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Dynamic Adaption of DCF and PCF mode of IEEE 802.11 WLAN

Abhishek Goliya01329012

Guided By:Prof. Sridhar Iyer

Dr. Leena-Chandran Wadia

MTech Dissertation

15th January 2003

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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

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Road Map

• IEEE 802.11MAC

• Problem Analysis

• Suggested Protocols

• DSP: Dynamic Switching Protocol

• PRRS: Priority Round Robin Scheduling

• CFP Adaption

• Conclusion & Future Research

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DCF operation

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PCF Mode

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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

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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.

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Theoretical Analysis and Simulation Study

Graph shows throughput degradation due to polling overhead

TPollFail = TPoll + SIFS + TNull + SIFS

TPollSuccess = TPoll + SIFS + TData + SIFS + TAck + SIFS

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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

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Node List Management

List Management in CFP

List Management in CP

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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

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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

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Node List Management in DCF

• CWavg=CWmin+ p* 2 * CWmin + …+ plevel * CWmax

• level= log2CWmax -log2CWmin

Event sequence and list management in

DCF

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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

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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

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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

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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

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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

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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

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Simulation Result for PRRS

32 Nodes with 8 active

32 Nodes with 32 active

32 Nodes with 16 active

64 Nodes with 16 active

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Delay Graphs for PRRS

32 Nodes with 8 active

32 Nodes with 32 active

32 Nodes with 16 active

64 Nodes with 16 active

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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

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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

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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

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Impact of configuration Parameter

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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

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CFP Adaption Criteria

• Approximate Load by CFP utilization• CFP utilization = Time spent from first poll to

CFPend

newCFPutili. = 0.8 * oldCFPutili. + 0.2*curCFPutili.• 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

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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

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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

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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

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Thanks…