2/17/20051 guest lecture for cs113, ucla medium access control in wireless sensor networks wei ye...
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2/17/20052/17/2005 11Guest lecture for CS113, UCLAGuest lecture for CS113, UCLA
Medium Access Control in Medium Access Control in Wireless Sensor NetworksWireless Sensor Networks
Wei YeWei Ye
USC Information Sciences InstituteUSC Information Sciences Institute
2/17/20052/17/2005 Guest lecture for CS113, UCLAGuest lecture for CS113, UCLA 22
OutlineOutline
• Introduction to MAC
• MAC attributes and trade-offs
• Scheduled MAC protocols
• Contention-based MAC protocols
• Case studies
• Summary
2/17/20052/17/2005 Guest lecture for CS113, UCLAGuest lecture for CS113, UCLA 33
Introduction to MACIntroduction to MAC
• The role of medium access control (MAC)– Controls when and how each node can transmit
in the wireless channel
• Why do we need MAC?– Wireless channel is a shared medium
– Radios transmitting in the same frequency band interfere with each other – collisions
– Other shared medium examples: Ethernet
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Where Is the MAC?Where Is the MAC?
• Network model from Internet
• A sublayer of the Link layer– Directly controls the radio– The MAC on each node only cares about its
neighborhood
Application layer
Transport layer
Network layer
Link/MAC layer
Physical layer
End-to-end reliability, congestion control
Routing
Per-hop reliability, flow control, multiple access
Packet transmission and reception
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What’s New in Sensor Networks?What’s New in Sensor Networks?
• 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
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Next…Next…
• Introduction to MAC
• MAC attributes and trade-offs
• Scheduled MAC protocols
• Contention-based MAC protocols
• Case studies
• Summary
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Primary MAC AttributesPrimary MAC Attributes
• Collision avoidance– Basic task of a MAC protocol
• Energy efficiency– One of the most important attributes for sensor
networks, since most nodes are battery powered
• Scalability and adaptivity– Network size, node density and topology change
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Other MAC AttributesOther MAC Attributes
• Channel utilization– How well is the channel used? Also called
bandwidth utilization or channel capacity
• Latency– Delay from sender to receiver; single hop or
multi-hop
• Throughput– The amount of data transferred from sender to
receiver in unit time
• Fairness– Can nodes share the channel equally?
2/17/20052/17/2005 Guest lecture for CS113, UCLAGuest lecture for CS113, UCLA 99
Energy Efficiency in MAC DesignEnergy Efficiency in MAC Design
• Energy is primary concern in sensor networks
• What causes energy waste?– Collisions
– Control packet overhead
– Overhearing unnecessary traffic
– Long idle time• bursty traffic in sensor-net apps• Idle listening consumes 50—100% of the power
for receiving (Stemm97, Kasten)
Dominant factor
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Classification of MAC ProtocolsClassification of MAC Protocols
• Schedule-based protocols– Schedule nodes onto different sub-channels
– Examples: TDMA, FDMA, CDMA
• Contention-based protocols– Nodes compete in probabilistic coordination
– Examples: ALOHA (pure & slotted), CSMA
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Next…Next…
• Introduction to MAC
• MAC attributes and trade-offs
• Scheduled MAC protocols
• Contention-based MAC protocols
• Case studies
• Summary
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Scheduled Protocols: TDMAScheduled Protocols: TDMA
• Time division multiple access– Divide time into subchannels
– Advantages• No collisions• Energy efficient — easily support low duty cycles
– Disadvantages• Difficult to accommodate node changes• Requires strict time synchronization
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• Master-slave configuration– The master node decides which slave can send
by polling the corresponding slave
– Only direct communication between the master and a slave
– A special TDMA without pre-assigned slots
– Examples• IEEE 802.11 infrastructure mode (CPF)• Bluetooth piconets
Scheduled Protocols: PollingScheduled Protocols: Polling
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Scheduled Protocols: BluetoothScheduled Protocols: Bluetooth
• Wireless personal area network (WPAN)– Short range, moderate bandwidth, low latency– IEEE 802.15.1 (MAC + PHY) is based on
Bluetooth
• Nodes are clustered into piconet– Each piconet has a master and up to 7 active
slaves – scalability problem– The master polls each slave for transmission– CDMA among piconets– Multiple connected piconets form a scatternet
• Difficult to handle inter-cluster communications
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• By Sohrabi and Pottie– Have a pool of independent channels
• Frequency band or spreading code• Potential interfering links select different channels
– Talk to neighbors in different time slots
– Sleep in unscheduled time slots
– Looks like TDMA, but actual multiple access is accomplished by FDMA or CDMA
• Any pair of two nodes can talk at the same time
– Low bandwidth utilization
Scheduled Protocols: Self-OrganizationScheduled Protocols: Self-Organization
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Scheduled Protocols: LEACHScheduled Protocols: LEACH
• Low-Energy Adaptive Clustering Hierarchy — by Heinzelman, et al.– Similar to Bluetooth
– CDMA between clusters
– TDMA within each cluster• Static TDMA frame• Cluster head rotation• Node only talks to cluster head• Only cluster head talks to base station (long dist.)
– The same scalability problem
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Next…Next…
• Introduction to MAC
• MAC attributes and trade-offs
• Scheduled MAC protocols
• Contention-based MAC protocols
• Case studies
• Summary
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• ALOHA– Pure ALOHA: send when there is data
– Slotted ALOHA: send on next available slot
– Both rely on retransmission when there’s collision
• CSMA — Carrier Sense Multiple Access– Listening (carrier sense) before transmitting
– Send immediately if channel is idle
– Backoff if channel is busy• non-persistent, 1-persistent and p-persistent
Contention Protocols: ClassicsContention Protocols: Classics
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• Hidden terminal problem
– CSMA is not enough for multi-hop networks (collision at receiver)
• CSMA/CA (CSMA with Collision Avoidance)– RTS/CTS handshake before send data
– Node c will backoff when it hears b’s CTS
Contention Protocols: CSMA/CAContention Protocols: CSMA/CA
a b cNode a is hidden from c’s carrier sense
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Contention Protocols: MACA and MACAWContention Protocols: MACA and MACAW
• MACA — Multiple Access w/ Collision Avoidance– Based on CSMA/CA
– Add duration field in RTS/CTS informing other node about their backoff time
• MACAW– Improved over MACA
– RTS/CTS/DATA/ACK
– Fast error recovery at link layer
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Contention Protocols: IEEE 802.11Contention Protocols: IEEE 802.11
• IEEE 802.11 ad hoc mode (DCF)– Virtual and physical carrier sense (CS)
• Network allocation vector (NAV), duration field
– Binary exponential backoff
– RTS/CTS/DATA/ACK for unicast packets
– Broadcast packets are directly sent after CS
– Fragmentation support• RTS/CTS reserve time for first (fragment + ACK)• First (fragment + ACK) reserve time for second…• Give up transmission when error happens
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Contention Protocols: IEEE 802.11 (cont.)Contention Protocols: IEEE 802.11 (cont.)
• Power save (PS) mode in IEEE 802.11 DCF– Assumption: all nodes are synchronized and can
hear each other (single hop)– Nodes in PS mode periodically listen for beacons
& ATIMs (ad hoc traffic indication messages)– Beacon: timing and physical layer parameters
• All nodes participate in periodic beacon generation– ATIM: tell nodes in PS mode to stay awake for Rx
• ATIM follows a beacon sent/received• Unicast ATIM needs acknowledgement• Broadcast ATIM wakes up all nodes — no ACK
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Contention Protocols: IEEE 802.11 (cont.)Contention Protocols: IEEE 802.11 (cont.)
• Unicast example of PS mode in 802.11 DCF
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Contention Protocols: Tx Rate ControlContention Protocols: Tx Rate Control
• By Woo and Culler– Based on a special network setup
• A base station tries to collect data equally from all sensors in the network
– CSMA + adaptive rate control
– Promote fair bandwidth allocation to all sensors• Nodes close to the base station forward more
traffic, and have less chances to send their own data
– Helps in congestion avoidance
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Contention Protocols: Piconet Contention Protocols: Piconet
• By Bennett, Clarke, et al.– Not the same piconet in Bluetooth– Low duty-cycle operation — energy efficient
• Sleep for 30s, beacon, and listen for a while• Sending node needs to listen for receiver’s beacon
first, then• CSMA before sending data
– May wait for long time before sending
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Contention Protocols: PAMASContention Protocols: PAMAS
• PAMAS: Power Aware Multi-Access with Signalling — by Singh and Raghavendra– Improve energy efficiency from MACA
– Avoid overhearing by putting node into sleep
– Use separate control and data channels• RTS, CTS, busy tone to avoid collision• Probe packets to find neighbors transmission time
– Increased hardware complexity• Two channels need to work simultaneously,
meaning two radio systems.
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Contention Protocols: ZigBee Contention Protocols: ZigBee
• Based on IEEE 802.15.4 MAC and PHY– Three types devices
• Network Coordinator• Full Function Device (FFD)
– Can talk to any device, more computing power
• Reduced Function Device (RFD)– Can only talk to a FFD, simple for energy conservation
– CSMA/CA with optional ACKs on data packets
– Optional beacons with superframes
– Optional guaranteed time slots (GTS), which supports contention-free access
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Contention Protocols: ZigBee (cont.)Contention Protocols: ZigBee (cont.)
• Low power, low rate (250kbps) radio
• MAC layer supports low duty cycle operation– Target node life time > 1 year
2/17/20052/17/2005 Guest lecture for CS113, UCLAGuest lecture for CS113, UCLA 2929
Next…Next…
• Introduction to MAC
• MAC attributes and trade-offs
• Scheduled MAC protocols
• Contention-based MAC protocols
• Case studies
• Summary
2/17/20052/17/2005 Guest lecture for CS113, UCLAGuest lecture for CS113, UCLA 3030
Case Study 1: S-MACCase Study 1: S-MAC
• By Ye, Heidemann and Estrin
• Tradeoffs
• Major components in S-MAC– Periodic listen and sleep
– Collision avoidance
– Overhearing avoidance
– Massage passing
Latency
FairnessEnergy
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Coordinated SleepingCoordinated Sleeping
• Problem: Idle listening consumes significant energy
• Solution: Periodic listen and sleep
• Turn off radio when sleeping• Reduce duty cycle to ~ 10% (120ms
on/1.2s off)
sleeplisten listen sleep
Latency Energy
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Coordinated SleepingCoordinated Sleeping
• Schedules can differ
• Prefer neighboring nodes have same schedule— easy broadcast & low control overhead
Border nodes: two schedules
or broadcast twice
Node 1
Node 2
sleeplisten listen sleep
sleeplisten listen sleep
Schedule 2
Schedule 1
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Coordinated SleepingCoordinated Sleeping
• Schedule Synchronization – New node tries to follow an existing schedule
– Remember neighbors’ schedules — to know when to send to them
– Each node broadcasts its schedule every few periods of sleeping and listening
– Re-sync when receiving a schedule update
• Periodic neighbor discovery– Keep awake in a full sync interval over long
periods
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Coordinated SleepingCoordinated Sleeping
• Adaptive listening– Reduce multi-hop latency due to periodic sleep
– Wake up for a short period of time at end of each transmission
41 2 3
CTS
RTS
CTS
Reduce latency by at least half
listen listenlisten
t1 t2
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Collision AvoidanceCollision Avoidance
• S-MAC is based on contention
• Similar to IEEE 802.11 ad hoc mode (DCF)– Physical and virtual carrier sense
– Randomized backoff time
– RTS/CTS for hidden terminal problem
– RTS/CTS/DATA/ACK sequence
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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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Message PassingMessage Passing
• Problem: Sensor net in-network processing requires entire message
• 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 and ExperimentsImplementation and Experiments
• Platform: Mica Motes• Topology: 10-hop
linear network
• S-MAC saved a lot of energy compared with a MAC without sleep
0 2 4 6 8 100
5
10
15
20
25
30
Message inter-arrival period (S)
En
erg
y co
nsu
mp
tion
(J)
10% duty cycle without adaptive listen
No sleep cycles
10% duty cycle with adaptive listen
Energy consumption at different traffic load
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Case Study 2: B-MACCase Study 2: B-MAC
• Another low-power MAC for sensor networks
• B-MAC design considerations– Simplicity: based on simple CSMA
– Configurable options
– Minimize idle listening
– Based on model of periodic sensor data transfer
• B-MAC components– CSMA without RTS/CTS
– Optional Low-power listening (LPL)
– Optional ACK
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Low-Power ListeningLow-Power Listening
• Determine channel status by quick sampling– Very low overhead on wake-up
Joe Polastre, et al., SenSys’04
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Low Duty Cycle with LPLLow Duty Cycle with LPL
• Nodes periodically sleep and perform LPL
• Nodes do not synchronized on listen time
• Sender uses a long preamble before each packet to wake up the receiver
• Shift most burden to the sender
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Comparison of S-MAC and B-MACComparison of S-MAC and B-MAC
S-MAC B-MAC
Collision avoidance CSMA/CA CSMA
ACK Yes Optional
Message passing Yes No
Overhearing avoidance Yes No
Listen period Pre-defined + adaptive listen Pre-defined
Listen interval Long Very short
Schedule synchronization Required Not required
Packet transmission Short preamble Long preamble
Code size 6.3KB 4.4KB (LPL & ACK)
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Next…Next…
• Introduction to MAC
• MAC attributes and trade-offs
• Scheduled MAC protocols
• Contention-based MAC protocols
• Case studies
• Summary
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MAC Design for Sensor NetworksMAC Design for Sensor Networks
• MAC protocols can be classified as scheduled and contention-based
• Major considerations– Energy efficiency
– Scalability and adaptivity to number of nodes
• Major ways to conserve energy– Low duty cycle to reduce idle listening
– Effective collision avoidance
– Overhearing avoidance
– Reducing control overhead
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Scheduled vs. Contention ProtocolsScheduled vs. Contention Protocols
Scheduled Protocols
Contention Protocols
Collisions No Yes
Energy efficiency GoodNeed
improvement
Scalability and adaptivity
Bad Good
Multi-hop communication
Difficult Easy
Time synchronization
StrictLoose or not
required
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