Adaptive Topology Discovery in Hybrid Wireless Networks

Ranveer ChandraCornell University

Joint work with Christof Fetzer and Karin Högstedt

AT&T Labs-Research

Problem Description

• Hybrid Wireless Network: – Ad hoc network with slow moving nodes– Presence of at least one relatively static node

• Nearly all the links are bidirectional

• A coordinator wants to know the topology of the network.– All the nodes– All the links

Hybrid Wireless Network

• Applications: Home, office networks, mesh networking

CoordinatorA

B

C

D

E

(A, D)(A, B)(B, D)(B, C)(D, E)(E, C)

Issues In Ad Hoc Networks

• Communication is expensive

• Unreliable links

• No a priori knowledge of neighbors

• Possibility of unidirectional links

• Links could become stale due to mobility

Outline of the Talk

• Introduction

• Basic protocol

• Enhancements for:– Unreliable broadcasts– Low mobility (low overhead)– High mobility (overhead depends on mobility)

• Performance

• Conclusion

Protocol Overview

• Diffusion Phase– Propagates request through network– Establishes neighborhood information– Builds tree structure

• Gathering Phase– Propagates neighborhood information back

along tree structure

Diffusion Phase

Acoord

D

B

C

Parent:Children:

Parent:Children:

Parent:Children:

Parent:Children:

Parent:Children:

Diffusion Phase

Acoord

D

B

C

Parent:Children:

Parent:Children:

Parent:Children:

Parent:Children:

Parent:Children:

coord

Diffusion Phase

Acoord

D

B

C

Parent:Children:

Parent:Children:

Parent:Children:

Parent:Children:

Parent: coordChildren:

coord

coord

coord

A

A

A

Diffusion Phase

Acoord

D

B

C

Parent:Children: A

Parent:Children:

Parent: AChildren:

Parent: AChildren:

Parent: coordChildren:

A

A

B

B

Diffusion Phase

Acoord

D

B

C

Parent:Children: A

Parent: BChildren:

Parent: AChildren:

Parent: AChildren:

Parent: coordChildren: B

B

D

B

Diffusion Phase

Acoord

D

B

C

Parent:Children: A

Parent: BChildren:

Parent: AChildren:

Parent: AChildren: D

Parent: coordChildren: B,

A

A

C

Gathering Phase

Acoord

D

B

C

LA: coord, B, C

dLA:

Lcoord: A

dLcoord:

LB: A, D

dLB:

LD: B, C

dLD:

LC: A, D

dLC:

Gathering Phase

Acoord

D

B

C

LDLC

LA: coord, B, C

dLA: LC

Lcoord: A

dLcoord:

LB: A, D

dLB: LD

LD: B, C

dLD:

LC: A, D

dLC:

Gathering Phase

Acoord

D

B

C

LB, LD

LA: coord, B, C

dLA: LB, LC, LD

Lcoord: A

dLcoord:

LB: A, D

dLB: LD

LD: B, C

dLD:

LC: A, D

dLC:

Gathering Phase

Acoord

D

B

C

LA: coord, B, C

dLA: LB, LC, LD

Lcoord: A LB: A, D

dLB: LD

LD: B, C

dLD:

LC: A, D

dLC:

LA, LB, LC, LD

dLcoord: LA, LB, LC, LD

Gathering Phase

C

E D

A

B

Problem:What if node B fails?How long should A wait?

?

Node Or Link Failures• Node sends its response if:

– it is a child, or– all its children have replied, or– it waited for (height - depth)*(b_time + u_time)

amount of time.

(height is the total height of the tree,

depth is the current distance to the root

b_time is the time to bcast a message to a nbr

u_time is the time to unicast a message to a nbr)

Bad Performance

Broadcasts in 802.11 are unreliable

Fraction of Links not Discovered

0

0.05

0.1

0.15

0.2

0.25

-10 dBm -6 dBm -4 dBm

Using GloMoSim:- 200m x 200m- 50 nodes- stationary

Robust Broadcast

• Modified RTS/CTS scheme for broadcasts

• Retry with back-off until parent ACKed

ACKBCAST

A

DB Cretry withbackoff

retry withbackoff

Bad Performance In Mobile Networks

Discovers nearly all links when nodes arestationary

Performance suffers in a mobile network

Building A Mesh

• Use the robustness of a mesh– A node can have multiple parents, max ‘k’

• Modified Algorithm:– On receiving the the first ‘k’ distinct

broadcasts, mark them as parents– Use distance from coordinator to avoid

loops– A node sends ‘k’ broadcasts if it has ‘k’

parents robustness

Diffusion Phase

Acoord

D

B

C

Parent:Children: A

Parent: B, CChildren:

Parent: AChildren:

Parent: AChildren: D

Parent: coordChildren: B,

A

A

C

Diffusion Phase

Acoord

D

B

C

Parent:Children: A

Parent: B, CChildren:

Parent: AChildren: D

Parent: AChildren: D

Parent: coordChildren: B,

B, C

B, C

C

Mesh Advantages

• Tolerates lossy links

• Tolerates unreliable nodes

• Handles mobility

A message from ‘A’reaches ‘S’ through3 different routes

A

S

Mesh Failures

What if links to all parents fail?

Send information along alternate paths

• If a node is unable to send to any parents, it enters Panic mode and broadcasts its message to all its nbrs.

Panic Mode

When a node receives a bcast from a node in panic mode,

• it removes sender from its list of parents, and• if message gives new information, and

– if its parent list is non-empty,• it proceeds as before

– if its parent list is empty,• it enters “Panic Mode” and bcasts to all nbrs

Panic Mode: Example

CB

FE

D

A

Note:B does not resendinfo from D if italready received it from E.

Optimized Algorithm: Performance

• Simulation Environment : GloMoSim– 200m x 200m area– 50 nodes– Random waypoint mobility model– 802.11 MAC protocol– Varying speeds and power of nodes– Assume bidirectional links

Links Discovered: -10dBmWithout Panic Mode

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Speed(m/s)

Fra

cti

on

1-parent

2-parents

3-parents

4-parents

5-parents

Links Discovered: -10dBm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Speed(m/s)

Frac

tion

1-parent

2-parents

3-parents

4-parents

5-parents

With Panic Mode

Message Overhead: -10dBm

0

10

20

30

40

50

60

70

80

90

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Speed(m/s)

#M

es

sa

ge

s 1-parent

2-parents

3-parents

4-parents

5-parents

Without Panic

Message Overhead: -10dBm

0

50

100

150

200

250

300

350

400

450

500

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Speed(m/s)

#M

es

sa

ge

s 1-parent

2-parents

3-parents

4-parents

5-parents

With Panic Mode

Links Discovered: -4dBm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

Speed(m/s)

Fra

cti

on 1-parent

2-parents

3-parents

4-parents

5-parents

With Panic Mode

Message Overhead: -4dBm

0

50

100

150

200

250

300

350

400

450

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

Speed(m/s)

#M

es

sa

ge

s 1-parent

2-parents

3-parents

4-parents

5-parents

With Panic Mode

Conclusions• Algorithm

– Discovers all links & nodes in most slow moving networks

– O(N) message complexity in such networks– Adaptive: The robustness of the mesh is a parameter

of the algorithm

• Target scenario: – Not all nodes are moving… Some moving slowly,

most of them stationary O(N)

Questions?