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Wireless Access

Wireless Access. avoid collisions: 2 + nodes transmitting at same time CSMA - sense before transmitting don’t collide with ongoing transmission by other node no collision detection! difficult to receive (sense collisions) when transmitting (what you hear is not he hear)

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Wireless Access

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  1. Wireless Access • avoid collisions: 2+ nodes transmitting at same time • CSMA - sense before transmitting • don’t collide with ongoing transmission by other node • no collision detection! • difficult to receive (sense collisions) when transmitting (what you hear is not he hear) • can’t sense all collisions in any case: hidden terminal, fading • The next best thing to do: avoid collisions CSMA/C(ollision)A(voidance)

  2. RTS(B) RTS(A) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) ACK(A) ACK(A) Collision Avoidance: RTS-CTS exchange A B AP defer time

  3. 802.11b 2.4-5 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer widely deployed, using base stations 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps All use CSMA/CA for multiple access Base-station and ad-hoc network versions IEEE 802.11 Wireless LAN

  4. AP AP Internet 802.11 LAN architecture • wireless host communicates with base station • base station (BS) = access point (AP) switch or router BSS 1 BSS 2

  5. 802.11 Operating Modes • DCF: Distributed Coordination Function • aka Ad-hoc mode • CSMA/CA • RTS/CTS signaling • Two stations can communicate • All stations have the same logic • No infrastructure, Suitable for small area • PCF: Point Coordination Function • Infrastructure Based: • Must have a base station (or access point) • BS periodically broadcast a beacon • MS can only transmit when it is being polled

  6. 802.11 Channels • 802.11b: Use 11 channels at different frequencies • AP admin select operating frequency for AP • interference possible: channel can be same as that chosen by neighboring AP! • Planning: decide which AP use which channels to provide good coverage. • host: must associate with an AP • scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address • selects AP to associate with • may perform authentication • will typically run DHCP to get IP address in AP’s subnet

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