شبکههای بیسیم (873-40 ) مقدمه
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شبکههای بیسیم (873-40 ) مقدمه. دانشکده مهندسی کامپیوتر. نیمسال دوّم 93-92 افشین همّت یار. References. A. Kumar, D. Manjunath, and J. Kuri, Wireless Networking, Morgan Kaufmann Publishers, 2008. - PowerPoint PPT PresentationTRANSCRIPT
92-93نیمسال دوّم یار افشین هّمت
دانشکده مهندسی کامپیوتر
(40-873شبکه های بی سیم )مقدمه
References
A. Kumar, D. Manjunath, and J. Kuri, Wireless Networking, Morgan Kaufmann Publishers, 2008.
C.M. Cordeiro and D.P. Agrawal, Ad Hoc and Sensor Networks: Theory and Applications, 2nd Ed,
World Scientific, 2011.
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Contents
Background Wireless communication: concepts, techniques,
and models Application models and performance issues Cellular FDM-TDMA Cellular CDMA Random Access and Wireless LANs (WLANs) Mobile Ad-hoc Networks (MANETs) Wireless Mesh Networks (WMNs) Wireless Sensor Networks ( WSNs)
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Wired Vs. Wireless Communications
Wired WirelessEach cable is a different channel One media shared by all
Low signal attenuation High signal attenuation
No interference High interferencenoise; co-channel interference; adjacent channel interference
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Advantages• Sometimes it is impractical to lay cables• User mobility• Cost
Limitations• Bandwidth• Fidelity• Power• Security
Why Wireless?
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Propagation Principle
electricfield
magneticfield
propagation direction
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Electromagnetic Spectrum
Propagation characteristics are different in each frequency band.
UV
1 MHz1 kHz 1 GHz 1 THz 1 PHz 1 EHz
infrared visible
X raysGamma rays
LF HF VHF UHF SHF EHFMF
AM radio
S/W ra
dioFM
radio
TV TV cellu
lar
902 – 928 Mhz2.4 – 2.4835 Ghz
5.725 – 5.785 Ghz
ISM band
30kHz 300kHz 3MHz 30MHz 300MHz 30GHz 300GHz
10km 1km 100m 10m 1m 10cm 1cm 100mm
3GHz
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Unlicensed Radio Spectrum (ISM: Industrial, Science, Medicine)
902 Mhz
928 Mhz
26 Mhz 83.5 Mhz 125 Mhz
2.4 Ghz2.4835 Ghz
5.725 Ghz5.850 Ghz
cordless phonesbaby monitorsWaveLan
802.11bBluetoothMicrowave oven
802.11a
33cm 12cm 5cm
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Propagation Mechanisms
Non Line-of-Sight
Reflection
λ << D
Diffraction
λ D
Scattering
λ >> D
S DLine-of-Sight
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Propagation in the “Real World”
a wave can be absorbed
reflect
penetrate
bend
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Path-loss Models
Path-Loss Exponent Depends on environment:
L(d) = L(d0)(d/d0)n
Free space n = 2Urban area cellular n = 2.7 to 3.5Shadowed urban cell n = 3 to 5In building LOS n = 1.6 to 1.8Obstructed in building n = 4 to 6Obstructed in factories n = 2 to 3
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Networking as Resource Allocation
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Resource Allocation
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Wirelineo Static bit-carrier infrastructureo High quality digital transmission over copper or optical
mediao Bit pipes with a certain bit rate and very small bit error
rateo Dynamically reconfigured based on traffic demands
Wirelesso Point-to-point Line-of-sight (same as wireline or higher bit rate)o Time-varying channel impairmentso Adaptable PHY layer
Wireless Networking
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Our view:All the mechanisms, procedures, or algorithms for efficient sharing of a portion of the radio spectrum so that all instances of communication between the various devices obtain their desired Quality of Service (QoS).
Wireless Networks
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Fixed Networks
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Point to point Long distance transmission High gain antennas Tall masts Higher bit rate and also higher bit error rate than wireline
Mobile and Ad-hoc Networks
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Access Networks Mesh Network
Mobile Networks: Circuit Multiplexing
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GSM (2G)o Narrowbando FDM-TDMAo High SINR o Careful frequency planning to avoid co-
channel interferenceo Call admission control
GSM-GPRS (2.5G) Combining TDM Time slots GSM-EDGE (2.75G) Combining TDM Time slots
and higher order modulation schemes
Mobile Networks: Centralized Statistical Multiplexing (1)
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CDMA (IS-95)o Widebando CDMA (Spread spectrum)o Correlation receiverso No frequency planningo Interference limitedo Call admission control
WCDMA (CDMA-2000) o Most widely adapted standard for 3G
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WiMAX (IEEE 802.16 series)o Wireless access to Interneto Fixed subscriber stationso OFDMAo TDD (uplink & downlink)
o Specifications now have been extended to include broadband access to mobile users.
Mobile Networks: Centralized Statistical Multiplexing (2)
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WLAN (IEEE 802.11 series)o Wireless access to Interneto Limited mobilityo Statistical TDMAo Few Mbps (over 100s of meter) up to 100Mbps (over a few meters)o MIMO-OFDM (enhancement)
Mobile Networks: Distributed Statistical Multiplexing
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No infra-structure Multi-hop communication Point-to-point store and forward traffic
Miniature devices for nodes Low power, low bit rate digital radio transceiver, and small battery
Ad hoc Networks: Internet Access and Sensor Networks
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General: Transport of the user’s bits over the shared radio spectrum
Neighbor discovery, association and topology formation, routing
Transmission scheduling (cross layer)
Only in ad hoc sensor networks: Location determination Distributed computation
Technical Elements