cannot send
to C.

Classification of MAC protocols :
These are as following below.
1.Contention-based protocols without reservation/scheduling –
1. Bandwidth are not reserved.
2. No guarantees.
2.Contention-based protocols with reservation mechanisms –
1. Bandwidth is reserved for transmission.
2. Guarantees can be given.
3.Contention-based protocols with scheduling mechanisms –
1. Distributed scheduling is done between nodes.
2. Guarantees can be given.

End of module -1

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