MAC Layer Design for Wireless
Sensor Networks
Wei Ye
USC Information Sciences Institute
1
Introduction
Wireless sensor network
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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
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Introduction
Important attributes of MAC protocols
 Collision avoidance
Basic task — medium access control
 Energy efficiency
 Scalability and adaptivity
Number of nodes changes overtime
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Latency
Fairness
Throughput
Bandwidth utilization
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Overview of MAC protocols
Contention-based protocols
 CSMA — Carrier Sense Multiple Access
Ethernet
Not enough for wireless (collision at receiver)
C
A
B
Hidden terminal: A is hidden from C’s CS
 MACA — Multiple Access w/ Collision Avoidance
RTS/CTS for hidden terminal problem
RTS/CTS/DATA
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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
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Example: IEEE 802.11 DCF
Distributed coordinate function: ad hoc mode
 Virtual and physical carrier sense (CS)
Network allocation vector (NAV), duration field
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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
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Example: IEEE 802.11 DCF
Timing relationship
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Energy Efficiency in MAC Design
Energy is primary concern in sensor networks
What causes energy waste?
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Collisions
Control packet overhead
Overhearing unnecessary traffic
Dominant in sensor nets
Long idle time
bursty traffic in sensor-net apps
Idle listening consumes 50—100% of the power
for receiving (Stemm97, Kasten)
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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
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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
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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
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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
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 Example
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4
2
6 different channels
Each node has its own scheduled time slots —
superframe
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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
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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.
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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
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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
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Contention-Based Protocols
Example of PS mode in IEEE 802.11 DCF
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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
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S-MAC: Introduction
S-MAC — by Ye, Heidemann and Estrin
Tradeoffs
Latency
Fairness
Energy
Major components in S-MAC
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Periodic listen and sleep
Collision avoidance
Overhearing avoidance
Massage passing
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Periodic Listen and Sleep
Problem: Idle listening consumes significant
energy
Solution: Periodic listen and sleep
listen
sleep
listen
sleep
• Turn off radio when sleeping
• Reduce duty cycle to ~ 10% (150ms on/1.5s off)
Latency
Energy
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Periodic Listen and Sleep
Schedules can differ
Node 1
Node 2
listen
sleep
listen
listen
sleep
sleep
listen
sleep
• Prefer neighboring nodes have same schedule
— easy broadcast & low control overhead
Schedule 1
Schedule 2
Border nodes:
two schedules
broadcast twice
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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
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Collision Avoidance
Problem: Multiple senders want to talk
Options: Contention vs. TDMA
Solution: Similar to IEEE 802.11 ad hoc
mode (DCF)
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Physical and virtual carrier sense
Randomized backoff time
RTS/CTS for hidden terminal problem
RTS/CTS/DATA/ACK sequence
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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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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
Fairness
Energy
Msg-level latency
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Msg Passing vs. 802.11 fragmentation
 S-MAC message passing
Data 19
RTS 21
ACK 18
CTS 20
...
Data 17
ACK 16
Data 1
...
ACK 0
 Fragmentation in IEEE 802.11
• No indication of entire time — other nodes keep listening
• If ACK is not received, give up Tx — fairness
Data 3
RTS 3
CTS 2
Data 3
ACK 2
...
ACK 2
Data 1
...
ACK 0
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Implementation on Testbed Nodes
Platform
Mica Motes (UC Berkeley)
8-bit CPU at 4MHz,
128KB flash, 4KB RAM
433MHz radio
TinyOS: event-driven
Compared MAC modules
1. IEEE 802.11-like protocol w/o sleeping
2. Message passing with overhearing avoidance
3. S-MAC (2 + periodic listen/sleep)
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Experiments: two-hop network
Topology and measured energy consumption
on source nodes
Average energy consumption in the source nodes
1800
Source 2
Sink 1
Sink 2
• Each source node sends
10 messages
— Each message has 400B
in 10 fragments
• Measure total energy over
time to send all messages
802.11-like protocol w/o sleep
Overhearing avoidance
S-MAC
1600
Energy consumption (mJ)
Source 1
1400
1200
1000
800
600
400
200
0
2
4
6
8
10
Message inter-arrival period (second)
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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
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
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Recent Progress: 10-hop Experiments
Compare S-MAC in 3 different modes
10-hop linear network with 11 nodes
Energy consumption on radios in the entire network
Average message latency under the lowest traffic load
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10% duty cycle without adaptive listen
10% duty cycle with adaptive listen
No periodic sleep
10
20
15
6
10
4
5
2
0
0
10% duty cycle without adaptive listen
10% duty cycle with adaptive listen
No periodic sleep
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Latency (S)
Energy consumption (J)
25
12
2
4
6
8
Message inter-arrival period (S)
10
0
0
2
4
6
8
10
Number of hops
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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)
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