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) Node exchanging info via beacons Overhead: 3 bits per beacon (550 bits) Assumption: unique fastest clock

window

beacon interval

DS, Bridges, Covered, Uncovered Nodes

DS

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

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

A

B

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 Issues1. 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.

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

Am I the Fastest?

1:00

12:01

3:45

8.16

1.00

1.01

7:591.01

0:591:33

1:32

1:31 1:31

1:301.0010:011:35

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 with a far smaller timing.

Rule 4: Let gateways enter DS.

12:05 12:04 12:0312:32 12:30 12:20

How fast can gateways be recognized?

Depends on the drift rate difference 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.

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

Maximum Clock Drift – 802.11b vs. DS-based

802.11b

DS-based

Summary

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

Conclusion?