연구 목표 Idle Sense: An Optimal Access Method for High Throughput and Fairness in Rate Diverse Wireless LANs. M. Heusse, F. Rousseau, R. Guillier, and A. Duda LSR-IMAG, Grenoble, France In proceedings of SIGCOMM 2005. Presented by Chaegwon Lim

Idle Sense Optimal Contention Window using an analytical model 연구 목표 Idle Sense Optimal Contention Window using an analytical model Fixed CW based on idle slots AIMD CW adjustments Short term fairness Time fairness among different rate devices. 2019-04-06

Contents Shortcomings of DCF Analysis of channel contention 연구 목표 Contents Shortcomings of DCF Analysis of channel contention Optimal CW and idle slots CW control mechanism Time Fairness Performance Conclusions 2019-04-06

연구 목표 IEEE 802.11 DCF 2019-04-06

Shortcomings of 802.11 DCF Inefficient channel usage 연구 목표 Shortcomings of 802.11 DCF Inefficient channel usage Fixed CW control If N is small, many idle slots If N is large, many collisions Poor short-term fairness Exponential backoff methods Performance anomaly among multi-rate nodes Slow host limits the throughput of faster hosts 2019-04-06

Ignorance of physical capture effect 연구 목표 Contention Control based on collisions If a host looses frames due to bad transmission conditions, it performs exponential backoffs Increased CW lowers the transmission attempt probability Ignorance of physical capture effect 2019-04-06

Towards a better access method 연구 목표 Towards a better access method Keep good aspects of DCF No explicit information exchange Keep backoff procedure: random backoff Modifications No exponential backoff make hosts use similar values of CW ⇒ fairness Adapt CW to varying traffic conditions more hosts, bigger CW; less hosts smaller CW 2019-04-06

Idle Sense Observe the number of idle slots Control CW 연구 목표 Idle Sense Observe the number of idle slots Channel load indicator Control CW Adjust CW to the current state Optimal operation in all conditions What is the optimal CW? How does it relate to the number of idle slots? 2019-04-06

Analysis of channel contention 연구 목표 Analysis of channel contention Two-host contention model Three states 1. idle contention slot, 2. success transmission 3. collison 2019-04-06

Pe : Attempt p. for a slot per node 연구 목표 Pe : Attempt p. for a slot per node Pt : Successful Tx p. for a given slot Pc : Collision p. for a given slot Pi : Slot Idle p. __ ni : No. of consecutive idle slots 2019-04-06

연구 목표 Approximate Pe Throughput Cost Function 2019-04-06

Cost function w.r.t Contention window (for different numbers of hosts) 연구 목표 Cost function w.r.t Contention window (for different numbers of hosts) 2019-04-06

 Optimal value of CW which has an smaller cost. 연구 목표  Optimal value of CW which has an smaller cost.  The first derivative of the cost function  By denoting 802.11b 11Mbps N  ∞  Optimal idle prob.  Optimal idle slots 2019-04-06

연구 목표 Optimal CW and ni  If N is given 2019-04-06

Principles of Idle Sense 연구 목표 Principles of Idle Sense Hosts track ni and make it converge to the target value (5.68) Each host estimates ni Rises/Lowers CW when ni too small/big compared to Adjusting CW is done according to AIMD ⇒ all hosts converge to a similar value of CW 2019-04-06

CW Control Algorithm AIMD : Additive Increase, Multiplicative Decrease 연구 목표 CW Control Algorithm AIMD : Additive Increase, Multiplicative Decrease Using Pe=2/CW 2019-04-06

연구 목표 2019-04-06

Channel Adaptation , : higher/lower bit rate , : frame error rate 연구 목표 Channel Adaptation , : higher/lower bit rate , : frame error rate , : proportion of useful throughput when using the higher/lower bit rate Which condition is the boundary? 2019-04-06

연구 목표 Assume and = 0 2019-04-06

연구 목표 Frame Error Rate Perr Pc < 10 % for IEEE 802.11b 2019-04-06

Time Fairness Channel Adaptation Time-fairness 연구 목표 Time Fairness Channel Adaptation Transmission bit rate (necessity to vary) P [collision/transmission] ( << 10% ) Frame error rate Perr~1- Pc –Pok Time-fairness CW’=CW * (rmax/rcurrent) Hence, slower hosts get the channel less. 2019-04-06

연구 목표 Performance Throughput 2019-04-06

연구 목표 Fairness 2019-04-06

연구 목표 Convergence speed 5 nodes + 5 nodes - 5 nodes 2019-04-06

연구 목표 Time Fairness 2019-04-06

Conclusions Idle Sense Near optimal distributed convergence of CW 연구 목표 Conclusions Idle Sense Near optimal distributed convergence of CW AIMD adjustments Short term fair Fixes performance anomaly (Time fairness) No hardware modification required Future work Access point adaptations: downlink traffic Better solution to the problem of exposed terminals Some slides are borrowed by these two materials in parts [1] http://www.sigcomm.org/sigcomm2005/slides-HeuRou.pdf [2] idle sense.ppt made by Jayesh Seshadri 2019-04-06

연구 목표 Question? 2019-04-06