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Is IEEE 802.11 Scalable?

Is IEEE 802.11 Scalable?. IEEE 802.11: how large can it be?. Bandwidth: Up to 54 Mbps Good for a few hundred nodes Timing Synchronization Function Not scalable. Basic Service Set (BSS). BSS: building block of 802.11 LAN Infrastructure BSS Independent BSS (IBSS) -- Ad Hoc. AP. IBSS.

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Is IEEE 802.11 Scalable?

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  1. Is IEEE 802.11 Scalable?

  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

  3. Basic Service Set (BSS) • BSS: building block of 802.11 LAN • Infrastructure BSS • Independent BSS (IBSS) -- Ad Hoc AP IBSS Infrastructure BSS

  4. 802.11 Timers (Clocks) • Timer: 64 bits, ticking in microseconds. • Accuracy: within + 0.01%, or +100 ppm. • Time synchronization needed for: • Frequency hopping • Power management • ∆ = max tolerable difference between clocks.

  5. Frequency Hopping f1 f2 f3 f4 f5

  6. Power Saving Beacon interval sleep sleep time Beacon window ATIM window

  7. 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

  8. 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

  9. 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

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

  11. 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 links are out of sync. • Questions: How often? For how long?

  12. 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. • In /(d*T) intervals, fastest-station will be out of sync with all others. T

  13. Fastest-station out-of-sync (2) • n = number of stations. w = size of beacon window. • P’(n,w) = prob(fastest station wins beacon contention)

  14. Prob(Fastest station sends a beacon)

  15. Fastest-station out-of-sync (3) • H = # beacon intervals with F.S. out-of-sync. • L = # beacon intervals between async periods. • E(R) = E(H)/[E(H)+E(L)] = percent of time in which the fastest station is out of sync with all others. H L

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

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

  18. Global Asynchronism • Lower bound on E(H) • Upper bound on E(L) H L

  19. How often does 25%-async occur?

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

  21. Percentage of channels out of sync

  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 of Modified TSF • 802.11 Performance of TSF • ATSP ATSP.pdf • TATSP Performance of Modified TSF

  30. Performance of TSF

  31. Performance of Modified TSF

  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? • IBSS: single-hop • MANET: multihop ?? transmission range

  34. 欲知後事如何, 且待下回分解.

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