1 medium access control in sensor networks huaming li electrical and computer engineering michigan...

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Medium Access Control in Medium Access Control in Sensor NetworksSensor Networks

Huaming Li

Electrical and Computer Engineering

Michigan Technological University

Computer Engineering Seminar

Department of ECE

OutlineOutline

Overview S-MAC: an energy-efficient MAC protocol for

wireless sensor networks Other MAC Techniques References


Department of ECE

Medium Access Control in Sensor NetworksMedium Access Control in Sensor Networks

Sensor networks Consist of a set of sensor nodes; Each node is equipped with one or more sensors and is

normally battery operated; Nodes communicate with each other via wireless

connection. Medium Access Control (MAC)

Fundamental task is to avoid collisions so that two interfering nodes do not transmit at the same time


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Characteristics of Sensor NetworkCharacteristics of Sensor Network

A special wireless ad hoc network Large number of nodes Battery powered Topology and density change Nodes for a common task In-network data processing

Sensor-net applications Sensor-triggered bursty traffic Can often tolerate some delay

Speed of a moving object places a bound on network reaction time

Energy efficiency

Scalability & Self-configuration

Fairness not importantMsg-level

Latency

Trade for energy

Adaptivity

Adaptivity


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MAC Protocols ClassificationMAC Protocols Classification

Scheduling-Based MAC Protocols Contention-Based MAC Collision Free Real Time MAC Hybrid MAC


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Scheduling Based MACScheduling Based MAC

Time is divided into slots Each node knows when to transmit Schedule is predetermined TDMA Synchronization problems Adaptability problems


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Contention Based MACContention Based MAC

Carrier sensing & collision avoidance In-band, out-band handshaking Busy-tone multiple access (BTMA) Multiple access with collision avoidance

(MACA) High priority packets


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Common MAC Protocol RequirementsCommon MAC Protocol Requirements

Quality of service (QoS) Tolerate message loss Support real time guarantees

Decentralized Global information may not be available

Flexibility Diversity of applications


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MAC Requirements in Sensor NetworksMAC Requirements in Sensor Networks

Important requirements of MAC protocols Collision avoidance Energy efficiency Scalability & Adaptivity Latency Fairness Throughput Bandwidth utilization

Primary

Secondary


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Energy Efficiency in MAC DesignEnergy Efficiency in MAC Design

Energy is primary concern What causes energy waste on radio?

Long idle time Control packet overhead Overhearing unnecessary traffic Collisions

bursty traffic in sensor-net apps Idle listening consumes 50—100% of the power for receiving

(Stemm97, Kasten)

Dominant in sensor networks


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S-MAC Design OverviewS-MAC Design Overview

Tradeoffs

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

Latency

FairnessEnergy


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Periodic Listen and SleepPeriodic Listen and Sleep

Reduce long idle time Reduce duty cycle to ~ 10% (120ms on/1.2s off)

Schedules can differ

Latency Energy

Node 1 sleeplisten listen sleep

Node 2 sleeplisten listen sleep

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


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Periodic Listen & SleepPeriodic Listen & Sleep

Nodes are in idle for a long time if no sensing event happens

Put nodes into periodic sleep mode i.e. in each second, sleep for half second and listen

for other half second


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Schedule 2Schedule 1

Coordinated SleepingCoordinated Sleeping

Nodes coordinate on sleep schedules Nodes periodically broadcast schedules New node tries to follow an existing schedule

Nodes on border of two schedules follow both Periodic neighbor discovery

Keep awake in a full sync interval over long time

12


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Choose & Maintain ScheduleChoose & Maintain Schedule

Each node maintains a schedule table that stores schedules of all its neighbors

Nodes exchange schedules by broadcasting them to its neighbors Try to synchronize neighboring nodes together


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Choose ScheduleChoose Schedule

If not hear a schedule from others, the node randomly chooses a schedule and broadcast the schedule

If receive a schedule, the node follows that schedule, wait for a random delay then rebroadcast this schedule

If receive a different schedule, the node adopt both, broadcast its own schedule


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Maintain SynchronizationMaintain Synchronization

Listen/sleep scheme requires synchronization among neighboring nodes

Looser synchronization (compared to TDMA) Listen period is significantly longer than clock error

or drift Use relative time rather than absolute Update schedule by sending SYNC packets


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Maintain Sync (contd.)Maintain Sync (contd.)

Divide listen time into two parts: For receiving SYNC packets For receiving data packets

Each part is further divided into many time slots for senders to perform carrier sense


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Maintain Sync (contd.)Maintain Sync (contd.)

CS: carrier sense


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Collision AvoidanceCollision Avoidance

Adopt IEEE 802.11 collision avoidance Virtual carrier sense

During field Network allocation vector (NAV)

Physical carrier sense RTS/CTS exchange (for hidden terminal problem)

Broadcast packets (SYNC) are sent without RTS/CTS Unicast packets (DATA) are sent with RTS/CTS


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Overhearing AvoidanceOverhearing Avoidance

Problem: Receive packets destined to others Solution: Sleep when neighbors talk

Basic idea from PAMAS (Singh, Raghavendra 1998) But we only use in-channel signaling

Who should sleep?• All immediate neighbors of sender and

receiver

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

other nodes the sleep interval


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ExampleExample

Who should sleep when node A is transmitting to B?

All immediate neighbors of both sender & receiver should go to sleep


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Message PassingMessage Passing

How to efficiently transmit a long message? Single packet vs. fragmentations

Single packet: high cost of retransmission if only a few bits have been corrupted

Fragmentations: large control overhead (RTS & CTS for each fragment), longer delay

Problem: Sensor network in-network processing requires entire message


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Message PassingMessage Passing

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 Other nodes sleep for whole message time

FairnessEnergy

Msg-level latency


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Implementation on Testbed NodesImplementation on Testbed Nodes

Configurable S-MAC optionsLow duty cycle with adaptive listenLow duty cycle without adaptive listenFully active mode (no periodic sleeping)

PlatformMica Motes (UC Berkeley)

8-bit CPU at 4MHz,128KB flash, 4KB RAM20Kbps radio at 433MHz

TinyOS: event-driven


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Implementation on Testbed NodesImplementation on Testbed Nodes

Layered model on Motes MAC layer: S-MAC Physical layer

Radio state control, Carrier sense CRC checking, Channel coding, Byte buffering

Nested headers Avoid memory copy across layers

ApplicationTransportRouting

MAC/LinkPhysical


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Test BedTest Bed

Three test MAC modules Simplified IEEE 802.11 DCF Message passing with overhearing avoidance Complete S-MAC

Topology in experiments

Source 1

Source 2

Sink 1

Sink 2


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Experiment ResultExperiment Result

Average source nodes energy consumption

• S-MAC consumes much less energy than 802.11-like protocol w/o sleeping

• At heavy load, overhearing avoidance is the major factor in energy savings

• At light load, periodic sleeping plays the key role

0 2 4 6 8 10

200

400

600

800

1000

1200

1400

1600

1800Average energy consumption in the source nodes

Message inter-arrival period (second)

Ene

rgy

cons

umpt

ion

(mJ)

802.11-like protocolwithout sleep

Overhearing avoidance

S-MAC w/o adaptive listen


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Experiment Result (contd.)Experiment Result (contd.)

Percentage of time source nodes in sleep


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Experiment Result (contd.)Experiment Result (contd.)

Energy consumption in the intermediate node


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S-MAC ConclusionsS-MAC Conclusions

Advantages: Periodically sleep reduces energy consumption in

idle listening Sleep during transmissions of other nodes Message passing reduces contention latency and

control packet overhead Disadvantages:

Reduction in both per-node fairness & latency


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Other MAC TechniquesOther MAC Techniques

Timeout-MAC (T-MAC) S-MAC has fixed duty cycle and not optimal Reduce idle listening by transmitting data in

bursts Sleep in between bursts to save power End the active time in an intuitive way Timeout on hearing nothing


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T-MACT-MAC

Every node periodically wakes up and communicates with its neighbors

A node will keep listening and potentially transmitting, as long as it is in active period

An active period ends when no activation event has occurred for time TA


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Activation eventActivation event

The firing of periodic timer The reception of any data on radio The sensing of communication on the radio The end of transmission of a node’s own data

packet The knowledge through prior RTS and CTS

packets


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T-MACT-MAC

A node will sleep if it is not in an active period TA determines the minimum amount of idle

listening per frame All communication occurs as a burst in the

beginning of the frame Buffer capacity determines the upper bound on

the maximum frame time


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EvolutionEvolution

CSMA/CD CSMA/CA SMAC TMAC

IEEE 802.11

Carrier Sense Multiple Access with Collision

Detection

IEEE 802.3

Carrier Sense Multiple Access with Collision

Avoidance

Fixed duty cycle

Adaptive duty cycle

ARC

Adaptive Rate

Control

DMAC/MMAC

Directional

Antennas


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Performance Analysis of 802.15.4 in WPANPerformance Analysis of 802.15.4 in WPANPerformance Analysis of 802.15.4 in WPANPerformance Analysis of 802.15.4 in WPAN

One promising kind of sensor network: Wireless Personal Area Network (WPAN)

Medical sensing and control

Wearable computing

Location awareness and identification

Implanted medical sensors (Focus)

Coronary care

Diabetes

Optical aids

Drug delivery


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Critical Metric : Battery LifeCritical Metric : Battery LifeCritical Metric : Battery LifeCritical Metric : Battery Life

Implanted medical sensors (Main concern)

Objective

Make Batteries work 10-15 years

Method

Ensure that all sensors are powered down or in sleep mode when not in active use

Tradeoff

Battery life VS. latency


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Protocol OptionsProtocol OptionsProtocol OptionsProtocol Options

Market Name

Standard

GPRS/GSM

1xRTT/CDMA

Wi-Fi™

802.11b

Bluetooth™

802.15.1

ZigBee™

802.15.4

Application Focus

Wide Area Voice & Data

Web, Email, Video

Cable Replacement

Monitoring & Control

System Resources

16MB+ 1MB+ 250KB+ 4KB - 32KB

Battery Life (days)

1-7 .5 - 5 1 - 7 100 - 1,000+

Network Size 1 32 7 255 / 65,000

Bandwidth (KB/s)

64 - 128+ 11,000+ 720 20 - 250

Transmission Range (meters)

1,000+ 1 - 100 1 - 10+ 1 - 100+

Success MetricsReach, Quality

Speed, Flexibility

Cost, Convenience

Reliability, Power, Cost

Possible options


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802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN)

IEEE 802.15.4 MAC

Upper Layers

IEEE 802.2 LLC Other LLC

IEEE 802.15.4

2400 MHz

PHY

IEEE 802.15.4

868/915 MHz

PHY

Physical Medium


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802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) MAC Layer (prefer star topology)

Why star topology here?


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802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN) 802.15.4 (LR-WPAN)

Coordinator is external to the body

PDA, mobile phone or bedside monitor station

Easy to replace of charge batteries

Easy to communicate with other networks

Coordinator defines the start and end of a superframe and is charge of the association and disassociation of the other nodes

Why star topology here?


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IEEE 802.15.4 superframe structure IEEE 802.15.4 superframe structure


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Two Communication methods Two Communication methods

Beacon mode

Pros: Coordinator can communicate at will

Cons: Listeners have to keep awake

Non-beacon mode

Pros: Nodes can sleep more

Cons: Communication latency


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Network scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysis

Sensor power consumption with beacon reception

Problem: The sensor devices within a beacon network have to wake up to receive the beacon from the coordinator (Power consuming)

Timebase Tolerances

Warm-up time


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Network scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysisData Transfer Mechanisms (Beacon)

Data transfer to a coordinator (upload)

Is the upload period


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Network scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysisNetwork scenarios and power analysis

Data Transfer Mechanisms (Beacon)

Data transfer from a coordinator (download)

Is the download period


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ResultsResultsResultsResults

Average Back-off

With a small number of sensors that are effectively off most of the time, the probability of a channel being free is greater than 99 %. Therefore, for the relatively small number of sensors used in the WBAN networks explored here, it would be more economical to keep the CSMA/CA switched off.

This is to ensure that the automatic initial back-off is avoided.


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Node Lifetime in Beacon NetworksNode Lifetime in Beacon Networks


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Node Lifetime in Beacon NetworksNode Lifetime in Beacon Networks

15-year lifetime may only be obtained for very low upload rates.

It is under very limited data rate conditions and a tight tolerance crystal, which typically must be better than 25 ppm.


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GTS OptionGTS Option

The main drawback of using

GTS is that the receiver in the

sensor remains on for the duration of

the timeslot regardless of the size of the data

packet.


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Non-Beacon NetworksNon-Beacon Networks


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ConclusionConclusionConclusionConclusion

As a solution to the challenge of the personal area network, the IEEE 802.15.4 standard would provide a limited answer in its non-beacon form.

Sensors that do not have large amounts of data to transfer could be used, i.e., small packets of data several times per hour.


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Questions and CommentsQuestions and CommentsQuestions and CommentsQuestions and Comments


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1 medium access control in sensor networks huaming li electrical and computer engineering michigan...

Documents

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based mac time

energy adaptivity slide

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latency fairness energy

smac periodic listen

wireless sensor networks