1. Efficient MAC Protocols for Wireless Sensor Networks
Mahendra Kumar

2. Properties of a Well Defined MAC Protocol
Energy Efficient
Scalability
Adaptability to changes in network topology
Latency, throughput, bandwidth
Fairness –not so important

3. Reason of Energy Waste Collision Overhearing Control Packet overhead
Idle Listening
Overremitting

4. Existing MAC Protocols
Sensor-MAC (S-MAC) : Listen-sleep
Timeout-MAC (T-MAC) : Activation event
WiseMAC : Preamble Sampling

5. S-MAC Main goal –reduce power consumption Three major components:
Periodic sleep-listen
Collision and overhearing avoidance
Message passing

6. 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.

7. 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.

8. S-MAC

9. Maintaining Synchronization
The listen/sleep scheme requires synchronization among neighboring nodes.
Updating schedules is accomplished by sending a SYNC packet.

10. Advantages/Disadvantages
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.

11. 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.

12. S-MAC Vs T-MAC

13. Advantages/Disadvantages
Gives better result under variable load.
Suffers from early sleeping problem –node goes to sleep when a neighbor still has messages for it.

14. 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.

15. WiseMAC

16. Advantages/Disadvantages
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

17. Other MAC Protocols SIFT :Event Driven
TRAMA : Traffic Adaptive MAC, TDMA Based

18. 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
–based
Gives better results under
variable loads
Early sleeping problem.
WiseMAC
Preamble based
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
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
CSMA/CA,
Window-based
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.