An Adaptive Energy-Efficient MAC Protocol

for Wireless Sensor Networks

Tijs van DamKoen Langendoen

Presenter: Michael Curcio

ACM SenSys 2003

Sensor Networks

• Low message rate

• Insensitive to latency

• Low processing power and memory capacity

• Lots of redundancy

• Often battery operated!

Goals

• Focus has moved away from maximizing throughput and fairness; minimizing latency

• Power consumption kept to a minimum

• Memory/Network processing kept low

Energy Sinks

• Processor

• Radio

• Receiving/Transmitting

• Idle Listening

• Collisions

• Protocol Overhead

• Overhearing

What else addresses this?

• TDMA

• 802.11 (CSMA)

• Extra wake-up radio

• TinyOS

• S-MAC

• Radio-triggered wake-up hardware

How do they do that?

• TDMA

• Built-in duty cycle; eliminates collisions

• But-- Hard to do for ad-hoc network

• Scheduling, slot allocation, coordination

• Clock drift (especially for small slots)

• Extra radio

• Solve problems with more hardware

• Add a radio on a different frequency that can wake up other nodes

How do they do that?• 802.11

• Has power-saving features, but not good enough for sensor networks

• Was designed for nodes all existing in one cell (no multi-hop)

• TinyOS

• Instead of listening really long for a short transmission, listen realy short for a long transmission

• Makes the transmitter, not the receiver pay the energy bill

How do they do that?

• S-MAC

• Fixed duty cycle

• Compress spread out transmits and receives into a shorter amount of time so we can sleep the rest

• Event, issue, request-based transfer of information hop-to-hop

• Radio-triggered wake-up

• Stay awake for Soji’s presentation

• Teaser: On-demand node wake-up

T-MAC Approach

• “Timeout”-MAC

• Adaptive duty cycle

• Period of radio activity can be ended dynamically

• Reduce idle listening to a minimum

• Better handle variable network load

Sensor Network Communication Patterns

• Local uni-/broadcast

• Event processed in network among nodes

• Nodes to sink(s) reporting

• Messages move through the network in a generally unified direction (to the sink(s))

• May or may not be aggregated/processed en-route

Sensor Network Communication Patterns• Traditional, multi-hop routing not used

• Might there be a case where it would be useful?

• Time dependence

• Nothing to do if no events occur

• Location dependence

• Network in proximity to sink nodes experiences heavier traffic than at remote edge of network

EYES Nodes• 16-bit, 5MHz,

variable clock rate processor

• 2KB RAM

• 60KB FLASH

• 2MB EEPROM

• JTAG, RS232, 2 LEDs, 16 GPIO (8 ADC) pins

• Runs on 2 AA batteries

Photo Courtesy: Eyes - Energy Efficient Sensor Networks,

http://www.eyes.eu.org/

T-MAC Duty Cycle

• Variable length duty cycle

• Transmit in bursts

• Maintain optimal active time under variable load

• Sleep after time of hearing nothing

S-MAC Duty Cycle

• Fixed duty cycle

• Frame time - limited by latency requirements and buffer space

• Active time - configured to be long enough to handle highest expected load

T-MAC Protocol Basics

• Burst communication schedule

• Messages are queued while node is asleep

• Buffer capacity determines Frame Time

• RTS/CTS/<Data>/ACK

• Collision avoidance

• Reliability

• Active Period (Active Time)

T-MAC Active Period

• Starts at scheduled intervals

• Ends when no Activation Event is heard for a time = TA

• firing of a periodic frame timer

• reception of any data

• sensing of any communication activity

• end-of-transmission of own data or acknowledgment

• overhearding end of neighbor’s data exchange

T-MAC Considerations

• Clustering and Synchronization

• RTS Operation and Choosing TA

• Overhearing Avoidance

• Asymmetric Communication

Clustering and Synchronization

• From S-MAC protocol

• Virtual Clustering

• Frame schedules and SYNC packets define a node’s active time

• Shared with neighbors to ensure transmissions go to nodes that are awake

RTS and TA

• Fixed contention window at beginning of active time for RTS signaling

• RTS retry after loss (max of two times)

• TA > C + R + T

Overhearing Avoidance

• Results in energy savings, but decreases max throughput (do not use if speed is required)

• Experiments show going to sleep to avoid overhearing makes nodes miss other RTS/CTS transmissions. When they wake-up, they cause interference collisions.

Asymmetric Communication

• Most communication is unidirectional (node-to-sink)

• Early Sleeping Problem

• Future-Request-to-Send (FTRS)

• Full-buffer Priority

Future RTS

• Nodes that lose RTS/CTS contention have opportunity to send FRTS

• Collides with empty DS packet of contention winner

• Requires increase in TA (increased energy usage)

• 75% throughput gain

Full-buffer Priority• When a node’s

sender buffer is or is almost full, it can decide to ignore an incoming RTS, i.e., refuse to send a CTS reply, and sends its own RTS to a different node

• But... heavy load increases collisions rapidly

Experimental Setup

• OMNeT++ discrete event simulation package

• EYES nodes modeling

• 100 nodes on 10 x 10 grid; non-edge nodes have 8 neighbors each

• Local Unicasts

• Nodes-to-sink communication; shortest-path routing

Results

• Simulations

• Homogenous local unicast

• Nodes-to-sink communication

• The effects of early sleep

• Event-based local unicast and node-to-sink reporting

• Real Implementation

• Energy use

Homogenous Local Unicast

Nodes-to-sink Communication

Early Sleeping

Event-based Unicast and Node-to-sink

Energy Consumption

Future Work

• Experimentation with FRTS and full-buffer priority to solve early sleeping problem

• Node mobility

• Virtual clustering and multi-hop synchronization

Conclusions

• Power consumption reductions achieved

• As much as 96% with low loads as compared to traditional protocols

• Improves upon S-MAC performance in volatile environments where message rates change with both time and place