Transcript Document 7416028

Efficient MAC Protocols for
Wireless Sensor Networks
Mahendra Kumar
Properties of a Well Defined
MAC Protocol
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Energy Efficient
Scalability
Adaptability to changes in network
topology
Latency, throughput, bandwidth
Fairness –not so important
Reason of Energy Waste
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Collision
Overhearing
Control Packet overhead
Idle Listening
Overremitting
Existing MAC Protocols
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Sensor-MAC (S-MAC) : Listen-sleep
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Timeout-MAC (T-MAC) : Activation event
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WiseMAC : Preamble Sampling
S-MAC
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Main goal –reduce power consumption
Three major components:
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Periodic sleep-listen
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Collision and overhearing avoidance
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Message passing
Periodic Sleep-Listen
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Each node goes to sleep for some time,
and then wakes up and listens to see if any
other node wants to talk to it.
During Sleep it turn off its radio.
Collision and Overhearing
Avoidance
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Interfering nodes go to sleep after they hear an RTS
or CTS packet.
Duration field in each transmitted packet indicates
how long the remaining transmission will be.
S-MAC
Maintaining Synchronization
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The listen/sleep scheme requires
synchronization among neighboring nodes.
Updating schedules is accomplished by
sending a SYNC packet.
Advantages/Disadvantages
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Energy waste caused by idle listening is reduced by
sleep schedules.
Sleep and listen periods are predefined and constant
which decreases the efficiency of the algorithm under
variable traffic load.
Timeout-MAC (T-MAC)
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Proposed to enhance the poor results of S-MAC protocol under
variable traffic load.
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Listen period ends when no activation event has occurred for a
time threshold TA.
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Reduce idle listening by transmitting all messages in bursts of
variable length, and sleeping between bursts.
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times out on hearing nothing.
S-MAC Vs T-MAC
Advantages/Disadvantages
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Gives better result under variable load.
Suffers from early sleeping problem –node goes to
sleep when a neighbor still has messages for it.
WiseMAC
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All nodes defined to have two communication
channels.
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Data channel uses TDMA
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Control channel uses CSMA
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Preamble sampling used to decrease idle listening
time.
Nodes sample the medium periodically to see if any
data is going to arrive.
WiseMAC
Advantages/Disadvantages
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Dynamic preamble length adjustment results in
better performance.
Conflict when one node starts to send the preamble
to a node that is already receiving another node’s
transmission where the preamble sender is not within
range. Hidden terminal problem
Other MAC Protocols
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SIFT :Event Driven
TRAMA : Traffic Adaptive MAC, TDMA
Based
MAC
Protocol
Time Sync
Needed
Type
Adaptivity to
Changes
Advantages
Disadvantages
S-MAC
No
CSMA,
Contention
-based
Good
Energy waste caused by
idle listening is reduced by
sleep schedules.
Simplicity.
Sleep and listen periods are
predefined and constant, which
decreases the efficiency of the
algorithm under variable traffic
load.
T-MAC
No
CSMA,
Contention
–based
Good
Gives better results under
variable loads
Early sleeping problem.
WiseMAC
No
CSMA,
Preamble based
Good
Dynamic preamble length
adjustment results in better
performance under
variable traffic conditions.
Decentralized sleep-listen
scheduling results in different
sleep and wake-up times for
each neighbor of a node. Hidden
terminal problem
TRAMA
Yes
TDMA/CSMA
Good
Higher percentage of sleep
time and less collision
probability is achieved
compared to CSMA based
protocols.
Without considering the
transmissions and receptions, the
duty cycle is at least 12.5 %,
which is a considerably high
value.
SIFT
No
CSMA/CA,
Contention
Window-based
Good
Very low latency is
achieved with many traffic
sources.
Increased idle listening caused
by listening to all slots before
sending. System-wide time
synchronization is needed for
slotted contention windows.