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UNIVERSITY OF CALIFORNIASANTA CRUZ

Energy-Efficient ChannelAccess Protocols

Venkatesh Rajendran

[email protected]


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Introduction

Sensor networks are a special class ofmultihop wireless networks.

Ad-hoc deployment.

Self-configuring.

Unattended.

Battery powered.


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Network Architecture

Sink Sink

Thousands of nodes.

Short radio range (~10m).

Event-driven.

Hierarchical deployment.

Fault tolerance.

Information Processing


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MAC Protocols

Regulate channel access in a sharedmedium.

A

C

B Collision at B

X

No coordination(ALOHA)


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CSM

A

Listen before transmittingStations sense the channel before transmittingdata packets.

S R H

X

Collision at R

Hidden Terminal Problem


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CSM

A with Collision Avoidance Stations carry out a handshake to

determine which one can send a data

packet (e.g.,M

ACA, FAM

A,IEEE802.11, RIMA).

S R H

RTSCTS

Data

ACK Backoff due to CTS


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Idle Listening

Tx

Rx

Idle Listen

Idle Listen

Tx

Rx

Sleep

Sleep

Sleep Scheduling

Medium Access Energy-efficient channel access is important to prolong the

life-time of sensor nodes.

Conventional media-access control protocols waste energyby collisions, and idle listening.

Radios have special sleep mode for energy conservation.


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Achieving energy efficiency

When a node is neither transmitting orreceiving, switch to low-power sleep

mode. Prevent collisions and retransmissions.

Need to know Tx, Rx and whentransmission event occurs.

Scheduled-based(time-slotted) MAC Protocols


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Contention-based Channel AccessProtocols: S-MAC & T-MAC


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S-M

AC [Ye etal] Features Collision Avoidance

Similar to 802.11 (RTS/CTS handshake).

Overhearing Avoidance

All the immediate neighbors of the sender andreceiver goes to sleep.

Message Passing Long messages are broken down in to smaller

packets and sent continuously once the channelis acquired by RTS/CTS handshake.

Increases the sleep time, but leads to fairnessproblems.


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S-M

AC Overview Time is divided in to cycles of listen

and sleep intervals.

Schedules are established such thatneighboring nodes have synchronoussleep and listen periods.

SYNC packets are exchanged

periodically to maintain schedulesynchronization.


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S-M

AC Operation


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Schedule Establishment Node listens for certain amount of time. If it does not hear a schedule, it chooses a

time to sleep and broadcast this

information immediately. This node is called the Synchornizer. If a node receives a schedule before

establishing its schedule, it just follows thereceived schedule.

If a node receives a different schedule,after it has established its schedule, itlistens for both the schedules.


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S-M

ACIllustration


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T-M

AC[Dam etal]: S-M

AC Adaptive Listen


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T-M

AC: Early Sleeping Problem


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T-M

AC/S-M

AC Summary Simple contention-based channel

access with duty cycle-based

sleeping. Restricting channel contention to a

smaller window negative effect onenergy savings due to collisions.

Requires schedule co-ordination withone hop neighbors.


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Scheduling-based Channel AccessProtocols: TRAMA and FLAMA


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TRAMA: TRaffic-Adaptive Medium Access

Establish transmission schedules in away that:

is self adaptive to changes in traffic, nodestate, or connectivity.

prolongs the battery life of each node.

is robust to wireless losses.


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TRAMA - Overview

Single, time-slotted channel access. Transmission scheduling based on two-hop

neighborhood information and one-hoptraffic information.

Random access period Used for signaling: synchronization and updating

two-hop neighbor information.

Scheduled access period:

Used for contention free data exchange betweennodes. Supports unicast, multicast and broadcast

communication.


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TRAM

A Features Distributed TDMA-based channel

access.

Collision freedom by distributedelection based on Neighborhood-Aware Contention Resolution (NCR).

Traffic-adaptive scheduling to

increase the channel utilization. Radio-mode control for energy

efficiency.


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Time slot organization


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Each node maintains two-hop neighborinformation.

Based on the time slot ID and node ID, node

priorities are calculated using a random hashfunction.

A node with the highest two-hop priority isselected as the transmitter for the particular

time slot.

Neighborhood-aware contentionresolution (NCR)[Bao et al., Mobicom00]


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A

C

D

B E

F

3

2

15

8

9

11C Winner

G

H

I

13

3

14NCR does not elect receivers and hence, nosupport for radio-mode control.

NCR - Example


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TRAMA Components

Neighbor Protocol (NP). Gather 2-hop neighborhood information.

Schedule Exchange Protocol (SEP). Gather 1-hop traffic information.

Adaptive Election Algorithm (AEA). Elect transmitter, receiver and stand-by

nodes for each transmission slot. Remove nodes without traffic from

election.


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Neighbor Protocol Main Function: Gather two-hop

neighborhood information by using

signaling packets. Incremental neighbor updates to

keep the size of the signaling packetsmall.

Periodically operates during randomaccess period.


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Packet Formats


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Schedule Exchange Protocol

(SEP) Schedule consists of list of intended

receivers for future transmission slots.

Schedules are established based on thecurrent traffic information at the node.

Propagated to the neighborsperiodically.

SEP maintains consistent schedules forthe one-hop neighbors.


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Schedule Packet Format


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Adaptive Election Algorithm

(AEA) Decides the node state as either

Transmit, Receive or Sleep.

Uses the schedule informationobtained by SEP and a modified NCRto do the election.

Nodes without any data to send are

removed from the election process,thereby improving the channelutilization.


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Simulation Results

Delivery Ratio

Synthetic broadcasttraffic using Poissonarrivals.

50 nodes, 500x500 area.

512 byte data.

Average node density: 6


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Energy Savings

Percentage Sleep Time Average Length of sleep interval


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TRAMA Limitations

Complex election algorithm and datastructure.

Overhead due to explicit schedulepropagation.

Higher queueing delay.

Flow-aware, energy-efficient framework.


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TRAMA Summary

Significant improvement in delivery ratio inall scenarios when compared to contention-based protocols.

Significant energy savings compared to S-MAC (which incurs more switching).

Acceptable latency and traffic adaptive.


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Flow-aware Medium Access

Framework Avoid explicit schedule propagation.

Take advantage of application.

Simple election algorithm to suit systemswith low memory and processing power(e.g., 4KB ROM in Motes).

Incorporate time-synchronization, flow

discovery and neighbor discovery duringrandom-access period.


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Flow Information

Flow information characterizesapplication-specific traffic patterns.

Flows can be unicast, multicast orbroadcast.

Characterized by source, destination,duration and rate.


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Example: Data Gathering

Application

A

C

E

BD

Sink

Fb

Fc

Fd

Fe


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Flow Discovery Mechanism

Combined with neighbor discoveryduring random-access period.

Adapted based on the application. for data gathering application, flow

discovery is essentially establishing thedata forwarding tree.


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Example

S

Sink initiates neighbor discovery,flow discovery and timesynchronization.

Broadcasts periodic SYNC packets.

Potential children reply with

SYNC_REQ.

Source reinforces with another

SYNC packet.

Once associated with a parent,nodes start sending periodic SYNC

broadcasts.

A

B

C

Sink

SYNC

SYNC_REQ


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Election Process

Weighted election toincorporate traffic adaptivity.

Nodes are assigned weightsbased on their incoming and

outgoing flows. Highest priority 2-hop node is

elected as the transmitter.

A node listens if any of itschildren has the highest 1-hop priority.

Can switch to sleep modeif no transmission isstarted.

S

A

B

C

Sinkws=0

wc=1

wc=1

wa=3


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Simulation Results (16 nodes, 500x500 area,CC1000 radio, grid topology, edge sink)

Delivery Ratio Queueing Delay


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FLAMA Summary

Simple algorithm that can beimplemented on a sensor platform.

Significant performance improvementby application awareness.

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