Energy-Efficient Medium Access Control

Energy-EfficientEnergy-Efficient Medium Access Control Medium Access Control

Koen Langendoen and Gertjan Koen Langendoen and Gertjan HalkesHalkes

Delft University of TechnologyDelft University of Technology

The Embedded Systems HandbookThe Embedded Systems Handbook

RoadmapRoadmap

• IntroductionIntroduction• Energy efficiencyEnergy efficiency• Looking into some existing MAC Looking into some existing MAC

protocolsprotocols– Contention-based protocolsContention-based protocols– Slotted protocolsSlotted protocols– TDMA-based protocolsTDMA-based protocols

• Simulation and benchmarksSimulation and benchmarks• Observation and conclusionObservation and conclusion

RoadmapRoadmap




What is the role of MACWhat is the role of MAC in sensor networks? in sensor networks?

• The unattended operation of sensor The unattended operation of sensor networks stresses the importance of energy networks stresses the importance of energy efficiency and reduce the significance of efficiency and reduce the significance of performance considerations such as low performance considerations such as low latency, high throughput, and fairness.latency, high throughput, and fairness.

• The task of MAC layer in sensor networks is The task of MAC layer in sensor networks is to use the radio, with limited resources, as to use the radio, with limited resources, as efficient as possible to send and receive efficient as possible to send and receive data generated by upper layers.data generated by upper layers.

Some characteristics in Some characteristics in hardwarehardware• Sending or receiving data consumes Sending or receiving data consumes

two to three orders of magnitude two to three orders of magnitude more than standby state.more than standby state.– Turn the radio off!Turn the radio off!

• The time needed to switch the radio The time needed to switch the radio between transmit and receive mode between transmit and receive mode is non-negligible.is non-negligible.– Do not switch the radio if you don’t need Do not switch the radio if you don’t need

one!one!

Some characteristics in Some characteristics in communicationcommunication

• The data rates are very low.The data rates are very low.– Typically in the order of 1-200 bytes per second.Typically in the order of 1-200 bytes per second.

• Two distinct important communication patterns.Two distinct important communication patterns.– Convergecast and local gossip.Convergecast and local gossip.

• The traffic is not distributed evenly over the The traffic is not distributed evenly over the network.network.– Nodes in the vicinity of the sink relay much more Nodes in the vicinity of the sink relay much more

traffic than other nodes.traffic than other nodes.– Fluctuations in network load is triggered by events.Fluctuations in network load is triggered by events.

What are convergecast and local What are convergecast and local gossip?gossip?• Convergecast:Convergecast:

– Information needs to be sent to a sink node to be Information needs to be sent to a sink node to be processed.processed.

– Small packets and the need to travel across the network.Small packets and the need to travel across the network.• Do aggregation!Do aggregation!

• Local gossip:Local gossip:– We expect when a sensor node observes an event, so We expect when a sensor node observes an event, so

does its neighbor.does its neighbor.– They can collaborate to obtain a better estimate by They can collaborate to obtain a better estimate by

unicast or broadcast.unicast or broadcast.– By sharing (gossip) their sensor readings (rumors), By sharing (gossip) their sensor readings (rumors),

nodes can reduce false positives and report significant nodes can reduce false positives and report significant events.events.

RoadmapRoadmap




How to achieve energy How to achieve energy efficiency?efficiency?

• Individual node failures do not harm the Individual node failures do not harm the overall functioning of a sensor network.overall functioning of a sensor network.– Optimize for network lifetime, that is, the time Optimize for network lifetime, that is, the time

until the network gets partitioned.until the network gets partitioned.• Optimization is easier at higher layers, since Optimization is easier at higher layers, since

they have global view.they have global view.– Solution: MAC layer ensures that the energy it Solution: MAC layer ensures that the energy it

spends is directly related to the amount of traffic it spends is directly related to the amount of traffic it handles.handles.

• Trade off :Trade off :– Performance (latency, throughput, fairness) v.s. Performance (latency, throughput, fairness) v.s.

Cost (energy efficiency, reduced algorithmic Cost (energy efficiency, reduced algorithmic complexiy)complexiy)

Sources of overheadsSources of overheads• Idle listeningIdle listening

– Since a node does not know when it will be the receiver of a Since a node does not know when it will be the receiver of a message from one of its neighbors, it must keep its radio in message from one of its neighbors, it must keep its radio in receive mode all the time.receive mode all the time.

• CollisionCollision– Energy used during transmission and reception is wasted.Energy used during transmission and reception is wasted.– RTS/CTSRTS/CTS

• OverhearingOverhearing– A node may receive packets that are not destined for it.A node may receive packets that are not destined for it.– Turn off the radio.Turn off the radio.

• Protocol overheadProtocol overhead– MAC headers and control packets do not contain application data.MAC headers and control packets do not contain application data.– The data is short.The data is short.

• Traffic fluctuationsTraffic fluctuations– A sudden peak in activity raises the probability of collision.A sudden peak in activity raises the probability of collision.– Much time and energy are spent on waiting in the random Much time and energy are spent on waiting in the random

backoff.backoff.– The performance collapse when the load approaches the channel The performance collapse when the load approaches the channel

capacity.capacity.

Some existing MAC Some existing MAC protocolsprotocols

Some trade offs that people Some trade offs that people used.used.

• The number of the physical channel The number of the physical channel used.used.

• The degree of organization between The degree of organization between nodesnodes

• The way in which a node is notified of The way in which a node is notified of an incoming message.an incoming message.

Is it good to use multiple Is it good to use multiple channels?channels?

• Multiple channelsMultiple channels– FDMA or CDMAFDMA or CDMA

• It requires a rather complicated radio consming It requires a rather complicated radio consming considerable of energy.considerable of energy.

– Tone or control signalTone or control signal• Use a second, extremely low power radio that Use a second, extremely low power radio that

can be used for signaling an intended receiver to can be used for signaling an intended receiver to wake up.wake up.

Do networks need to be Do networks need to be organized?organized?

• Contention based protocol (random)Contention based protocol (random)– Low implementation complexityLow implementation complexity– Ad-hoc natureAd-hoc nature– Flexible to accommodate mobile nodes and traffic Flexible to accommodate mobile nodes and traffic

fluctuationfluctuation• Frame-based TDMA protocolFrame-based TDMA protocol

– Lack of collisionLack of collision– Lack of overhearingLack of overhearing– No idle-listeningNo idle-listening

• Slotted SystemSlotted System– E.g. S-MAC, T-MACE.g. S-MAC, T-MAC– Duty cycleDuty cycle

• Nodes are awake in the first part of each slot and go to sleep Nodes are awake in the first part of each slot and go to sleep in the second part.in the second part.

How do the receivers get How do the receivers get notified?notified?

• Schedule-based protocolsSchedule-based protocols– ScheduleSchedule

• Data transfers are scheduled ahead of time, so receiving Data transfers are scheduled ahead of time, so receiving nodes know exactly when to turn on the radio.nodes know exactly when to turn on the radio.

• Contention-based protocolsContention-based protocols– ListeningListening

• Idle-listening Idle-listening – Use duty cycleUse duty cycle

Individually (e.g. Low power listening)Individually (e.g. Low power listening) Collectively per slot (e.g. S-MAC)Collectively per slot (e.g. S-MAC)

– Use a cellular topology with access points (e.g. Sift)Use a cellular topology with access points (e.g. Sift)

– WakeupWakeup• Send a wake up tone over a second, very low-power radio.Send a wake up tone over a second, very low-power radio.

RoadmapRoadmap




Characteristics ofCharacteristics of contention-based protocols contention-based protocols

• Needs to reduce the energy Needs to reduce the energy consumption caused by collisions, consumption caused by collisions, overhearing, and idle-listening.overhearing, and idle-listening.

Low Power Listening Low Power Listening and Preamble Samplingand Preamble Sampling

• Used in Aloha and B-MACUsed in Aloha and B-MAC• Turn the radio off repeatedlyTurn the radio off repeatedly• Header starts off with a preamble that is used to Header starts off with a preamble that is used to

notify receivers of the upcoming transfer.notify receivers of the upcoming transfer.– Shift the cost from the receiver (the frequent case) to Shift the cost from the receiver (the frequent case) to

the transmitter (the rare case).the transmitter (the rare case).

• No collision avoidance to save protocol overhead.No collision avoidance to save protocol overhead.• Slight increase in latencySlight increase in latency• Minor reduction in throughputMinor reduction in throughput

WiseMACWiseMAC

• Long preamble are not necessary when the Long preamble are not necessary when the sender sender knowsknows the sampling schedule of the the sampling schedule of the intended receiver.intended receiver.

• Nodes maintain the schedule offsets of their Nodes maintain the schedule offsets of their neighbors through piggy backed information.neighbors through piggy backed information.

• To account for clock drift, the preamble is To account for clock drift, the preamble is extended with a time proportional to the extended with a time proportional to the length of the interval since the last message length of the interval since the last message exchange.exchange.

• Not effective for broadcast.Not effective for broadcast.– Preamble must span the sampling points of all Preamble must span the sampling points of all

neighbors and account for drift.neighbors and account for drift.

RoadmapRoadmap




Characteristics of slotted Characteristics of slotted protocolsprotocols

• They address the inherent idle They address the inherent idle listening overhead by listening overhead by synchronizingsynchronizing the nodes, and implementing a duty the nodes, and implementing a duty cycle within each slot.cycle within each slot.

S-MACS-MAC

• Broadcast SYNC packets at the beginning Broadcast SYNC packets at the beginning of a slot, so other nodes receiving these of a slot, so other nodes receiving these packets can adjust their clock.packets can adjust their clock.

• Collision avoidance and overhearing Collision avoidance and overhearing avoidance.avoidance.– RTS/CTS handshakeRTS/CTS handshake

T-MACT-MAC

• Improvement of S-MACImprovement of S-MAC

• Use adaptive duty cycle to account for Use adaptive duty cycle to account for traffic fluctuation.traffic fluctuation.

• Fixed slot time and variable active time.Fixed slot time and variable active time.– Use time-out to determine the end of active Use time-out to determine the end of active

time.time.– Time out value is 15 msTime out value is 15 ms

• Early sleep problemEarly sleep problem– S wants to send data to RS wants to send data to R

N S RSending msg

Keep silence

Go to sleep

DMACDMAC

• Convergecast onlyConvergecast only

• Each node repeatedly executes a basic Each node repeatedly executes a basic sequence of 1 receive, 1 send, n sleep slots.sequence of 1 receive, 1 send, n sleep slots.

• Ensures that the sequence are staggered to Ensures that the sequence are staggered to match the structure of the convergecast match the structure of the convergecast tree rooting at the sink node.tree rooting at the sink node.

RoadmapRoadmap




Characteristics of Characteristics of TDMA-based protocolsTDMA-based protocols

•Collision free Collision free

•No idle listening.No idle listening.

Scheduling of Scheduling of TDMA-based protocolsTDMA-based protocols• Sink-based schedulingSink-based scheduling

– The traffic within each cluster is scheduled by a sink node who is connected to the wired backbone The traffic within each cluster is scheduled by a sink node who is connected to the wired backbone network.network.

– Sink node will decide which node will sense, relay or sleep.Sink node will decide which node will sense, relay or sleep.– All nodes need to directly communicate with the sink node.All nodes need to directly communicate with the sink node.

• Static schedulingStatic scheduling– Fixed schedule.Fixed schedule.– Acceptable performance for typical communication patterns.Acceptable performance for typical communication patterns.

• Rotation dutiesRotation duties– The cost of serving as an access point may be amortized over multiple nodes by The cost of serving as an access point may be amortized over multiple nodes by

rotating duties.rotating duties.

• Partitioned schedulingPartitioned scheduling– Each slot serves as a mini TDMA frame and consists of a contention phase, a traffic Each slot serves as a mini TDMA frame and consists of a contention phase, a traffic

control section and a data section.control section and a data section.– Active nodes owns a slot and always transmit in its slot.Active nodes owns a slot and always transmit in its slot.– Passive nodes must content for a slot.Passive nodes must content for a slot.– All nodes listen to their neighbors’ control section.All nodes listen to their neighbors’ control section.

• Replicated schedulingReplicated scheduling– Node broadcasts its traffic and the identity of their one-hop neighborNode broadcasts its traffic and the identity of their one-hop neighbor– Use a distributed hash function to compute the winner of each slot.Use a distributed hash function to compute the winner of each slot.– Collision-free assignment.Collision-free assignment.– High channel utilization but at cost of high algorithmic complexity.High channel utilization but at cost of high algorithmic complexity.

LMACLMAC

• Each active node is in control of a slot.Each active node is in control of a slot.• Does not acknowledge on the reception of the Does not acknowledge on the reception of the

data.data.• Ensure collision-free transmission by having Ensure collision-free transmission by having

nodes select a slot number that is not in use nodes select a slot number that is not in use within a two-hop neighborwithin a two-hop neighbor

• Broadcast a bit-set detailing which slots are Broadcast a bit-set detailing which slots are occupied by the one-hop neighbors.occupied by the one-hop neighbors.

• Nodes must always listen to the control section of Nodes must always listen to the control section of all slots in a frame.all slots in a frame.– Sample the unused slots first to determine whether Sample the unused slots first to determine whether

there is any activitythere is any activity

RoadmapRoadmap




Performance in empty Performance in empty networknetwork

• LatencyLatency– LMACLMAC

• A node want to send pkts must wait until A node want to send pkts must wait until its slotits slot

– S-MAC/T-MACS-MAC/T-MAC• Source node must wait for the next active Source node must wait for the next active

periodperiod• Early sleep in T-MACEarly sleep in T-MAC

• ThroughputThroughput– LPLLPL

• Collision generated by hidden nodesCollision generated by hidden nodes– S-MACS-MAC

• Loss is caused at contention periodLoss is caused at contention period• RTS/CTS overheadRTS/CTS overhead

– T-MACT-MAC• Adaptive duty cycleAdaptive duty cycle• Fixed contention windowFixed contention window

Homogeneous unicastHomogeneous unicast

• Delivery ratio collapsed at Delivery ratio collapsed at some point for LPL, S-MAC and some point for LPL, S-MAC and T-MACT-MAC

• The energy consumption jump The energy consumption jump in T-MAC and LPL is due to in T-MAC and LPL is due to breakdown. Most of the breakdown. Most of the energy is used in energy is used in retransmissionretransmission

• The energy consumption of The energy consumption of the 802.11 protocol decreases the 802.11 protocol decreases for higher load.for higher load.– Overhearing avoidance Overhearing avoidance

mechanismmechanism

Homogeneous broadcastHomogeneous broadcast• LMAC guarantees collision-free LMAC guarantees collision-free

communicationscommunications

• Delivery rates degrade at light Delivery rates degrade at light load, but improve for high loadload, but improve for high load– No retransmissionNo retransmission

• Slotted protocol perform worse Slotted protocol perform worse than contention-based protocolsthan contention-based protocols– In a rather short active period, the In a rather short active period, the

probability of a collision is increased.probability of a collision is increased.– Overhearing is no longer applicable, Overhearing is no longer applicable,

forcing the radio to be on all the timeforcing the radio to be on all the time

• For LPLFor LPL– Longer preamble cause more Longer preamble cause more

collisioncollision

Local gossipLocal gossip

• In essence local gossip is In essence local gossip is a mixture of empty a mixture of empty workload and workload and homogeneous workloadhomogeneous workload

• The delivery rate The delivery rate associated with local associated with local gossip are completely gossip are completely determined by determined by homogeneous unicast homogeneous unicast component.component.

• Collision occur less Collision occur less frequently due to a frequently due to a relatively large number of relatively large number of edge nodes with inactive edge nodes with inactive neighbors.neighbors.

ConvergecastConvergecast• The generated load that The generated load that

can be handled is much can be handled is much lower than with lower than with homogeneous unicast, homogeneous unicast, since the average hop since the average hop length is large.length is large.

• The performace do not The performace do not simply scale with the path-simply scale with the path-length vector.length vector.– LMAC cannot handle heavy LMAC cannot handle heavy

load around the sink, since load around the sink, since each node can use only one each node can use only one slot.slot.

RoadmapRoadmap




ObservationObservation• Communication grouping considered Communication grouping considered

harmfulharmful– Slotted protocolsSlotted protocols

• Very low duty cycles can be obtained, but at the expense of high Very low duty cycles can be obtained, but at the expense of high latency and a collapse under high loadlatency and a collapse under high load

– TDMA-based protocolsTDMA-based protocols• High latency and reduced throughput under high loadHigh latency and reduced throughput under high load• Energy consumption does not deteriorateEnergy consumption does not deteriorate

– Contention-based protocolsContention-based protocols• LPL is most flexibleLPL is most flexible• Sampling approach depends on the radio’s ability to switch quicklySampling approach depends on the radio’s ability to switch quickly

• Collision avoidance considered Collision avoidance considered prohibitiveprohibitive– RTS/CTS prevents collision due to hidden nodesRTS/CTS prevents collision due to hidden nodes– Reduce the effective channel capacityReduce the effective channel capacity– Small messages in sensor networks. (A high overhead)Small messages in sensor networks. (A high overhead)

• Adapteivity considered essentialAdapteivity considered essential– Static protocol either consume too much energy under low load, or Static protocol either consume too much energy under low load, or

throttle throughput too much under high loads.throttle throughput too much under high loads.

ConclusionConclusion

• No MAC protocol outperform the No MAC protocol outperform the others in all ways.others in all ways.

• Adaptivity is mandatory to handle Adaptivity is mandatory to handle the generic local gossip and the generic local gossip and convergecast communication, since convergecast communication, since these patterns showing traffic these patterns showing traffic fluctuation both in time and space.fluctuation both in time and space.

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Energy-Efficient Medium Access Control

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