1 / 33

Is IEEE 802.11 TSF Scalable?

Is IEEE 802.11 TSF Scalable?. L. Huang, T.H. Lai, On the scalability of IEEE 802.11 ad hoc networks , MobiHoc 2002. IEEE 802.11: how large can it be?. Bandwidth: Up to 54 Mbps Good for a few hundred nodes Timing Synchronization Function Not scalable How to fix it?.

edan
Download Presentation

Is IEEE 802.11 TSF Scalable?

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. Is IEEE 802.11 TSF Scalable? L. Huang, T.H. Lai, On the scalability of IEEE 802.11 ad hoc networks, MobiHoc 2002.

  2. IEEE 802.11: how large can it be? • Bandwidth: • Up to 54 Mbps • Good for a few hundred nodes • Timing Synchronization Function • Not scalable • How to fix it?

  3. 802.11’s Time Sync Function (I) • Time divided into beacon intervals, each containing a beacon generation window. • Each station: • waits for a random number of slots; • transmits a beacon (if no one else has done so). • Beacon: several slots in length. beacon interval window

  4. 802.11’s Time Sync Function (II) • Beacon contains a timestamp. • On receiving a beacon, STA adopts beacon’s timing if T(beacon) > T(STA). • Clocks move only forward. 12:01 12:02 12:01 12:00 12:01 faster slower adopts not adopts

  5. Problems with 802.11’s TSF • Faster clocks synchronize slower clocks. • Equal opportunity for nodes to generate beacons. 1:16 1:17 1:18 1:19 1:21 1:23 1:21 1:22 1:23 1:25 1:28 1:31 1:18 1:18 1:18 1:19 1:21 1:23 1:23 1:23 1:23 1:25 1:28 1:31 1:10 1:11 1:12 1:13 1:14 1:15 1:13 1:13 1:13 1:13 1:14 1:15 +3 +4 +5 +6 +7 +8 +3 +4 +5 +6 +7 +8

  6. The Out-of-Sync Problem When number of stations increases Fastest station sends beacons less frequently Stations out of synchronization

  7. Two Types of Out-of-Sync • Fastest-station out-of-sync – fastest station is out of sync with all others. • k-global out-of-sync – k percent of the n(n-1)/2 links/pairs are out of sync. • Questions: How often? For how long?

  8. Fastest-station out-of-sync (1) • Clock1 and Clock2: two fastest clocks • d = their difference in accuracy • T = length of beacon interval (0.1 sec.) • Clock drift: d*T per beacon interval. • If there is no beacon from fastest station in /(d*T) intervals, fastest-station out of sync occur. T

  9. Fastest-station out-of-sync (2) • How often may it occur? • Once occurs, how long may it last? • H = # beacon intervals with F.S. out-of-sync. • L = # beacon intervals between async periods. • E(H) = ? E(L)? H L

  10. Fastest-station out-of-sync (3) • n = number of stations. • W + 1 = size of beacon window. • P = P(n,w) = prob(fastest station wins beacon contention) 01 w    W + 1

  11. p = P(n,W) = ? P(n,W,k) = prob(F.S. succeeds | it sends at slot k) k 0 w      

  12. P(n,w,k) = prob(F.S. succeeds | it sends at k) = ? k 0 w       b n-1 stations k slot 0 i i+b-1 w             #stations x≥2 y n-x-y             #stations y x≥2 n-x-y P(n-x-y, w-i-b, k-i-b)

  13. P(n,W,k) = ?

  14. E(H) = ? • H = # beacon intervals with F.S. out-of-sync. H L

  15. E(L) = ? • ei: F.S. sends another beacon after i intervals. τ = /(d*T) i H L

  16. E(L) = ?

  17. Prob(Fastest station sends a beacon)

  18. How often does fastest-node get out of sync with others?

  19. Percentage of time fastest station out of sync with all others 802.11a 54 Mbps ∆ = 224 s d = 0.003%

  20. How often does 25%-async occur?

  21. Percentage of time with 25 percent of links out-of-sync 802.11a 54 Mbps ∆ = 224 s d = 0.01%

  22. How to fix it? • Desired properties: simple, efficient, and compatible with current 802.11 TSF. • Causes of out-of-sync • Unidirectional clocks • Equal beacon opportunity • Single beacon per interval • Beacon contention (collision)

  23. Improve fastest station’s chance • Let the fastest station contend for beacon generation more frequently than others.

  24. Adaptive Clock Sync Protocol • Station x participates in beacon contention once every C(x) intervals. • Initially, C(x) =1. Always, 1 < C(x) < Cmax. • Dynamically adjust C(x): x x C(x) +1 faster C(x) -1 slower

  25. Once the protocol converges Fastest station, C(x) =1 Other stations, C(x) = Cmax (Cmax= ?)

  26. What if the fastest node leaves the IBSS? • The previously second fastest now becomes the fastest. Its C(x) will decrease to 1.

  27. What if a new fastest node enters the IBSS? • The previously fastest now no longer the fastest. Its C(x) will increase to Cmax.

  28. Compatible with current TSF • Suppose some nodes do not implement the new protocol.

  29. Performance • 802.11 Performance of TSF • ATSP ATSP.pdf

  30. Performance of TSF

  31. Performance of ATSP

  32. Summary • Showed: the IEEE 802.11 Timing Sync Function (TSF) is not scalable. • Proposed: a simple remedy compatible with the current TFS.

  33. What’s Next? • It’s 25 μs, stupid! • How to deal with MANET? transmission range

More Related