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CSCI 465 D ata Communications and Networks Lecture 18

CSCI 465 D ata Communications and Networks Lecture 18. Martin van Bommel. Wireless Phone Networks. Prior to cellular was Mobile radio telephone service Provided by one high-power transmitter/receiver Support 25 channels Effective radius of 80 km Move to Cellular radio network

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CSCI 465 D ata Communications and Networks Lecture 18

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  1. CSCI 465Data Communications and NetworksLecture 18 Martin van Bommel CSCI 465Data Communications & Networks

  2. Wireless Phone Networks Prior to cellular was • Mobile radio telephone service • Provided by one high-power transmitter/receiver • Support 25 channels • Effective radius of 80 km Move to • Cellular radio network • Lower-power systems with shorter radius • Numerous transmitters/receivers CSCI 465Data Communications & Networks

  3. Cellular Network Organization • Area is divided into cells • uses tiling pattern to provide full coverage • each with own antenna (max 100 Watts) • each with own range of frequencies • served by base station • consisting of transmitter, receiver, and control unit • adjacent cells use different frequencies to avoid crosstalk • cells sufficiently distant can use same frequency band CSCI 465Data Communications & Networks

  4. Cellular Geometry • Divide area into cells • Square cells easy, but distances not equal • Hexagonal pattern permits equidistant antennas • Distance between cell centers • In reality, slight variation dueto topographical limitations,local signal propagation, andlimits on antennae CSCI 465Data Communications & Networks

  5. Frequency Reuse • Objective • Share nearby (but not adjacent) cell frequencies without interfering with each other • Power of base transceiver controlled • Permit communications within cell on given frequency • Limit escaping power to adjacent cells • Allow multiple simultaneous conversations in cell • Generally 10 to 50 frequencies assigned per cell CSCI 465Data Communications & Networks

  6. Frequency Reuse Patterns • Pattern of N cells • K frequencies • Cell has K/N freqs. • AMPS • Advance MobilePhone System K = 395, N = 7 < 57 freqs/cell

  7. Increasing Cellular Capacity • Add new channels • Add more frequencies to the system • Frequency borrowing • Move frequencies into the congested cells from less used adjacent cells • Dynamically assign frequencies • Cell splitting • Non-uniform topography and traffic distribution • Use smaller cells in high use areas • less power but more handoffs CSCI 465Data Communications & Networks

  8. Operation of Cellular System CSCI 465Data Communications & Networks

  9. Cellular System Channels • Two types of channels between mobile unit and base station (BS) • Traffic channels • Carry voice and data • Control channels • Set up and maintain calls • Establish relationship between mobile unit and nearest base station (BS) CSCI 465Data Communications & Networks

  10. Typical Scenario • Mobile unit initialization (power on) • Scan and select strongest control channel • Repeated periodically to account for mobility • Handshake between unit and MTSO via BS • Identify user and register location • Mobile-originated call • Unit checks if preselected channel is free • Unit sends number on channel • BS sends request to MTSO CSCI 465Data Communications & Networks

  11. Typical Scenario (2) • Paging • MTSO attempts to complete connection by sending paging message to certain BSs • BS sends paging signal on setup channel • Call accepted • Called unit recognizes its number • Responds to BS – passed on to MTSO • MTSO sets up circuit by assigning channels CSCI 465Data Communications & Networks

  12. Typical Scenario (3) • Ongoing call • Two mobile units exchange voice or data by going through BS and the MTSO • Handoff • If mobile unit moves out of range of one cell, finds new BS and establishes channel for it, switching call to new BS • Hangup • Either unit hanging up sends signal to MTSO which instructs BS channels to be released CSCI 465Data Communications & Networks

  13. First Generation: Analog • Original cellular telephone networks • Analog traffic channels • Advanced Mobile Phone Service (AMPS) • Early 1980s in North America • Also common in South America, Australia, China • Replaced by later generation systems • Still in use in some areas CSCI 465Data Communications & Networks

  14. AMPS Parameters CSCI 465Data Communications & Networks

  15. AMPS Operation • AMPS-capable phone has read-only memory with numeric assignment module (NAM) • NAM contains • Telephone number of phone – assigned by provider • Serial number of phone – assigned by manufacturer • When power on, transmit both to MTSO • MTSO has database of stolen units - blocked • MTSO uses phone number for billing CSCI 465Data Communications & Networks

  16. AMPS Call Sequence • Subscriber initiates call keying in number • MTSO validates number and checks if user authorized to call • MTSO issues message indicating traffic channels to use • MTSO sends ringing signal to called party • When answered, MTSO • establishes circuit • initiates billing information • When one party hangs up, MTSO • releases circuit, • frees radio channels, and • completes billing information CSCI 465Data Communications & Networks

  17. AMPS Control Channels • 21 full-duplex 30-kHz control channels • Transmit digital data using FSK • Data transmitted in frames • Control information can be transmitted over voice channel during conversation • Mobile unit or base station inserts burst of data • Turn off voice FM transmission for about 100 ms • Replace it with an FSK-encoded message • Used to exchange urgent messages • Change power level • Handoff to another base station CSCI 465Data Communications & Networks

  18. 2nd Generation: GSM • Global System for Mobile Communication • First appeared in 1991 in Europe • Similar to working of AMPS • Designed to support phone, data, and image • Rates up to 9.6 kbps • GSM transmission is encrypted using secret keys CSCI 465Data Communications & Networks

  19. GSM SIM • Subscriber Identity Module • Smart card or plug-in module to activate unit • stores • subscriber’s identification number • networks subscriber is authorized to use • encryption keys • Can use any unit anywhere with your SIM CSCI 465Data Communications & Networks

  20. Second Generation: Digital • Provide higher quality signals, higher data rates for support of digital services, with overall greater capacity • Key differences • Digital traffic channels – data or digitized voice • Encryption – easy to encrypt digital traffic • Error detection and correction • Channel access – channels are shared in CDMA • Time division multiple access (TDMA) • Code division multiple access (CDMA) CSCI 465Data Communications & Networks

  21. CDMA • Each cell allocated frequency bandwidth which is split in two • Half for reverse, half for forward • Uses direct-sequence spread spectrum (DS-SS) • Each bit encoded using pre-assigned code which represents it using several bits • Orthogonal codes permit multiple access CSCI 465Data Communications & Networks

  22. CDMA Advantages • Frequency diversity • Spread over larger bandwidth • Noise bursts and fading have less effect • Multipath resistance • Signal delay and echo does not interfere • Privacy • Unique coding implies privacy • Graceful degradation • More users means more noise and more errors • Leads to slow signal degradation until unacceptable CSCI 465Data Communications & Networks

  23. CDMA Disadvantages • Self-jamming • Some cross correlation between users • Arriving signals not perfectly aligned • Near-far problems • Signals closer to receiver have less attenuation • Transmissions from remote units more likely to fail CSCI 465Data Communications & Networks

  24. Third Generation: 3G • high-speed wireless communications to support multimedia, data, and video in addition to voice • 3G capabilities: • voice quality comparable to PSTN • 144 kbps available to users over large areas • 384 kbps available to pedestrians over small areas • support for 2.048 Mbps for office use • symmetrical and asymmetrical data rates • packet-switched and circuit-switched services • adaptive interface to Internet • more efficient use of available spectrum • support for variety of mobile equipment • allow introduction of new services and technologies CSCI 465Data Communications & Networks

  25. 3-G Driving Forces • trend toward universal personal telecommunications access • GSM (Global System for Mobile communications) telephony with subscriber identity module • personal communications services (PCSs) • personal communication networks (PCNs) • technology is digital • time division multiple access (TDMA) or • code-division multiple access (CDMA) • PCS handsets low power, small, and lightweight CSCI 465Data Communications & Networks

  26. Fourth Generation: 4G • Rapid increase in data traffic on wireless • More terminals can access Internet • Permanent connections to e-mail • Multimedia services • Support for real-time services • Instant messaging • Two standards • LTE – Long Term Evolution • WiMax 4G – IEEE 802.16 committee standard CSCI 465Data Communications & Networks

  27. 4G - OFDM • Both standards based on OFDM • Orthogonal Frequency Division Multiplexing • Uses multiple carrier signals at different frequencies • Sends some bits on each channel • All subcarriers dedicated to single data source • Extends symbol period on each signal • Intersymbol interference higher at higher bit rates • Each subcarrier carries lower bit rate CSCI 465Data Communications & Networks

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