1-1 Topology Control. 1-2 What’s topology control?
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Transcript of 1-1 Topology Control. 1-2 What’s topology control?
1-1
Topology Control
1-2
What’s topology control?
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What’s topology control?
When nodes are deployed, how do they organize into a network? And how do they maintain this organization over the lifetime of the system?
Neighbor-discovery protocol is important. If neighborhood is sparse, use all
neighbors. What if neighborhood is dense?
Use a subset of neighbors. How?
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Two Important Goals
Coverage: ensures critical events can be detected/monitored.
Connectivity: ensures data can be propagated over the network.
Tunable parameters: Node mobility affects both coverage and
connectivity. Transmission power control. Sleep schedules.
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Over-Deployed Networks
Redundant nodes. Nodes are inexpensive. Deployment is remote. Position of sensors is not critical.
Advantages: Longer lifetime. Higher robustness. Adjustable connectivity/coverage.
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Approaches to topology control Adjust transmit power. Turn nodes on/off.
Approaches that follow are sleep-based approaches that target connectivity.
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ASCENT
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ASCENT: scenario
Ad hoc deployment. Energy limitations. Arbitrarily large scale. Unattended operation.
Assume CSMA.
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ASCENT: goals
Self-organization of nodes into topology that allows sensing coverage and communication under tight energy constraints.
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ASCENT: approach
Nodes turn themselves on/off depending on assessment of operating conditions. Neighborhood density. Data loss.
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State diagram
Test Active
Passive Sleep
After Tt
After Tt: Nbors > NT orLoss > LT
After Tp:Nbors<NTAndLoss>LT orHelp After Tp
After Ts
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In “test” state:
Signaling (e.g., neighbor announcements).
After Tt, goes to “active”. Or, if before Tt, number of
neighbors>NT or average data loss (Tt) > average data loss (T0), go to “passive”.
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In “passive” state:
After Tp, go to “sleep” or, If neighborhood is sparse, loss > LT, or
“help” from “active” neighbor, go to “test”.
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In “sleep”:
Turn off radio. After Ts, go to “passive”.
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In “active”:
Node does routing and forwarding. Sends “help” if data loss > LT. Stays on until runs out of battery!
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Considerations
Why passive and test states? Why once in active, a node runs until
battery dies? How to set parameters?
NT, LT. Tt, Tp, Ts.
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Neighborhood and loss
Node is neighbor if directly connected and link packet loss < NLS.
NLS is adjusted according to node’s number of neighbors.
Average loss date uses data packets only.
Packet is lost if not received from any neighbors.
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Performance evaluation
Modeling, simulation, experimentation. Metrics:
Packet loss. Delivery ratio. Energy efficiency. Lifetime.
• Time till 90% of transit nodes die.
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PEAS
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PEAS
Probing Environment, Adaptive Sleeping.
“Extra” nodes are turned off. Nodes keep minimum state.
No need for neighborhood-related state. PEAS considers very high node density
and failures are likely to happen.
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Bi-modal operation
Probing environment. Adaptive sleeping.
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PEAS state diagram
Working
Sleeping Probing
No reply for probe
Wakes up
Hears probe reply.
Sleep->Probe: randomized wake-up timer with exponential distribution.
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Probing
When node wakes up, enters probing mode.
Is there working node in range? Broadcasts PROBE to range Rp. Working nodes send REPLY (randomly
scheduled). Upon receiving REPLY, node goes back to sleep.
• Adjusts sleeping interval accordingly. Else, switches to working state.
Probing rate is adjusted over time based on the probe replies.
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Considerations
Probing range is application-specific. Robustness (sensing and communication)
versus energy-efficiency. Location-based probing as a way to
achieve balance between redundancy and energy efficiency.
Randomized sleeping time. Better resilience to failure. Less contention. Adaptive based on “desired probing rate”.
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Evaluation
Simulations. Simulated failures: failure rate and
failure percentage. Metrics:
Coverage lifetime. Delivery lifetime.
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Cross-Layer Issues
Relationship to routing and to MAC. Topology control <-> Routing:
Topology control provides network substrate for routing.
Topology control below routing layer. Routing considers only “active” nodes.
Topology control <-> MAC: Co-existence of MAC sleep schedules with
topology control sleep schedules.