Medium Access Control in Sensor Networks

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Medium Access Control in Sensor Networks

• Huaming Li
• Electrical and Computer Engineering
• Michigan Technological University

2
Outline

• Overview
• S-MAC an energy-efficient MAC protocol for
  wireless sensor networks
• Other MAC Techniques
• References

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Medium Access Control in Sensor Networks

• Sensor networks
• Consist of a set of sensor nodes
• Each node is equipped with one or more sensors
  and is normally battery operated
• Nodes communicate with each other via wireless
  connection.
• Medium Access Control (MAC)
• Fundamental task is to avoid collisions so that
  two interfering nodes do not transmit at the same
  time

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Characteristics of Sensor Network

• 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

Msg-level Latency
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MAC Protocols Classification

• Scheduling-Based MAC Protocols
• Contention-Based MAC
• Collision Free Real Time MAC
• Hybrid MAC

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Scheduling Based MAC

• Time is divided into slots
• Each node knows when to transmit
• Schedule is predetermined
• TDMA
• Synchronization problems
• Adaptability problems

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Contention Based MAC

• Carrier sensing collision avoidance
• In-band, out-band handshaking
• Busy-tone multiple access (BTMA)
• Multiple access with collision avoidance (MACA)
• High priority packets

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Common MAC Protocol Requirements

• Quality of service (QoS)
• Tolerate message loss
• Support real time guarantees
• Decentralized
• Global information may not be available
• Flexibility
• Diversity of applications

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MAC Requirements in Sensor Networks

• Important requirements of MAC protocols
• Collision avoidance
• Energy efficiency
• Scalability Adaptivity
• Latency
• Fairness
• Throughput
• Bandwidth utilization

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Energy Efficiency in MAC Design

• Energy is primary concern
• What causes energy waste on radio?
• Long idle time
• Control packet overhead
• Overhearing unnecessary traffic
• Collisions
• bursty traffic in sensor-net apps
• Idle listening consumes 50100 of the power for
  receiving (Stemm97, Kasten)

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S-MAC Design Overview

• Tradeoffs

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

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Periodic Listen and Sleep

• Reduce long idle time
• Reduce duty cycle to 10 (120ms on/1.2s off)

• Schedules can differ

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

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Periodic Listen Sleep

• Nodes are in idle for a long time if no sensing
  event happens
• Put nodes into periodic sleep mode
• i.e. in each second, sleep for half second and
  listen for other half second

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Coordinated Sleeping

• Nodes coordinate on sleep schedules
• Nodes periodically broadcast schedules
• New node tries to follow an existing schedule

Schedule 1
Schedule 2
1
2

• Nodes on border of two schedules follow both
• Periodic neighbor discovery
• Keep awake in a full sync interval over long time

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Choose Maintain Schedule

• Each node maintains a schedule table that stores
  schedules of all its neighbors
• Nodes exchange schedules by broadcasting them to
  its neighbors
• Try to synchronize neighboring nodes together

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Choose Schedule

• If not hear a schedule from others, the node
  randomly chooses a schedule and broadcast the
  schedule
• If receive a schedule, the node follows that
  schedule, wait for a random delay then
  rebroadcast this schedule
• If receive a different schedule, the node adopt
  both, broadcast its own schedule

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Maintain Synchronization

• Listen/sleep scheme requires synchronization
  among neighboring nodes
• Looser synchronization (compared to TDMA)
• Listen period is significantly longer than clock
  error or drift
• Use relative time rather than absolute
• Update schedule by sending SYNC packets

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Maintain Sync (contd.)

• Divide listen time into two parts
• For receiving SYNC packets
• For receiving data packets
• Each part is further divided into many time slots
  for senders to perform carrier sense

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Maintain Sync (contd.)
CS carrier sense
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Collision Avoidance

• Adopt IEEE 802.11 collision avoidance
• Virtual carrier sense
• During field
• Network allocation vector (NAV)
• Physical carrier sense
• RTS/CTS exchange (for hidden terminal problem)
• Broadcast packets (SYNC) are sent without RTS/CTS
• Unicast packets (DATA) are sent with RTS/CTS

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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

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Example

• Who should sleep when node A is transmitting to
  B?
• All immediate neighbors of both sender receiver
  should go to sleep

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Message Passing

• How to efficiently transmit a long message?
• Single packet vs. fragmentations
• Single packet high cost of retransmission if
  only a few bits have been corrupted
• Fragmentations large control overhead (RTS CTS
  for each fragment), longer delay
• Problem Sensor network in-network processing
  requires entire message

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Message Passing

• 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

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Implementation on Testbed Nodes

• Configurable S-MAC options
• Low duty cycle with adaptive listen
• Low duty cycle without adaptive listen
• Fully active mode (no periodic sleeping)

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Implementation on Testbed Nodes

• Layered model on Motes
• MAC layer S-MAC
• Physical layer
• Radio state control, Carrier sense
• CRC checking, Channel coding, Byte buffering
• Nested headers
• Avoid memory
• copy across
• layers

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Test Bed

• Three test MAC modules
• Simplified IEEE 802.11 DCF
• Message passing with overhearing avoidance
• Complete S-MAC
• Topology in experiments

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Experiment Result

• Average source nodes energy consumption

• S-MAC consumes much less energy than 802.11-like
  protocol w/o sleeping
• At heavy load, overhearing avoidance is the major
  factor in energy savings
• At light load, periodic sleeping plays the key
  role

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Experiment Result (contd.)

• Percentage of time source nodes in sleep

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Experiment Result (contd.)

• Energy consumption in the intermediate node

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S-MAC Conclusions

• Advantages
• Periodically sleep reduces energy consumption in
  idle listening
• Sleep during transmissions of other nodes
• Message passing reduces contention latency and
  control packet overhead
• Disadvantages
• Reduction in both per-node fairness latency

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Other MAC Techniques

• Timeout-MAC (T-MAC)
• S-MAC has fixed duty cycle and not optimal
• Reduce idle listening by transmitting data in
  bursts
• Sleep in between bursts to save power
• End the active time in an intuitive way
• Timeout on hearing nothing

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T-MAC

• Every node periodically wakes up and communicates
  with its neighbors
• A node will keep listening and potentially
  transmitting, as long as it is in active period
• An active period ends when no activation event
  has occurred for time TA

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Activation event

• The firing of periodic timer
• The reception of any data on radio
• The sensing of communication on the radio
• The end of transmission of a nodes own data
  packet
• The knowledge through prior RTS and CTS packets

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T-MAC

• A node will sleep if it is not in an active
  period
• TA determines the minimum amount of idle
  listening per frame
• All communication occurs as a burst in the
  beginning of the frame
• Buffer capacity determines the upper bound on the
  maximum frame time

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Evolution
Adaptive Rate Control
ARC
IEEE 802.11
IEEE 802.3
CSMA/CD
CSMA/CA
SMAC
TMAC
Carrier Sense Multiple Access with Collision
Detection
Carrier Sense Multiple Access with Collision
Avoidance
Fixed duty cycle
Adaptive duty cycle
DMAC/ MMAC
Directional Antennas
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Performance Analysis of 802.15.4 in WPAN

• One promising kind of sensor network Wireless
  Personal Area Network (WPAN)
• Medical sensing and control
• Wearable computing
• Location awareness and identification
• Implanted medical sensors (Focus)
• Coronary care
• Diabetes
• Optical aids
• Drug delivery

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Critical Metric Battery Life

• Implanted medical sensors (Main concern)
• Objective
• Make Batteries work 10-15 years
• Method
• Ensure that all sensors are powered down or in
  sleep mode when not in active use
• Tradeoff
• Battery life VS. latency

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Protocol Options
Possible options
Market Name Standard GPRS/GSM 1xRTT/CDMA Wi-Fi 802.11b Bluetooth 802.15.1 ZigBee 802.15.4
Application Focus Wide Area Voice Data Web, Email, Video Cable Replacement Monitoring Control
System Resources 16MB 1MB 250KB 4KB - 32KB
Battery Life (days) 1-7 .5 - 5 1 - 7 100 - 1,000
Network Size 1 32 7 255 / 65,000
Bandwidth (KB/s) 64 - 128 11,000 720 20 - 250
Transmission Range (meters) 1,000 1 - 100 1 - 10 1 - 100
Success Metrics Reach, Quality Speed, Flexibility Cost, Convenience Reliability, Power, Cost
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802.15.4 (LR-WPAN)
Upper Layers
Other LLC
IEEE 802.2 LLC
IEEE 802.15.4 MAC
IEEE 802.15.4
IEEE 802.15.4
2400 MHz
868/915 MHz
PHY
PHY
Physical Medium
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802.15.4 (LR-WPAN)
MAC Layer (prefer star topology)
Why star topology here?
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802.15.4 (LR-WPAN)
Why star topology here?

• Coordinator is external to the body
• PDA, mobile phone or bedside monitor station
• Easy to replace of charge batteries
• Easy to communicate with other networks
• Coordinator defines the start and end of a
  superframe and is charge of the association and
  disassociation of the other nodes

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IEEE 802.15.4 superframe structure
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Two Communication methods

• Beacon mode
• Pros Coordinator can communicate at will
• Cons Listeners have to keep awake
• Non-beacon mode
• Pros Nodes can sleep more
• Cons Communication latency

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Network scenarios and power analysis
Sensor power consumption with beacon reception

• Problem The sensor devices within a beacon
  network have to wake up to receive the beacon
  from the coordinator (Power consuming)

Timebase Tolerances
Warm-up time
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Network scenarios and power analysis
Data Transfer Mechanisms (Beacon)

• Data transfer to a coordinator (upload)

Is the upload period
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Network scenarios and power analysis
Data Transfer Mechanisms (Beacon)

• Data transfer from a coordinator (download)

Is the download period
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Results
Average Back-off
With a small number of sensors that are
effectively off most of the time, the probability
of a channel being free is greater than 99 .
Therefore, for the relatively small number of
sensors used in the WBAN networks explored here,
it would be more economical to keep the CSMA/CA
switched off. This is to ensure that the
automatic initial back-off is avoided.
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Node Lifetime in Beacon Networks
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Node Lifetime in Beacon Networks
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Node Lifetime in Beacon Networks

• 15-year lifetime may only be obtained for very
  low upload rates.
• It is under very limited data rate conditions and
  a tight tolerance crystal, which typically must
  be better than 25 ppm.

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GTS Option
The main drawback of using GTS is that the
receiver in the sensor remains on for the
duration of the timeslot regardless of the size
of the data packet.
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Non-Beacon Networks
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Non-Beacon Networks
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Conclusion

• As a solution to the challenge of the personal
  area network, the IEEE 802.15.4 standard would
  provide a limited answer in its non-beacon form.
• Sensors that do not have large amounts of data to
  transfer could be used, i.e., small packets of
  data several times per hour.

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Questions and Comments
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