IEEE 802.11:multiple access avoid collisions:2*nodes transmitting at same time 802.11:CSMA-sense before transmitting o don't collide with ongoing transmission by other node 口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) A's signal C's signal 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 - 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