2/17/2005 2/17/2005 1 Guest lecture for CS113, UCLA Guest lecture for CS113, UCLA Medium Access Control in Medium Access Control in Wireless Sensor Networks Wireless Sensor Networks Wei Ye Wei Ye USC Information Sciences USC Information Sciences Institute Institute
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2/17/20051 Guest lecture for CS113, UCLA Medium Access Control in Wireless Sensor Networks Wei Ye USC Information Sciences Institute.
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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
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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
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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?
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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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• 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
2/17/20052/17/2005 Guest lecture for CS113, UCLAGuest lecture for CS113, UCLA 1818
• 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
• 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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