adaptive topology discovery in hybrid wireless networks ranveer chandra cornell university joint...
Post on 21-Dec-2015
215 views
TRANSCRIPT
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?