1 / 27

Idle Sense: Optimal Access Method for High Throughput and Fairness in Rate Diverse WLANs

This paper presents Idle Sense, an optimal access method for wireless LANs that achieves high throughput and fairness by using an analytical model to determine the optimal contention window and idle slots. It addresses the shortcomings of IEEE 802.11 DCF, such as inefficient channel usage, fixed CW control, and poor short-term fairness. The Idle Sense algorithm adjusts the CW based on the number of idle slots, resulting in improved performance and time fairness among different rate devices. No hardware modification is required for implementing Idle Sense.

ramirezr
Download Presentation

Idle Sense: Optimal Access Method for High Throughput and Fairness in Rate Diverse WLANs

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. 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.

  3. Contents • Shortcomings of DCF • Analysis of channel contention • Optimal CW and idle slots • CW control mechanism • Time Fairness • Performance • Conclusions

  4. IEEE 802.11 DCF

  5. 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

  6. 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

  7. 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

  8. 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?

  9. Analysis of channel contention • Two-host contention model • Three states • 1. idle contention slot, • 2. success transmission • 3. collison

  10. 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

  11. Approximate Pe Throughput Cost Function

  12. Cost function w.r.t Contention window (for different numbers of hosts)

  13. 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

  14. Optimal CW and ni  If N is given

  15. Principles of Idle Sense • Hosts track niand make it converge to thetarget value (5.68) • Each host estimates ni • Rises/Lowers CW when nitoo small/bigcompared to • Adjusting CW is done according to AIMD • ⇒ all hosts converge to a similar value of CW

  16. AIMD : Additive Increase, Multiplicative Decrease Using Pe=2/CW CWControl Algorithm

  17. 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?

  18. Assume and = 0

  19. Frame Error RatePerr Pc < 10 % for IEEE 802.11b

  20. 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.

  21. Performance • Throughput

  22. Fairness

  23. Convergence speed 5 nodes + 5 nodes - 5 nodes

  24. Time Fairness

  25. 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

  26. Question?

More Related