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Topology Control

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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.