Efficient MAC Protocols for
Wireless Sensor Networks
Mahendra Kumar
Properties of a Well
Defined MAC Protocol
Energy Efficient
Scalability
Adaptability to changes in network
topology
Latency, throughput, bandwidth
Fairness not so important
Reason of Energy Waste
Collision
Overhearing
Control Packet overhead
Idle Listening
Overremitting
Existing MAC Protocols
Sensor-MAC (S-MAC) : Listen-sleep
Timeout-MAC (T-MAC) : Activation event
WiseMAC : Preamble Sampling
S-MAC
Main goal reduce power consumption
Three major components:
Periodic sleep-listen
Collision and overhearing avoidance
Message passing
Periodic Sleep-Listen
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
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
The listen/sleep scheme requires
synchronization among neighboring nodes.
Updating schedules is accomplished by
sending a SYNC packet.
Advantages/Disadvantage
s
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)
Proposed to enhance the poor results of S-MAC protocol
under variable traffic load.
Listen period ends when no activation event has
occurred for a time threshold TA.
Reduce idle listening by transmitting all messages in
bursts of variable length, and sleeping between bursts.
times out on hearing nothing.
S-MAC Vs T-MAC
Advantages/Disadvantage
s
Gives better result under variable load.
Suffers from early sleeping problem node
goes to sleep when a neighbor still has
messages for it.
WiseMAC
All nodes defined to have two communication
channels.
Data channel uses TDMA
Control channel uses CSMA
Preamble sampling used to decrease idle
listening time.
Nodes sample the medium periodically to see if
any data is going to arrive.
WiseMAC
Advantages/Disadvantage
s
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 nodes transmission where the
preamble sender is not within range. Hidden
terminal problem
Other MAC Protocols
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.
