A Glimpse at Three Wireless A Glimpse at Three Wireless Networking Problems Networking Problems Bob Kinicki Bob Kinicki Computer Science Department Computer Science Department [email protected][email protected]Colloquium October 5, 2007 Colloquium October 5, 2007
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A Glimpse at Three A Glimpse at Three Wireless Networking Wireless Networking
ProblemsProblems Bob KinickiBob Kinicki Computer Science DepartmentComputer Science Department [email protected]@cs.wpi.edu
Colloquium October 5, 2007Colloquium October 5, 2007
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OutlineOutlineOutlineOutline Thoughts and Mini-MotivationThoughts and Mini-Motivation Wireless Networking PrimerWireless Networking Primer #1#1 Dynamic Rate AdaptationDynamic Rate Adaptation
– Performance problems with ARFPerformance problems with ARF– Rate Adaptation AlgorithmsRate Adaptation Algorithms
• RBAR, CARA, RFT and CARAF
Wireless Sensor Networks (WSNs)Wireless Sensor Networks (WSNs) #2 #2 Dynamic Cluster FormationDynamic Cluster Formation #3#3 Power-Aware MAC ProtocolsPower-Aware MAC Protocols
– SMAC, TMAC, WiseMAC, SCP-MAC andSMAC, TMAC, WiseMAC, SCP-MAC and
CrankshaftCrankshaft
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My Research SpaceMy Research SpaceMy Research SpaceMy Research Space
NetworkingNetworking
Wireless NetworkingWireless Networking
WirelessWirelessSensorSensor
NetworksNetworks
19831983
20032003
20062006
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The Future of Sensor The Future of Sensor Networks? Networks?
The Future of Sensor The Future of Sensor Networks? Networks?
Wireless PrimerWireless Primer
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LAN TerminolgyLAN TerminolgyLAN TerminolgyLAN Terminolgy802.3::802.3::
ZigBeeZigBee 802.11-based802.11-basedlower data rates, lower powerlower data rates, lower power
Bluetooth::Bluetooth::TDMATDMA
- wireless - wireless Personal Area Networks (PANs)Personal Area Networks (PANs) that that provide secure, globally unlicensed short-range radio provide secure, globally unlicensed short-range radio communication.communication.
– Clusters with max of 8: cluster head + 7 nodesClusters with max of 8: cluster head + 7 nodes
WSNs
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Wireless LAN ProtocolsWireless LAN ProtocolsWireless LAN ProtocolsWireless LAN Protocols
(a)(a) A sending an RTS to B. A sending an RTS to B.
(b)(b) B responding with a CTS to A. B responding with a CTS to A.
Tanenbaum slide
node D isnode D ispossible hidden terminalpossible hidden terminal
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Virtual Channel Sensing in Virtual Channel Sensing in CSMA/CACSMA/CA
Virtual Channel Sensing in Virtual Channel Sensing in CSMA/CACSMA/CA
C (in range of A) C (in range of A) receives the RTS and based receives the RTS and based on information in RTS creates a on information in RTS creates a virtual channel virtual channel busy busy NAV(Network Allocation Vector).NAV(Network Allocation Vector).
D (in range of B) D (in range of B) receives the CTS and receives the CTS and creates a shorter creates a shorter NAV.NAV.
Tanenbaum slide
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RTS/CTS can reduce collisions.RTS/CTS can reduce collisions. RTS/CTS can guard against and RTS/CTS can guard against and reduce hidden terminals.reduce hidden terminals.
RTS/CTS adds overhead that RTS/CTS adds overhead that reduces throughput.reduces throughput.
Normally, RTS/CTS is turned off!Normally, RTS/CTS is turned off!
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Uses recent history and probes:Uses recent history and probes: ARF,ARF, AARF, AARF, SampleRateSampleRate
Long interval smoothing:Long interval smoothing: ONOE,ONOE, SampleRateSampleRateMultiple rates:Multiple rates: MultiRateRetry, AMRR, RRAA MultiRateRetry, AMRR, RRAAUses RTS/CTS:Uses RTS/CTS: RBAR,RBAR, OAR, CROAR, OAR, CROAR, CARACARAUses RSSI to approximate SNR, each node Uses RSSI to approximate SNR, each node
maintains 12 dynamic RSS thresholds:maintains 12 dynamic RSS thresholds: LALAPuts checksum on header and use NACK to signal Puts checksum on header and use NACK to signal
link loss error:link loss error: LD-ARFLD-ARFTable lookup with thresholds:Table lookup with thresholds:
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Auto Rate Fallback (ARF) Auto Rate Fallback (ARF) Auto Rate Fallback (ARF) Auto Rate Fallback (ARF)
If If twotwo consecutive ACK frames are not consecutive ACK frames are not received correctly, the second retry and received correctly, the second retry and subsequent transmissions are done at a subsequent transmissions are done at a lower rate and a timer is started.lower rate and a timer is started.
When the number of successfully When the number of successfully received ACKs reaches received ACKs reaches 1010 or the timer or the timer goes off, a goes off, a probe frame probe frame is sent at the is sent at the next higher rate. However, if an ACK is next higher rate. However, if an ACK is NOT received for this frame, the rate is NOT received for this frame, the rate is lowered back and the timer is restarted. lowered back and the timer is restarted.
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ARF and AARFARF and AARFARF and AARFARF and AARF
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Receiver Based Auto Rate Receiver Based Auto Rate (RBAR) (RBAR)
Receiver Based Auto Rate Receiver Based Auto Rate (RBAR) (RBAR)
Receivers control sender’s Receivers control sender’s transmission rate.transmission rate.
RTS and CTS are modified to RTS and CTS are modified to contain info on size and rate contain info on size and rate {not {not 802.11 compatible}.802.11 compatible}.
Uses analysis of RTS reception Uses analysis of RTS reception (RSSI) to estimate SNR and send (RSSI) to estimate SNR and send choice back to sender in CTS.choice back to sender in CTS.
Receiver picks rate based on apriori Receiver picks rate based on apriori SNR thresholds in a lookup table.SNR thresholds in a lookup table.
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RTS ProbingRTS Probing{Idea: Assume all RTS/CTS transmission {Idea: Assume all RTS/CTS transmission failures after a successful RTS/CTS failures after a successful RTS/CTS exchange must be due to channel exchange must be due to channel errors.errors.
(Note – this assumes hidden terminals (Note – this assumes hidden terminals are not possible) are not possible) }}
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RTS ProbingRTS ProbingRTS ProbingRTS Probing
CARA-1CARA-1 Data frame transmitted without Data frame transmitted without RTS/CTS.RTS/CTS.
If the transmission fails, RTS/CTS If the transmission fails, RTS/CTS exchange is activated for the next exchange is activated for the next retransmission. If this retransmission retransmission. If this retransmission fails fails {assume channel quality {assume channel quality problem}problem}, then the rate is lowered., then the rate is lowered.
If retransmission with RTS/CTS is If retransmission with RTS/CTS is successful successful {assume collision {assume collision occurred},occurred}, stay at same rate and send stay at same rate and send next frame without RTS/CTS. next frame without RTS/CTS.
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Fragmenting a frame can increase Fragmenting a frame can increase the probability of the fragment the probability of the fragment being received successfully.being received successfully.
Propose a dynamic fragmentation Propose a dynamic fragmentation scheme with different scheme with different fragmentation thresholds based fragmentation thresholds based on different channel conditions.on different channel conditions.
Namely, fragment sizes vary with Namely, fragment sizes vary with the chosen adaptation rate.the chosen adaptation rate.
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CARAF (CARA with CARAF (CARA with Fragmentation)Fragmentation)
CARAF (CARA with CARAF (CARA with Fragmentation)Fragmentation)
Dan Courcey’s MS thesis:Dan Courcey’s MS thesis:Combine CARA with Fragmentation.Combine CARA with Fragmentation.Top Level Scheme:Top Level Scheme: Upon CCA determination of collision, Upon CCA determination of collision, use fragmentation.use fragmentation.
If CCA shows idle, initiate RTS/CTS If CCA shows idle, initiate RTS/CTS probe.probe.
{Investigate how to vary fragment size {Investigate how to vary fragment size to maximize throughput and increase to maximize throughput and increase the likelihood of CCA case 2}the likelihood of CCA case 2}
A distributed connection of nodes that A distributed connection of nodes that coordinate to perform a common task.coordinate to perform a common task.
In many applications, the nodes are In many applications, the nodes are battery poweredbattery powered and it is often very and it is often very difficult to recharge or change the difficult to recharge or change the batteries.batteries.
Prolonging network lifetime is a critical Prolonging network lifetime is a critical issue.issue.
Sensors often have long period between Sensors often have long period between transmissions (e.g., in seconds).transmissions (e.g., in seconds).
Thus, a good WSN MAC protocol needs Thus, a good WSN MAC protocol needs to be to be energy efficient.energy efficient.
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Another attribute is Another attribute is scalability to scalability to changechange in network size, node in network size, node density and topology.density and topology.– In general, nodes can die, join later or In general, nodes can die, join later or
be mobile.be mobile. Often high bandwidth is Often high bandwidth is not not importantimportant..
Nodes can take advantage of short-Nodes can take advantage of short-range, mulit-hop communication to range, mulit-hop communication to conserve energy.conserve energy.
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Sources of energy waste:Sources of energy waste:– Idle listening, collisions, Idle listening, collisions,
overhearing and control overhead.overhearing and control overhead.– Idle listening dominates Idle listening dominates
(measurements show idle listening (measurements show idle listening consumes between 50-100% of the consumes between 50-100% of the energy required for receiving.)energy required for receiving.)
Idle listening:: listen to receive Idle listening:: listen to receive possible traffic that is not sent.possible traffic that is not sent.
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Power MeasurementsPower MeasurementsPower MeasurementsPower Measurements
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Duty cycle:: ratio between listen time Duty cycle:: ratio between listen time and the full listen-sleep cycle.and the full listen-sleep cycle.
central approach – lower the duty central approach – lower the duty cycle by turning the radio off part of cycle by turning the radio off part of the time.the time.
• Three techniques to reduce the Three techniques to reduce the duty cycle:duty cycle:• TDMATDMA• Schedule contention periodsSchedule contention periods• LPL (Low Power Listening)LPL (Low Power Listening)
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Techniques to Reduce Idle Techniques to Reduce Idle ListeningListening
Techniques to Reduce Idle Techniques to Reduce Idle ListeningListening
TDMATDMA requires cluster-based or requires cluster-based or centralized control.centralized control.
SchedulingScheduling – ensures short listen period – ensures short listen period when transmitters and listeners can when transmitters and listeners can rendezvous and other periods where rendezvous and other periods where nodes sleep (turn off their radios).nodes sleep (turn off their radios).
LPLLPL – nodes wake up briefly to check for – nodes wake up briefly to check for channel activity without receiving data.channel activity without receiving data.
– If channel is idle, node goes back to sleep.If channel is idle, node goes back to sleep.– If channel is busy, node stays awake to If channel is busy, node stays awake to
receive data.receive data.– A long preamble (longer than poll period) is A long preamble (longer than poll period) is
used to assure than preamble intersects used to assure than preamble intersects with polls.with polls.
Periodically select Periodically select new cluster heads new cluster heads to minimize power to minimize power consumption and consumption and maximize WSN maximize WSN lifetime.lifetime.
More complex More complex problem when size problem when size of cluster changes of cluster changes dynamically.dynamically.
As time goes by, As time goes by, some sensor some sensor nodes die!nodes die!
Not worried about Not worried about coverage issues!coverage issues!
X
X
X
X
X
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Rick Skowyra’s MS thesis:Rick Skowyra’s MS thesis:
‘ ‘Energy Efficient Dynamic Energy Efficient Dynamic Reclustering Strategy for WSNs’Reclustering Strategy for WSNs’– ‘‘Leach-like’ with a fitness function Leach-like’ with a fitness function
and periodic reclustering.and periodic reclustering.– He hopes to design a distributed He hopes to design a distributed
genetic algorithm to speed the genetic algorithm to speed the recluster time.recluster time.
3.3. Low Power Listening Low Power Listening: LPL, BMAC, : LPL, BMAC, WiseMACWiseMAC
Cross-LayeringCross-Layering: SP, BSD: SP, BSD
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SMACSMACSMACSMAC All nodes periodically listen, sleep and wakeup. All nodes periodically listen, sleep and wakeup.
Nodes listen and send during the active period Nodes listen and send during the active period and turn off their radios during the sleep and turn off their radios during the sleep period.period.
The beginning of the active period is a SYNC The beginning of the active period is a SYNC period used to accomplish periodic period used to accomplish periodic synchronization and remedy clock drift.synchronization and remedy clock drift.
Following the SYNC period, data may be Following the SYNC period, data may be transferred for the remainder of the active transferred for the remainder of the active period using RTS/CTS for unicast transmissions.period using RTS/CTS for unicast transmissions.
Long frames are fragmented and transmitted Long frames are fragmented and transmitted as a burst.as a burst.
SMAC controls the duty cycle to tradeoff energy SMAC controls the duty cycle to tradeoff energy for delay.for delay.
However, as density of WSN grows, SMAC However, as density of WSN grows, SMAC incurs additional overhead in maintaining incurs additional overhead in maintaining neighbors’ schedules.neighbors’ schedules.
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SMACSMACSMACSMAC
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TMACTMACTMACTMAC
TMAC employs an adaptive duty TMAC employs an adaptive duty cycle by using a very short cycle by using a very short listening window at the beginning listening window at the beginning of each active period.of each active period.
After the SYNC portion of the After the SYNC portion of the active period, RTS/CTS is used in active period, RTS/CTS is used in listening window. If no activity listening window. If no activity occurs, the node goes to sleep.occurs, the node goes to sleep.
TMAC saves power at the cost of TMAC saves power at the cost of reduced throughput and additional reduced throughput and additional delay.delay.
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TMACTMACTMACTMAC
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WiseMACWiseMACWiseMACWiseMAC
Algorithm focused on Algorithm focused on downlink protocoldownlink protocol
for infrastructure WSNs:for infrastructure WSNs:– Access Point (AP) is assumed to have wired Access Point (AP) is assumed to have wired
link to Internet and not battery-powered.link to Internet and not battery-powered. Based on preamble sampling.Based on preamble sampling. WiseMac regularly samples (via WiseMac regularly samples (via listening) for a short duration during listening) for a short duration during preamble.preamble.– All sensor nodes sample with same All sensor nodes sample with same
constant period constant period TTWW ..
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WiseMACWiseMACWiseMACWiseMAC Normally, wake-up preamble needs to be of Normally, wake-up preamble needs to be of size size TTWW . .
– This implies low power use when WSN is idle.This implies low power use when WSN is idle.– But this yields large power consumption overhead But this yields large power consumption overhead
for reception.for reception. WiseMAC AP learns and keeps the sampling WiseMAC AP learns and keeps the sampling schedule of all sensors in a up-to-date table.schedule of all sensors in a up-to-date table.
Sensors’ ACKs provide info for the table.Sensors’ ACKs provide info for the table. WiseMAC then minimizes the preamble WiseMAC then minimizes the preamble duration, duration, TTPP . .
Needs to deal with clock drift to get this Needs to deal with clock drift to get this right.right.
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WiseMACWiseMACWiseMACWiseMAC
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Zigbee MACZigbee MACZigbee MACZigbee MAC
802.11 MAC PSM (Power Save 802.11 MAC PSM (Power Save Mode) uses beacon frames to Mode) uses beacon frames to coordinate and periodic wake-up coordinate and periodic wake-up by sensor nodes.by sensor nodes.
Mike Putnam’s thesis:Mike Putnam’s thesis:
‘‘A Beaconless Protocol for A Beaconless Protocol for Improving Energy Efficiency in Improving Energy Efficiency in Wireless Sensor Networks’Wireless Sensor Networks’
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WiseMACWiseMACWiseMACWiseMAC
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With channel polling (LPL scheme), With channel polling (LPL scheme), receiver efficiency is gained receiver efficiency is gained through cost to sender.through cost to sender.
LPLs are very sensitive to tuning LPLs are very sensitive to tuning for neighborhood size and traffic for neighborhood size and traffic rate.rate.
By synchronizing channel polling By synchronizing channel polling times of all neighbors, long times of all neighbors, long preambles are eliminated and preambles are eliminated and ultra-low duty cycles (below the ultra-low duty cycles (below the LPL 1-2% limits) are possible.LPL 1-2% limits) are possible.
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The issue is knowing my neighbors’ The issue is knowing my neighbors’ schedule information.schedule information.
SCP piggybacks schedule info on SCP piggybacks schedule info on data packets when possible or a data packets when possible or a node broadcasts its schedule in a node broadcasts its schedule in a SYNC packet in synch period (as in SYNC packet in synch period (as in SMAC)SMAC)
Knowing schedules Knowing schedules short wakeup short wakeup tone.tone.
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SCP-MACSCP-MACSCP-MACSCP-MAC
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CrankshaftCrankshaftCrankshaftCrankshaft
Designed specifically for dense WSNs.Designed specifically for dense WSNs. Employs channel polling mechanism similar to Employs channel polling mechanism similar to
SCP-MAC.SCP-MAC. Employs synchronization, framing and slotting Employs synchronization, framing and slotting
mechanisms similar to TDMA-based LMAC.mechanisms similar to TDMA-based LMAC. Unlike LMAC, Crankshaft schedules receivers Unlike LMAC, Crankshaft schedules receivers
rather than senders.rather than senders. Basic principle: Basic principle: nodes are only awake to receive nodes are only awake to receive
messages at fixed offsets from the start of a messages at fixed offsets from the start of a frame.frame.
The hope is to develop modified version of The hope is to develop modified version of Crankshaft.Crankshaft.
I need students interested in exploring this I need students interested in exploring this idea!!idea!!
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CrankshaftCrankshaftCrankshaftCrankshaft
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Focused on two traffic types that Focused on two traffic types that are common in WSNs:are common in WSNs:
Convergecast – monitoring trafficConvergecast – monitoring traffic– All sensor nodes periodically send All sensor nodes periodically send
data to a sink node (either AP or data to a sink node (either AP or sensor cluster head)sensor cluster head)
Broadcast floods – packets sent Broadcast floods – packets sent in the other direction to either in the other direction to either send routing update or to send routing update or to distribute queries over the WSN.distribute queries over the WSN.
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Energy Conservation ResultsEnergy Conservation ResultsEnergy Conservation ResultsEnergy Conservation Results
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LatencyLatencyLatencyLatency
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