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/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.

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/Disadvantages 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/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

Other MAC Protocols

SIFT :Event Driven TRAMA : Traffic Adaptive MAC,

TDMA Based

MAC Protocol

Time SyncNeeded

Type Adaptivity toChanges

Advantages Disadvantages

S-MAC No CSMA, Contention-based

Good Energy waste caused by idle listening is reduced bysleep schedules.Simplicity.

Sleep and listen periods are predefined and constant, whichdecreases the efficiency of thealgorithm under variable trafficload.

T-MAC No CSMA,Contention–based

Good Gives better results undervariable loads

Early sleeping problem.

WiseMAC No CSMA,Preamble based

Good Dynamic preamble length adjustment results in betterperformance undervariable traffic conditions.

Decentralized sleep-listen scheduling results in differentsleep and wake-up times foreach neighbor of a node. Hiddenterminal problem

TRAMA Yes TDMA/CSMA Good Higher percentage of sleeptime and less collisionprobability is achievedcompared to CSMA basedprotocols.

Without considering the transmissions and receptions, theduty cycle is at least 12.5 %,which is a considerably highvalue.

SIFT No CSMA/CA, ContentionWindow-based

Good Very low latency is achieved with many trafficsources.

Increased idle listening caused by listening to all slots before sending. System-wide time synchronization is needed for slotted contention windows.