1. 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

2. 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.

3. 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

4. 연구 목표
IEEE DCF

5. Shortcomings of 802.11 DCF Inefficient channel usage
연구 목표
Shortcomings of 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. 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

7. 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

8. 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?

9. 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

10. 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

11. 연구 목표
Approximate Pe
Throughput
Cost Function

12. 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)

13.  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

14. 연구 목표
Optimal CW and ni
 If N is given

15. 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

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

17. 연구 목표

18. 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?

19. 연구 목표
Assume and = 0

20. 연구 목표
Frame Error Rate Perr
Pc < 10 % for IEEE b

21. 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.

22. 연구 목표
Performance
Throughput

23. 연구 목표
Fairness

24. 연구 목표
Convergence speed
5 nodes
+ 5 nodes
- 5 nodes

25. 연구 목표
Time Fairness

26. 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]
[2] idle sense.ppt made by Jayesh Seshadri

27. 연구 목표
Question?