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Transcript of 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?