S-MAC S-MAC Sensor Medium Access Sensor Medium Access

Control ProtocolControl Protocol

An Energy Efficient MAC protocol An Energy Efficient MAC protocol for Wireless Sensor Networksfor Wireless Sensor Networks

OutlineOutline

IntroductionIntroductionDesign ConsiderationsDesign ConsiderationsMain sources of energy inefficiencyMain sources of energy inefficiencyCurrent MAC designCurrent MAC designS-MACS-MACProtocol implementation in a test-bedProtocol implementation in a test-bedDiscussionDiscussionConclusion and future workConclusion and future work

Wireless Sensor NetworksWireless Sensor Networks

Application specific wireless networks for Application specific wireless networks for monitoring, smart spaces, medical monitoring, smart spaces, medical systems and robotic exploration systems and robotic exploration

Battery operated and power limited sensor Battery operated and power limited sensor devicesdevices

Large number of distributed nodes Large number of distributed nodes deployed in an ad-hoc fashiondeployed in an ad-hoc fashion

Design ConsiderationsDesign Considerations

Primary attributes:Primary attributes:Energy EfficiencyEnergy Efficiency

often difficult to recharge or replace batteriesoften difficult to recharge or replace batteries

prolonging the network lifetime is importantprolonging the network lifetime is important

ScalabilityScalability Some nodes may die or new nodes may joinSome nodes may die or new nodes may join

Secondary attributes:Secondary attributes:Fairness, latency, throughput and bandwidthFairness, latency, throughput and bandwidth

Sources of Energy InefficiencySources of Energy Inefficiency

CollisionCollision

OverhearingOverhearing

Control packet overheadControl packet overhead

Idle listeningIdle listening

Existing MAC DesignExisting MAC Design

Contention-based protocolsContention-based protocols• IEEE 802.11 – Idle listeningIEEE 802.11 – Idle listening• PAMAS – heavy duty cycle of the radio, PAMAS – heavy duty cycle of the radio,

avoids overhearing, idle listening avoids overhearing, idle listening

TDMA based protocolsTDMA based protocols

Advantages - Reduced energy consumptionAdvantages - Reduced energy consumption

Problems – requires real clusters, Problems – requires real clusters,

and does not support scalability and does not support scalability

Design goal of S-MACDesign goal of S-MAC

Reduce energy consumptionReduce energy consumption

Support good scalability Support good scalability

Self-configurableSelf-configurable

S-MACS-MAC

Tries to reduce wastage of energy from all Tries to reduce wastage of energy from all four sources of energy inefficiencyfour sources of energy inefficiency

Collision – by using RTS and CTSCollision – by using RTS and CTS Overhearing – by switching the radio off Overhearing – by switching the radio off

when transmission is not meant for that when transmission is not meant for that nodenode

Control Overhead – by message passingControl Overhead – by message passing Idle listening – by periodic listen and sleepIdle listening – by periodic listen and sleep

Is the improvement free of cost?Is the improvement free of cost?

NoNo

In exchange there is some reduction in In exchange there is some reduction in per-hop fairness and latencyper-hop fairness and latency

But does not reduce end-to-end fairness But does not reduce end-to-end fairness and latencyand latency

Is it important for sensor networks?Is it important for sensor networks?

Network AssumptionsNetwork Assumptions

Composed of many small nodes deployed Composed of many small nodes deployed in ad hoc fashionin ad hoc fashion

Most communication will be between Most communication will be between nodes as peers, rather than a single base nodes as peers, rather than a single base stationstation

Nodes must self-configureNodes must self-configure

Application AssumptionsApplication Assumptions

Dedicated to a single applicationDedicated to a single applicationor a few collaborative applicationor a few collaborative application

Involves in-network processing to reduce Involves in-network processing to reduce traffic and increase life timetraffic and increase life time

Applications will have long idle periods Applications will have long idle periods and can tolerate some latencyand can tolerate some latency

Components of S-MACComponents of S-MAC

Periodic listen and sleepPeriodic listen and sleep

Collision and Overhearing avoidanceCollision and Overhearing avoidance

Message passingMessage passing

Periodic Listen and SleepPeriodic Listen and Sleep

Each node goes into periodic sleep mode Each node goes into periodic sleep mode during which it switches the radio off and during which it switches the radio off and sets a timer to awake latersets a timer to awake later

When the timer expires, it wakes up When the timer expires, it wakes up

Selection of sleep and listen duration is Selection of sleep and listen duration is based on the application scenariosbased on the application scenarios

Neighboring nodes are synchronized Neighboring nodes are synchronized togethertogether

Contd….Contd….

Nodes exchange schedules by broadcastNodes exchange schedules by broadcast

Multiple neighbors contend for the mediumMultiple neighbors contend for the medium

Once transmission starts, it does not stop Once transmission starts, it does not stop until completeduntil completed

A B C D

Choosing and Maintaining Choosing and Maintaining SchedulesSchedules

Each node maintains a schedule tableEach node maintains a schedule table

Initial schedule is establishedInitial schedule is established SynchronizerSynchronizer FollowerFollower

Rules for joining a new nodeRules for joining a new node

Maintaining SynchronizationMaintaining Synchronization

Needed to prevent clock driftNeeded to prevent clock driftPeriodic updating using a SYNC packetPeriodic updating using a SYNC packet

Receivers adjust their timer countersReceivers adjust their timer countersListen interval divided into two partsListen interval divided into two parts

Each part further divided into time slots Each part further divided into time slots

Sender Node ID Next-Sleep Time

SYNC Packet

Timing Relationship Timing Relationship

Collision AvoidanceCollision Avoidance

Similar to IEEE 802.11 using RTS/CTS Similar to IEEE 802.11 using RTS/CTS mechanismmechanism

Perform virtual and physical carrier sense Perform virtual and physical carrier sense before transmissionbefore transmission

• RTS/CTS addresses the hidden terminal RTS/CTS addresses the hidden terminal problemproblem

• NAV –indicates how long the remaining NAV –indicates how long the remaining transmission will be.transmission will be.

Overhearing AvoidanceOverhearing Avoidance

Interfering nodes go to sleep after they hear Interfering nodes go to sleep after they hear the RTS or CTS packetthe RTS or CTS packet

The medium is busy when the NAV value is The medium is busy when the NAV value is not zero not zero

All immediate neighbors of sender and All immediate neighbors of sender and receiver should go to sleepreceiver should go to sleep

Message PassingMessage Passing

What is a message?What is a message?

Transmitting a message as a long packetTransmitting a message as a long packet

High retransmission costHigh retransmission cost

Fragmentation into small packetsFragmentation into small packets

High control overheadHigh control overhead Solution Solution DisadvantageDisadvantage

Protocol ImplementationProtocol Implementation

Test bedTest bed• Rene motes developed at UCBRene motes developed at UCB• They run TinyOS, an event–driven operating They run TinyOS, an event–driven operating

systemsystem Two types of packetsTwo types of packets• Fixed size data packets with header(6B), Fixed size data packets with header(6B),

payload(30B) and CRC(2B)payload(30B) and CRC(2B)• Control packets (RTS and CTS), header(6B)Control packets (RTS and CTS), header(6B)

(2B) CRC(2B) CRC

MAC modules implementedMAC modules implemented

Simplified IEEE 802.11 DCF – physical Simplified IEEE 802.11 DCF – physical and virtual carrier sense, backoff and retry, and virtual carrier sense, backoff and retry, RTS/CTS/DATA/ACK packet exchange RTS/CTS/DATA/ACK packet exchange and fragmentation supportand fragmentation support

Message passing with overhearing Message passing with overhearing avoidanceavoidance

The complete S-MAC – all the features are The complete S-MAC – all the features are implementedimplemented

Conclusions and Future workConclusions and Future work

S-MAC has good energy conserving S-MAC has good energy conserving properties comparing to IEEE 802.11properties comparing to IEEE 802.11

Future workFuture work

Analytical study on the energy Analytical study on the energy consumption and latencyconsumption and latency

Analyze the effect of topology changesAnalyze the effect of topology changes

Our ProjectOur Project

Implementing S-MAC on TinyOS 1.0Implementing S-MAC on TinyOS 1.0

Incorporating multicasting with S-MACIncorporating multicasting with S-MAC

Directed Diffusion and S-MACDirected Diffusion and S-MAC

S-MAC can be incorporated into the S-MAC can be incorporated into the directed diffusion paradigmdirected diffusion paradigm