Wireless Sensor Networks The MAC layer

Wireless Sensor NetworksThe MAC sublayerSupervised by professor: Ivan StojmenovicDone by: Yaser Fouad

OutlineIntroductionThe Medium Access Control sublayerProblem definition different Medium Access protocols overviewConclusionReferences

Characteristics of sensor networksheavily deploymentLimited powerChanging network topologyMultihop routing and low range communicationsNon uniform data distributionDifferent from convential mobile networks as it has short range and multihop communications and limited power Different from ordinary ad-hoc as we have dense deployment, changing network topology due to dying nodes or environmental effects like nodes changing their positions due to wind or flood it is also infrastructure based as there is a common source or sink to absorb the collected dataAs usually there is a very long waiting time and then periods of very dense data3

The MAC sublayerMedium Access ControlSubdivision of the data link layerDifferent from Multiple accessMeasures of Performance for MAC protocolsThroughput efficiencyStabilityScalabilityAccess delay Power conservation

The data link layer consists of mac layer + error control protocols , however their work is completely different .Its protocols are designed for scheduling how the nodes access the medium for efficient transmission without collisions and minimum power.Throughput efficiency (how much is data)Stability(abilitiy to work in harsh conditions and changing tropoligies)Delay time(between first transmission and reception)Power conservation(how much power is consumed for data tranmission reception,routing )4The MAC sublayer for WSNChanges due to WSNs characteristics:Throughput efficiency

Latency

Power consumption

Problem definitionHow to preserve the energy to increase the lifetime of the WSN ? Use Protocols that aim mainly to increase the sleep periods as much as possible

Other problems Hidden/Exposed terminal problem

Deadlock/Livelock problem

MAC layer protocols for WSNsActive research area for three decadesDifferent categorization criteriaOne criteria is based on type of assignmentSome protocols use a combination

Sparce Topology and Energy Managemnt(STEM)Reduction of power consumption is the main targetDesigned mainly for networks with bursts of dataUses two transceivers (one low power and one high power(

The data channel remains asleep and the wake up channel oscillates between wake up and sleep periods and when it receives a wake up order it wakes up the data channel and starts to follow a standard MAC procedure.9STEM continuedSTEM-TSingle tone(simple)High latencyResults in overhearingSTEM-BMore complexLower LatencyHigh energy consumptionTransmitter sends a wak up tone and all the neighbours wake up and when others interpret that it is not theirs they shut down again . Trnasmitted over a full wake up period therefore high latency however the transceiver is far more simpler and consumes less power. STEM-B the wake up is a beacon containing the Tx and Rx addresses , replies with ACK (the wait time is on average T/2) . Transceiver is more complex and consumes more power.10Carrier Sense Multiple Access (CSMA)Based on channel sensingNo controller neededTwo major categories:Persistent and non persistentTwo variation of persistent came along afterP persistent 1 persistent

Sense the channel first and if it is idle, I can start transmitting the dataKeeps listening till the channel becomes idle and this results in large energy lossDue to the fact that many sensors might be transmitting at the same time because in the wsn the channel remains idle for a long time then many sensors suddenly start transmitting data(bursty channel)However they give priority to the routed data as it cost more energy (went through many hopes)Distribute the load and avoid congestions (and data collisions due to hidden nodes)When the node senses the channel and find it busy it closes the Tx then reopen it again after a certain delay .(also helps in avoiding collisions).A certain packet that can propagate back to the deep back nodes to reduce the rates to balance the ratio between originated and rate through traffic to achieve fairness for nodes close to the gateway.

11CSMA continuedMajor problem is the hidden/exposed terminalA busy tone was suggested as a solutionResults in higher complexity Another technique was suggested(CSMA/CA)Based on RTS ,CTS and random back offMight still have some collisions in RTS

Low Energy Adaptive Cluster Hierarchy (LEACH)Based on some assumptionsHeavy deploymentAbility to contact the sink in one hopReduce Cluster head and following nodesMore load on cluster heads

LEACH continuedWe have two phases for this protocol

Self-organizing Medium Access Control for Sensor networks(SMACS)Distributed infrastructure algorithmBased on some assumptionsAvailable spectrum is much larger Most of the nodes are stationary Uses unidirectional linksUses four messages format for link buildingSMACS continuedHow links are built:

SMACS continuedSMACS protocol matlab simulation:

SMACS improvementProblems with algorithm: should accommodate largest depth(Latency and limitation on number of nodes)Nodes wake up in their assigned slots even with no data (wasted power)Suggestion( adaptive frame usage)XYCBDAFEACBEDFCode 1Code 2

802.15.4Expected to have commercial impactFeaturesBased on low data rate communicationsScalable and self maintainedEnable low power and cost operationCombines scheduled and contention based schemesDivides nodes into two main categoriesReduced Function DeviceFull Function Device802.15.4 continuedCharacteristicsRFDs can only communicate with the FFDsRouting is done through FFDs communicationsSlots are unidirectional FFDs are awake much more than RFDsHas two main modes of operationBeaconed modeNonbeaconed mode802.15.4 (Beaconed mode)Frame structure

802.15.4 (Nonbeaconed mode)No beacons and therefore no GTSDepends mainly on CSMA/CAChecks the channel once for being idleCoordinators are always awakeSmall active period after transmission for devices

ConclusionDifferent protocols available Medium Access is different from Multiple AccessAll MAC protocols for WSN aim at reducing energy usageSleeping is the only state that consumes almost no energyThe 802.15.4 protocol has a potential to hit the commercial market

23ReferencesIan F.Akyildiz, Weilian Su, Yogesh Sankarasubramaniam and Erdal Cayirci, Georgia institute of technology,A survey on sensor networks " IEEE communications magazine , August 2002Holger Karl, Andreas Willig, Protocols and Architectures for wireless sensor networks, John Wiley &sons , Ltd , 2005G. Lu, B. Krishnamachari, and C. S. Raghavendra. Performance Evaluation of the IEEE 802.15.4 MACfor Low-Rate Low-Power Wireless Networks. pages 701706, Phoenix, AZ, April 2004.K. Sohrabi, J. Gao, V. Ailawadhi, and G. J. Pottie. Protocols for Self-Organization of a Wireless SensorNetwork. IEEE Personal Communications, 2000.W. B. Heinzelman, A. P. Chandrakasan, and H. Balakrishnan. An Application-Specific Protocol Architecturefor Wireless Microsensor Networks. IEEE Transactions on Wireless Networking, 1(4): 660670,2002.Thank you for your attention

My questionsWhy did the 802.15.4 protocol adopt Carrier Sensing Multiple Access to work with?

Because CSMA doesnt have significant overhead , suitable for scalability and doesnt waste energy if no data is to be sent

My questions continuedIf A FFD consumes 1 watt per day to remain active and receive data from nodes when working in the beaconed mode with an active period of 10 slots and the available power in its battery is 15 watts and it has on average 10 followers with an activity probability of 40%. what would you suggest to enable the FFD to function for at least 20 days period?

Solution:1 day 1 watts20 days 20 watts >15

we should reduce the activity period to preserve power10 slots 1 watts?? slots 15/20 (total available power/number of days of operation)

Number of slots needed=10*(15/20)=7.5 which should be floored to 7 because we have at least 20 daysnumber of active followers at a time=40%* number of followers =0.4*10=4 nodes

then we should assign 4 slots to the GTS and 3 slots for the GAP period

My questions continuedfor 802.14.5 with beacon mode. assuming the slot size to be able to accommodate a 50 kbyte frame define the procedure the RFD will follow to send:A) 25Kbyte frame to the FFD B) 100Kbyte frame to the FFD

SolutionCase Asince frame < available slot sizeRFD sends a transmit request in the CAP periodWait for the assigned slot number in the beacon Wake up in the assigned slot and start transmitting

Case BSince frame size >available slot size A node uses slotted CSMA to transmit the frame in the CAP period wait for immediate acknowledgment as it is not a preserved slot for it