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)

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

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

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

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

Channels b: 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