Post on 20-Dec-2015
transcript
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Wireless Networks
Wireless networks
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Lecture Outline
• Wireless Networks Background
• Overview of Cellular Networks
• Overview of Satellite Networks
• Overview of Wireless Local Area Networks
• Other Wireless Networks
Based on the book: “Wireless communication and networks”
by © William Stallings, 2002 Prentice Hall
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Communication Frequency Spectrum
• Electromagnetic spectrum and applications (Tanenbaum 2003)
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• Aspects of Wireless Networks
– mobility and convenient deployment
– scarce frequency spectrum
– wireless implications such as transmission problems (e.g. interference, path loss, fading), security, battery, installation, health
• Wireless networks
– Cellular: GSM, PCS, IMT 2000
– Satellite: IRIDIUM, Globalstar
– WLAN: IEEE 802.11, HiperLAN
– Ad-Hoc, PAN, HAN
Wireless Networks Background
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• Extensive evolution and fast deployment
• First-Generation Mobile Phones: analog voice
(e.g. AMPS, NMT)
• Second-Generation Mobile Phones: digital voice
and some data (e.g. GSM, IS-95)
• Third-Generation Mobile Phones: digital voice
and data (e.g. 3G)
Wireless Cellular Networks
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• Mobile Terminals and Base Stations
• Communication area divided in hexagonal cells
• Cell dimensions from hundreds of meters till
tens of kilometers (e.g. GSM: 100m to 35 Km)
• Each cell served by a base station formed by a
transceiver and a control unit
• Each cell allocated a frequency band for
communication
• Communication from MS to BS -> reverse link
• Communication from BS to MS -> forward link
Principles of cellular networks [1]
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Principles of cellular networks [2]
• Frequency reuse: use the same frequency
spectrum in different set of cells
• Cells that reuse the same frequency must
be distant enough for avoiding
interference
• Transmission power control
• Migration of a mobile station from one
cell to another with continuance of
communication -> handoff
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Methods for increase capacity in cellular networks
• Adding new channels
• Frequency borrowing: congested cells use frequencies taken from adjacent cells
• Cell splitting:
– due to initial network design
– high-used cells are divided in smaller cells and frequencies are reallocated
• Cell sectoring:
– cells divided in sectors (e.g. 3, 6 sectors)
– each sector has allocated its own set of channels
– base stations use directional antennas for covering sectors
• Microcells and picocells: very small cells
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Cellular systems - general architecture
• General cellular system:– Mobile Station (MS)
– Base Station (BS)
– Mobile telecommunication switching office (MTSO)
• Communication between mobile station and base station use:– Control channels: exchange control data for calls management
– Traffic channels: data or voice connections between users
– Dominant switching mode: circuit-switch
MS
MS
BS
BS
BS
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Cellular systems operation example [1](a) Mobile station initialization (b) Mobile-originated call
(c) Paging (d) Call accepted
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(e) Ongoing call (f) Handoff
Cellular systems operation example [2]
• Other operations:
– call blocking
– call termination
– call drop
– calls to/from fixed and remote mobile subscriber
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Aspects of Cellular Networks [1]
• Radiowave propagation: – signal strength
– fading
– diverse propagation patterns
• Handoff: assigning a MS to a BS other than the current one when MS move from one cell toward other cell
• Different handoff parameters: cell blocking probability, call dropping, call completion, handoff success, handoff blocking, ...
• Handoff strategies:– relative signal strength
– relative signal strength with threshold
– relative signal strength with hysteresis
– relative signal strength with hysteresis and threshold
– prediction techniques
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Aspects of Cellular Networks [2]
• Power control requirements:
– reduce interference
– increase battery life
– overcome transmission conditions
– equalize received power (SS)
– other...
• Open-loop power control (a)
– depends on mobile station
• Closed-loop power control (b)
– depends on base station(a) Open-loop power control
(b) Closed-loop power control
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Aspects of Cellular Networks [3] - Access Methods• Frequency Division Multiple Access ->FDMA
– two (frequency) channels assigned per user, one for forward and one for reverse
– used in first generation cellular (e.g. AMPS)
• Time Division Multiple Access ->TDMA
– each physical channel divided in logical subchannels
– two logical channels assigned for user, for forward and reverse links
– transmission in repetitive sequence of frames divided in time slots
– each time slot position forms a logical channel that is assigned to the user
– used in second generation cellular (e.g. GSM)
• Code Division Multiple Access -> CDMA
– direct-sequence spread spectrum transmission -> use a chipping code for data
– two logical channels per user
– used in second and third generation cellular
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Second Generation Cellular Networks - Example
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GSM Cellular Network
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First, Second, Third Generation Cellular Networks
• More in 2G than 1G: – 2G have digital traffic channels, 1G pure analog– encryption – error detection and correction– dynamic channel access -> users share dynamically channels
• 3G capabilities:– voice quality comparable with switched telephone network– up to 384 kbps data rate outdoor– support for up to 2.048 Mbps indoor– symmetrical/asymmetrical data transmission rates– support for circuit switched and packet switched data services– support for wide variety of equipment– efficient spectrum usage– Internet interface– flexibility
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Satellite Communication
• Communication between earth stations and satellites
• Uplink-> earth station to satellite
• Downlink-> satellite to earth station
• Satellite categorization:
– Coverage area: global, regional, national
– Service type: fixed service satellite (FSS), broadcast service satellite (BSS), mobile service satellite (MSS)
– General usage: commercial, military, amateur, experimental
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Satellite Networks Configurations
• Point-to-point link
• Broadcast link
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Satellite Communication and Wireless Terrestrial Communication
• Advantages of satellite communication:– extensive area of coverage
– relative slowly variant conditions for communication between satellites
– transmission cost independent of distance
– support for broadcast, multicast and point-to-point communication
– high bandwidth and high data rates
– high quality of transmission
• Drawbacks of satellite communication :– expensive installation
– transmission delay
– needed terminals power
– needed number of satellites for global coverage
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Satellite and Orbits
• GEO - geostationary orbit satellites
• LEO - low earth orbit satellites
• MEO - medium earth orbit satellites
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GEO and LEO
• GEO characteristics:– no problems with the frequency change due to satellite movement
– simplified tracking of satellite
– very large area coverage (e.g. 3 satellite for almost whole Earth)
– weak signal and extensive delay (e.g. 0.5 s) due to long distance
– polar region poorly served
• LEO characteristics:– small coverage area-> big number of satellites
– satellite visibility cca. 20 minutes
– frequency changes due to satellite movement
– significant atmospheric drag
– small latency
– high data rates, up to few Mbps for Big LEOs
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Frequency Bands for Satellite Communication
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Issues in Satellite Communication
• Problems:
– Distance between earth station antenna and satellite antenna
– Downlink: terrestrial distance between earth antenna and the “aim point” of the satellite
– Atmospheric attenuation: oxygen, water, higher frequencies
• Capacity Allocation Strategies:
– frequency division multiple access (FDMA)
– time division multiple access (TDMA)
– code division multiple access (CDMA)
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Satellite Communication Examples
IRIDIUM LEO Satellite System
TDMA Satellite Transmission
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Wireless Local Area Networks (WLAN)
• General considerations:– limited utilization till last decade but extensive development lately
– usually in spaces where wired networks were difficult or not appropriate to deploy
– may increase reliability
– cost effective
– different standards
– different transmission medium used
• Some WLAN implications:– transmission problems: connection, multipath propagation, path loss and radio
signal interference
– network security
– system interoperability
– installation issues and health risks
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WLAN Applications [1]
• LAN Extension: – wireless extensions to fixed LANs
– stations in large open areas
– single or multiple cells
• Cross-Building Interconnect: – connect nearby building
– usually point-to-point communication
– connected devices: usually routers and bridges
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WLAN Applications [2]
• Nomadic Access: – connect mobile terminals with a LAN hub• Ad-hoc Networking: – spontaneous established temporary networks
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WLAN Requirements [1]
• Throughput: efficient use of the transmission medium
• Number of nodes: large number of nodes may be needed
• Connection to backbone LAN: usually a connection with a
wired networks is needed
• Service area: typical 100 to 300 m
• Battery life time: efficient management of mobile station
battery
• Transmission robustness and security: WLAN may be
interference prone and may be eavesdropped
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• Collocated network operations: more than one WLAN in the
same area
• License free operation: user oriented approach
• Handoff and roaming: moving between cells and even
networks may be needed
• Dynamic configuration: addition, deletion and reallocation
of end systems without affecting the network functionality
WLAN Requirements [2]
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WLAN Technologies [1]
Categorized according with the used transmission technology:
• Infrared (IR) LAN :
– IR does not penetrate walls
– limited to a single room
• Spread Spectrum LAN:
– use spread spectrum
– usually operate in ISM band
• Narrowband microwave LAN:
– operate at microwave frequency (e.g. above 1 GHz)
– can use ISM or licentiate frequency spectrum
– do not use spread spectrum
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WLAN Technologies [2]
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Infrared (IR) WLAN
• Directed Beam IR (a)
– point-to-point links
– range depend on power and wave focus
• Diffused (c)– all transmitter focus at a point on ceiling
– IR radiation is retransmitted (reradiate)
– reradiated IR waves are received by all
stations in the area
• Omnidirectional IR (b)
– single base station in LOS of all stations
– the base station acts as a repeater
(a)
(b)
(c)
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Infrared (IR) WLAN vs. Microwave WLAN
• Strengths
– virtual unlimited spectrum
– unregulated spectrum
– simple equipment needed -> inexpensive
– reflected by light-colored objects
– does not penetrate walls: more secure against eavesdropping and does not
introduce interference
• Drawbacks:
– sunlight, indoor lighting and other ambient radiation perceived as noise
– high-power transmitter required -> may introduce health problem (i.e. eye
safety) and power consumption problems
– limited range
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Spread Spectrum and Narrowband WLAN
• Spread Spectrum WLAN – usually multi-cell with different frequencies
– hub or peer-to-peer topology in a cell
– hub topology: the hub may provide access control and repeater operations and is
usually connected to a backbone network
– peer-to-peer topology: use ad-hoc connectivity without any hub
– usually unlicensed spectrum
– interference prone
• Narrowband WLAN – usually use narrow microwave band for transmission
– unlicensed as well as licensed frequency spectrum
– does not use spread spectrum
– licensed -> interference-free
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Other Wireless Networks - Cordless Systems• Characteristics– Residential: single base station with support for voice and data– Office: single base station for small offices, multiple base stations deployed using a cellular configuration for larger offices– Telepoint: a base station set up in a public place (e.g. an airport)– Small range for hand set -> low power– Inexpensive base station and hand set -> simple technical approaches– Limited frequency flexibility -> must work in different places
• Cordless Systems Example: DECT– Band: 1.88 - 1.9 GHz; Bandwidth: 20 MHz; Number of channels 120 – Access Method : TDMA/FDMA– Data rate: 1.152 Mbps; Speech rate: 32 kbps– Mean power: 10 mW– Maximum cell radius: 30 to 100 m– Provide handoff
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Other Wireless Networks - Wireless Local Loops (WLL)
• Narrowband WLL - replacement for telephony services
• Broadband WLL - high-speed voice and data services
• Usually use milimetric waves (e.g. above 10 GHz)• Advantages: cost, installation time, selective installation• Propagation problems: free space loss, rainfall attenuation, atmospheric and gaseous absorption, mutipath losses, vegetation effects• Multichannel Multipoint Distribution Service: MMDS (ex. 2.67-2.68 GHz)
• Local Multipoint Distribution Service: LMDS, standardized as IEEE 802.16(ex. 27.5-28.3 GHz)
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Other Wireless Networks - Personal Area Networks (PAN) and Home Area Networks (HAN)
HAN • Broadband smart house with intelligent appliances• HANs over phone lines, powerline and wireless HANs: diverse combination possible • Examples: HomeRF, Home Audio Video interoperability HAVi• Control Networks: low-speed powerline networks – specify protocols that are used via the power line– examples: LonWorks, X10, CEBus
PAN• Network serving a single person or a small group• Usually accommodate diverse mobile devices• Provide support for virtual docking station, peripheral sharing• Examples: Bluetooth, Infrared Data Association (IrDA), HomeRF
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Lecture Summary
• Overview of wireless networks and frequency spectrum for wireless
communication
• Cellular networks, cellular principles, channel access, different
generations of cellular networks
• Satellite communication, GEO, LEO, MEO
• WLAN, infrared, spread spectrum, microwave
• Other wireless networks: WLL, HAN, PAN, ad-hoc