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MAC Layer Protocols for Sensor Networks. Prasun Sinha Department of Computer Science and Engineering Ohio State University April 25 th , 2007. (some slides adapted from authors presentations found on the Internet). Introduction. Wireless sensor network Special ad hoc wireless network

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mac layer protocols for sensor networks

MAC Layer Protocols for Sensor Networks

Prasun Sinha

Department of Computer Science and Engineering

Ohio State University

April 25th, 2007

(some slides adapted from authors presentations found on the Internet)

introduction
Introduction
  • Wireless sensor network
    • Special ad hoc wireless network
    • Large number of nodes w/ sensors & actuators
    • Battery-powered nodes energy efficiency
    • Unplanned deployment self-organization
    • Node density & topology change robustness
  • Sensor-net applications
    • Nodes cooperate for a common task
    • In-network data processing
slide3

Some Applications of Sensor Networks

  • Data Collection Networks
    • Sensing Movement of Glaciers
    • Environment Monitoring
    • Habitat Monitoring
      • Habitat Monitoring of Storm Petrels in Great Duck Island
    • Microsoft’s Effort to put every sensor on the web
  • Event Triggered Networks
    • Structural Monitoring
      • Golden Gate Bridge
    • Precision Agriculture
      • Oregon and British Columbia Vineyards
    • Condition based Maintenance
      • Hardware Manufacturing facilities
    • Military Applications
    • Environment Monitoring
      • Poisonous gas, pollutants etc.
    • National Asset Protection
      • Coastline, Border Patrol, Roadways, Oil/gas pipelines, Secure facilities
talk outline
Talk Outline
  • SMAC: http://www.isi.edu/~weiye/pub/smac_ton.pdf
    • “Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks”, Wei Ye, John Heidemann, and Deborah Estrin, Transactions on Networking, 2004, (also Infocom 2002)
  • BMAC: http://www.polastre.com/papers/sensys04-bmac.pdf
    • “Versatile Low Power Media Access for Wireless Sensor Networks”, Joseph Polastre, Jason Hill and David Culler, ACM SENSYS 2004
  • CMAC: http://www.cse.ohio-state.edu/~prasun/publications/conf/secon07-cmac.pdf
    • “CMAC: An Energy Efficient MAC Layer Protocol Using Convergent Packet Forwarding for Wireless Sensor Networks”, Sha Liu, Kai-Wei Fan and Prasun Sinha, IEEE SECON 2007
medium access control in sensor nets

Primary

Secondary

Medium Access Control in Sensor Nets
  • Important attributes of MAC protocols
    • Collision avoidance
    • Energy efficiency
    • Scalability in node density
    • Latency
    • Fairness
    • Throughput
    • Bandwidth utilization
energy efficiency in mac

Dominant in sensornets

Common to all wireless networks

Energy Efficiency in MAC
  • Major sources of energy waste (cont.)
    • Idle listening
      • Long idle time when no sensing event happens
    • Collisions
    • Control overhead
    • Overhearing
  • We try to reduce energy consumption from all above sources
  • Combine benefits of TDMA + contention protocols
sensor mac s mac design

Latency

Fairness

Energy

Sensor-MAC (S-MAC) Design
  • Tradeoffs
  • Major components in S-MAC
    • Periodic listen and sleep
    • Collision avoidance
    • Overhearing avoidance
    • Massage passing
periodic listen and sleep

sleep

listen

listen

sleep

Energy

Latency

Periodic Listen and Sleep
  • Problem: Idle listening consumes significant energy
  • Solution: Periodic listen and sleep
  • Turn off radio when sleeping
  • Reduce duty cycle to ~ 10% (200ms on/2s off)
periodic listen and sleep1

Node 1

sleep

sleep

listen

listen

Node 2

sleep

sleep

listen

listen

Schedule 1

Schedule 2

Periodic Listen and Sleep
  • Schedules can differ
  • Prefer neighboring nodes have same schedule
    • — easy broadcast & low control overhead

Border nodes:

two schedules

broadcast twice

periodic listen and sleep2
Periodic Listen and Sleep
  • Schedule Synchronization
    • 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
    • Schedule packets also serve as beacons for new nodes to join a neighborhood
collision avoidance
Collision Avoidance
  • Problem: Multiple senders want to talk
  • Options: Contention vs. TDMA
  • Solution: 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
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
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
msg passing vs 802 11 fragmentation

...

...

...

Data 1

Data 19

Data 17

Data 3

Data 1

Data 3

RTS 3

CTS 20

RTS 21

CTS 2

...

ACK 0

ACK 2

ACK 16

ACK 18

ACK 0

ACK 2

Msg Passing vs. 802.11 fragmentation
  • S-MAC message passing
  • Fragmentation in IEEE 802.11
    • No indication of entire time — other nodes keep listening
    • If ACK is not received, give up Tx — fairness
implementation on testbed nodes

Platform

Motes (UC Berkeley)

8-bit CPU at 4MHz,

8KB flash, 512B RAM

916MHz radio

TinyOS:event-driven

Implementation on Testbed Nodes
  • Compared MAC modules
    • IEEE 802.11-like protocol w/o sleeping
    • Message passing with overhearing avoidance
    • S-MAC (2 + periodic listen/sleep)
experiments

Source 1

Sink 1

Sink 2

Source 2

Experiments
  • Topology and measured energy consumption on source nodes
  • Each source node sends 10 messages
    • — Each message has 400B in 10 fragments
  • Measure total energy over time to send all messages
s mac conclusions
S-MAC Conclusions
  • S-MAC offers significant energy efficiency over always-listening MAC protocols
  • S-MAC can function at 10% duty cycle
talk outline1
Talk Outline
  • SMAC: http://www.isi.edu/~weiye/pub/smac_ton.pdf
    • “Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks”, Wei Ye, John Heidemann, and Deborah Estrin, Transactions on Networking, 2004, (also Infocom 2002)
  • BMAC: http://www.polastre.com/papers/sensys04-bmac.pdf
    • “Versatile Low Power Media Access for Wireless Sensor Networks”, Joseph Polastre, Jason Hill and David Culler, ACM SENSYS 2004
  • CMAC: http://www.cse.ohio-state.edu/~prasun/publications/conf/secon07-cmac.pdf
    • “CMAC: An Energy Efficient MAC Layer Protocol Using Convergent Packet Forwarding for Wireless Sensor Networks”, Sha Liu, Kai-Wei Fan and Prasun Sinha, IEEE SECON 2007
slide19

BMAC Objectives

  • Information sharing with higher layers
  • Control and reconfiguration of link protocol
  • Tradeoffs in link protocols
slide20

B-MAC Design

  • Principles
    • Reconfigurable MAC protocol
    • Flexible control
    • Hooks for sub-primitives
      • Backoff/Timeouts
      • Duty Cycle
      • Acknowledgements
    • Feedback to higher protocols
    • Minimal implementation
    • Minimal state
  • Primary Goals
    • Low Power Operation
    • Effective Collision Avoidance
    • Simple/Predicable Operation
    • Small Code Size
    • Tolerant to Changing RF/Networking Conditions
    • Scalable to Large Number of Nodes
  • Implementation is on Mica2 motes with CC1000
slide21

B-MAC Link Protocol Interaction

  • Reconfiguration and control of link layer protocol parameters
    • Acknowledgements, Backoff/Timeouts, Power Management,
  • Ability to choose tradeoffs – “knobs”
    • Fairness, Latency, Energy Consumption, Reliability
  • Power consumption estimation through analytical and empirical models
    • Feedback to network protocols
    • Lifetime estimation
  • Mechanisms to achieve network protocols’ goals
slide22

wakeup

wakeup

wakeup

wakeup

wakeup

wakeup

wakeup

wakeup

wakeup

Low Power Listening (LPL)

  • Higher level communication scheduling
    • Energy Cost = RX + TX + Listen
    • Start by minimizing the listen cost
  • Example of a typical low level protocol mechanism
  • Periodically
    • wake up, sample channel, sleep
  • Properties
    • Wakeup time fixed
    • “Check Time” between wakeups variable
    • Preamble length matches wakeup interval
  • Overhear all data packets in cell
    • Duty cycle depends on number of neighbors and cell traffic

TX

sleep

sleep

sleep

Node 1

time

RX

sleep

sleep

sleep

Node 2

time

slide23

Effect of Neighborhood Size

  • Neighborhood Size affects amount of traffic in a cell
    • Network protocols typically keep track of neighborhood size
    • Bigger Neighborhood  More traffic
slide24

B-MAC Performance

  • Experimental Setup:
    • n nodes send as quickly as possible to saturate the channel
  • B-MAC never worse than traditional approach
    • Often much better
  • Flexible configuration yields efficient:
    • Reliable transport (Acks)
    • Hidden Terminal support (RTS-CTS)

topology

slide25

A

E

C

B

D

Fragmentation Support

  • S-MAC
    • RTS-CTS Fragmentation Support
  • B-MAC w/app control
    • Network protocol sends initial data packet with number of fragments pending
    • Disable backoff & LPL for rest of fragments
  • Measure energy consumption at C(bottleneck node)
  • Minimizing power relieson controlling link layer primitives

10 packets every 10 seconds

10 packets every 100 seconds

bmac conclusions
BMAC Conclusions
  • Coordination with higher protocols is essential for long lived operation
  • Feedback allows protocols to make informed decisions
talk outline2
Talk Outline
  • SMAC: http://www.isi.edu/~weiye/pub/smac_ton.pdf
    • “Medium Access Control With Coordinated Adaptive Sleeping for Wireless Sensor Networks”, Wei Ye, John Heidemann, and Deborah Estrin, Transactions on Networking, 2004, (also Infocom 2002)
  • BMAC: http://www.polastre.com/papers/sensys04-bmac.pdf
    • “Versatile Low Power Media Access for Wireless Sensor Networks”, Joseph Polastre, Jason Hill and David Culler, ACM SENSYS 2004
  • CMAC: http://www.cse.ohio-state.edu/~prasun/publications/conf/secon07-cmac.pdf
    • “CMAC: An Energy Efficient MAC Layer Protocol Using Convergent Packet Forwarding for Wireless Sensor Networks”, Sha Liu, Kai-Wei Fan and Prasun Sinha, IEEE SECON 2007
existing mac layer approaches
Existing MAC Layer Approaches
  • Synchronized Solutions
    • SMAC, TMAC, DMAC
  • Unsynchronized Solutions
    • BMAC, GeRaF
synchronized approaches
Synchronized Approaches
  • Unnecessary power consumption on synchronization message exchanges
    • Can be improved if synchronization is infrequent
  • Can not achieve very low duty cycles
    • 10% level
unsynchronized approaches bmac
Unsynchronized Approaches - BMAC
  • Long Preamble Approach
  • Wasteful if the receiver wakes up early

Sleep

Long Preamble

Packet

Sender

Sleep

Receiving Preamble

Packet

Receiver

our approach cmac
Our Approach - CMAC
  • Unsynchronized Duty Cycling
  • Flow Initialization
    • Aggressive RTS
    • Anycasting for Packet Forwarding
  • Flow Stabilization
    • Convergent Packet Forwarding
cmac aggressive rts
CMAC: Aggressive RTS
  • Aggressive RTS

Sleep

RTS

RTS

RTS

RX

Packet

Sleep

Sender

Sleep

RX

CTS

Packet

Sleep

Receiver

cmac aggressive rts double channel check
CMAC: Aggressive RTS(Double Channel Check)
  • The receiver only needs to check if the channel is busy after waking up
  • Check the channel twice to avoid missing activities
  • Time between the two checks
    • Larger than inter-RTS separation
    • Smaller than RTS duration

RTS

RTS

RTS

RTS

(a)

(b)

Channel check

Channel check

RTS

RTS

(c) (shouldn’t happen)

Channel check

cmac anycasting
CMAC: Anycasting
  • Anycast Packet Forwarding
    • Exploits network density
  • Nodes other than the target receiver may
    • wake up earlier
    • can make some progress toward the sink
contention among anycast receivers
Contention Among Anycast Receivers
  • Anycast to nodes which are
    • awake
    • closer to the destination
  • More than one potential participants
    • Nodes closer to the sink send CTS’s earlier
contention among anycast receivers1

mini-slot

CTS slot

RTS

Canceled RTS

Sender

CTS

Node in R1

Node in R1

Canceled CTS

Canceled CTS

Node in R2

Canceled CTS

Node in R3

Contention Among Anycast Receivers
  • Anycast candidate prioritization
cmac convergent forwarding
CMAC: Convergent Forwarding
  • Anycast has higher overhead than unicast
  • Nodes stay awake for a short duration after receiving a packet
    • For how long?
  • Switch from anycast to unicast if
    • Node is able to communicate with a node in R1
    • Cannot find a better next hop than current one
slide38

CMAC: Convergent Forwarding Illustration

Time 1

Time 2

Time 3

Active nodes

Unicast links

Sleeping nodes

Anycast links

experiments1
Experiments
  • Testbed: Kansei Testbed
    • 7 x 15 XSM nodes
  • Metrics
    • Normalized Energy Consumption
      • Average energy consumption to deliver one packet
    • Throughput: Number of packets received by sink
    • Latency
  • Scenarios:
    • Static Event
    • Moving Event
experimental results static scenario
Experimental Results: Static Scenario
  • Sink is at one corner of the network
  • The node that is diagonally opposite to sink sends data to the sink
  • Vary data rates
experimental results moving event
Experimental Results: Moving Event
  • One node generates data at any point for the sink
  • The node generating data (event) moves along one side of the network that does not include the sink.
  • Vary moving speeds
cmac conclusion
CMAC Conclusion
  • CMAC supports high throughput, low latency and consumes less energy than existing solutions.
  • CMAC’s performance difference from existing approaches increases with speed of the moving event.
slide43
Thanks for your attention!

For more information on my research please check my webpage at

http://www.cse.ohio-state.edu/~prasun