IEEE 802.11 Wireless LANs Presented by Peng Ge September 12, 2001.
IEEE 802.11: Wireless LANs
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Transcript of IEEE 802.11: Wireless LANs
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IEEE 802.11: Wireless LANs
ALOHA, Slotted ALOHA
Carrier Sense Multiple Access (CSMA), CSMA/CD
MACA, MACAW, FAMA, DFWMAC
Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) Difficult to detect packet collisions (near-far problem) Optional RTS-CTS handshaking to avoid hidden node problem
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IEEE 802.11 MAC
• Mandatory Distributed Coordination Function (DCF)
– For distributed contention-based channel
• Optional Point Coordination Function (PCF) – For centralized contention-free channel access
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Distributed Coordination Function (DCF)
RTS
CTS
DATA
ACK
ACK... RTS ...NAV (RTS)
NAV (CTS)
DIFS
SIFS SIFS SIFS
DIFS
BACKOFF BACKOFF
STATION A
STATION B
OTHER STATIONS
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Random Backoff ProcedureChoose a Backoff Time over the interval [0, CW] Backoff Time = Random() × aSlotTime
6
CW = 31(e.g., Backoff Time = 6 × 20 = 120 µs)
• If the medium is idle for a backoff slot, the backoff time is decremented by aSlotTime
• If the medium is determined to be busy during a backoff slot, the backoff procedure is suspended until the medium is determined to be idle for DIFS period
• Whenever the Backoff Timer reaches zero, a packet transmission begins
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Random Backoff Procedure
• Successful Packet Transmissions : minimum contention window size
Frame
Frame
Frame
Frame
STATION A
STATION B
STATION C
DIFS DIFS DIFS
CW = 7, BT = 5 slots
CW = 7, BT = 2 slots
CW = 7, BT = 3 slots CW = 7, BT = 1 slot
CW = 7, BT = 2 slotsCW = 7, BT = 3 slots
Idle Backoff Slot
Deferred Backoff Slot
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Random Backoff Procedure
• Retransmission Case (i.e., Collisions or Transmission Failures) : increase contention window size
Frame
Frame
STATION A
STATION B
STATION C
DIFS DIFS DIFS
CW = 7, BT = 2 slots
Frame
DIFS
CW = 7, BT = 2 slots
CW = 7, BT = 3 slots
CW = 15, BT = 5 slotsFirst Retransmission
CW = 15, BT = 3 slotsFirst Retransmission
CW = 7, BT = 1 slot
CW = 15, BT = 4 slots
CW = 15, BT = 2 slots
CW = 15, BT = 2 slots
CW = 7
PACKET COLLISION
Frame
Frame
Frame
CW = 7
Idle Backoff Slot
Deferred Backoff Slot
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Binary Exponential Backoff
31 63127
255
511
1023CW max
CW min
More than 5 RetransmissionsFifth Retransmission
Fourth RetransmissionThird Retransmission
Second RetransmissionFirst Retransmission
Initial Attempt
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Wasting Factors in Backoff Procedure
IDLE SLOTS : small # of active stations with large contention window size (ex, 2 stations in DSSS MinCW=31)
STATION A
STATION B
COLLISIONS : large # of active stations with small contention window size (ex, 100 stations in HSSS MinCW=15)
STATION A
STATION B
STATION C
STATION D
STATION E
STATION F
STATION G
STATION H
STATION I
STATION J
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Example for Idle Slots
Frame
Frame
Frame
CW = 63, BT = 50, Idle Slots = 29
CW = 63, BT = 29, Idle Slots = 29
CW = 63, BT = 21, Idle Slots = 21
CW = 63, BT = 60, Idle Slots = 21 CW = 63, BT = 39, Idle Slots = 39
CW = 63, BT = 58, Idle Slots = 39
• Large minimum contention window size + small number of stations large wasting idle slots
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Point Coordination Function (PCF)
• Two consecutive frames are separated by SIFS
• CFP lengths depend on traffic amount – Maximum length announced by
Beacon D1+Poll
NAV
SIFS
SIFS
U1+Ack
D2+Ack+Poll
SIFS
U2+Ack
SIFS
SIFS
CF-End
Uplink
Downlink
Contentio Free Period (CFP) for PCF
ContentionPeriod (CP)
for DCF
Contention Free Period Repetition Interval (CFPRI) or Superframe
Reset NAV
CF_MAX_DurationDx - downlink frame to STA xUx - uplink frame from STA x
PIFS
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Problems of Legacy MAC
No notion of QoS and related signalingRestricted polling schedulingSuperframe with alternating CFP and CP needs to be short for short delay boundAP assuming the full control over the medium during CFP: overlapping WLANs?Uncontrollable/unpredictable frame transmission timesLarge wasting ilde slots when # of active stations is smallRapid performance degradation when # of active stations is large too slow to resolve collisions
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Hybrid Coordination Function (HCF)
• Contention-based channel access– Enhanced Distributed Coordination Function (EDCF) for prioriti
zed QoS
– Variation of legacy DCF
– provide differentiated, distributed access to the WM for 8 user priorities
– By using different AIFS, CWmin,CWmax values,
• Controlled channel access– QoS is characterized by a set of parameters
– A traffic stream (TS) is set up between transmitter and receiver (and HC – located within QoS AP)
– Polling mode plus HC’s prioritized channel access for parameterized QoS
– Variation of legacy PCF
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Access Category
AC0 AC1 AC2 AC3
Virtual Collision Handler
Backo
ff A
IFS[0]
BO
[0]
Backo
ff A
IFS[1]
BO
[1]
Backo
ff A
IFS[2]
BO
[2]
Backo
ff A
IFS[3]
BO
[3]
Transmission Attempt
• Access category (AC) as a virtual DCF
• 4 ACs implemented within a QSTA to support 8 user priorities
• Multiple ACs contend independently
• The winning AC transmits a frame
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AIFS (Arbitration-time inter-frame space)
• AIFS is the deferral time for backoff count-down that is used to achieve QoS differentiation
• AIFS is an actual IFS of priority-dependent duration
For stations with classification i= 0,1,…AIFSi = aSIFSTime + aAIFSi x aSlotTime
where aAIFSi is the AIFS slot count for class i
Example: For the top-priority class aAIFS0 = 1 and AIFS0 = PIFS
For legacy stations aAIFS0 = 2 and AIFS = DIFS *
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Default QoS Parameter Set
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EDCF Inter-Frame Space
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HC Controlled Channel Access
• Traffic Specification (TSPEC) Element
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HC Controlled Channel Access
• During CFP– HC assumes the full control over the medium
– Similar to PCF
• During CP– HC can grab the channel after a PIFS idle time
• Polled TXOP can exist in both CFP and CP– Superframe size needs not be very small anymore
• QoS (+)CF-Poll specifies the polled TXOP limit• During a polled TXOP, the TXOP holder can transmi
t whatever frames it wants • NAV protects a polled TXOP
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HC Controlled Channel Access
• HC scheduling– Mixture of downlink and polled TXOP scheduling
• QSTA scheduling– During a polled TXOP, schedule frame transmissions
• Admission control by HC– To decide whether to admit a TS or not