IEEE 802. 11 Wireless lan 802.11b 802.11a o 24-5 GHz unlicensed o 5-6 GHz range radio spectrum o up to 54 mbps o up to 11 Mbps 802.11 o direct sequence spread spectrum(DSSS)in o 2.4-5 GHz range physical layer o up to 54 Mbps all hosts use same o All use CSMA/CA for chipping code multiple access o widely deployed, using All have base-station base stations and ad-hoc network versions 6: Wireless and Mobile Networks 6-16
6: Wireless and Mobile Networks 6-16 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 • all hosts use same chipping code 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 All have base-station and ad-hoc network versions
802.11 LAN architecture g wireless host communicates Internet with base station o base station access point(AP) o Basic Service Set(BSS) hub. switch (aka"cell )in infrastructure or router mode contains AP o wireless hosts bss1 o access point(AP): base station AP o ad hoc mode: hosts only BSS2 6: Wireless and Mobile Networks 6-17
6: Wireless and Mobile Networks 6-17 802.11 LAN architecture wireless host communicates with base station base station = access point (AP) Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: wireless hosts access point (AP): base station ad hoc mode: hosts only BSS 1 BSS 2 Internet hub, switch or router AP AP
802.11: Channels association o 802.11b: 2.4GHz-2 485GHz spectrum divided into 11 channels at different frequencies o aP admin chooses frequency for ap o interference possible: channel can be same as that chosen by neighboring ap g host: must associate with an AP o scans channels, listening for beacon frames containing AP's name (SSID)and MAc address o selects ap to associate with o may perform authentication [Chapter 8] o will typically run DHCP to get IP address in AP's subnet 6: Wireless and Mobile Networks 6-18
6: Wireless and Mobile Networks 6-18 802.11: Channels, association 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! 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 [Chapter 8] will typically run DHCP to get IP address in AP’s subnet
IEEE 802.11: multiple access O avoid collisions: 2+ nodes transmitting at same time 0802. 11: CSMA(Carrier Sensing for multiple access) sense before transmitting o don,'t collide with ongoing transmission by other node g 802.11: no collision detection o difficult to receive(sense collisions)when transmitting due to weak received signals(fading o can't sense all collisions in any case: hidden terminal, fading o goal: avoid collisions: CSMA/C(ollision)A(voidance) C A's sianal s signa B strength strength space 6: Wireless and Mobile Networks 6-19
6: Wireless and Mobile Networks 6-19 IEEE 802.11: multiple access avoid collisions: 2+ nodes transmitting at same time 802.11: CSMA (Carrier Sensing for Multiple Access) - sense before transmitting don’t collide with ongoing transmission by other node 802.11: no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can’t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) A B C A B C A’s signal strength space C’s signal strength
IEEE 802. 11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for difs (a short time known as Distributed Inter-frame Space)then sender recelver transmit entire frame(no CD) 2 if sense channel busy then DIFS start random backoff time timer counts down while channel idle 3 transmit when timer expires data 4 if received ACK, it begins at step 2 to send another frame if no AcK increase random backoff interval, repeat 2 SIFS 802.11 receiver if frame received OK return ACk after siFs (a short period of time known as Short Inter-frame Spacing, ACK needed due to hidden terminal problem 6: Wireless and Mobile Networks 6-20
6: Wireless and Mobile Networks 6-20 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS (a short time known as Distributed Inter-frame Space) then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle 3 transmit when timer expires 4 if received ACK, it begins at step 2 to send another frame; if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS (a short period of time known as Short Inter-frame Spacing, ACK needed due to hidden terminal problem) sender receiver DIFS data SIFS ACK