mobile computing basic concepts - notesvillagenotesvillage.com/upload/module1.pdf · – wireless...

MOBILE COMPUTING –

BASIC CONCEPTS

Mobile Computing-Definitions

• Mobile computing is a form of human–computer

interaction by which a computer is expected to be

transported during normal usage. Mobile computing has

three aspects: mobile communication, mobile hardware,

and mobile software.

Mobile Computing-Definitions

• The first aspect addresses communication issues in ad-

hoc and infrastructure networks as well as

communication properties, protocols, data formats and

concrete technologies.

• The second aspect is on the hardware, e.g., mobile

devices or device components.

• The third aspect deals with the characteristics and

requirements of mobile applications.

Computers for the next decades?

• Computers are integrated

– small, cheap, portable, replaceable - no more separate

devices

• Technology is in the background

– computer are aware of their environment and adapt

(“location awareness”)

– computer recognize the location of the user and react

appropriately (e.g., call forwarding, fax forwarding,

“context awareness”))

Computers for the next decades?

• Advances in technology

– more computing power in smaller devices

– flat, lightweight displays with low power

consumption

– Fast network access on fast moving vehicles like

flight, car etc.

– more bandwidth per cubic meter

– multiple wireless interfaces: wireless LANs, wireless

WANs, regional wireless telecommunication

networks etc.

Mobile communication

• Two aspects of mobility:

– user mobility: users communicate (wireless)

“anytime, anywhere, with anyone”

– device portability: devices can be connected

anytime, anywhere to the network

• Wireless vs. mobile Examples stationary computer

notebook in a hotel

wireless LANs in historic buildings

Personal Digital Assistant (PDA)

Mobile communication

• The demand for mobile communication

creates the need for integration of wireless

networks into existing fixed networks:

– local area networks: standardization of IEEE

802.11, ETSI (HIPERLAN)

– Internet: Mobile IP extension of the internet

protocol IP

– wide area networks: e.g., internetworking of

GSM and ISDN

Device Portability

• The communication devices moves with or

without user

– Mechanisms inside the network make sure

that communication is still possible while the

device is moving

– Example: Mobile Phone system- Handover

Characteristics of a

Communication Device

• Fixed and Wired

• Mobile and Wired

• Fixed and Wireless

• Mobile and Wireless

Applications

• Vehicles

– transmission of news, road condition,

weather, music via DAB

– personal communication using GSM

– position via GPS

– local ad-hoc network with vehicles close-by

to prevent accidents, guidance system,

redundancy

– vehicle data (e.g., from busses, high-speed

trains) can be transmitted in advance for

maintenance

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Applications II

• Emergencies

– early transmission of patient data to the

hospital, current status, first diagnosis

– replacement of a fixed infrastructure in case

of earthquakes, hurricanes, fire etc.

– crisis, war, ...

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Typical application: road trafficA

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Applications

• Travelling salesmen

– direct access to customer files stored in a

central location

– consistent databases for all agents

– mobile office

• Replacement of fixed networks

– remote sensors, e.g., weather, earth activities

– flexibility for trade shows

– LANs in historic buildings

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Applications

• Entertainment, education, ...

– outdoor Internet access

– intelligent travel guide with up-to-date

location dependent information

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Location dependent services• Location aware services

– what services, e.g., printer, fax, phone, server

etc. exist in the local environment

• Follow-on services

– automatic call-forwarding, transmission of the

actual workspace to the current location

• Information services

– „push“: e.g., current special offers in the

supermarket

– „pull“: e.g., where is the Black Forrest Cherry

Cake?

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Location dependent services

• Support services

– caches, intermediate results, state information

etc. „follow“ the mobile device through the

fixed network

• Privacy

– who should gain knowledge about the

location

– We can hide our current location

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Mobile devices

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

• V: supply voltage

• f: clock frequency

• Loss of data

– higher probability, has to be included in advance into

the design (e.g., defects, theft)

Effects of device portability

• 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

Wireless networks in

comparison to fixed networks• Higher loss-rates due to interference

– emissions of, e.g., engines, lightning

• Restrictive regulations of frequencies

– frequencies have to be coordinated, useful

frequencies are almost all occupied

• Low transmission rates

– local some Mbit/s

• Higher delays, higher jitter

– connection setup time with GSM in the second range,

several hundred milliseconds for other wireless

systems

Wireless networks in

comparison to fixed networks• 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

• Ad-hoc Networking

– Wireless and mobile computing allows for

spontaneous networking without infrastructure

Areas of research in mobile

communication• Wireless Communication

– transmission quality (bandwidth, error rate, delay)

– modulation, coding, interference

– media access, regulations

– ...

• Mobility

– location dependent services

– location transparency

– quality of service support (delay, jitter, security)

– ...

• Portability

– power consumption

– limited computing power, sizes of display, ...

– Usability……….

Simple Reference Model


• In the above figure, PDA communicate with base station consists of

radio transceiver.

• The end system communication done by using intermediate systems.

• Different Layers are

– Physical Layer

• Lowest layer, Conversion of stream of bits into signals

• Responsible for frequency selection, generation of carrier

frequency, signal detection, modulation of data and

encryption.

– Data Link Layer

• Accessing the medium

• Multiplexing of different data streams

• Correction of transmission errors and synchronization

• Responsible for reliable point-to-point connection , to

multipoint connection.


– Network Layer

• Responsible for routing packets through a network

• Establishing a connection between two entities over many

other intermediate systems

• It handles addressing, routing, device location, and handover

between different networks.

– Transport Layer

• It is for establishing an end to end connection

• It maintains quality of service, flow and congestion control

• Two protocols- TCP and UDP

– Application Layer

• Applications on application layer

• Example: Multimedia application

Overlay Networks - the global goal

Signal Propagation

– Wireless communication networks also have senders

and receivers of signal

– Signal has no wires to determine the direction of

propagation

– In wire, we can predict the characteristics of signal at

each point as long as it is not damaged.

– But in wireless, predictable behavior is only in

vacuum.

Signal Propagation Ranges

• Transmission range

– communication possible

– low error rate

– Receiver can also act as sender

• Detection range

– detection of the signal possible

– no communication possible

– Error rate is high to establish a connection

• Interference range

– signal may not be detected

– signal adds to the background noise

Path loss of radio signals (free

space loss)• In free space signal propagate as light does

• So sender and receiver is in line of sight

• But signal still experiences the Free Space Loss

– The received power pr is proportional to 1/d2 ( d – is the distance between sender and receiver)-inverse square law

– Sender emit the signal with certain energy

– The signal travels away from the sender at the speed of light as a wave with a spherical shape.

– As the surface area „s‟ grows with increasing distance „d‟ from the center (s=4∏ d2)

Path loss of radio signals (Path loss)

• Received power is also depends on the wavelength and gain of

RVR and TMR.

• Radio transmission take place through atmosphere- signals

travels through air, rain, snow, fog, dust particles etc..

• This loss is known as path loss or attenuation.

• The atmosphere heavily influences transmission over long

distances (e.g. Satellite communication).

• Rain can absorb much of the radiated energy of the antenna. So

communication links may break down as soon as the rain set in.

Additional Signal Propagation Effects

• Shadowing

– Extreme form of attenuation is blocking or

shadowing of radio signal due to large obstacles.

– The higher frequency of a signal behave like a

light, even small obstacles like simple wall, truck

on the street or trees may block the signal.

• Reflection

– If the object is large compared to the wavelength

(huge building, mountains..) the signal is reflected.

– Reflected signal is weak, object can absorb some

of the signal power.

– Reflection helps transmitting signals as soon as no

LOS exists.

– Signal transmitted from the sender may bounce of

the wall of building several time before they reach

the RVR.


• Refraction

– Refraction occurs because the velocity of the

electromagnetic waves depends on the density of

medium through which it travels.

– This is the reason for LOS radio waves bent

towards the earth: density of the atmosphere is

higher close to the ground.

• Scattering

– If the size of the obstacle is in the order of

wavelength or less, then waves can be scattered.

– All incoming signal is scattered into several

weaker outgoing signal.


• Diffraction

– Radio waves will be reflected at an edge and

propagate in different direction.

Multipath propagation

• Signal can take many different paths between sender and

receiver due to reflection, scattering, diffraction

• Time dispersion: signal is dispersed over time

– interference with “neighbor” symbols, Inter Symbol

Interference (ISI)

• The signal reaches a receiver directly and phase shifted

– distorted signal depending on the phases of the different

parts

Cellular System

• Each transmitter typically called a base station , covers certain

area –a cell

• Mobile stations communicate only via the base station

• Advantages of cell structures

– higher capacity, higher number of users, frequency reuse.

– less transmission power needed

– more robust, decentralized ( if one antenna fails, this only

effects within small area)

– base station deals with interference, transmission area etc.

locally

• Problems

– fixed network needed for the base stations

– handover (changing from one cell to another) necessary

– interference with other cells (Frequency planning required)

Cellular System-Frequency planning

• Cell sizes from some 100 m in cities to, e.g., 35 km on the

country side (GSM) - even less for higher frequencies

• Frequency reuse only with a certain distance between the base stations

• Standard model using 7 frequencies:

• The above is a cell pattern with minimal interferences.

• The group of cells is known as clusters

• All cells within a cluster use disjoint set of frequencies.

f4

f5

f1f3

f2

f6

f7

f3f2

f4

f5

f1

THANK YOU

mobile computing basic concepts - notesvillagenotesvillage.com/upload/module1.pdf · – wireless...

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