1 ieee 802.11 wlan. 2 ieee 802.11 protocol stack for centralized contention-free channel access for...
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IEEE 802.11 WLANIEEE 802.11 WLAN
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IEEE 802.11 Protocol StackIEEE 802.11 Protocol Stack
For centralized contention-free channel access
For distributed contention-based channel access
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Possible Network TopologiesPossible Network Topologies
BSS mode ESS mode
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802.11: Channels, association802.11: Channels, association
802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequenciesAP admin chooses frequency for APinterference 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 withmay perform authenticationwill typically run DHCP to get IP address in AP’s
subnet
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Distributed Coordination Function Distributed Coordination Function (DCF)(DCF)
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Binary Exponential BackoffBinary 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
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Virtual Carrier SensingVirtual Carrier Sensing
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).
A B C DRTSRTSCTSDATAACK
E
CTSDATA
ACK
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802.11 Timeline802.11 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.
t
SIFS
DIFSACK
Defer access
Nodes that hear transmitter
Receiver
TransmitterDATA
Randombackoff
RTS
CTS
SIFS SIFS
NAV (RTS)NAV (CTS)
DIFS
Nodes that hear receiver Another
transfer
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RTS/CTS Mechanism (Optional)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
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Collisions are not completely avoidedCollisions are not completely avoidedin IEEE 802.11 !!in IEEE 802.11 !!
H does not sense any signal during D’s DATA tx H may transmit Collision in E’s reception
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Energy Conservation: PowerEnergy Conservation: Powercontrolcontrol Power control has two potential benefit
Reduced interference & increased spatial reuseEnergy saving
If C reduces transmit power, it can still communicate with DReduces energy consumption at node CAllows B to receive A’s transmission (spatial reuse)
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framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
802.11 frame: addressing802.11 frame: addressing
Address 2: MAC addressof wireless host or AP transmitting this frame
Address 1: MAC addressof wireless host or AP to receive this frame
Address 3: MAC addressof router interface to which AP is attached
Address 4: used only in ad hoc mode
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Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addr
address 1 address 2 address 3
802.11 frame
R1 MAC addr AP MAC addr
dest. address source address
802.3 frame
802.11 frame: addressing802.11 frame: addressing
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framecontrol
durationaddress
1address
2address
4address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seqcontrol
TypeFromAP
SubtypeToAP
More frag
WEPMoredata
Powermgt
Retry RsvdProtocolversion
2 2 4 1 1 1 1 1 11 1
802.11 frame: more802.11 frame: more
duration of reserved transmission time (RTS/CTS)
frame seq #(for reliable ARQ)
frame type(RTS, CTS, ACK, data)
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The Other IEEE 802.11 EffortsThe Other IEEE 802.11 Efforts 802.11e
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