Lecture notes on Mobile Computing

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Published Date:11-07-2017
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A Course Material on MOBILE COMPUTING By Mr. D.PRABHAKARAN ASSISTANT PROFESSOR DEPARTMENT OF INFORMATION TECHNOLOGY & COMPUTER APPLICATIONS SASURIE COLLEGE OF ENGINEERING VIJAYAMANGALAM – 638 056Introduction – Wireless transmission – Frequencies for radio transmission – Signals – Antennas – Signal Propagation – Multiplexing – Modulations – Spread spectrum – MAC – SDMA – FDMA – TDMA – CDMA – Cellular Wireless Networks. UNIT II TELECOMMUNICATION NETWORKS 9 Telecommunication systems – GSM – GPRS – DECT – UMTS – IMT-2000 – Satellite Networks - Basics – Parameters and Configurations – Capacity Allocation – FAMA and DAMA – Broadcast Systems – DAB - DVB. UNIT III WIRLESS LAN 9 Wireless LAN – IEEE 802.11 - Architecture – services – MAC – Physical layer – IEEE 802.11a - 802.11b standards – HIPERLAN – Blue Tooth. UNIT IV MOBILE NETWORK LAYER 9 Mobile IP – Dynamic Host Configuration Protocol - Routing – DSDV – DSR – Alternative Metrics. UNIT V TRANSPORT AND APPLICATION LAYERS 7 Traditional TCP – Classical TCP improvements – WAP, WAP 2.0. TOTAL : 45 REFERENCE BOOKS: 1. Jochen Schiller, “Mobile Communications”, PHI/Pearson Education, Second Edition, 2003. (Unit I Chap 1,2 &3- Unit II chap 4,5 &6-Unit III Chap 7.Unit IV Chap 8- Unit V Chap 9&10.) 2. William Stallings, “Wireless Communications and Networks”, PHI/Pearson Education, 2002. (Unit I Chapter – 7&10-Unit II Chap 9) 3. Kaveh Pahlavan, Prasanth Krishnamoorthy, “Principles of Wireless Networks”, PHI/Pearson Education, 2003. 4. Uwe Hansmann, Lothar Merk, Martin S. Nicklons and Thomas Stober, “Principles of Mobile Computing”, Springer, New York, 2003. YCS012 -MOBILE COMPUTING UNIT I WIRELESS COMMUNICATION FUNDAMENTALS Introduction – Wireless transmission – Frequencies for radio transmission – Signals – Antennas – Signal Propagation – Multiplexing – Modulations – Spread spectrum – MAC – SDMA – FDMA – TDMA – CDMA – Cellular Wireless Networks. INTRODUCTION Mobile computing means different things to different people. Ubiquitous, wireless and remote computing Wireless and mobile computing are not synonymous. Wireless is a transmission or information transport methodthat enables mobile computing. Aspects of mobility: user mobility: users communicate (wireless) “anytime, anywhere, with anyone” device portability: devices can be connected anytime, anywhere to the network Mobility Issues • Bandwidth restrictions and variability • Location-aware network operation o User may wake up in a new environment o Dynamic replication of data • Querying wireless data & location-based responses • Busty network activity during connections & handling disconnections • Disconnection o OS and File System Issues - allow for disconnected operation o Database System Issues - when disconnected, based on local data Portability Issues • Battery power restrictions • Risks to data - Physical damage, loss, theft - Unauthorized access - encrypt data stored on mobiles - Backup critical data to fixed (reliable) hosts • Small user interface - Small displays due to battery power and aspect ratio constraints - Cannot open too many windows - Difficult to click on miniature icons - Input - Graffiti, (Dictionary-based) Expectation - Gesture or handwriting recognition with Stylus Pen Voice matching or voice recognition 2APPLICATIONS 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 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, ... 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 Entertainment, education, outdoor Internet access intelligent travel guide with up-to-date location dependent information ad-hoc networks for multi user games 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? Support services caches, intermediate results, state information etc. „follow“ the mobile device through the fixed network Privacy who should gain knowledge about the location Effects of device portability Power consumption limited computing power, low quality displays, small disks due to limited battery capacity CPU: power consumption CV2f 3• 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 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, 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 Early history of wireless communication Many people in history used light for communication heliographs, flags („semaphore“), ... 150 BC smoke signals for communication; (Polybius, Greece) 1794, optical telegraph, Claude Chappe Here electromagnetic waves are of special importance: 1831 Faraday demonstrates electromagnetic induction J. Maxwell (1831-79): theory of electromagnetic Fields, wave equations (1864) H. Hertz (1857-94): demonstrateswith an experiment the wave character of electrical transmission through space(1886, in Karlsruhe, Germany, at the location of today’s University of Karlsruhe) 4Wireless systems: overview of the development cordless wireless cellular phones satellites phones LAN 1980: 1981: CT0 NMT 450 1982: 1984: Inmarsat - A 1983: CT1 1986: AMPS 1987: NMT 900 1988: CT1+ Inmarsat - C 1991: 1991: 1989: 199x: CDMA D - AMPS CT 2 proprietary 1991: 1992: 1992: DECT GSM Inmarsat - B 1995/96/97: 1993: Inmarsat - M IEEE 802.11, PDC HIPERLAN 1994: DCS 1800 1998: Iridium analog 2005?: 2005?: MBS, WATM digital UMTS/IMT - 2000 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 5Simple reference model used here Application Application Transport Transport Network Network Network Network Data Link Data Link Data Link Data Link Physical Physical Physical Physical Medium Radio Influence of mobile communication to the LAYER MODEL Application layer service location new applications, multimedia adaptive applications Transport layer congestion and flow control quality of service Network layer addressing, routing, device location hand-over Data link layer authentication media access multiplexing media access control 6Physical layer encryption modulation interference attenuation frequency WIRELESS TRANSMISSION - FREQUENCIES FOR RADIO TRANSMISSION Frequencies for communication twisted coax cable optical transmission pair 1 Mm 10 km 100 m 1 m 10 mmm 1m 100 300 Hz 30 kHz 3 MHz 300 MHz 30 GHz 3 THz 300 THz MF HF VHF UHF SHF EHF infrared VL LF UV visible F light • VLF = Very Low Frequency UHF = Ultra High Frequency • LF = Low Frequency SHF = Super High Frequency • MF = Medium Frequency EHF = Extra High Frequency • HF = High Frequency UV = Ultraviolet Light • VHF = Very High Frequency • Frequency and wave length:  = c/f 8 • wave length , speed of light c 3x10 m/s, frequency f Frequencies for mobile communication • VHF-/UHF-ranges for mobile radio • simple, small antenna for cars • deterministic propagation characteristics, reliable connections • SHF and higher for directed radio links, satellite communication • small antenna, focusing • large bandwidth available • Wireless LANs use frequencies in UHF to SHF spectrum • some systems planned up to EHF • limitations due to absorption by water and oxygen molecules (resonance frequencies) • Weather dependent fading, signal loss caused by heavy rainfall etc. Frequencies and regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Europe USA Japan Conferences) Mobile NMT 453 - 457MHz, AMPS , TDMA , CDMA PDC phones 463 -467 MHz; 824 -849 MHz, 810 -826 MHz, GSM 890 -915 MHz, 869 -894 MHz; 940 -956 MHz; 935 -960 MHz; TDMA , CDMA , GSM 1429 - 1465 MHz, 1710 - 1785 MHz, 1850 - 1910 MHz, 1477 - 1513 MHz 1805 - 1880 MHz 1930 - 1990 MHz; Cordless CT1+ 885 - 887 MHz, PACS 1850 - 1910 MHz, PHS telephones 930 -932 MHz; 1930 - 1990 MHz 1895 - 1918 MHz 7 CT2 PACS - UB 1910 - 1930 MHz JCT 864 - 254 -380 MHz 868 MHz DECT W ireless LANs SIGNALS physical representation of data function of time and location signal parameters: parameters representing the value of data classification o continuous time/discrete time o continuous values/discrete values o analog signal = continuous time and continuous values o digital signal = discrete time and discrete values signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift j sine wave as special periodic signal for a carrier: s(t) = At sin(2 p ft t + jt) Fourier representation of periodic signals      1 nftnft g t sin( 2 ) cos( 2 ) n n c a b ( ) n1 n1 2 81 1 0 0 t t Ideal periodic signal based on Real composition ( harmonics ) Different representations of signals amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase j in polar coordinates) Composed signals transferred into frequency domain using Fourier transformation Digital signals need infinite frequencies for perfect transmission Modulation with a carrier frequency for transmission (analog signal) ANTENNAS Isotropic radiator Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna Real antennas always have directive effects (vertically and/or horizontally) Radiation pattern: measurement of radiation around an antenna Ideal isotropic radiator 9z y z y x x Simple dipoles Real antennas are not isotropic radiators but, e.g., dipoles with lengths l/4 on car roofs or l/2 as Hertzian dipole, shape of antenna proportional to wavelength /4 /2 Example: Radiation pattern of a simple Hertzian dipole 10• Real antennas are not isotropic radiators but, e.g., dipoles wit h lengths/4 on car shape of antenna proportional to wavelength roofs or/2 as Hertzian dipole /4 /2 • Example: Radiation pattern of a simple Hertzian dipole z y y Simple dipole x z x side view ( xy - plane) side view ( yz - plane ) top view ( xz - plane) • Gain: maximum power in the direction of the main lobe compared t o the power of an isotropic radiator (with the same average power) Directed and Sectorized Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley) y y z Directed antenna x z x side view ( xy-plane) side view ( yz-plane) top view ( xz-plane) z z Sectorized antenna x x top view, 3 sector top view, 6 sector 11Antennas: diversity Grouping of 2 or more antennas o multi-element antenna arrays Antenna diversity o switched diversity, selection diversity receiver chooses antenna with largest output diversity combining combine output power to produce gain cophasing needed to avoid cancellation /2/2 /4/4/2 + + SIGNAL PROPAGATION Transmission range communication possible low error rate Detection range detection of the signal possible no communication possible Interference range signal may not be detected signal adds to the background noise 12Sende r Transmission Distance Detection Interferenc e Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d² (d = distance between sender and receiver) Receiving power additionally influenced by fading (frequency dependent) shadowing reflection at large obstacles scattering at small obstacles diffraction at edges 13Shadowing Reflection Scattering Diffraction Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction Time dispersion: signal is dispersed over time è Interference with “neighbor” symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted è Distorted signal depending on the phases of the different parts Effects of mobility Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts èQuick changes in the power received (short term fading) Additional changes in distance to sender obstacles further away è Slow changes in the average power received (long term fading) MULTIPLEXING Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c) Frequency Division Multiplexing - FDM The oldest used technique used for multiplexing. Possible when the useful bandwidth of the medium exceeds that of the signals it has to carry. Each signal is modulated on a different carrier frequency. This results in shifting the spectrum of the signal around the carrier frequency. Sufficient guard-band is given so those neighboring signals do not overlap in the frequency domain. At the receiving end each signal is extracted by first passing it through a band-pass filter and then demodulating with the same carrier frequency that was used to modulate the signal. The signals carried using FDM may be analog signals or may be analog signals representing digital data. However FDM is mostly a technique from the 14era of analog communications. In FDM a device uses some of the channel all of the time. FDM is used in radio and television broadcasting. FDM is also used in high capacity long distance links in the telephone network. Frequency division multiplexing (FDM) achieves multiplexing by using different carrier frequencies .Receiver can "tune" to specific frequency and extract modulation for that one channel .Frequencies must be separated to avoid interference - “Wastes” potential signal bandwidth for guard channels.Only useful in media that can carry multiple signals with different frequencies - high-bandwidth required . Used in: The standard of the analog telephone network The standard in radio broadcasting The standard for video 1. Broadcast 2. Cable 3. Satellite Frequency Division Multiplexing Diagram Time Division Multiplexing - TDM Time division multiplexing is more suitable for digital data. TDM can be used when the data rate available on a communication link exceeds the data rate required by any one of the sources. In TDM each source that is to use the link fills up a buffer with data. A TDM multiplexer scans the buffers in some predetermined order and transmits bits from each source one after the other. Requires digital signaling & transmission Requires data rate = sum of inputs + framing Data rate much higher than equivalent analog bandwidth uses Separates data streams in time not frequency The standard of the modern digital telephone system 15Code Division Multiplexing - CDM Each channel has a unique code. 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 more complex signal regeneration 16C k k k k k k 1 3 4 5 2 6 F T MODULATIONS Digital modulation o digital data is translated into an analog signal (baseband) o ASK, FSK, PSK - main focus in this chapter o differences in spectral efficiency, power efficiency, robustness Analog modulation o shifts center frequency of baseband signal up to the radio carrier Motivation 17o smaller antennas (e.g., l/4) o Frequency Division Multiplexing o medium characteristics Basic schemes o Amplitude Modulation (AM) o Frequency Modulation (FM) o Phase Modulation (PM) Modulation and demodulation analog baseband digital signal data digital analog Radio transmitter modulation modulation 101101001 radio carrier analog baseband digital signal data synchronization analog Radio receiver demodulation decision 101101001 radio carrier Digital modulation Modulation of digital signals known as Shift Keying. Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference Frequency Shift Keying (FSK): needs larger bandwidth Phase Shift Keying (PSK): 18more complex robust against interference 1 0 1 t ASK 1 0 1 FSK t 1 0 1 t PSK Advanced Frequency Shift Keying bandwidth needed for FSK depends on the distance between the carrier frequencies special pre-computation avoids sudden phase shifts è MSK (Minimum Shift Keying) bit separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen the frequency of one carrier is twice the frequency of the other even higher bandwidth efficiency using a Gaussian low-pass filter è GMSK (Gaussian MSK), used in GSM. Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used e.g. in satellite systems 19

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