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An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor Networks

Tijs van Dam, Koen LangendoenIn ACM SenSys 2003.

8/1/2005Hong-Shi Wang

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Contents

IntroductionRelated workDrawbacks of S-MACT-MACExperimentsConclusions and Future Work

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Introductions

Traditional MAC Protocols– Design to maximize packet throughput, minimize latency and

provide fairness

Protocol design for wireless sensor networks– focuses on minimizing energy consumption

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Related Work

TDMA-based protocol– Have advantage of energy conservation compared to

contention protocols, because there is no contention-introduced overhead and collisions

– But requires the nodes to form real communication clusters like LEACH Managing inter-cluster communication and interference is not an

easy task.

Contention-based protocol– simplicity– Energy consumption using this MAC is very high when nodes

are in idle mode

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Drawbacks of S-MAC

Active (Listen) interval – If message rate is less – energy is still wasted in idle-listening

S-MAC’ fixed duty cycle is NOT OPTIMAL

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T-MAC : Preliminaries (1)

Basic idea– To utilize an active and a sleep cycle, similar to S-MAC– To introduce an adaptive duty cycle by dynamically ending

the active part– An active period ends when no activation event has occurred

for a time TA Activation event

– The reception of any data on the radio (RTS, CTS, DATA, ACK)– The sensing of communication on the radio (overhearing)

– Difference in the duty cycle S-MAC - fixed duty cycle T-MAC – Dynamic duty cycle

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T-MAC : Preliminaries (2)

With normal MAC protocols, messages are spread out over the whole time frame

With S-MAC, active time is fixed With T-MAC, the active time is dynamically adjusted (i.e., be

shorten) by timing out on hearing nothing during some time period (TA)– TA determines the minimal amount of idle listening per frame

Active Active Active

Sleep Sleep

S-MAC

Active Active Active

Sleep Sleep

TA TATAT-MAC

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T-MAC : RTS Operation (1)

Contention Interval

In contention-based protocols, like IEEE 802.11, a back-off scheme is used: – Contention interval increases when traffic is higher.

In the T-MAC protocol, waiting and listening for a random time within a fixed contention interval – Tuned for maximum load.

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T-MAC : RTS Operation (2)

RTS Retries

No CTS reply for RTS?– Collision– The receiving node is prohibited from replying due to an

overhead RTS or CTS– Receiving node is asleep

Solutions:– Retransmit RTS if no answer– If there is still no reply after two retries, it should give up and

go to sleep

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T-MAC : Choosing TA

Determining TA

The interval TA must be long enough to receive at least the start of the CTS packet

TA > C+R+T– C –contention interval length; R–RTS packet length;– T –turn-around time, time between RTS end & CTS start– Larger TA increases the energy used– In experiments, used TA = 1.5 x (C + R + T)

A

B

C

contend

contend

RTS CTS DATA ACK

TA

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T-MAC : Overhearing Avoidance

~= S-MAC But implemented as an option in T-MAC Node goes to sleep after overhearing RTS/CTS of

other nodes communication– miss other RTS/CTS while sleeping– throughput decreases

Although overhearing avoidance saves energy, it must not be used when maximum throughput is required

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T-MAC: Asymmetric Communication (1)Early-Sleeping Problem – unidirectional (A to D) If node C looses contention because it overhears a CTS packet

from B to A, C must remain silent. Since D does not know of the communication between A and B,

its active time will end, and node D will go to sleep. Only at the start of the next frame will node C have a new

chance to send to node D Early-Sleeping Problem

– Node goes to sleep when a neighbor still has messages for it

A

B

C

D

contend

contend

RTS CTS DATA ACK

RTS?

active sleep

TA

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T-MAC: Asymmetric Communication (2)Future request-to-send (FRTS) Let others know that we still have a message for it, but cannot

access the medium; – C sends FRTS to future target of an RTS packet

– FRTS has duration field

FRTS might affect data; so, DATA postponed until FRTS is over; To prevent others from taking medium, A send DS packet;

A

B

C

D

contend

contend

RTS CTS DATA ACK

RTS

active

TA

DS

FRTS

active

TA > C+R+T+CTS_length

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T-MAC: Asymmetric Communication (3)Taking priority on full buffers When a node’s transmit/routing buffers are almost full, it may

prefer sending than receiving Receive RTS, send its own RTS to others instead of CTS limited form of flow control

A

B

C

D

contend

contend

RTS

CTS DATA ACK

active

RTS

contend

TA

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Experiments

S-MAC Vs. T-MAC

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Simulation setup and parameters

Simulator: OMNeT++ Built a network of 100 nodes in a 10 by 10 grid Energy consumption

–

S-MAC protocol– A frame length of one second, and with several lengths of the active

time, varying from 75 ms to 915 ms.

T-MAC protocol– Always used a frame length of 610ms and an interval TA with a len

gth of 15 ms

– Can optionally be deployed with overhearing avoidance, full-buffer priority, and FRTS

20 A while sleeping, 4mA while receiving and 10 mA while transmitting

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Homogeneous local unicast

Nodes send packets to one of their neighbors at random T-MAC: Used overhearing avoidance, but no FRTS or full-buffer

priority mechanisms

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Nodes-to-sink communication

Nodes send messages to a single sink node – Shortest path routing, no data aggregation

T-MAC: Used overhearing avoidance, FRTS or full-buffer priority mechanisms

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Early-sleeping Problem Nodes send messages to a single sink node

– Shortest path routing, no data aggregation

T-MAC: FRTS Vs. PriorityVs. FRTS + PriorityVs. No measures

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Event-Based Local Unicast When no events happen, nodes exchange local messages of 10 bytes

with each other every 20 seconds. Also report to a sink node every 100 seconds.

When an event happens, nodes start sending local unicast messages of 30 bytes. Then also send messages of 50 bytes to the sink.

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Conclusions And Future Work

Conclusions T-MAC dynamically adapts a listen/sleep duty cycle T-MAC Protocol

– During a high load, nodes communicate without sleeping– During a very low load, nodes will use their radios for as little

as 2.5% of the time, saving as much as 96% of the energy compared to a traditional

Future Work Throughput and Early-Sleeping Problem

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The End

Thanks for your listening !