Enablers - Enabling Technologies CS-6777 Mobile Ad Hoc...

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Enabling Technologies CS-6777 Mobile Ad Hoc Networking Memorial University of Newfoundland
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Why Enable? Reliable point-to-point communication via media access control (MAC) Challenges in medium share Categories of MAC protocols Random access principles Case studies IEEE 802.11, Bluetooth More PHY/MAC standards WiMax, ZigBee, UWB
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Digital Radio Properties Higher bit error rate (BER) Several orders compared to wired medium Path loss – signal attenuation At least quadratic of distance; could be 4 th power Fading effects Shadow fading (large-scale) Doppler and multipath fading (small-scale)
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MAC in Wireless Networks Contention resolution and collision handling ALOHA & CSMA/CD – recall CSMA/CD in Ethernet not working here Half duplex transceiver More challenges Hidden terminal problem Exposed terminal problems
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Hidden Terminal Problem A, B, C – three nodes B can talk to A and C directly, but A and C can’t talk to each other When A transmits to B, node C, not aware of the ongoing transmission, may transmit and collide with A’s transmission A B C
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Exposed Terminal Problem A, B, C, D – four nodes Only AB, BC, and CD can talk to each other directly When B transmits to A, node C, aware of the ongoing transmission, can’t transmit to D, even though there will not be interference at D A B C D
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A Simple Categorization Wireless MAC Distributed Guaranteed or Controlled access Random access Centralized
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How to Solve Hidden Terminals? Your inputs …
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Busy Tone [Tobagi75,Haas98] A receiver transmits busy tone when receiving data All nodes hearing busy tone keep silent Avoids interference from hidden terminals Requires a separate channel for busy tone
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MACA [Karn90] Three-way handshake When A wants to transmit to B, it first sends an RTS (Request-To-Send) packet After receiving A’s RTC, B sends a CTS (Clear-To-Send) packet After receiving B’s CTS, B sends the DATA packet A B C Data RTS CTS
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MACA (Cont’d) When should C reactivate? Both RTS and CTS contain duration information for overhearing nodes, e.g. C, to reactivate after specified silent period A B C RTS CTS Data
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MACA (Cont’d) When should C reactivate? Both RTS and CTS contain duration information for overhearing nodes, e.g. C, to reactivate after specified silent period A B C
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MACAW [Bharghavan94] Simple modification to improve reliability Four-way handshake RTS/CTS/DATA/ ACK Essential ingredient of IEEE802.11’s DCF A B C D RTS CTS Data ACK
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MACAW [Bharghavan94] Simple modification to improve reliability Four-way handshake RTS/CTS/DATA/ ACK Essential ingredient of IEEE802.11’s DCF A B C D
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802.11 WLAN 802.11 WLAN components Access Point (AP) + Mobile Stations (MS) + Distribution System (DS) 802.11 WLAN Modes BSS – basic service set (infrastructure mode) ESS – extended service set (multiple AP’s) IBSS – independent BSS (ad hoc mode)
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IEEE 802.11– Scope of Standard Logic Layer Control (LLC) – 802.2 Medium Access Control (MAC) PHY FH, DS, IR, OFDM Data link layer Physical layer IEEE 802.11 Upper layer protocols of OSI
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IEEE 802.11 Family 802.11(legacy)-1997 2Mbps/2.4GHz
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