IEEE 802.11 WLAN

IEEE 802.11 Protocol Stack For centralized contention-free channel access For distributed contention-based channel access

Possible Network Topologies BSS mode ESS mode

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 will typically run DHCP to get IP address in AP’s subnet

Distributed Coordination Function (DCF)

Binary Exponential Backoff Backoff Counter is randomly selected from [0,CW],where CW is contention window For each unsuccessful frame transmission, CW doubles (from CWmin to CWmax) CW  2 (CW+1)-1 If successful transmission, CW  CWmin Reduces the collision probability

Virtual Carrier Sensing B C D E RTS RTS CTS CTS DATA DATA ACK ACK Any node hearing RTS or CTS sets up their NAV (network allocation vector) until end of ACK. NAV set -> node silent (act as if carrier busy).

802.11 Timeline If carrier busy (physical or virtual), schedule transmission after a random backoff when carrier is free. Average backoff interval is doubled for each failed attempt. RTS DATA Transmitter SIFS SIFS DIFS CTS ACK Receiver SIFS Nodes that hear transmitter DIFS NAV (RTS) NAV (CTS) t Nodes that hear receiver Defer access Random backoff Another transfer

RTS/CTS Mechanism (Optional) RTC/CTS solves HTP But, non-negligible overhead If frame size > RTSthreshhold, RTS-CTS-DATA-ACK Otherwise, DATA-ACK 802.11b tslot 20usec SIFS 10usec PIFS SIFS + tslot DIFS SIFS + 2*tslot EIFS > DIFS

Collisions are not completely avoided in IEEE 802.11 !! H does not sense any signal during D’s DATA tx H may transmit Collision in E’s reception

Energy Conservation: Power control Power control has two potential benefit Reduced interference & increased spatial reuse Energy saving If C reduces transmit power, it can still communicate with D Reduces energy consumption at node C Allows B to receive A’s transmission (spatial reuse)

802.11 frame: addressing frame control duration address 1 2 4 3 payload CRC 6 0 - 2312 seq Address 4: used only in ad hoc mode Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame

802.11 frame: addressing Internet router H1 R1 AP AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame H1 R1 R1 MAC addr AP MAC addr dest. address source address 802.3 frame

802.11 frame: more frame seq # (for reliable ARQ) duration of reserved transmission time (RTS/CTS) frame control duration address 1 2 4 3 payload CRC 6 0 - 2312 seq Type From AP Subtype To More frag WEP data Power mgt Retry Rsvd Protocol version 2 4 1 frame type (RTS, CTS, ACK, data)

The Other IEEE 802.11 Efforts 802.11e 802.11h 802.11i 802.11n 802.11r Provides QoS support by differentiating traffic streams Applicable to 802.11 PHY a, b, and g 802.11h Supplementary to MAC layer so as to comply with European regulations for 5 GHz WLAN 802.11i Security enhancement 802.11n Enhancement for higher throughput (> 100 Mbps ) Decrease overhead within 802.11 protocol Packet preamble, CW, ACK, IFS parameters 802.11r Speed up handoff between APs (Fast BSS-Transition) Important for VoWLAN 802.11s Support mesh networks