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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?.

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Is IEEE 802.11 TSF Scalable?

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  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

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