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Cross-Layer Scheduling for Power Efficiency in

Wireless Sensor Networks

Mihail L. SichitiuDepartment of Electrical and Computer Engineering

North Carolina State University

IEEE INFOCOM 2004

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Outline

Wireless Sensor NetworksAssumptionsThe Steady State PhaseThe Setup and Reconfiguration PhaseSimulation ResultsConclusion

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Wireless Sensor Networks

There are several power saving scheduling algorithms proposed in different layers.

The main power savings in these papers results from eliminating (almost completely) idle listening and collisions in the sensor network.

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Sources of power wastage in WSNs

Idle listening Retransmissions resulting from collisions Control packet overhead

RTS, CTS, ACK…

Unnecessarily high transmission power Results in higher power consumption and also

increase the interference at other nodes. Sub-optimal utilization of the available

resources

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Assumptions

Periodic monitoring (i.e., one sample taken every T seconds).

Common period T known in the entire WSN. Neighboring nodes are synchronized to each

other. Rate of changes in network topology or WSN

objective is low compared to the monitoring period.

Each node generates one flow toward the base station.

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State Diagram for each data flow

A different query or query parameters

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Proposed Scheduling Approach

The setup and reconfiguration phase Its goal is to set up the schedules that will be

used during the steady state phase.

The steady state phase Similar to the forwarding phase. It utilizes the schedule established in the setup

and reconfiguration phase to forward the data to the base station.

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The Steady State Phase

3 actions can be scheduled by each sensor node: Sample (read sensor)

This sample will be forwarded along the (generally multihop) path to the base station.

Transmit (either own sensor reading or forward) All nodes on the path of a flow, except for the base st

ation, have a transmit action associated with the flow. Receive (to be further forwarded)

All nodes on the path of a flow, except for the source node, have a receive action associated with the flow.

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The Steady State Phase

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Example of 10 node sensor and a possible distributed schedule

Node 1 :

take a sample(5ms), send it(5ms), then go to sleep for 990ms.

Node 2 : needs to be awake for 10ms to forward the message from node 1 (receive for 5ms, then forward for 5ms), then take its measurement and forward it ( another 10ms), then go to sleep for 980ms.

Perfect synchronization

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The Steady State Phase

When a transmission fails If the application tolerates it, we can simply igno

re lost packets. In case the MAC layer supports retransmissions,

enough time can be reserved in the schedule for several (but a limited number of) retries.

The schedule can have one (or several) special spare cycle(s) reserved specifically for retransmissions.

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The Setup and Reconfiguration Phase

Proceeds in 2 steps The Route Select Step

A route to the base station is selected by the routing algorithm.

The Route Setup Step Schedules are being setup along the chosen

route. If it fails, a new route is selected. A special route setup (RSETUP) packet will be

sent on the selected route from the source of the flow to the base station.

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The Setup and Reconfiguration Phase

If the packet is postponed for more than a period, a route error (RERR) packet is sent back to the source.

If the route setup (RSETUP) packet arrives at the base station, a route acknowledge (RACK) packet will be sent from the base station to the source on the reverse path.

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Comments

The route setup phase, while being a typical Layer-3 function, relies heavily on Layer-2 information and properties.

The schedule can take full advantage of the spatial reuse inherent in ad-hoc networks. In Fig. 3, nodes 1 and 7 are scheduled to send at the

same time, as their transmissions do not interfere.

Schedule tables should be able to maintain “per flow” information (two entries for every flow).

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Simulation Results

Custom simulator (existing simulators cannot simulate hundreds of nodes for periods of months and years).

Network lifetime = when 50% of the nodes cannot report to the base station (either the batteries are depleted or no available routes)

Will compare 3 technologies: Always On (sensors in

receive mode when not transmitting).

802.11 power savings mode

Proposed scheduling approach.

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Simulation Setup

Will use a base case and vary one parameter at a time. Base case parameters:

Nodes: 100 Transition from “off” to “on”: 3ms

Area: 100x100m

Transmission Radius: 25m

Current in TX mode: 17mA

One sample sent every: 60s

802.11 PSM beacon interval: 500ms

Current in RX mode: 10mA

Current in idle mode: 10µA

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Simulation Results

Network Lifetime

Mean Std. deviation

No Power Savings

8.3 days 4 minutes

802.11 PSM 3.2 months 7.5 days

Power scheduling

24.2 months 5 months

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Dependency of the network lifetimes on the number of nodes for a constant deployment area.

Adding new nodes in thesame area does not increase the depth of the network, and thus the forwarding overhead remains constant with the number of nodes.

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Dependency of the network lifetimes on the number of nodes for a constant density.

(1) The nodes close to the base station tend to deplete their batteries first.

(2) The rest of the network cannot bridge the gap around the base station.

(3) Resulting in premature system failure (most nodes still have plentiful energy supplies, but no routes to the base station).

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Dependency of the network lifetimes on the measurement period of the network.

A larger period enables longer sleep times and, correspondingly, increased improvements in the lifespan of the sensor network.

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Dependency of the network lifetimes on the power consumption in idle mode.

(1) The power savings of the method result directly from the difference between the power of the system in idlemode and the power in receive mode.

(2) When the gap betweenidle mode and receive mode closes, the power savings of the proposed approach decrease.

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Conclusion

Present a new distributed scheduling algorithm for stationary, continuous monitoring wireless sensor networks.

The algorithm requires the collaboration of both the routing with MAC layers.