adaptive topology discovery in hybrid wireless networks ranveer chandra cornell university joint...

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?