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Improving Scalability of IEEE 802.11 TSF with Adaptive Clock Sync Protocol

This study explores the scalability issues in IEEE 802.11 TSF and proposes an Adaptive Clock Sync Protocol as a remedy. By enhancing beacon generation opportunities and dynamically adjusting timing intervals, the protocol aims to address synchronization discrepancies in large ad hoc networks. The findings shed light on optimizing TSF performance and maintaining synchronization amongst diverse nodes. The research offers valuable insights for enhancing the scalability and efficiency of IEEE 802.11 networks.

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Improving Scalability of IEEE 802.11 TSF with Adaptive Clock Sync Protocol

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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 win 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) = ? k 0 w       n-1 stations b k slot 0 i i+b-1 w             #stations x>2 y n-x-y

  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) 1 n Prob <

  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 • TATSP Performance of Modified TSF

  30. Performance of TSF

  31. Performance of ATSP

  32. Performance of Modified TSF

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

  34. What’s Next? • IBSS: single-hop better solution • MANET: multihop transmission range

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