Dynamic Adaption of DCF and PCF mode of IEEE 802.11 WLAN - PowerPoint PPT Presentation

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Dynamic Adaption of DCF and PCF mode of IEEE 802.11 WLAN

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Active Node : Node having high probability to transmit, when polled ... Prioritize nodes that have not been polled in CFP by using DCF priority schemes ... – PowerPoint PPT presentation

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Title: Dynamic Adaption of DCF and PCF mode of IEEE 802.11 WLAN


1
Dynamic Adaption of DCF and PCF mode of IEEE
802.11 WLAN
MTech Dissertation
  • Abhishek Goliya
  • 01329012
  • Guided By
  • Prof. Sridhar Iyer
  • Dr. Leena-Chandran Wadia

15th January 2003
2
Aim
  • To optimize overall performance of IEEE 802.11
    MAC in WLAN
  • Devise protocols that dynamically adapt DCF and
    PCF mode of 802.11 Wireless LAN

3
Road Map
  • IEEE 802.11MAC
  • Problem Analysis
  • Suggested Protocols
  • DSP Dynamic Switching Protocol
  • PRRS Priority Round Robin Scheduling
  • CFP Adaption
  • Conclusion Future Research

4
DCF operation
5
PCF Mode
6
Problem Area
  • Polling overhead in PCF due to unsuccessful
    polling attempts
  • Need for Dynamic Switching between PCF and DCF to
    exploit better features of both
  • Need of Dynamic adaptation of configuration
    parameters both of PCF and DCF

7
Polling Overheads
  • When most stations have pending data, sequential
    polling provides ordered channel access and
    reduces collisions.
  • But when few stations have pending data polling
    mechanism become significant overhead.
  • Unnecessary delay for stations with data
  • Results in throughput degradation.

8
Theoretical Analysis and Simulation Study
  • TPollFail TPoll SIFS TNull SIFS
  • TPollSuccess TPoll SIFS TData SIFS TAck
    SIFS

Graph shows throughput degradation due to polling
overhead
9
Network Monitoring Layer
  • PRRS and DSP are based on monitoring layer at PC.
  • Eavesdrops packets to classify nodes as Active
    and Passive

Active Node Node having high probability to
transmit, when polled Passive Node Node
having low probability to transmit, when polled
10
Node List Management
List Management in CFP
List Management in CP
11
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12
Need of Dynamic Switching
  • DCF perform well till networks size is small
  • Suffers from throughput degradation at high load
    and in large-size networks
  • PCF results in larger delay in small-size
    networks due to polling overheads

13
Learning In DCF
  • Criteria of classifying node as Active is
    directly applicable
  • How much time node should be kept in Active List?
  • Station needs to transmit, take random backoff
    after DIFS
  • Random backoff lies between 0 to Current CWsize

14
Node List Management in DCF
  • CWavgCWmin p 2 CWmin plevel CWmax
  • level log2CWmax -log2CWmin

Event sequence and list management in DCF
15
Switching Criteria (PCF to DCF)
  • Keep track of number of active and passive nodes
    in BSS
  • Switch on the basis of number of active nodes
  • Approximate traffic load by keeping track of CFP
    utilization

No. of stations
16
Switching Criteria (DCF to PCF)
  • Keeping track of number of active nodes
  • Network Utilization
  • Number of free slots after DIFS
  • RTS Failure
  • Fail to receive CTS in response to RTS
  • Number of Collision
  • Assuming center position for PC. It can hear
    collision and approximate contention in network

17
Restricted DSP
  • Need fixed central coordinator (PC)
  • Station need to associate with PC
  • They need to follow same rules for association,
    deassociation, etc. as in PCF
  • Network starts in PCF
  • Switching decision are made at the time of beacon
    transmissions

18
PCF to DCF switching
  • Exploits the virtual carrier sense mechanism
  • Transmit beacon, as it does normally, but with
    different CF parameter set
  • Set CFPmaxDuration, CFPremainingDur to zero and
    CFPcount to non zero
  • PC sets its local variable cfp to zero mode to
    DCF

CF parameter set
19
DCF to PCF switching
  • Beacon generation is centralized in DCF and done
    by PC
  • PC waits for PIFS instead of DIFS for sending
    beacons
  • This prioritize PC for sending beacons
  • To transit into PCF mode, PC sets its CFPrate
    parameter to its normal value and mode to PCF
  • Beacons sent by it now has actual value of
    CFPmaxDuration, CFPremainingDuration and CFPcount

20
Simulation Setup
  • Studies confined to single cell of radius 240m
  • We added support for sending Null data frame and
    broadcast packet in CFP, in existing PCF patch

Throughput measured as total number of bits
passed up from the MAC sublayer at each
destination Laod measured as total number of bits
offered to the MAC sublayer at each source Delay
is measured as end to end delay at agent layer
21
Simulation Result for PRRS
32 Nodes with 16 active
32 Nodes with 8 active
32 Nodes with 32 active
64 Nodes with 16 active
22
Delay Graphs for PRRS
32 Nodes with 8 active
32 Nodes with 16 active
32 Nodes with 32 active
64 Nodes with 16 active
23
Result Analysis
  • When 25 nodes are active, throughput increases
    by 10 with 32 nodes and by 15 with 64 nodes
  • Mean packet delay reduces when few nodes are
    active
  • But it increases slightly when number of active
    node reaches total node.
  • If load is moderate and few nodes are active then
    PRRS gives better result

24
Alternative proposals to reduce Delay further
  • Use Service Differentiation mechanism in CP
  • Prioritize nodes that have not been polled in CFP
    by using DCF priority schemes
  • Use Bi-Level Feedback scheduling
  • Multilevel Feedback Scheduling (MFS)
  • Number of levels needed depends upon network size
  • Issues regarding MFS are still open for further
    research

25
Simulation Results for DSP
  • We vary number of active stations from 4 to 16
    while simulating DSP
  • Both throughput and delay value improve as
    compared to existing PCF and DCF

26
Impact of configuration Parameter
27
Effect of CFP repetition interval on PRRS
  • Good value for CFP repetition interval depends
    upon network size
  • Graph shows result with 16 and 64 nodes active
    respectively from the top

16 Nodes active
64 Nodes active
28
CFP Adaption Criteria
  • Approximate Load by CFP utilization
  • CFP utilization Time spent from first poll to
    CFPend
  • newCFPutili. 0.8 oldCFPutili.
    0.2curCFPutili.
  • Linear update is addition or subtraction of
    beacon transmission interval
  • Exponential update halves the current rate
  • Ensuring minimum interval and keep it as multiple
    of beacon interval

C is CFP repetition count
29
(No Transcript)
30
Simulation Results
  • CFP adapted PRRS shows better result than PRRS
    with arbitrary configured CFP repetition interval
  • Graph shows result with 16 and 32 active nodes

16 Nodes active
32 Nodes active
31
Conclusion
  • We suggested network monitoring based approach to
    dynamically adapt MAC in IEEE 802.11
  • PRRS improves throughput and delay values by
    about 10-15 especially at moderate load and few
    stations have data
  • DSP improves network capacity and reduces delay
    under all load regime
  • We showed the need to dynamically adapt the
    configuration parameters of both PCF and DCF
  • Our protocol for CFP adaption successfully adapts
    CFP rate to suit current network load

32
Future Work
  • PRRS suffers from larger delay in some scenario
  • Need to implement DCF priority schemes
  • Analyse the performance of Bi-level and
    multilevel feedback scheduling
  • Thorough experimentation and simulation study is
    required to define switching points for DSP
  • Idea of Distributed DSP is open for further
    research
  • Design algorithm to adapt other configuration
    parameters

33
  • Thanks
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