ALAK ROY.Assistant ProfessorDept. of CSENIT AgartalaEmail-alakroy.nerist@gmail.com

MOBILE COMPUTINGNIT Agartala, Dept of CSE

Jan-May,2012

Introduction to Introduction to Mobile ComputingMobile Computing

SLIDE #1

WHAT IS MOBILE COMPUTING ?

According to a dictionary:Mobile: Able to move freely

Computing: The activity of using a computer

Wireless network: Communications without a wire.

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WHAT IS MOBILITY?Mobility: 2 types

User Mobility:Users can access the same network services at different placesUser can be mobile and the services will follow him/herExamples: Call forwarding and Computer Desktop supporting

roaming

Device Portability:Communication device can move and mechanisms in the network

and inside the device have to make sure that communication is still possible.

Example: Mobile phone system

In most of the cases, both user mobility and device portability are available.

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MOBILE COMPUTING

Desktop Laptop

Mobile Computing?

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COMPUTER APPLICATIONS

Information Processing word processing, spreadsheet, database, etc.

Networking email, web browsing, etc.

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MOBILE COMPUTING

Able to communicate (or to access information) anytime, and anywhere

Portable Devices

Wireless Access

NetworkData Server, Other People, etc.

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OUR FOCUS

Mobile Cellular Systems GSM, CDMA

Wireless LANBluetooth

Portable Devices

Wireless Access

NetworkData Server, Other People, etc.

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MOBILE/PORTABLE DEVICES

performanceperformance

Pager• receive only• tiny displays• simple text messages

Mobile phones• voice, data• simple graphical displays

PDA• simpler graphical displays• character recognition• simplified WWW

Palmtop• tiny keyboard• simple versions of standard applications

Laptop• fully functional• standard applications

Sensors,embeddedcontrollers

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EFFECTS OF DEVICE PORTABILITY Power consumption

limited computing power, low quality displays, small disks due to limited battery capacity

CPU: power consumption ~ CV2f C: internal capacity, reduced by integration V: supply voltage, can be reduced to a certain limit f: clock frequency, can be reduced temporally

Loss of data higher probability, has to be included in advance into the design (e.g.,

defects, theft)

Limited user interfaces compromise between size of fingers and portability integration of character/voice recognition, abstract symbols

Limited memory limited value of mass memories with moving parts flash-memory or ? as alternative

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COMMUNICATION DEVICESFixed and Wired

Examples: Typical desktop computers connected to the fixed network

Mobile and Wired Examples: carrying laptops from one place to another and

connecting to the organizations network through different network access methods

Fixed and Wireless Examples: Installing Networks in historical buildings where

installing wires is not allowed. Trade shows Mobile and Wireless

Examples: Users can roam between different wireless networks. Cellular networks (GSM).

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WIRELESS COMES OF AGE

Marconi invented the wireless telegraph in 1897 (2 yrs after Prof. J. C. Bose demonstrated existence of milimetre range microwave) Communication by encoding alphanumeric characters

in analog signal Sent telegraphic signals across the Atlantic Ocean

Communications satellites launched in 1960sAdvances in wireless technology

Radio, television, mobile telephone, communication satellites

More recently Satellite communications, wireless networking,

cellular technology 11

BROADBAND WIRELESS TECHNOLOGY

Higher data rates obtainable with broadband wireless technology Graphics, video, audio

Shares same advantages of all wireless services: convenience and reduced cost Service can be deployed faster than fixed

service No cost of cable plant Service is mobile, deployed almost anywhere

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LIMITATIONS AND DIFFICULTIES OF WIRELESSTECHNOLOGIES

Wireless is convenient and less expensive Limitations and political and technical

difficulties inhibit wireless technologiesLack of an industry-wide standardDevice limitations

E.g., small LCD on a mobile telephone can only displaying a few lines of text

E.g., browsers of most mobile wireless devices use wireless markup language (WML) instead of HTML

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WIRELESS NETWORKS IN COMPARISON TOFIXED NETWORKS

Higher loss-rates due to interference Restrictive regulations of frequencies

frequencies have to be coordinated, useful frequencies are almost all occupied

Low transmission rates local some Mbit/s, regional currently, e.g., 9.6kbit/s with GSM

Higher delays, higher jitter connection setup time with GSM in the second range, several hundred

milliseconds for other wireless systems Lower security, simpler active attacking

radio interface accessible for everyone, base station can be simulated, thus attracting calls from mobile phones

Always shared medium secure access mechanisms important

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Layered Architecture

Application

Transport

Network

Data Link

Physical

Medium

Data Link

Physical

Application

Transport

Network

Data Link

Physical

Data Link

Physical

Network Network

Radio

Scope of this course:

Anything above and including the data link layer.15

Effect of Mobility on Protocol Stack • Application

– new applications and adaptations• Transport

– congestion and flow control• Network

– addressing and routing• Link

– media access and handoff• Physical

– transmission errors and interference 16

Why study this course?

• Reason 1:Mobile computing is popular?

More and more people use mobile phones, wireless technology is built into many cars, wireless data services are available in many regions, and WLAN are used in many places.

e.g. no of mobile phone subscribers more than 12 billions (in 2002)

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Why study this course?

• Reason 2:Mobile computing is interesting?

Hope all of you find it interesting after studying this course.

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Why study this course?

• You have studied Computer Networks before. What makes wireless different?

• In principle, TCP/IP can work on top of any physical and data link layer.– Can we simply transport IP packets over a

wireless channel?

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Challenges in Mobile Computing

• Three major challenges:– Wireless Channel– Mobility– Device Limitation

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Wireless Channel

The 1st challenge

Communication Channel

Transmitter ReceiverThe medium used to transmit the signal from the transmitter to the receiver

Wireline / Wireless channel

Channel

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Wireline Channel

Transmitter Receiver

Wireline Channel, e.g. copper wire

Too many noises? Shielded against electromagnetic noise

Large signal attenuation? Use repeaters

Data speed too low? Upgrade to coaxial cable

Data speed still too low? Upgrade to optical fiber23

Fading Effect• Typical Indoor Wireless Environment– Signal strength

fluctuates significantly

• Wireless channel cannot be engineered.– You can only

improve your transmission and reception techniques.

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Bit Error Rate

• Optical fiber: 10-11 or 10-12

• Mobile channel: – Good quality: 10-6

– Actual condition: 10-2 or worse

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Implication

• For wireline systems, it is assumed that the channel is error free

• Many protocols are designed with this assumption

• These protocols do not work well in a wireless environment – e.g. TCP (why? Packet loss is only due to

congestion not due to error in channel)

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What if more than 1 transmitter?

Switching Center

or

Network Access Point

Every user accesses the network by means of a dedicated channel

New user is served by a new wireline circuit

Access capacity is “unlimited”.

Dedicated Channel

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How about wireless networks?

Base Station

Wireless users access the network by means of a shared channel

Access capacity is inherently limited.

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Implication

• For wireline systems, we can simply install new cables to increase capacity.

• For wireless systems, the channel can only be shared by the users.– Capacity does not increase.

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Interference

• Multiuser Interference– Radio signals of different users interfere with

each other• Self-Interference

– Multipath effect– Phase-shifted images of the signal at the

receiver interact and may cancel the entire signal, (i.e. destructive interference).

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Interference Management

• How to manage multiuser interference?– i.e. how to share the channel?– Multiple Access Problem

• FDMA, TDMA, CDMA, etc.– Media Access Control

• Aloha, CSMA, etc.

• How to manage self-interference?– Physical layer issue

• Equalization, coding, diversity, etc.– This issue will NOT be considered in this course

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Mobility

The 2nd challenge

User Mobility

• Location Management Problem– How does the network know where the

intended recipient of a message is currently located?

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Cellular ScenarioWhere is 9435055555?

Send broadcast messages from every base station?

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Ad Hoc Network Scenario

B

A

S EF

H

J

D

C

G

IK

M

N

L

How to find a suitable path from source S to destination D?

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Device Limitation

The 3rd challenge

Device Limitation

• Resource Poor– Limited memory– Limited computational power– Small display– Limited battery life

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End of Introduction

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Wireless Networks• Need: Access computing and communication

services, on the move

• Infrastructure-based Networks– traditional cellular systems (base station infrastructure)

• Wireless LANs– Infrared (IrDA) or radio links (Wavelan)– very flexible within the reception area; ad-hoc networks

possible– low bandwidth compared to wired networks (1-10 Mbit/s)

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Wireless Networks

• Ad hoc Networks– useful when infrastructure not available, impractical, or

expensive– military applications, rescue, home networking

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Limitations of Mobile Environments Limitations of the Wireless Network

heterogeneity of fragmented networks frequent disconnections limited communication bandwidth

Limitations Imposed by Mobility lack of mobility awareness by system/applications route breakages

Limitations of the Mobile Computer short battery lifetime limited capacities

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Mobile Applications• Vehicles

– transmission of news, road condition etc– ad-hoc network with near vehicles to prevent accidents

• Emergencies– early transmission of patient data to the hospital– ad-hoc network in case of earthquakes, cyclones– military ...

• Traveling salesmen– direct access to central customer files– consistent databases for all agents– mobile office

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Mobile Applications• Web access

– outdoor Internet access – intelligent travel guide with up-to-date

location dependent information• Location aware services

– find services in the local environment, e.g. printer• Information services

– push: e.g., stock quotes– pull: e.g., nearest cash ATM

• Disconnected operations– mobile agents, e.g., shopping

• Entertainment– ad-hoc networks for multi user games 43

Topic 1(Part 2)

Wireless Networks basics

Signals

•Physical representation of data•Data exchange through the transmission of signals•Signal parameters (amplitude, freq and phase shift) represents data values•g(t)= Atsin (2πftt +φt)•A signal can be represented in time domain (oscilloscope), frequency domain (spectrum analyzer) and phase domain (signal constellation diagram)

Frequency BandsBand Frequency

RangePropagationCharacteristics

Typical Use

ELF (Extremely Low Freq)

30 – 300Hz Ground Wave (GW)

Power Line frequencies; used by some home control systems

VF (Voice Freq)

300 – 3KHz GW Telephone system for analog subscriber lines

VLF (Very Low Freq)

3 – 30KHz GW Long-range navigation; submarine commn

LF (Low Freq)

30 – 300KHz GW Long-range navigation; marine comm radio beacon

MF(MediumFreq)

300 – 3MHz GW and night SW (Sky Wave)

Maitime radio; direction finding; AM broadcasting

HF (High Freq)

3 – 30MHz SW Amateur Radio; int’l broadcasting; military comm; long dist aircraft and ship comm

Frequency Bands (Cont’d)Band Frequency

RangePropagationCharacteristics

Typical Use

VHF (Very High Freq)

30 – 300MHz Line-of-Sight (LOS)

VHF Television; FM Radio and 2-way radio, AM aircraft comm; aircraft navigation aids

UHF (Ultra High Freq)

300 – 3GHz LOS UHF Television; Cell phone; radar; microwave links; PCS

SHF (Very Low Freq)

3 – 30GHz LOS Satellite commn;radar; terrestrial microwave links; Wireless Local Loop

EHF (Extremely High Freq)

30 – 300GHz LOS Experimental; WLL

Infrared 300GHz –400THz

LOS Infrared LANs; Consumer electronic applns

Visible Light 400THz –900THz

LOS Optical Communication

Decibel Gain• As signal propagates along a trans medium, there will

be a loss, attenuation, of signal strength.• To compensate, amplifiers may be inserted at various

points to impart a gain in signal strength.• Decibel gain G(db)=10 log10 (Pout/Pin)• Prob: If a signal with a power level of 10mW is

inserted onto a transmission line and the measured power some distance away is 5mW, the loss can be expressed as :

• L(db)= - G(db) = -10 log(5/10)= 10 log (10/5)= 3 (db)

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Decibel Gain• Prob: Consider a series in which the input is at

a power level of 4 mW, the 1st element is a trans line with a 12 dB loss, 2nd element is an amplifier with a 35 dB gain, and 3rd element is a trans line with a 10 dB loss. The net gain/loss is = ?, Output power = ?:

• Net gain = -12+35 -10 = 13 dB• Therefore, Pout = 79.8 mW

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