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MAC Layer Design for Wireless Sensor Networks. Wei Ye USC Information Sciences Institute. Introduction. Wireless sensor network Large number of densely distributed nodes Battery powered Multi-hop ad hoc wireless network Node positions and topology dynamically change Self-organization

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mac layer design for wireless sensor networks

MAC Layer Design for Wireless Sensor Networks

Wei Ye

USC Information Sciences Institute

introduction
Introduction
  • Wireless sensor network
    • Large number of densely distributed nodes
    • Battery powered
    • Multi-hop ad hoc wireless network
    • Node positions and topology dynamically change
    • Self-organization
  • Sensor-net applications
    • Nodes cooperate for a common task
    • In-network data processing
introduction1
Introduction
  • Important attributes of MAC protocols
    • Collision avoidance
      • Basic task — medium access control
    • Energy efficiency
    • Scalability and adaptivity
      • Number of nodes changes overtime
    • Latency
    • Fairness
    • Throughput
    • Bandwidth utilization
overview of mac protocols

C

A

B

Hidden terminal: A is hidden from C’s CS

Overview of MAC protocols
  • Contention-based protocols
    • CSMA — Carrier Sense Multiple Access
      • Ethernet
      • Not enough for wireless (collision at receiver)
    • MACA — Multiple Access w/ Collision Avoidance
      • RTS/CTS for hidden terminal problem
      • RTS/CTS/DATA
overview of mac protocols1
Overview of MAC Protocols
  • Contention-based protocols (contd.)
    • MACAW — improved over MACA
      • RTS/CTS/DATA/ACK
      • Fast error recovery at link layer
    • IEEE 802.11 Distributed Coordination Function
      • Largely based on MACAW
  • Protocols from voice communication area
    • TDMA — low duty cycle, energy efficient
    • FDMA — each channel has different frequency
    • CDMA — frequency hopping or direct sequence
example ieee 802 11 dcf
Example: IEEE 802.11 DCF
  • Distributed coordinate function: ad hoc mode
    • 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 (frag + ACK)
      • First (frag + ACK) reserve time for second…
      • Give up tx when error happens
example ieee 802 11 dcf1
Example: IEEE 802.11 DCF
  • Timing relationship
energy efficiency in mac design

Dominant in sensornets

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)
energy efficiency in mac design1
Energy Efficiency in MAC Design
  • TDMA vs. contention-based protocols
    • TDMA can easily avoid or reduce energy waste from all above sources
    • Contention protocols needs to work hard in all directions
    • TDMA has limited scalability and adaptivity
      • Hard to dynamically change frame size or slot assignment when new nodes join
      • Restrict direct communication within a cluster
    • Contention protocols easily accommodate node changes and support multi-hop communications
tdma protocols
TDMA Protocols
  • Bluetooth
    • Clustering (piconet)
    • FH-CDMA between clusters
    • TDMA within each cluster
      • Centralized control by cluster head
      • Only master-slave communication
      • Master polling slave and schedule tx
    • At most 8 active nodes can be in a cluster — scalability problem
tdma protocols1
TDMA 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
tdma protocols2

3

1

4

2

TDMA Protocols
  • Self-Organaiztion — by Sohrabi and Pottie
    • Have a pool of independent channels
      • Frequency band or spreading code
      • Potential interfering links select different channels
    • Talk to each neighbor in different time slots
    • Sleep during unscheduled time
    • Example
      • 6 different channels
      • Each node has its own scheduled time slots — superframe
tdma protocols3
TDMA Protocols
  • Self-Organaiztion (Contd.)
    • Result (if a node has n neighbors)
      • Uses n different channels to talk to each of them
      • Schedules n time slots to send to each of them and n time slots to receive from each of them
    • Looks like TDMA, but actually FDMA or CDMA
      • Any pair of two nodes can talk at the same time
    • Low bandwidth utilization
contention based protocols
Contention-Based Protocols
  • PAMAS: Power Aware Multi-Access with Signalling — by Singh and Raghavendra
    • Improve energy efficiency from MACA
    • Avoid overhearing by putting node into sleep
    • Control channel separates from data channel
      • 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.
contention based protocols1
Contention-Based Protocols
  • Piconet — by Bennett, Clarke, et al.
    • 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
      • Carrier sense before sending data
    • May wait for long time before sending
  • Tx rate control —by Woo and Culler
    • Carrier sense + adaptive rate control
    • Promote fair bandwidth allocation
    • Helps for congestion avoidance
contention based protocols2
Contention-Based Protocols
  • 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
contention based protocols3
Contention-Based Protocols
  • Example of PS mode in IEEE 802.11 DCF
energy efficiency contention
Energy Efficiency: Contention
  • Extend 802.11 PS mode for Multi-hops

— By Tseng, et al. at IEEE Infocom 2002

    • Nodes do not synchronize with each other
    • Designed 3 sleep patterns — ensure nodes listen intervals overlap, example:
      • Periodically fully-awake interval: similar to S-MAC
      • Problem on broadcast — wake up each neighbor
s mac introduction

Latency

Fairness

Energy

S-MAC: Introduction
  • S-MAC — by Ye, Heidemann and Estrin
  • 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% (150ms on/1.5s 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
    • 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
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

Mica Motes (UC Berkeley)

8-bit CPU at 4MHz,

128KB flash, 4KB RAM

433MHz 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 two hop network

Average energy consumption in the source nodes

1800

802.11-like protocol w/o sleep

1600

Overhearing avoidance

S-MAC

1400

1200

1000

Energy consumption (mJ)

800

600

400

200

0

2

4

6

8

10

Message inter-arrival period (second)

Source 1

Sink 1

Sink 2

Source 2

Experiments: two-hop network
  • 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
recent progress adaptive listen

C

A

B

Recent Progress: Adaptive Listen
  • Reduces latency due to periodic sleep
  • At end of each transmission, nodes wake up for a short time
  • Next transmission can start right away
  • Example: data flow ABC
    • A sends RTS, B replies CTS, C overhears CTS
    • C will wake up when AB is done
    • BC can start right away
recent progress 10 hop experiments
Recent Progress: 10-hop Experiments
  • Compare S-MAC in 3 different modes
  • 10-hop linear network with 11 nodes
s mac information
S-MAC Information
  • URL: http://www.isi.edu/scadds/
  • Released S-MAC source code (for TinyOS 0.6.1)
  • Currently porting to nesC environment (TinyOS 1.0)

Thank You!