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 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.