802'11 Performance Tuning

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802.11 Performance Tuning

• Tuning radio management
• Tuning power management
• Timing operations

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Tuning radio management

• Beacon interval
• Decreasing
• passive scanning more reliable faster
• mobility
• Increasing
• Power-saving capability (listen, DTIM intervals)
• Throughput

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2347
2346
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Simulation Environment for RTS threshold (1/2)

• Poisson distribution was used to determine the
  number of MAC service data unit (MSDU) arrivals
  and the lengths of the MSDUs were decided by the
  exponential distribution function.
• In the simulation all nodes had direct radio
  contact which means that each source to
  destination had only 1 hop distance.

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Simulation Environment for RTS threshold (2/2)

• Assumptions
• (1) all stations support the 2 Mb/s data rate.
• (2) all data and control frame were sent at 2
  Mb/s.
• (3) the propagation delay was neglected
• (4) the channel was error-free
• (5) there was no interference from nearby basic
  service sets (BSS)

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Simulation Results (1/3)

• The load of the network is determined by three
  factors
• The number of contending nodes (denoted as N) in
  the BSS
• The packet arrival rate per slot time per node
  (denoted as l)
• The mean data length (MDL) of the packets
• The network load is equal to
• (N x l x MDL) / (aSlotTime x Data rate)

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Simulation Results (2/3)

• The Simulations are split into four parts
• Influence of various mean data lengths Find the
  optimal RT value for different MDLs while keeping
  the packet arrival rate and the number of
  contending nodes fixed.
• Influence of various packet arrival rates Find
  the optimal RT for different packet arrival rates
  while keeping the mean packet length and the
  number of contending nodes fixed.

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Simulation Results (3/3)

• Influence of various contending nodes Find the
  optimal RT for different numbers of contending
  nodes while keeping the packet arrival rate and
  the mean packet length fixed.
• Trend of throughput Show the trend of throughput
  for different RT values while keeping the three
  load factors the same.

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Figure 2.Throughput vs. RT for various MDLs asN
25 and l 0.0001 packets/slot/node
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Figure 3.Throughput vs. RT for various MDLs asN
25 and l 0.0002 packets/slot/node
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Figure 4.Throughput vs. RT for various MDLs asN
25 and l 0.001 packets/slot/node
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Figure 5.Throughput vs. RT for various ls asN
25 and MDL 150 bytes
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Figure 6.Throughput vs. RT for various ls asN
25 and MDL 1k bytes
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Figure 7.Throughput vs. RT for various numbers
of nodes asl 0.001 packets/slot/node and MDL
150 bytes
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Figure 8.Throughput vs. RT for various numbers
of nodes asl 0.001 packets/slot/node and MDL
1k bytes
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Figure 9.Throughput vs. MDL for various RTs asN
25, l 0.001 packets/slot/node
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Figure 10.Throughput vs. MDL for various RTs
asN 25, l 0.0001 packets/slot/node
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Tuning radio management
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Tuning power management
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• Two drawbacks for long Listen Interval
• More buffer space
• Larger delivery delay
• Non-real time application is OK
• Real time application may not acceptable

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ATIM window

• Review Power management in IBSS
• Figure
• TBTT, beacon transmission, clock adjusting,
  CWmin,.. be skipped here
• STAs stay in active mode
• During ATIM (all nodes)
• After ATIM (ATI frame senders and receivers)

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802.11 PS mode in IBSS
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Enhanced IBSS Power Management - Issues and
Observations on 802.11 Standard (I)

• Mismatch and contentions
• Lengths on ATIM window and the beacon interval
  are fixed
• mismatch of the number stations in each
  contention period
• Two contention transmissions (one for ATIM and
  one for data packet)
• More collisions occur
• As simple simulation result SEE

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Success rate under fix TX window
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Enhanced IBSS Power Management - Issues and
Observations on 802.11 Standard (II)

• If ATIM window too short
• High contention within ATIM and fewer stations
  can successfully send their ATIMs.
• Contenting causes low-utilized in data windows
• As simple simulation result SEE
• If ATIM window too long
• Many stations can enter the data transmission
• Higher contention in data window
• Stations which get a chance to transfer (within
  ATIM) waste energy to stay in AM throughout the
  beacon interval.

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Stations of success tx data under fix TX CW
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Enhanced IBSS Power Management - Issues and
Observations on 802.11 Standard (III)

• Solutions to mismatch and contentions
• Relationship of ATIM length and the beacon
  interval
• ATIM takes ¼ beacon interval 16
• Dynamic ATIM according to loading 17

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Enhanced IBSS Power Management Proposed
Algorithm (I)

• for each beacon interval
• Contend to send beacon
• Upon receiving a beacon, enter ATIM window
• if have packet to transmit then
• Contend to send ATIM
• Listen to every ATIM and gather
  scheduling information
• Sort stations who successfully send ATIM
  based on the number of packets they will transfer
  and their ID (as tie-break)

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Enhanced IBSS Power Management Proposed
Algorithm (I)

• After packet transmission
• if the remaining time of current beacon
  interval is longer than threshold then
• Go to PS until next beacon interval
• else
• Stay in AM
• else
• Go to PS until next beacon interval

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Simulations and results

• IEEE 802.11 and its enhancement algorithm results
• The enhanced algorithm, because of the contention
  free data transmission and variable beacon
  interval length, the mismatch problem is no
  longer exist.
• The more stations enter data transmission, the
  more packets are transferred, while the system
  throughput remains at a steady level.

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IEEE 802.11
IEEE 802.11
the number of successful stations in ATIM window
number of data packets transferred,
Enhanced
Enhanced
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Other figures of 802.11
the number of successful stations in data
window Enhance one is fixed
the number of collision slots in data
window Enhance one is zero
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Other figures of enhanced
data window size
the average number of PS slots per PS station
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Conclusions

• An enhanced algorithm for power saving was
  proposed for ad hoc mode
• Beacon interval is modified on-demand
• Short packets transfer before the long one.
• Simulation shows its good.

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802.11 PS mode in IBSS
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Timing Operations
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Handoff Procedure with Active Scanning
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Handoff Latency

• Probe delay (scanning delay), authentication
  delay, (re)associaiton delay
• scanning delay the duration taken from the first
  Probe Request message to the last Probe Response
  message
• Scanning delay is the most time consuming part of
  the handoff process, taking over 90 of the total
  handoff delay

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Physical Operation

• Transmit Power Level

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Adaptive Transmit Power Control (1/2)

• The optimal transmit power between a
  sender-receiver pair is give by
• PTxOpt Path Loss(t) RSSmin
• Where Path Loss includes multipath fading and
  shadowing.
• RSSmin is the minimum threshold that a packet can
  be correctly decoded by the radio.

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Adaptive Transmit Power Control (2/2)
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Physical Operations
Transmit Power Level (802.11a)
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