1. Is IEEE 802.11 TSF Scalable? L. Huang, T.H. Lai,
On the scalability of IEEE 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
12:01
12:00
12:01
12:02
faster
adopts
slower
not adopts

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

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) w
  
W + 1

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

12. P(n,W,k) = ? k w       n-1 stations b k slot i i+b-1 w   w
  
  
n-1 stations b k
slot i
i+b-1 w
  
  
  
  
#stations
x> 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)

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
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
54 Mbps
∆ = 224 s
d = 0.01%

22. How to fix it?
Desired properties: simple, efficient, and compatible with current 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
faster
C(x) +1
slower
C(x) -1

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 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