1 A Novel Topology-blind Fair Medium Access Control for Wireless LAN and Ad Hoc Networks Z. Y. Fang and B. Bensaou Computer Science Department Hong Kong University of Science and Technology ICC 2003

2 Outline Introduction Introduction IEEE DCF IEEE DCF Improving the Fairness of the Backoff Algorithm Improving the Fairness of the Backoff Algorithm Simulations Simulations Conclusion Conclusion

3 Introduction DCF DCF Carrier sensing / Random backoff / Virtual backoff / Collision avoidance Carrier sensing / Random backoff / Virtual backoff / Collision avoidance Guarantees that ultimately a node will have a chance of attempting to access the channel Guarantees that ultimately a node will have a chance of attempting to access the channel Since the backoff timers are frozen when a node defers Since the backoff timers are frozen when a node defers No guarantee the success of such attempts No guarantee the success of such attempts In presence of hidden terminals In presence of hidden terminals When the traffic load is high and the node density is non uniform When the traffic load is high and the node density is non uniform Station succeeding a large number of transmission consecutively in an asymmetric network Station succeeding a large number of transmission consecutively in an asymmetric network

4 Introduction Problem of fairness Problem of fairness There are N network nodes, optimal success probability of 1/N There are N network nodes, optimal success probability of 1/N This unfairness can be very marked when the traffic load is high This unfairness can be very marked when the traffic load is high ABC 1/21/3 1/2

5 Introduction Scheduled schemes Scheduled schemes Need coordinators to dictate the behavior of nodes Need coordinators to dictate the behavior of nodes Minimum bandwidth guarantee Minimum bandwidth guarantee Need to incorporate a weight allocation method as well as the means to distribute such information among nodes Need to incorporate a weight allocation method as well as the means to distribute such information among nodes Measurement-base fair backoff algorithm Measurement-base fair backoff algorithm Can alleviate the fairness problem Can alleviate the fairness problem Need to exchange Need to exchange Weights allocation information Weights allocation information Maintain topology knowledge Maintain topology knowledge

6 Introduction Blind backoff scheme Blind backoff scheme Without need for any exchange of information Without need for any exchange of information Without any assumption Without any assumption Self adapting to changes in the traffic load as well as topological changes Self adapting to changes in the traffic load as well as topological changes Novel backoff algorithm Novel backoff algorithm To propose a new backoff mechanism for IEEE for a fair channel access

7 Improving the Fairness of the Backoff Algorithm Deals with how to estimate t p: Counts the number of successful transmissions in a period Increases contention window size in case of more successful transmission Decreases contention window size to be more aggressive in case of successive failures Adjusts t p when contention window hits the maximum or the minimum bound since it is either too large or too small

8 Improving the Fairness of the Backoff Algorithm t packet RTS CTS ACK DATA t unit tptp = k * t unit DIFS SIFS If k=2 RTS CTS ACK DATA Other STA packet

9 if ( >1 ) if ( == CWmax ) = - +1 else = 2 * else if ( == 0) if( == CWmin ) = + 1 else = / 2 endif = 0 Timer Event: ( T P ) win i t i unit t i p = k i * win i i i i i n i n i n i n i i k i k i k i k Use current contention window to access channel if ( successfully received ACK) = + 1 Update Init : t i packet = 0 k i = 1 t i unit t i p = k i * Channel Access : t i unit t i packet win i n i n i n i = 0 // the number of successful transmission

10 AABAA B STA A : CW=32 slot, Tp=1 STA B : CW=32 slot, Tp=1 STA A STA B CW=32 Tp = 1 CW=32 Tp = 1 CW=32 Tp = 2 CW=32 Tp = 1 CW=32 Tp = 1 CW=32 Tp = 2 CW=64 Tp = 2 CW=32 Tp = 2 CW=32 Tp = 2 CW=64 Tp = 2 CW=32 Tp = 3 A B ABC 1/21/3 1/2 Monitor 1 Monitor 1 CW=32 Tp = 2 CW=32 Tp = 2 T P T UINT AA

11 Simulations Network topology

12 FMAC FMAC

13 FMAC FMAC FMAC

14 Conclusion Improve fairness of IEEE would have to pay a price for this is terms of total network throughput degradation Improve fairness of IEEE would have to pay a price for this is terms of total network throughput degradation Most of the time less than 50% throughput is lost due to introduction of fairness Most of the time less than 50% throughput is lost due to introduction of fairness Satisfaction is the driving force behind any fairness oriented work Satisfaction is the driving force behind any fairness oriented work

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