Architectures and Architectures and Applications for Applications for Wireless Sensor Wireless Sensor

Networks (01204525)Networks (01204525)

Medium Access ControlMedium Access Control

Chaiporn JaikaeoChaiporn Jaikaeochaiporn.j@ku.ac.thchaiporn.j@ku.ac.th

Department of Computer EngineeringDepartment of Computer EngineeringKasetsart UniversityKasetsart University

Materials taken from lecture slides by Karl and Willig

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OverviewOverview Principal options and difficultiesPrincipal options and difficulties Contention-based protocolsContention-based protocols Schedule-based protocolsSchedule-based protocols IEEE 802.15.4IEEE 802.15.4

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DifficultiesDifficulties Medium access in wireless networks Medium access in wireless networks

is difficult, mainly because ofis difficult, mainly because of Half-duplex communicationHalf-duplex communication High error ratesHigh error rates

RequirementsRequirements As usual: high throughput, low As usual: high throughput, low

overhead, low error rates, …overhead, low error rates, … Additionally: energy-efficient, handle Additionally: energy-efficient, handle

switched off devices!switched off devices!

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Requirements for Energy-Requirements for Energy-Efficient MAC ProtocolsEfficient MAC Protocols RecallRecall

Transmissions are costlyTransmissions are costly Receiving about as expensive as transmittingReceiving about as expensive as transmitting Idling can be cheaper but is still expensive Idling can be cheaper but is still expensive

Energy problemsEnergy problems CollisionsCollisions OverhearingOverhearing Idle listeningIdle listening Protocol overhead Protocol overhead

Always wanted: Low complexity solutionAlways wanted: Low complexity solution

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Main OptionsMain OptionsWireless medium access

Centralized Distributed

Contention-based

Schedule-based

Fixedassignment

Demandassignment

Contention-based

Schedule-based

Fixedassignment

Demandassignment

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Centralized Medium Centralized Medium AccessAccess A central station controls when a A central station controls when a

node may access the mediumnode may access the medium E.g., Polling, computing TDMA schedulesE.g., Polling, computing TDMA schedules Advantage: Simple, efficientAdvantage: Simple, efficient

Not directly feasible for non-trivial Not directly feasible for non-trivial wireless network sizeswireless network sizes

But: Can be quite useful when But: Can be quite useful when network is somehow divided into network is somehow divided into smaller groupssmaller groups

Distributed approach still preferableDistributed approach still preferable

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Schedule- vs. Schedule- vs. Contention-BasedContention-Based Schedule-basedSchedule-based protocols protocols

FDMA, TDMA, CDMAFDMA, TDMA, CDMA Schedule can be Schedule can be fixed fixed or computed or computed on on

demanddemand Usually mixedUsually mixed

Collisions, overhearing, idle listening no issuesCollisions, overhearing, idle listening no issues Time synchronization neededTime synchronization needed

Contention-basedContention-based protocols protocols Hope: coordination overhead can be savedHope: coordination overhead can be saved Mechanisms to handle/reduce Mechanisms to handle/reduce

probability/impact of collisions required probability/impact of collisions required Randomization used somehowRandomization used somehow

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OverviewOverview Principal options and difficultiesPrincipal options and difficulties Contention-based protocolsContention-based protocols

MACAMACA S-MAC, T-MACS-MAC, T-MAC Preamble sampling, B-MACPreamble sampling, B-MAC PAMASPAMAS

Schedule-based protocolsSchedule-based protocols IEEE 802.15.4IEEE 802.15.4

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A

Distributed, Contention-Distributed, Contention-Based MACBased MAC Basic ideasBasic ideas

Receivers need to tell surrounding Receivers need to tell surrounding nodes to shut upnodes to shut up

Listen before talk Listen before talk (CSMA) (CSMA) Suffers from Suffers from sendersender not knowing what is not knowing what is

going on at going on at receiverreceiver

B C D

Hidden terminal

scenario: Also: recall exposed terminal scenario

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How To Shut Up How To Shut Up Senders Senders Inform potential interferers Inform potential interferers duringduring

receptionreception Cannot use the same channelCannot use the same channel So use a different oneSo use a different one

Busy tone Busy tone protocolprotocol

Inform potential interferers Inform potential interferers before before receptionreception Can use same channelCan use same channel Receiver itself needs to be informed, by Receiver itself needs to be informed, by

sender, about impending transmission sender, about impending transmission Potential interferers need to be aware of such Potential interferers need to be aware of such

information, need to store itinformation, need to store it

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MACAMACA MMultiple ultiple AAccess with ccess with

CCollision ollision AAvoidancevoidance Sender B issues Sender B issues

Request to Send Request to Send (RTS)(RTS)

Receiver C agrees with Receiver C agrees with Clear to Send Clear to Send ((CTSCTS))

Potential interferers Potential interferers learns from RTS/CTSlearns from RTS/CTS Store this information Store this information

in a in a Network Network Allocation Vector Allocation Vector (NAV)(NAV)

B sends, C acksB sends, C acks Used in Used in IEEE 802.11IEEE 802.11

A B C D

RTS

CTS

Data

Ack

NAV indicates busy medium

NAV indicates busy medium

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RTS/CTS RTS/CTS RTS/CTS helps, but do not solve RTS/CTS helps, but do not solve

hidden/exposed terminal problemshidden/exposed terminal problemsA B C D

RTS

CTS

Data

A B C D

RTS

RTS

CTS

RTS

RTSCTS

CTSData

Data

Ack

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MACA Problem: Idle MACA Problem: Idle listeninglistening Need to sense carrier for RTS or CTS Need to sense carrier for RTS or CTS

packetspackets In some form shared by many CSMA In some form shared by many CSMA

variants; but e.g. not by busy tonesvariants; but e.g. not by busy tones Simple sleeping will break the protocolSimple sleeping will break the protocol

IEEE 802.11 solution: ATIM windows IEEE 802.11 solution: ATIM windows & sleeping& sleeping Idea: Nodes that have data buffered for Idea: Nodes that have data buffered for

receivers send receivers send traffic indicatorstraffic indicators at at prearranged points in timeprearranged points in time

Receivers need to wake up at these Receivers need to wake up at these points, but can sleep otherwisepoints, but can sleep otherwise

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Sensor-MAC (S-MAC)Sensor-MAC (S-MAC) MACA unsuitable if average data rate is MACA unsuitable if average data rate is

lowlow Most of the time, nothing happensMost of the time, nothing happens

Idea: Switch off, ensure that neighboring Idea: Switch off, ensure that neighboring nodes turn on simultaneously to allow nodes turn on simultaneously to allow packet exchangepacket exchange Need to also exchange Need to also exchange

wakeup schedule wakeup schedule between neighborsbetween neighbors

When awake, When awake, perform RTS/CTSperform RTS/CTS

Wakeup period

Active period

Sleep period

For SYNCH For RTS For CTS

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Listen for SYNC

td

Schedule AssignmentSchedule Assignment Synchronizer

Listen for a mount of time

If hear no SYNC, select its own SYNC

Broadcasts its SYNC immediately

Follower Listen for amount

of time Hear SYNC from

A, follow A’s SYNC

Rebroadcasts SYNC after random delay td

Sleep

Listen

Go to sleep after time t

Sleep

Listen

Broadcasts

A

B

Broadcasts

Go to sleep after time t- td

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S-MAC Synchronized S-MAC Synchronized IslandsIslands Nodes learn schedule from other nodesNodes learn schedule from other nodes Some node might learn about two Some node might learn about two

different schedules from different nodesdifferent schedules from different nodes ““Synchronized islands”Synchronized islands”

To bridge this gap, it has to follow both To bridge this gap, it has to follow both schemesschemes

Time

A A A A

C C C C

A

B B B B

D D D

A

C

B

D

E E E EE E E

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Timeout-MAC (T-MAC)Timeout-MAC (T-MAC) In S-MAC, active In S-MAC, active

period is of constant period is of constant lengthlength

Idea: Prematurely Idea: Prematurely go back to sleep go back to sleep mode after timeoutmode after timeout Adaptive duty cycleAdaptive duty cycle

One ensuing One ensuing problem: Early problem: Early sleepingsleeping C wants to send to C wants to send to

D, but is hindered by D, but is hindered by transmission Atransmission ABB

A B C DRTS

CTS

DATA

May not send

Timeout, go back tosleep asnothing

happened

ACK

RTS

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Preamble SamplingPreamble Sampling Alternative option: Don’t try to explicitly Alternative option: Don’t try to explicitly

synchronize nodessynchronize nodes Have receiver sleep and only periodically Have receiver sleep and only periodically

sample the channelsample the channel Use Use long preambleslong preambles to ensure that to ensure that

receiver stays awake to catch actual receiver stays awake to catch actual packet packet Example: B-MAC, WiseMAC Example: B-MAC, WiseMAC

Check channel

Check channel

Check channel

Check channel

Start transmission:Long preamble Actual packet

Stay awake!

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B-MACB-MAC Very simple MAC protocolVery simple MAC protocol EmploysEmploys

Clear Channel Assessment (CCA) and Clear Channel Assessment (CCA) and backoffs for channel arbitrationbackoffs for channel arbitration

Link-layer acknowledgement for Link-layer acknowledgement for reliabilityreliability

Low-power listening (LPL)Low-power listening (LPL) I.e., preamble sampling I.e., preamble sampling

Currently: Often considered as the Currently: Often considered as the default WSN MAC default WSN MAC protocolprotocol

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B-MACB-MAC B-MAC does not haveB-MAC does not have

SynchronizationSynchronization RTS/CTSRTS/CTS Results in simpler, leaner Results in simpler, leaner

implementation implementation Clean and simple interfaceClean and simple interface

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Clear Channel Clear Channel AssessmentAssessment "Carrier Sensing" in wireless "Carrier Sensing" in wireless

networksnetworks

Thresholding Thresholding CCA algorithmCCA algorithm

Outlier detection Outlier detection CCA algorithmCCA algorithm

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PAMAS PAMAS PPower ower AAware ware MMulti-ulti-AAccess with ccess with

SSignalingignaling Idea: combine busy tone with Idea: combine busy tone with

RTS/CTSRTS/CTS Avoid overhearingAvoid overhearing Does not address idle listeningDoes not address idle listening Uses separate Uses separate data data and and control control

channelschannels Time

Controlchannel

Datachannel

RTS A B

CTS B A

Data A B

Busy tone sent by B

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PAMASPAMAS Suppose a node C in vicinity of A is Suppose a node C in vicinity of A is

already receiving a packet when A already receiving a packet when A initiates RTS initiates RTS

A

BC

?

Time

Controlchannel

Datachannel

RTS A B

CTS B A

No data!

Busy tone by C

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OverviewOverview Principal options and difficultiesPrincipal options and difficulties Contention-based protocolsContention-based protocols Schedule-based protocolsSchedule-based protocols

LEACHLEACH SMACSSMACS TRAMATRAMA

IEEE 802.15.4IEEE 802.15.4

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LEACHLEACH LLow-ow-EEnergy nergy AAdaptive daptive CClustering lustering

HHierarchyierarchy AssumptionsAssumptions

Dense network of nodesDense network of nodes Direct communication with central sinkDirect communication with central sink Time synchronizationTime synchronization

Idea: Group nodes into “Idea: Group nodes into “clustersclusters”” Each cluster controlled by Each cluster controlled by clusterheadclusterhead About 5% of nodes become clusterhead About 5% of nodes become clusterhead

(depends on scenario)(depends on scenario) Role of clusterhead is rotatedRole of clusterhead is rotated

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LEACH ClusterheadLEACH Clusterhead Each CH organizesEach CH organizes

CDMA code for its clusterCDMA code for its cluster TDMA schedule to be used within a TDMA schedule to be used within a

clustercluster In steady state operationIn steady state operation

CHs collect & aggregate data from all CHs collect & aggregate data from all cluster memberscluster members

Report aggregated data to sink using Report aggregated data to sink using CDMACDMA

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LEACH rounds LEACH rounds

Setup phase Steady-state phase

Fixed-length round

……….. ………..

Advertisement phase Cluster setup phase Broadcast schedule

Time slot 1

Time slot 2

Time slot n

Time slot 1

…..….. …..

Clusterheads compete with CSMA

Members compete with CSMA

Self-election of clusterheads

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SMACSSMACS SSelf-Organizing elf-Organizing MMedium edium AAccess ccess

CControl for ontrol for SSensor Networksensor Networks AssumptionsAssumptions

Many radio channelsMany radio channels Most nodes are stationaryMost nodes are stationary Time synchronizationTime synchronization

Goal: set up Goal: set up directional links directional links between neighboring nodesbetween neighboring nodes

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SMACS LinksSMACS Links Each link is directionalEach link is directional

A pair of nodes needs two links to A pair of nodes needs two links to exchange dataexchange data

Radio channel + time slot at both sender Radio channel + time slot at both sender and receiverand receiver

Free of collisions at receiverFree of collisions at receiver Channel picked randomly, slot is Channel picked randomly, slot is

searched greedily until a collision-free searched greedily until a collision-free slot is foundslot is found

Receivers only wake up in their Receivers only wake up in their assigned time slots, once per assigned time slots, once per superframe superframe

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TRAMATRAMA TrTraffic affic AAdaptive daptive MMedium edium AAccess ccess

ProtocolProtocol Assume nodes are time synchronizedAssume nodes are time synchronized Time divided into cycles, divided intoTime divided into cycles, divided into

Random access periodRandom access period Scheduled access periodScheduled access period

Random Access PeriodRandom Access Period Scheduled-Access PeriodScheduled-Access Period

time cycletime cycle

• Exchange and learn two-hop Exchange and learn two-hop neighborsneighbors

• Exchange schedulesExchange schedules

• Used by winning nodes to Used by winning nodes to transmit datatransmit data

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TRAMA – Adaptive TRAMA – Adaptive Election Election How to decide which slot (in scheduled How to decide which slot (in scheduled

access period) a node can use? access period) a node can use? For node id For node id xx and time slot and time slot tt, compute , compute p p == h h ((xx

tt)) hh is a global hash function is a global hash function

Compute Compute pp for next for next kk time slots for itself and time slots for itself and all two-hop neighborsall two-hop neighbors

Node uses those time slots for which it has the Node uses those time slots for which it has the highest priority highest priority

t = t = 00

t = t = 1 1

t = t = 22

t=3t=3 t = t = 44

t = t = 5 5

AA 1414 2323 99 5656 33 2626BB 3333 6464 88 1212 4444 66CC 5353 1818 66 3333 5757 22

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Comparison: TRAMA, S-Comparison: TRAMA, S-MAC MAC Comparison between TRAMA & S-MACComparison between TRAMA & S-MAC

Energy savings in TRAMA depend on load Energy savings in TRAMA depend on load situationsituation

Energy savings in S-MAC depend on duty Energy savings in S-MAC depend on duty cycle cycle

TRAMA (as typical for a TDMA scheme) has TRAMA (as typical for a TDMA scheme) has higher delay but higher maximum higher delay but higher maximum throughput than contention-based S-MAC throughput than contention-based S-MAC

TRAMA disadvantage: substantial TRAMA disadvantage: substantial memory/CPU requirements for schedule memory/CPU requirements for schedule computationcomputation

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OverviewOverview Principal options and difficultiesPrincipal options and difficulties Contention-based protocolsContention-based protocols Schedule-based protocolsSchedule-based protocols IEEE 802.15.4IEEE 802.15.4

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IEEE 802.15.4IEEE 802.15.4 IEEE standard for low-rate WPAN (LR-WPAN) IEEE standard for low-rate WPAN (LR-WPAN)

applicationsapplications Low-to-medium bit ratesLow-to-medium bit rates Moderate delays without too strict requirements Moderate delays without too strict requirements Low energy consumption Low energy consumption

Physical layerPhysical layer 20 kbps over 1 channel @ 868-868.6 MHz20 kbps over 1 channel @ 868-868.6 MHz 40 kbps over 10 channels @ 905 – 928 MHz 40 kbps over 10 channels @ 905 – 928 MHz 250 kbps over 16 channels @ 2.4 GHz 250 kbps over 16 channels @ 2.4 GHz

MAC protocolMAC protocol Single channel at any one timeSingle channel at any one time Combines contention-based and schedule-based Combines contention-based and schedule-based

schemesschemes Asymmetric: nodes can assume different rolesAsymmetric: nodes can assume different roles

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868MHz / 915MHz PHY

2.4 GHz

868.3 MHz

Channel 0 Channels 1-10

Channels 11-26

2.4835 GHz

928 MHz902 MHz

5 MHz

2 MHz

2.4 GHz PHY

IEEE 802.15.4 PHY IEEE 802.15.4 PHY OverviewOverview Operating frequency bandsOperating frequency bands

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IEEE 802.15.4 MAC IEEE 802.15.4 MAC OverviewOverview Device classesDevice classes

Full function device (FFD)Full function device (FFD) Any topologyAny topology Network coordinator capableNetwork coordinator capable Talks to any other deviceTalks to any other device

Reduced function device (RFD)Reduced function device (RFD) Limited to star topologyLimited to star topology Cannot become a network coordinatorCannot become a network coordinator Talks only to a network coordinatorTalks only to a network coordinator Very simple implementationVery simple implementation

Slide 36 Joe Dvorak, Motorola 9/27/05

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Network TopologiesNetwork Topologies

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Cluster Tree NetworkCluster Tree Network A special case of peer-to-peer A special case of peer-to-peer

topologytopology

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CoordinatorsCoordinators RolesRoles

Manage a list of associated devicesManage a list of associated devices Allocate a short address to each deviceAllocate a short address to each device Transmit beacons (in Transmit beacons (in beaconed modebeaconed mode)) Exchange data with devices and peer Exchange data with devices and peer

coordinatorscoordinators Devices Devices are associated with are associated with

coordinatorscoordinators Forming a PAN, identified by a PAN Forming a PAN, identified by a PAN

identifieridentifier

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Beaconed ModeBeaconed Mode Superframe structureSuperframe structure

GTS assigned to devices upon request GTS assigned to devices upon request

Active period Inactive period

Contention access period

Guaranteed time slots (GTS)

Beacon

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Data TransferData Transfer Device Device coordinator coordinator

If having allocated GTS, If having allocated GTS, wake up and sendwake up and send

Otherwise, send during Otherwise, send during CAPCAP Using slotted CSMAUsing slotted CSMA

Coordinator Coordinator device device If having allocated GTS, If having allocated GTS,

wake up and receivewake up and receive Otherwise, see pictureOtherwise, see picture

Coordinator Device

Beacon

Data request

Acknowledgement

Data

Acknowledgement

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Slotted CSMASlotted CSMA

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Further protocolsFurther protocols MAC protocols for ad hoc/sensor MAC protocols for ad hoc/sensor

networks is one the most active networks is one the most active research fields research fields Tons of additional protocols in the Tons of additional protocols in the

literatureliterature E.g., STEM, mediation device protocol, E.g., STEM, mediation device protocol,

many CSMA variants with different many CSMA variants with different timing optimizations, protocols for timing optimizations, protocols for multi-hop reservations (QoS for multi-hop reservations (QoS for MANET), protocols for multiple radio MANET), protocols for multiple radio channels, … channels, …

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SummarySummary Many different ideas exist for medium access Many different ideas exist for medium access

control in MANET/WSN control in MANET/WSN Comparing their performance and suitability is Comparing their performance and suitability is

difficultdifficult Especially, clearly identifying interdependencies Especially, clearly identifying interdependencies

between MAC protocol and other between MAC protocol and other layers/applications is difficultlayers/applications is difficult Which is the best MAC for which application?Which is the best MAC for which application?

Nonetheless, certain “common use cases” existNonetheless, certain “common use cases” exist IEEE 802.11 DCF for MANETIEEE 802.11 DCF for MANET IEEE 802.15.4 for some early “commercial” WSN IEEE 802.15.4 for some early “commercial” WSN

variantsvariants B-MAC for WSN research not focusing on MAC B-MAC for WSN research not focusing on MAC