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

Speaker： jenchi

Outline

Introduction Proposed approach

The steady state phase The setup and reconfiguration phase

Simulation results Conclusion

Introduction

The characteristic of sensor nodes Very small form Be inexpensive and deployed in very large

numbers Once deployed, the sensor networks are

usually unattended The life of the sensor network is determined by

the life of its batteries Such a network is typically expected to work

for extended periods of time

Introduction

The power consumption in sensor networks Idle listening Retransmissions resulting from collisions Control packet overhead Unnecessarily high transmitting power Sub-optimal utilization of the available

resources

Introduction Two modes of sensor networks

Event driven sensor network Sensor nodes do not send data until a certain event oc

curs A forest fire monitoring application

difficulty： the sensor network is able to wake up the entire network when the event occurs

Continuous monitoring sensor network Data is sampled and transmitted at regular intervals

A temperature monitoring station Indeed, in order to detect an event, a continuous monit

oring scheme is necessary

Introduction

This paper presents an approach tailored specifically to the needs of sensor networks with continuous monitoring capabilities

The proposed scheme derives its power efficiency from eliminating idle listening and collisions in the sensor network

The need for such a scheme is highlighted and prompted by recent habitat monitoring applications

Proposed approach

To develop a framework for deterministic optimal energy conservation while maintaining the network real-time characteristics

Sensor nodes dynamically create on-off schedules in such a way that the nodes will be awake only when needed and asleep the rest of the time

Proposed approach

The proposed scheme can be decoupled into two distinct phases for each flow in the network The setup and reconfiguration phase

To set up the schedules that will be used during the steady state phase

The steady state phase It utilizes the schedule established in the setup

and reconfiguration phase to forward the data to the base station

Proposed approach

Assumptions The traffic is periodic, with the same period in

the entire network Each node originates only one packet in each

period To ensure that the control packets necessary

to set up schedules do not collide with the data packets forwarded, a two-level priority scheme MAC layer has to be used

Proposed approach

State diagram for each data flow in the network

The steady state phase

Assume that the network is perfectly synchronized

The nodes on the path forward DATA packets at the appropriate times

Each node on the path stores a schedule table that specifies when various actions have to take place

The steady state phase The three different actions

Sample The source node taking a data sample Be forwarded along the path to the base station

Transmit The action of transmitting a packet of the flow to

the next node on the path to the base station Receive

The reception of a packet The packet will be further transmitted to the next

node

The steady state phase

The actions are stored in a schedule table that has two columns• the first column： what type of action that has to be performed• the section column： when a certain action has to take place

The steady state phase

□

The setup and reconfiguration phase

The schedule setup algorithm for any flow proceeds in two steps Route select

A route to the base station is selected Route setup

Schedules are being setup along the chosen route

If it fails, a new route is selected

The setup and reconfiguration phase

The route select step The setup and reconfiguration algorithm is

independent of the underlying routing algorithm Power aware routing algorithms may be

preferable

The setup and reconfiguration phase

The route setup step A route setup (RSETUP) packet will be sent

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

To find a time when a DATA packet can be scheduled without colliding with other nearby transmissions

The RSETUP packet contain the node source and the duration of the packets on that flow

The setup and reconfiguration phase

A lower priority (lower than data on established paths) RTS/CTS handshake ensures that only non-interfering transmissions are scheduled at overlapping times

The length of the route setup packet is larger than the length of a data packet

Proposed priority scheme similar to 802.11 (SIFS/DIFS)

The setup and reconfiguration phase

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 three technologies: Always On (sensors in

receive mode when not transmitting).

802.11 power savings mode

Proposed scheduling approach.

Simulation results

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

Nodes: 100 Transition from “off” to “on”: 3msArea: 100x100m

Transmission Radius: 25m

Current in TX mode: 17mA

One sample sent every: 60sSynchronization precision: 1ms802.11 PSM beacon interval: 500ms

Current in RX mode: 10mACurrent in idle mode: 10µA

Simulation results Simulation results for base case

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

Simulation results—Dependency of the network lifetimes on the number of nodes for a constant deployment area

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Initial number of nodes

Simulation results—Dependency of the network lifetimes on the number of nodes for a constant density

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Initial number of nodes

Simulation results—Dependency of the network lifetimes on the measurement period of the network

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Measurement period of the sensor network(s)

Simulation results—Dependency of the network lifetimes on the power consumption in idle mode

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Current drawn in idle mode (mA)

Simulation results—Dependency of the network lifetimes on the precision of the synchronization algorithm

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Synchronization precision Δ [ms]

Conclusion

Presented a new distributed scheduling algorithm for stationary continuous monitoring sensor networks

Is fully distributed and works in tight cooperation with popular sensor networks routing and MAC families of protocols

For the right type of networks, it is shown via simulations that the network lifetime can be increased by orders of magnitude