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Mobile Computing

Mobile Computing. Cellular Concepts. Cellular Networks. Wireless Transmission Cellular Concept Frequency Reuse Channel Allocation Call Setup Cell Handoffs Location Management. Basic Idea. Single hop wireless connectivity to the wired world

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Mobile Computing

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  1. Mobile Computing Cellular Concepts

  2. Cellular Networks • Wireless Transmission • Cellular Concept • Frequency Reuse • Channel Allocation • Call Setup • Cell Handoffs • Location Management

  3. Basic Idea • Single hop wireless connectivity to the wired world • Space divided into cells; A base station is responsible to communicate with hosts in its cell

  4. Wireless Transmission • Communication Frequencies • Frequencies in the VHF range are used • Regulation bodies • Antennas • Theoretically: equal radiation in all directions • Reality: directive effects, sectorized antennas

  5. Wireless Transmission • Signal Propagation • Classification: Analog/Digital, Periodic/Aperiodic • Parameters: Amplitude, Frequency and Phase shift • Modulation Techniques • Amplitude, Frequency, Phase • Multiplexing Mechanisms • Space (SDM), Frequency (FDM), Time (TDM), Code (CDM)

  6. Cellular Concept • Mobile hosts can change cells while communicating • Hand-off occurs when a mobile host changes base station • Factors for determining cell size • No. of users to be supported • Multiplexing and transmission technologies

  7. Cellular Concept • Limited number of frequencies => limited channels • Single high power antenna => limited number of users • Smaller cells => frequency reuse possible => more number of users • As demand increases (more channels needed) • Number of base stations is increased • Transmitter power is decreased correspondingly to avoid interference

  8. Cellular Concept • Base stations (BS): implement space division multiplex • Each BS covers a certain transmission area (cell) • Each BS is allocated a portion of the total number of channels available • Cluster: group of nearby BSs that together use all available channels • Mobile stations communicate only via the base station, using FDMA, TDMA, CDMA…

  9. Cellular Concept • Cell size: • 100 m in cities to 35 km on the country side (GSM) • even less for higher frequencies

  10. Cellular Concept • Cell shape: • Hexagonal is useful for theoretical analysis • Practical footprint (radio coverage area) is amorphous • BS placement: • Center-excited cell: BS near center of cell • omni-directional antenna • Edge-excited cell: BSs on cell vertices • sectored directional antennas

  11. Cellular Concept • Advantages: • higher capacity, higher number of users • less transmission power needed • more robust, decentralized • base station deals with interference, transmission area etc. locally

  12. Cellular Concept • Problems: • fixed network needed for the base stations • handover necessary • interference with co-channel, adjacent-channel • Important Issues: • Cell sizing; Frequency reuse planning • Channel allocation strategies Bottom line: Attempt to maximize availability of channels in an area

  13. Geometric Representation • Cells are commonly represented by hexagons. • Why hexagon? • How about circle? • How about square, or triangle?

  14. Hexagonal Cells

  15. Channel Reuse • The total number of channels are divided into K groups. • K is called reuse factor or cluster size. • Each cell is assigned one of the groups. • The same group can be reused by two different cells provided that they are sufficiently far apart.

  16. Example: K = 7

  17. Coordinate System Use (i,j) to denote a particular cell. Example: Cell A is represented by (2,1). A

  18. Distance Formula where R D Reuse factor Note: i and j are integers

  19. MSC MSC HLR VLR HLR VLR To other MSCs PSTN PSTN Cellular System Architecture

  20. Cellular System Architecture • Each cell is served by a base station (BS) • Each BS is connected to a mobile switching center (MSC) through fixed links • Each MSC is connected to other MSCs and PSTN

  21. Cellular System Architecture • Each MSC is a local switching exchange that handles switching of mobile user from one base station to another • Locating the current cell of a mobile user • Home Location Register (HLR): database recording the current location of each mobile that belongs to the MSC • Visitor Location Register (VLR): database recording the cell of “visiting” mobiles • Interfacing with other MSCs and PSTN

  22. Cellular System Architecture • One channel in each cell is set aside for signalling information between BS and mobiles • Mobile-to-BS: location, call setup for outgoing, response to incoming • BS-to-Mobile: cell identity, call setup for incoming, location updating

  23. Call Setup • Outgoing call setup: • User keys in the number and presses send (no dial tone) • Mobile transmits access request on uplink signaling channel • If network can process the call, BS sends a channel allocation message • Network proceeds to setup the connection

  24. Call Setup (contd) • Network activity: • MSC determines current location of target mobile using HLR, VLR and by communicating with other MSCs • Source MSC initiates a call setup message to MSC covering target area

  25. Call Setup (contd) • Incoming call setup: • Target MSC (covering current location of mobile) initiates a paging msg • BSs forward the paging message on downlink channel in coverage area • If mobile is on (monitoring the signaling channel), it responds to BS • BS sends a channel allocation message and informs MSC

  26. Hand-Offs Mobile moves from one BS into another • BS initiated: • BS monitors the signal level of the mobile • Handoff occurs if signal level falls below threshold • Increases load on BS • Monitor signal level of each mobile • Determine target BS for handoff

  27. Hand-Offs (contd) • Mobile assisted: • Each BS periodically transmits beacon • Mobile, on hearing stronger beacon from a new BS, sends it a greeting • changes routing tables to make new BS its default gateway • sends new BS identity of the old BS • New BS acknowledges the greeting and begins to route mobile’s call

  28. Hand-Offs • Intersystem: (Roaming) • Mobile moves across areas controlled by different MSC’s • Handled similar to mobile assisted case with additional HLR/VLR effort • Local call may become long-distance

  29. Cellular Implementations • First-generation: Analog cellular systems (450-900 MHz) • Frequency shift keying for signaling • FDMA for spectrum sharing • NMT (Europe), AMPS (US) • Second-generation: Digital cellular systems (900, 1800 MHz) • TDMA/CDMA for spectrum sharing • Circuit switching • GSM (Europe), IS-136 (US), PDC (Japan)

  30. Cellular Implementations • 2.5G: Packet switching extensions • Digital: GSM to GPRS • Analog: AMPS to CDPD • 3G: • High speed, data and Internet services • IMT-2000

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