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1 Wireless LAN 최양희서울대학교컴퓨터공학부

2 Radio-Based Wireless LANs Most widely used method Adv: penetrating walls and other obstacles with little attenuation. Disadv: security, interference, etc. 3 approaches: ISM band Narrow band Spread spectrum

3 Overview of Wireless LANs use wireless transmission medium issues of high prices, low data rates, occupational safety concerns, & licensing requirements now addressed key application areas: LAN extension cross-building interconnect nomadic access ad hoc networking

4 Wireless LAN Configuration

5 Cross-Building Interconnect connect LANs in nearby buildings point-to-point wireless link Not a LAN per se connect bridges or routers

6 Infrastructure Wireless LAN

7 Ad Hoc Networking temporary peer-to-peer network

8 Wireless LAN Requirements throughput - efficient use wireless medium no of nodes - hundreds of nodes across multiple cells connection to backbone LAN - using control modules service area to 300 m low power consumption - for long battery life on mobiles transmission robustness and security collocated network operation license-free operation handoff/roaming dynamic configuration - addition, deletion, and relocation of end systems without disruption to users

9 Wireless LAN - Physical Infrared 1Mbps and 2Mbps Wavelength nm Direct sequence spread spectrum 2.4GHz ISM band Up to 7 channels Each 1Mbps or 2Mbps Frequency hopping spread spectrum 2.4GHz ISM band 1Mbps or 2Mbps Others under development

10 ISM Bands In 1985, FCC modified part 15 to stimulate the use of wireless networks. ISM=Industrial, Scientific, and Medical Unlicensed, you can freely install and move.

11 ISM Spectrum Availability Only 2.4 GHz is the world-accepted ISM band. 902 MHz is easier in manufacturing.

12 IEEE Architecture

13 Physical Layer Available bandwidth Unlicensed frequency of operation Number of nonoverlapping channels Data rate per channel a b g 83.5 MHz 300 MHz 83.5 MHz 83.5 MHz GHz DSSS, FHSS 3 (indoor/outdoor) 1, 2 Mbps GHz OFDM GHz OFDM 4 indoor 4 (indoor/outdoor) 4 outdoor 6, 9, 12, 18, 24, 36, 48, 54 Mbps GHz DSSS 3 (indoor/outdoor) 1, 2, 5.5, 11 Mbps Compatibility Wi-Fi5 Wi-Fi GHz DSSS, OFDM 3 (indoor/outdoor) 1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36, 48, 54 Mbps Wi-Fi at 11 Mbps and below

14 Data Rate vs Distance (m) Data Rate (Mbps) b a g (b)/6(a/g) (b)/12(a/g)

15 IEEE BSS basic service set (BSS) building block may be isolated may connect to backbone distribution system (DS) through access point (AP) BSS generally corresponds to cell DS can be switch, wired network, or wireless network have independent BSS (IBSS) with no AP

16 Extended Service Set (ESS) possible configurations: simplest is each station belongs to single BSS can have two BSSs overlap a station can participate in more than one BSS association between station and BSS dynamic ESS is two or more BSS interconnected by DS appears as single logical LAN to LLC

17 Media Access Control

18 Media Access Control Distributed wireless foundation MAC (DWFMAC) Distributed coordination function (DCF) CSMA No collision detection Point coordination function (PCF) Polling of central master

19 Four Frame Exchange Four Frame Exchange can use four-frame exchange for better reliability source issues a Request to Send (RTS) frame to dest destination responds with Clear to Send (CTS) after receiving CTS, source transmits data destination responds with ACK RTS alerts all stations within range of source that exchange is under way CTS alerts all stations within range of destination other stations don t transmit to avoid collision RTS/CTS exchange is required function of MAC but may be disabled

20 IEEE Medium Access Control Logic

21 IEEE MAC Timing Basic Access Method

22 IEEE MAC Frame Format

23 Roaming (1) Internal Roaming (2) External Roaming 2008 Yanghee Choi

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