An Energy-Efficient MAC Protocol for Wireless Sensor Networks

Wei Ye, John Heidemann, Deborah Estrin

-- Adapted the authors’ Infocom 2002 talk

Introduction

Wireless sensor network• Special ad hoc wireless network• Large number of nodes w/ sensors & actuators• Battery-powered nodes energy efficiency• Unplanned deployment self-organization• Node density & topology change robustness

Sensor-net applications• Nodes cooperate for a common task• In-network data processing

Medium Access Control in Sensor Nets

Important attributes of MAC protocols1. Collision avoidance2. Energy efficiency3. Scalability in node density4. Latency5. Fairness6. Throughput7. Bandwidth utilization

Primary

Secondary

Major sources of energy waste• Idle listening

Energy consumption of typical 802.11 WLAN cardsidle:receive:send — 1:1.05:1.4, 1:2:2.5 (Stemm

1997)Example: directed diffusion (Intanagonwiwat 2000)

Energy Efficiency in MAC

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Over always-listening MAC Over energy-aware MAC

Reminder: IEEE 802.11 MAC

Very popular wireless MAC protocol Two modes: DCF (distributed coordination function) & PCF

(point coordination function) DCF is based on CSMA/CA ≈ CSMA + MACA

• RTS-CTS-DATA-ACK• Physical carrier sensing + NAV (network allocation

vector) containing time value that indicates the duration up to which the medium is expected to be busy due to transmissions by other nodes

• Every packet contains the duration info for the remainder of the message

• Every node overhearing a packet continuously updates its own NAV for virtual carrier sensing

IFS (inter frame spacing)• Short IFS (SIFS), PCF IFS (PIFS), DCF IFS (DIFS),

Extended IFS (EIFS)

Big problem with existing wireless MACs

Idle listening• Does anybody send me a RTS?• Does anyone send CTS to my neighbor

which I want to communicate with?• Huge energy consumption!

PAMAS (Power Aware Medium Access with Signaling)• Separate radio channel for RTS/CTS• Sleep for the duration of the transmission

indicated in the control packets S-MAC aims to achieve this without requiring a separate channel for RTS & CTS

Idle listening in 802.11

RTS & CTS only reserves the medium for the first data fragment & the first ACK

The 1st fragment & ACk reserves the medium for the 2nd fragment and so on

After overhearing a fragment or an ACK, a neighboring node knows that there is one more fragment to be sent • It has to keep listening until all the fragments are sent • Promote fairness; If the sender fails to get ACK after

sending a fragment, it must give up the transmission and recondtend for the medium

• For in-network processing, an entire message is needed in sensor networks 802.11 may cause a largee delay

Major sources of energy waste (cont.)• Idle listening

Long idle time when no sensing event happens

• Collisions• Control overhead• Overhearing

Reduce energy consumption from all above sources

Combine benefits of TDMA + contention protocols

Energy Efficiency in MAC

Common to all wireless

networks

Dominant in sensor nets

Sensor-MAC (S-MAC) Design

Tradeoffs

Major components in S-MAC• Periodic listen and sleep• Collision avoidance• Overhearing avoidance• Massage passing

Latency

FairnessEnergy

Periodic Listen and Sleep

Problem: Idle listening consumes significant energy

Solution: Periodic listen and sleep

• Turn off radio when sleeping• Reduce duty cycle to ~ 10% (200ms on/2s

off)

sleeplisten listen sleep

Latency Energy

Periodic Listen and Sleep

Schedules can differ

• Prefer neighboring nodes have same schedule— easy broadcast & low control overhead

Border nodes: two schedules broadcast twice

Node 1

Node 2

sleeplisten listen sleep

sleeplisten listen sleep

Schedule 2

Schedule 1

Each nodes has a schedule table- Stores the schedules of all its known neighbors

1. Listens for a certain amount of time - If it doesn’t hear a schedule, it becomes a synchronizer - It randomly chooses a schedule and broadcast it in SYNC message- SYNC message indicates that it will go to sleep after t seconds

Periodic Listen and Sleep - Choosing and Maintaining Schedules

2. If the node receives a schedule before choosing its own schedule, it follows that schedule

- follower- waits for a random delay td and re-broardcasts this schedule, indicating that it will sleep in t – td

3. If a node receives a different schedule after it selects and broadcast (border node)

- adopts both schedules - less time to sleep - consume more energy

Periodic Listen and Sleep - Choosing and Maintaining Schedules

Periodic Listen and Sleep - Schedule Synchronization

Remember neighbors’ schedules — to know when to send to them

Each node broadcasts its schedule for multiple periods of sleeping and listening• Update period can be long, e.g., tens of

secondsRe-sync when receiving a schedule

updateSync packets also serve as beacons for

new nodes to join a neighborhood

Listen

For SYNC For RTS Sleep

Receiver

Sender 1

Sender 2

Sender 3

Sleep

SYNC

CS

CS

CS CS

SYNC RTS

RTS

Send data if CTS received

Send data if CTS received

Figure 2. Timing relationship between a receiver and different senders

Periodic Listen and Sleep

Collision Avoidance

Problem: Multiple senders want to talkOptions: Contention vs. TDMASolution: Similar to IEEE 802.11 ad hoc

mode (DCF)• Physical and virtual carrier sense• Randomized backoff time• RTS/CTS for hidden terminal problem• RTS/CTS/DATA/ACK sequence

Overhearing Avoidance

Problem: Receive packets destined to others• In 802.11, each node keeps listening to all transmissions from its

neighbors for virtual carrier sensing• Each node should overhear a lot of packets not destined to itself

Solution: Sleep when neighbors talk• Basic idea from PAMAS (Singh, Raghavendra 1998)• But S-MAC only uses in-channel signaling

Who should sleep?• All immediate neighbors of sender and receiver• S-MAC lets interfering nodes go to sleep after they hear an RTS or

CTS DATA packets are normally much longer than control packets

How long to sleep?• The duration field in each packet informs other nodes the sleep

interval• After hearing the RTS/CTS packet destined to a node, all the other

immediate neighbors of both the sender and receiver should sleep until the NAV becomes zero

Message Passing

Problem: Sensor net in-network processing requires entire message

Solution: Don’t interleave different messages• Long message is fragmented & sent in burst• RTS/CTS reserve medium for entire message• Fragment-level error recovery — ACK

— Extend Tx time and re-transmit immediately if no ACK is received

Other nodes sleep for whole message time Fragmentation in IEEE 802.11

• No indication of entire time — other nodes keep listening• If ACK is not received, give up Tx & recontend for medium —

fairness

FairnessEnergy

Msg-level latency

Energy Savings vs. Increased Latency

General delay factors• Carrier sensor delay• Backoff delay• Transmission delay• Propagation delay• Processing delay• Queuing delay

Sleep delay: S-MAC specific• A sender has to wait until the receiver

wakes up

Sleep delay and Relative energy savings

Average sleep delay on the sender is 0.5Tframe = 0.5(Tlisten + Tsleep)

Relative energy saving• Es = Tsleep/Tframe = 1 – Tlisten/Tframe

Duty

Cycle

Implementation on Testbed Nodes

PlatformMotes (UC Berkeley)

8-bit CPU at 4MHz,8KB flash, 512B RAM916MHz radio

TinyOS

Compared MAC modules1. IEEE 802.11-like protocol w/o sleeping2. Message passing with overhearing

avoidance3. S-MAC (2 + periodic listen/sleep)

Experiments

Topology and measured energy consumption on source nodes

Source 1

Source 2

Sink 1

Sink 2

• Each source node sends 10 messages

— Each message has 400B in 10 fragments

• Measure total energy over time to send all messages

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Message inter-arrival period (second)

En

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802.11-like protocol Overhearing avoidanceS-MAC

Conclusions

S-MAC offers significant energy efficiency over always-listening MAC protocols

Sleep delay can be accumulated in intermediate hops Potential problem for real-time sensing in multiple environments

Future Plans• Measurement of throughput and latency

Throughput reduces due to latency, contention, control overhead and channel noise

• Experiments on large testbeds~100 Motes, ~30 embedded PCs w/ MoteNIC

URL: http://www.isi.edu/scadds/

Questions?