Time Synchronization
in 802.11-based MANETs
Ten H. Lai
Ohio State University
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)
Basic Ideas

Select a subset of nodes to generate beacons
more frequently than the rest.

What subset?
fastest node + (connected) dominating set
Dominating Sets

A set of nodes that covers the entire graph.
connected dominating set
Constructing CDS’s


Many existing algorithms.
Layer 3 algorithms – useful for routing,
useless for our purpose.
A New CDS Algorithm



Embedded in TSF (time sync function)
Nodes exchanging info via beacons
Overhead: 3 bits per beacon (550 bits)
beacon interval
window
DS, Bridges, Covered, Uncovered Nodes
DS
Constructing a CDS: basic idea


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Initially, DS contains a single node.
The fastest node enters DS.
Bridges keep entering DS until no more bridges.
DS
Design Issue #1

How to recognize the fastest node, bridges,
DS nodes, covered nodes, uncovered nodes
thru beacons?
SA
Timestamp
Beacon
Design Issue #2

How to minimize the number of bridges
entering DS?
Design Issue #3

Cope with topology change and node mobility.
B
A
B
A
Design Issue #4

How to merge two subnets? Easy & hard.
?
Design Issue #5: MANET Formation

How to form a MANET from scratch?
?
Another way of MANET formation
?
Assumptions


Formation: MANET initiated by a single node.
Connectivity: MANET remains connected.
Summary of Design Issues
1.
How to recognize the fastest node and
bridges?
2.
How to control the number of bridges
entering DS?
3.
How to cope with topology change and node
mobility?
4.
How to merge subnets?
Initialization
Rule 1:
 Let the starting node
enter the DS.
Am I the Fastest?
1:31
12:01
1.00
1.01
7:59
1:31
1:00
1:33
1.00
10:01
1:35
0:59
3:45
1:30
8.16
1.01
1:32
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
Solution for Design Issue #1

How to recognize fastest node and bridges,
DS nodes, covered nodes, uncovered nodes
thru beacons?
SA
Timestamp
Beacon
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
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.
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.
12:05
12:32
12:04
12:30
12:03
12:20
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
Is the resulting DS always connected?

Not necessarily
Not a problem as far as clock sync is
concerned.
What if we do need a connected DS?

Is it possible to always construct a CDS using
only beacons?

Yes

Complicated
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.
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.
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.
802.11 TSF max time drift
DS-Based TSF
Problem, Problem, Problem!
???
A different approach (ASP)



A Clock Synchronization Algorithm for
Multihop Wireless Ad Hoc networks
J.P. Sheu, C.M. Chao, C.W. Sun, NCU
ICDCS’04
Basic Idea 1




Adjust C(x) according to 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
Basic Idea 2


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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
t2
T
Clock A
T’
Clock B
t1’
t2’
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
t2
T
Clock A
T’
Clock B
t1’
t2’
Automatic Self-Time-Correcting
MATSF (to be presented at MASS’05)
ASP (ICDCS’04)
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?