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Medium access layer forWireless Sensor Networks

T.G.Venkatesh

Electrical Engineering

Indian Institute of Technology

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Issues in MAC protocols for WSN Issues

Energy efficiency Fairness of end to end flows

Related work IEEE802.11

High energy consumption when the nodes are in theidle mode

CSMA To improve the energy consumption by avoiding

overhearing among neighboring nodes TDMA

No contention-introduced overhead and collisions Not easy to manage the inter-cluster

communication and interference Not easy to dynamically change its frame length

and time slot assignment PAMAS

Power off radio when not actively transmitting andreceiving packet

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Sensor MAC Requirements

High energy efficiency (High Throughput/energy Ratio)

High channel utilization (High throughput)

Low latency

Reliability

Scalability

Robustness and adaptability to changes

Channel conditions (highly time varying)

Sensor node failure (energy depletion, environmentalchanges)

High clock drift

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Four important sources of wasted energy inWSN:

Idle Listening (consumes about 50 -100% of thepower)

Overhearing (since RF is a broadcast medium)

Collisions (Hidden Terminal Problem)

Control Overhead (e.g. RTS/CTS or DATA/ACK)

MAC Energy Usage

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Power Measurements

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Motivation

Duty cycle: ratio between listen timeto total listen sleep cycle

Central idea: reduce the duty cycle byturning off the radio for part of thetime

Approaches:

TDMA

Schedule contention periods

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Medium Access Paradigms

Contention Based (CSMA)

Random-backoff and carrier-sensing

Simple, no time synch, and robust to network changes

High control overhead (for two-hop collision avoidance) High idle listening and overhearing overheads

Solve this by duty cycling

TDMA Based

Nodes within interference range transmit during different

times, so collision free Requires time synch and not robust to changes.

Low throughput and high latency even during low contention.

Low idle listening and overhearing overheads Wake up and listen only during its neighbor transmission

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Existing MAC Protocols

Sensor-MAC (S-MAC) : Listen-sleep

Timeout-MAC (T-MAC) : Activation event

WiseMAC : Preamble Sampling

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S-MAC

Main goal reduce powerconsumption

Three major components:

Periodic sleep-listen

Collision and overhearing avoidance

Message passing

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S-MAC Design

Listen Period

Sleep/Wake schedule synchronization with neighbours

Receive packets from neighbours

Sleep Period

Turn OFF radio

Set timer to wake up later

Transmission

Send packets only during listen period of intended receiver(s)

Collision Handling

RTS/CTS/DATA/ACK

sleeplisten listen sleepsleeplistenlisten listenlisten sleep

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S-MAC Design

Node 1

Node 2

sleeplisten listen sleep

sleeplisten listen sleep

Schedule 2

Schedule 1

Schedules can differ, prefer neighbouring nodes to havesame schedule

Border nodes may have to maintain more than oneschedule.

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S-MAC Design

Maintaining Schedule

To update schedule by sending a SYNC packet periodically

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S-MAC Design

Collision Avoidance

Problem : Multiple senders want to talk

Options: Contention vs TDMA

Solution :Similar to IEEE 802.11 ad hocmode (DCF) Physical and virtual carrier sense

Randomozed backoff time

RTS/CTS for hidden terminal problem RTS/CTS/DATA/ACK sequence

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S-MAC Design

Overhearing Avoidance

Problem: Receive packets destined toothers

Solution : Sleep while neighbours talk

Who should sleep

All immediate neighbors of sender and receiver

How long to sleep The duration field in each packet informs the

other nodes the sleep interval

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S-MAC Design

Message Passing Only one RTS packet and one CTS packet are used

To avoid large control overhead and long delay

ACK would be sent after each data fragment

To avoid fragment loss or error

To Prevent hidden terminal problem

After the neighbor node hears the RTS and CTS, itwill go to sleep for the time that is needed totransmit all the fragments (using the duration field)

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Advantages/Disadvantages

Energy waste caused by idle listening is reduced bysleep schedules.

Sleep and listen periods are predefined and constantwhich decreases the efficiency of the algorithm under

variable traffic load.

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Timeout-MAC (T-MAC)

Proposed to enhance the poor results of S-MAC protocolunder variable traffic load.

Listen period ends when no activation event has occurredfor a time threshold TA.

Reduce idle listening by transmitting all messages in burstsof variable length, and sleeping between bursts.

times out on hearing nothing.

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S-MAC Vs T-MAC

The SMAC duty cycle ;The arrows indicatetransmitted and received messages; note that themessages come closer.

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Gives better result under variable load.

Suffers from early sleeping problem node goes tosleep when a neighbor still has messages for it.

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WiseMAC

All nodes defined to have two communicationchannels.

Data channel uses TDMA

Control channel uses CSMA

Preamble sampling used to decrease idle listeningtime.

Nodes sample the medium periodically to see if anydata is going to arrive.

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WiseMAC

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Dynamic preamble length adjustment results in betterperformance.

Conflict when one node starts to send the preamble to

a node that is already receiving another nodestransmission where the preamble sender is not withinrange. Hidden terminal problem

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Other MAC Protocols

SIFT :Event Driven

TRAMA : Traffic Adaptive MAC, TDMA

Based

TRAFFIC ADAPTIVE MAC

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TRAFFIC-ADAPTIVE MACPROTOCOLTRAMA

Time is divided into random-accessand scheduled-access (transmission)periods.

The random-access period is used toestablish two hop topologyinformation and

the channel access is contention-based within that period.

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Dynamic Sensor MAC

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D MAC

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WSN MAC Comparison

MAC

Protocol

Type Adaptivity toChanges

Advantages Disadvantages

S-MAC CSMA,Contention

-based

Good Energy waste caused byidle 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 CSMA,Contention

based

Good Gives better results undervariable loads

Early sleeping problem.

WiseMAC CSMA,

Preamble basedGood 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 TDMA/CSMA Good Higher percentage of sleeptime and less collision

probability is achieved

compared to CSMA based

protocols.

Without considering thetransmissions and receptions, the

duty cycle is at least 12.5 %,

which is a considerably high

value.

SIFT CSMA/CA,

Contention

Window-based

Good Very low latency isachieved 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.

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Network Layer for WirelessSensor Networks

T.G.Venkatesh

Electrical Engineering

Indian Institute of Technology

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Communication architecture ofsensor networks

Network layer:

Power efficiency is always an important

consideration. Sensor networks are mostly data

centric.

Data aggregation is useful only when it

does not hinder the collaborative effortof the sensor nodes.

An ideal sensor network has attribute-based addressing and location

awareness.

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Several Network Layer Schemes for Sensor Networks

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Maximum available power (PA) route:Route 2

Minimum energy (ME) route: Route 1Minimum hop (MH) route: Route 3Minimum PA node route: Route 3

Energy Efficient Routes

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Interest Dissemination

Sinks broadcast the interest Sensor nodes broadcast the advertisements

Attribute-based naming

The areas where the temperature is over 70oF

The temperature read by a certain node

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Data aggregation

Solve implosion and overlapProblem

Aggregation based on sameattribute of phenomenon

Specifics (the locations ofreporting sensor nodes) shouldnot be left out

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Open research issues

New protocols need to be developed to addresshigher topology changes and higher scalability.

New internetworking schemes should be developed

to allow easy communication between the sensor

networks and external networks.

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Protocol Classification (1)

Proactive First Compute all Routes;

Then RouteReactive

Compute Routes On-Demand

Hybrid First Compute all Routes;Then Improve While Routing

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Direct Node and Sink Communicate Directly

(Fast Drainage; Small Scale)

Flat (Equal) Random Indirect Route

(Fast Drainage Around Sink; Medium Scale)

Clustering (Hierarchical) Route Through Distinguished Nodes

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Unicast One-to-One Message Passing

Multicast (actually Local Broadcast)Node-to-Neighbors Message

Passing

Broadcast Full-Mesh Source to Everyone

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Low Energy Adaptive Clustering

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1 - LEACH (1)

Protocol Highlights

Self-Organizing Adaptive Clustering

Cluster-Heads elect themselvesrandomly

Nodes die in random

Stationary Sink Localized Coordination

Data Fusion

Low Energy Adaptive ClusteringHierarchy


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1 - LEACH (2)

Main Drawbacks

Hot Spot Problem

(Nodes on a path from an event-congested area to the sink may drain)

Inadequate for Time-Critical

Applications Stationary Sink Maybe Unpractical

Basic Algorithm assumes any node can

communicate with sink limited scale



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1 - LEACH (3)Main Procedures

Works in Rounds, each withSet-Up (Short) and Steady-State (Long)

Set-Up Phase - subdivided:

Advertisement (I am a Cluster-Head)

Cluster Set-Up (I am in your Cluster)

Schedule Creation (This is your slot)

Steady-State Phase:

Data Transmission using TDMA



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1 - LEACH (4)

Main Procedures

Everyone uses the same channel

Different clusters use different CDMAcodes

Code chosen in random

Cluster-Head communicate with Sink

Can be extended to Hierarchical

Clustering



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1 - LEACH (5)


I

llustra

tions


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1 - LEACH (6)


Illustrations

P Effi i t G th i i S I f ti S t

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2 - PEGASIS (1)

Protocol Highlights

Token-Passing Chain-Based

Nodes die in random

Stationary Nodes and Sink

Every node have a global networkmap

Data Fusion

Greedy chain construction

Power-Efficient Gathering in Sensor Information Systems

Power Efficient Gathering in Sensor Information Systems

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2 - PEGASIS (2)

Main Drawbacks

Stationary Nodes

Global Information

Limited Scale:

Information travels many nodes

Assumes any node cancommunicate with sink



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2 - PEGASIS (3)

Main Procedures:

Greedy Algorithm Construct Chain

Start at a node far from sink and gathereveryone neighbor by neighbor

Node i (mod N) is the leader in round i

Nodes passes token thru the chain to leaderfrom both sides

Each node fuse its data with the rest

Leader transmit to sink



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2 - PEGASIS (4)

Illustrations



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2 - PEGASIS (5)


Il

lustrat

ions Rounds Until Death

Sensor Protocol for Information via

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3 - SPIN (1)Protocol Highlights

Network-wide Broadcast Limited by

Negotiation and using Local Communication Flooding problems solved:

Implosion same data from many neighbors Detection of overlapping regions Excessive resources consumption (Blindness)

Needs only Localized Information Data Fusion Two main protocols SPIN-PP & SPIN-BC

Sensor Protocol for Information viaNegotiation


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3 - SPIN (2)

Main Procedures

Broadcast - Limited Scale

every node handles O(n) messagesData is updated throughout

network unnecessary in manycases

Network lifetime - not clear

High degree nodes = High powerneeds

Sensor Protocol for Information viaNegotiation


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3 - SPIN (3)

Main ProceduresSPIN-PP (Point-to-Point Communication)

Data is described by meta-data ADV msg.

Node has data sends ADV to neighbors If neighbor do not have data sends REQ Node responds by sending the DATA

This process continues around the network Nodes may aggregate their data to ADV

In a Lossy Network ADV may be repeatedperiodically and REQ if not answered

Se so otoco o o at o aNegotiation


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3 - SPIN (4)

Main Procedures

SPIN-BC (Local Broadcast Communication)

ADV and DATA sending like PP (but in B.C.) Since only one REQ answer is needed, any

node waits a random interval and B.C. REQonly if none was received yet.

The rest like SPIN-PP

Negotiation


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ADVNode with data

Node with data advertises to all its neighbors

3 - SPIN (5)

Illustr

ation

s

Negotiation


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REQNode with data

Neighbor requests for data and it is sent

Illustr

ation

s

Negotiation

3 - SPIN (5)


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DATA Node with data

Node with data advertises to all its neighbors

3 - SPIN (5)

Illustrations

Negotiation


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Node with dataADV

Receiving node sends ADV to neighbors

I

llustra

tions

Negotiation

3 - SPIN (5)


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Node with data

Receiving neighbors requests for data.

REQ

Illustrat

ions

Negotiation

3 - SPIN (5)

Already

has data

(or dead)


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Node with data

DATA

Receiving node sends DATA to neighbors

Illustrations

Negotiation

3 - SPIN (5)

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Documents

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