1. Time Synchronization in 802.11-based MANETs
Ten H. Lai
Ohio State University

2. Out-of-sync problem in MANETs
More sever than in IBSS because of hidden terminals.
Recall: causes of out-of-sync
Unidirectional clocks
Equal beacon opportunity
Single beacon per interval
Beacon contention (collision)

3. Basic Ideas
Select a subset of nodes to generate beacons more frequently than the rest.
What subset?
fastest node + (connected) dominating set

4. Dominating Sets A set of nodes that covers the entire graph. connected

5. Constructing CDS’s Many existing algorithms.
Layer 3 algorithms – useful for routing, useless for our purpose.

6. A New CDS Algorithm Embedded in TSF (time sync function)
Node exchanging info via beacons
Overhead: 3 bits per beacon (550 bits)
beacon interval
window

7. DS, Bridges, Covered, Uncovered Nodes

8. Constructing a CDS: basic idea
Initially, DS contains a single node.
The fastest node enters DS.
Bridges keep entering DS until no more bridges. DS

9. Design Issue #1
How to recognize the fastest node, bridges, DS nodes, covered nodes, uncovered nodes thru beacons? SA
Timestamp
Beacon

10. Design Issue #2
How to minimize the number of bridges entering DS?

11. Design Issue #3
Cope with topology change and node mobility. B A A B

12. Design Issue #4
How to merge two subnets? Easy & hard. ?

13. Design Issue #5: MANET Formation
How to form a MANET from scratch? ?

14. Another way of MANET formation?

15. Assumptions Formation: MANET initiated by a single node.
Connectivity: MANET remains connected.

16. Summary of Design Issues
How to recognize the fastest node and bridges?
How to control the number of bridges entering DS?
How to cope with topology change and node mobility?
How to merge subnets?

17. Initialization
Rule 1:
Let the starting node enter the DS.

18. Am I the Fastest? Rule 2: A node x recognizes itself as the fastest if
T(beacons) < T(x)
for the last k received beacons.
The fastest enters DS
12:01
1:31
1.00
1:31
1.01
7:59
1:33
1:00
1:35
10:01
1.00
0:59
1:30
3:45
1:32
8.16
1.01

19. Solution for Design Issue #1
How to recognize fastest node and bridges, DS nodes, covered nodes, uncovered nodes thru beacons? SA
Timestamp
Beacon

20. Adding Bridges to DS Rule 3:
In each beacon interval, let bridge i enter DS with probability P(i).
Desired properties of P(i)? DS

21. Does it construct a CDS? R1. The starting node enters DS.
R2. The fastest node enters DS.
R3. Each bridge enters DS with a probability.
DS, yes.
CDS, not necessarily.

22. How to make it connected?
Gateway: a covered node receiving a beacon from a DS node with a far smaller timing.
Rule 4:
Let gateways enter DS.
12: : :20
12: : :03

23. How fast can gateways be recognized?
Depends on the drift rate between fastest node and A.
The higher the drift rate, the easier and faster to recognize gateways. A

24. Is the resulting DS always connected?
Not necessarily
Not a problem as far as clock sync is concerned.

25. What if we do need a connected DS?
Is it possible to always construct a CDS using only beacons?
Yes.

26. A problem: entrance only, no exit.
R1. The starting node enters DS.
R2. The fastest node enters DS.
R3. Each bridge enters DS with a probability.
R4. Each gateway enters DS.

27. Exit Rules R1. The starting node enters DS.
R2. The fastest node enters DS.
R3. Each bridge enters DS with a probability.
R4. Each gateway enters DS.
R2’. If no longer the fastest,
leaves the DS.

28. Exit Rules R1. The starting node enters DS.
R2. The fastest node enters DS.
R3. Each bridge enters DS with a probability.
R4. Each gateway enters DS.
R3’ & R4’. Leaves DS after a random amount of time.

29. TSF max time drift

30. DS-Based TSF

31. Maximum Clock Drift – 802.11b vs. DS-based

32. A different approach
A Clock Synchronization Algorithm for Multihop Wireless Ad Hoc networks
J.P. Sheu, C.M. Chao, C.W. Sun, NCU
ICDCS’04

33. Basic Idea 1
Adjust C(x) according x’s rank in speed in its neighborhood.
N(x) = number of neighbors
Slower(x) = number of slower neighbors
C(x) = {max(1, N(x)) / max(1, Slower(x))}^α x

34. Basic Idea 2 Automatic self-time-correcting
Suppose t2-t1 > t2’-t1’ (T > T’)
B falls behind A by T-T’ per T’ μs, or 1 μs per k = T’ / T-T’ μs
B moves its clock 1 μs ahead per each k μs t1 T t2
Clock A T’
Clock B
t1’
t2’

35. A subtle detail
In measuring T, A should not be synchronized between t1 and t2. Same for B?
Variable SyncNum
Carries SyncNum in beacon t1 T t2
Clock A T’
Clock B
t1’
t2’

36. Automatic Self-Time-Correcting

37. Summary Proposed: a DS-based clock sync protocol
By-product: an algorithm for constructing DS.
DS: mostly connected, occasionally not.
A different approach
What’s next?