شبکههای بیسیم (873-40 ) مقدمه

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.

2

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)

3

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

4

Advantages• Sometimes it is impractical to lay cables• User mobility• Cost

Limitations• Bandwidth• Fidelity• Power• Security

Why Wireless?

5

Propagation Principle

electricfield

magneticfield

propagation direction

6

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

7

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

8

Propagation Mechanisms

Non Line-of-Sight

Reflection

λ << D

Diffraction

λ D

Scattering

λ >> D

S DLine-of-Sight

9

Propagation in the “Real World”

a wave can be absorbed

reflect

penetrate

bend

10

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

11

Networking as Resource Allocation

12

Resource Allocation

13

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

14

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

15

Fixed Networks

16

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

17

Access Networks Mesh Network

Mobile Networks: Circuit Multiplexing

18

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)

19

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

20

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)

21

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

22

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

23

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