MAC Layer Protocols for Sensor Networks

1
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)
2
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

3
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
• Microsofts 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

4
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/public
  ations/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

5
Medium Access Control in Sensor Nets

• Important attributes of MAC protocols
• Collision avoidance
• Energy efficiency
• Scalability in node density
• Latency
• Fairness
• Throughput
• Bandwidth utilization

6
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

7
Sensor-MAC (S-MAC) Design

• Tradeoffs

• Major components in S-MAC
• Periodic listen and sleep
• Collision avoidance
• Overhearing avoidance
• Massage passing

8
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)

9
Periodic Listen and Sleep

• Schedules can differ

• Prefer neighboring nodes have same schedule
• easy broadcast low control overhead

Border nodes two schedules broadcast twice
10
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

11
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

12
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

13
Message Passing

• Problem Sensor net in-network processing
  requires entire message
• Solution Dont 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

14
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

15
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)

16
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

17
S-MAC Conclusions

• S-MAC offers significant energy efficiency over
  always-listening MAC protocols
• S-MAC can function at 10 duty cycle

18
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/public
  ations/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

19
BMAC Objectives

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

20
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

21
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

22
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
23
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

24
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
25
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
26
BMAC Conclusions

• Coordination with higher protocols is essential
  for long lived operation
• Feedback allows protocols to make informed
  decisions

27
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/public
  ations/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

28
Existing MAC Layer Approaches

• Synchronized Solutions
• SMAC, TMAC, DMAC
• Unsynchronized Solutions
• BMAC, GeRaF

29
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

30
Unsynchronized Approaches - BMAC

• Long Preamble Approach
• Wasteful if the receiver wakes up early

Sleep
Long Preamble
Packet
Sender
Sleep
Receiving Preamble
Packet
Receiver
31
Our Approach - CMAC

• Unsynchronized Duty Cycling
• Flow Initialization
• Aggressive RTS
• Anycasting for Packet Forwarding
• Flow Stabilization
• Convergent Packet Forwarding

32
CMAC Aggressive RTS

• Aggressive RTS

Sleep
RTS
Packet
Sleep
RTS
RTS
RX
Sender
Sleep
RX
Packet
Sleep
CTS
Receiver
33
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) (shouldnt happen)
Channel check
34
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

35
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 CTSs earlier

36
Contention Among Anycast Receivers

• Anycast candidate prioritization

37
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

38
CMAC Convergent Forwarding Illustration
Time 1
Time 2
Time 3
Active nodes
Unicast links
Sleeping nodes
Anycast links
39
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

40
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

41
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

42
CMAC Conclusion

• CMAC supports high throughput, low latency and
  consumes less energy than existing solutions.
• CMACs performance difference from existing
  approaches increases with speed of the moving
  event.

43

• Thanks for your attention!
• For more information on my research please check
  my webpage at
• http//www.cse.ohio-state.edu/prasun