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

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer

IEEE 802.11 Wireless LAN

• 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

802.11 LAN architecture

• wireless host communicates with base station– base station (BS) =

access point (AP)

BSS 1

BSS 2

Internet

switchor routerAP

AP

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

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