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Medium Access Control in Wireless Sensor Networks. Wei Ye USC Information Sciences Institute. Outline. Introduction to MAC MAC attributes and trade-offs Scheduled MAC protocols Contention-based MAC protocols Case studies Summary. Introduction to MAC.

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Medium access control in wireless sensor networks

Medium Access Control in Wireless Sensor Networks

Wei Ye

USC Information Sciences Institute

Guest lecture for CS113, UCLA


Outline
Outline

  • Introduction to MAC

  • MAC attributes and trade-offs

  • Scheduled MAC protocols

  • Contention-based MAC protocols

  • Case studies

  • Summary

Guest lecture for CS113, UCLA


Introduction to mac
Introduction 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

Guest lecture for CS113, UCLA


Where is the mac

Application layer

Transport layer

End-to-end reliability, congestion control

Network layer

Routing

Link/MAC layer

Per-hop reliability, flow control, multiple access

Physical layer

Packet transmission and reception

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

Guest lecture for CS113, UCLA


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

Guest lecture for CS113, UCLA


Next…

  • Introduction to MAC

  • MAC attributes and trade-offs

  • Scheduled MAC protocols

  • Contention-based MAC protocols

  • Case studies

  • Summary

Guest lecture for CS113, UCLA


Primary mac attributes
Primary 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

Guest lecture for CS113, UCLA


Other mac attributes
Other 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?

Guest lecture for CS113, UCLA


Energy efficiency in mac design

Dominant factor

Energy 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)

Guest lecture for CS113, UCLA


Classification of mac protocols
Classification 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

Guest lecture for CS113, UCLA


Next…

  • Introduction to MAC

  • MAC attributes and trade-offs

  • Scheduled MAC protocols

  • Contention-based MAC protocols

  • Case studies

  • Summary

Guest lecture for CS113, UCLA


Scheduled protocols tdma
Scheduled 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

Guest lecture for CS113, UCLA


Scheduled protocols polling
Scheduled Protocols: Polling

  • 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

Guest lecture for CS113, UCLA


Scheduled protocols bluetooth
Scheduled 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

Guest lecture for CS113, UCLA


Scheduled protocols self organization
Scheduled Protocols: Self-Organization

  • 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

Guest lecture for CS113, UCLA


Scheduled protocols leach
Scheduled 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

Guest lecture for CS113, UCLA


Next…

  • Introduction to MAC

  • MAC attributes and trade-offs

  • Scheduled MAC protocols

  • Contention-based MAC protocols

  • Case studies

  • Summary

Guest lecture for CS113, UCLA


Contention protocols classics
Contention Protocols: Classics

  • 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

Guest lecture for CS113, UCLA


Contention protocols csma ca

c

a

b

Node a is hidden from c’s carrier sense

Contention Protocols: CSMA/CA

  • 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

Guest lecture for CS113, UCLA


Contention protocols maca and macaw
Contention 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

Guest lecture for CS113, UCLA


Contention protocols ieee 802 11
Contention 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

Guest lecture for CS113, UCLA


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

Guest lecture for CS113, UCLA


Contention protocols ieee 802 11 cont1
Contention Protocols: IEEE 802.11 (cont.)

  • Unicast example of PS mode in 802.11 DCF

Guest lecture for CS113, UCLA


Contention protocols tx rate control
Contention 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

Guest lecture for CS113, UCLA


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

Guest lecture for CS113, UCLA


Contention protocols pamas
Contention 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.

Guest lecture for CS113, UCLA


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

Guest lecture for CS113, UCLA


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

Guest lecture for CS113, UCLA


Next…

  • Introduction to MAC

  • MAC attributes and trade-offs

  • Scheduled MAC protocols

  • Contention-based MAC protocols

  • Case studies

  • Summary

Guest lecture for CS113, UCLA


Case study 1 s mac

Latency

Fairness

Energy

Case 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

Guest lecture for CS113, UCLA


Coordinated sleeping

sleep

listen

listen

sleep

Energy

Latency

Coordinated 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)

Guest lecture for CS113, UCLA


Coordinated sleeping1

Node 1

sleep

sleep

listen

listen

Node 2

sleep

sleep

listen

listen

Schedule 1

Schedule 2

Coordinated Sleeping

  • Schedules can differ

  • Prefer neighboring nodes have same schedule

    • — easy broadcast & low control overhead

Border nodes:

two schedules or

broadcast twice

Guest lecture for CS113, UCLA


Coordinated sleeping2
Coordinated 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

Guest lecture for CS113, UCLA


Coordinated sleeping3

RTS

CTS

CTS

listen

listen

t1

t2

Coordinated Sleeping

  • Adaptive listening

    • Reduce multi-hop latency due to periodic sleep

    • Wake up for a short period of time at end of each transmission

2

4

1

3

listen

  • Reduce latency by at least half

Guest lecture for CS113, UCLA


Collision avoidance
Collision 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

Guest lecture for CS113, UCLA


Overhearing avoidance
Overhearing 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

Guest lecture for CS113, UCLA


Message passing

Energy

Msg-level latency

Fairness

Message 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

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Implementation and experiments

Platform: Mica Motes

Topology: 10-hop linear network

S-MAC saved a lot of energy compared with a MAC without sleep

Energy consumption at different traffic load

30

25

No sleep cycles

20

Energy consumption (J)

15

10

10% duty cycle without adaptive listen

5

10% duty cycle with adaptive listen

0

0

2

4

6

8

10

Message inter-arrival period (S)

Implementation and Experiments

Guest lecture for CS113, UCLA


Case study 2 b mac
Case 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

Guest lecture for CS113, UCLA


Low power listening
Low-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 lpl
Low 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 mac
Comparison of S-MAC and B-MAC

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Next…

  • Introduction to MAC

  • MAC attributes and trade-offs

  • Scheduled MAC protocols

  • Contention-based MAC protocols

  • Case studies

  • Summary

Guest lecture for CS113, UCLA


Mac design for sensor networks
MAC 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

Guest lecture for CS113, UCLA


Scheduled vs contention protocols
Scheduled vs. Contention Protocols

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Questions?

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