mac wireless sensor networks

8/4/2019 MAC Wireless Sensor Networks

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Wireless Sensor Networks


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Wireless Sensor Networks:

A wireless sensor network is a collection of nodes organized into

a cooperative network.

Each node consists of processing capability (one or more

microcontrollers, CPUs or DSP chips),

May contain multiple types of memory (program, data and flash

memories), Have a RF transceiver (usually with a single omni-directional

antenna),

Have a power source (e.g., batteries and solar cells), and

accommodate various sensors and actuators. The nodes communicate wirelessly and often self-organize after

being deployed in an ad hoc fashion.


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Currently, wireless sensor networks are beginning tobe deployed

at an accelerated pace. It is not unreasonable toexpect that in

10-15 years that the world will be covered withwireless sensor

networks with access to them via the Internet. This new technology is exciting with unlimited

potential for numerous application areas including

Environmental, medical, military, transportation,

entertainment, crisis management, homelanddefense, and smart spaces.

Since a wireless sensor network is a distributed real-

time system.


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Todays Discussion:

MAC layer

Routing

Node localization

Clock synchronization

Power management


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MAC A medium access control (MAC) protocol

coordinates actions over a shared channel. Themost commonly used solutions are contention-based.

Node which has a message to transmit to test the

channel to see if it is busy, if not busy then ittransmits, else if busy it waits and tries again later.

If two or more nodes transmit at the same timethere is a collision and all the nodes colliding tryagain later.

Many wireless MAC protocols also have a dozemode where nodes not involved with sending orreceiving a packet in a given timeframe go intosleep mode to save energy.

In general, most MAC protocols optimize for the


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However, a wireless sensor network has morefocused requirements that include a local uni-orbroad-cast, traffic is generally from nodes to one

or a few sinks (most traffic is then in one direction)

An effective MAC protocol for wireless sensornetworks must consume little power, avoidcollisions,

Be implemented with a small code size andmemory requirements, be efficient for a singleapplication, and be tolerant to changing radiofrequency and networking conditions.

B-MAC


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B-MAC

B-MAC is highly configurable and can be

implemented with a small code and memory size. It has an interface that allows choosing various

functionality and only that functionality as neededby a particular application

B-MAC Consists of 4 main Parts:

Clear channel assessment (CCA),

Packet backoff,

Link layer acks,

Low power listening.


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For CCA, B-MAC uses a weighted moving average ofsamples when the channel is idle in order to assessthe background noise and better be able to detect

valid packets and collisions.

The packet backoff time is configurable and ischosen from a linear range as opposed to an

exponential backoff scheme typically used in otherdistributed systems.

This reduces delay and works because of the typicalcommunication patterns found in a wireless sensor

network.


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Contents

Basic Concepts

S-MAC

T-MAC

B-MAC

P-MAC

Z-MAC


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Basic Concepts

Problem

TDMA

CSMA RTS / CTS


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Hidden Nodes

A B C


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MAC Challenges

Traditionally

Fairness

Latency

Throughput

For Sensor Networks

Power efficiency

Scalability


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S-MAC - Sensor MAC

Nodes periodically sleep

Trades energy efficiency for lowerthroughput and higher latency

Sleep during other nodestransmissions

Listen Sleep tListen Sleep


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S-MAC

Listen significantly longer than clock drift Neighboring nodes exchange SYNC msgs Exchanged timestamps are relative

rather than absolute RTS/CTS avoids hidden terminal Message passing provided Packets contain expected duration of

message

Every packet must be acknowledged Adaptive listening can be used so that

potential next hop nodes wake up intime for possible transmissions


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S-MAC Results

Latency and throughput are problems,but adaptive listening improves itsignificantly


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S-MAC Results

Energy savings significantcompared to non-sleepingprotocols


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T-MAC - Timeout MAC

Transmit all messages in bursts ofvariable length and sleep betweenbursts

RTS / CTS / ACK Scheme

Synchronization similar to S-MAC


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T-MAC Operation


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T-MAC Results

T-MAC saves energy compared to S-MAC The early sleeping problem limits the

maximum throughput Further testing on real sensors needed


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B-MAC - Berkeley MAC

B-MACs Goals:

Low power operation

Effective collision avoidance

Simple implementation (small code)

Efficient at both low and high data rates

Reconfigurable by upper layers

Tolerant to changes on the network

Scalable to large number of nodes


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B-MACs Features

Clear Channel Assessment (CCA)

Low Power Listening (LPL) usingpreamble sampling

Hidden terminal and multi-packetmechanisms not provided, should beimplemented, if needed, by higherlayers

Sleep

t

ReceiveReceiver

Sleep

t

PreambleSender Message

Sleep


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B-MAC Interface

CCA on/off

Acknowledgements on/off

Initial and congestion backoff in a perpacket basis

Configurable check interval and

preamble length


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B-MAC Lifetime Model

E can be calculated if hardware constants, sample

rate, number of neighboring nodes and checktime/preamble are known

Better: E can be minimized by varying checktime/preamble if constants, sample rate andneighboring nodes are known


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B-MAC Results

Performs better than the otherstudied protocols in most cases

System model can be complicatedfor application and routing protocoldevelopers

Protocol widely used because hasgood results even with defaultparameters


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P-MAC - Pattern MAC

Patterns are 0*1 strings with size 1-N Every node starts with 1 as pattern

Number of 0s grow exponentially up to athreshold and then linearly up to N-1

TR = CW + RTS + CTS + DATA + ACK N = tradeoff between latency and energy


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Patterns vs Schedules

Local

Pattern Bit

Packet to

Send

Receiver

Pattern Bit

Local

Schedule

1 1 1 1

1 1 0 1-

1 0 * 1-

0 1 1 1

0 1 0 0

0 0 * 0


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P-MAC Evaluation

Simulated results are better thanSMAC

Good for relatively stable trafficconditions

Adaptation to changes on trafficmight be slow

Loose time synchronization required

Needs more testing and comparisonwith other protocols besides S-MAC


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Z-MAC - Zebra MAC

Runs on top of B-MAC Combines TDMA and CSMA features

CSMAPros

Simple Scalable

Cons Collisions due to hidden

terminals RTS/CTS is overhead

TDMAPros

Naturally avoidscollisions

Cons Complexity of scheduling Synchronization needed


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Z-MAC Initialization

Neighborhood discovery through pingmessages containing known neighbors

Two-hop neighborhood used as input for ascheduling algorithm (DRAND)

Running time and message complexity ofDRAND is O(), where is the two-hopneighborhood size

The idea is to compensate the initialization

energy consumption during the protocolnormal operation


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Z-MAC Time SlotAssignment


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Z-MAC Transmission Control

The Transmission Rule:

If owner of slot

Take a random backoff within To

Run CCA and, if channel is clear,transmit

Else

Wait for To

Take a random backoff within [To,Tno]

Run CCA and, if channel is clear,

transmit


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Z-MAC HCL Mode

Nodes can be in High Contention Level(HCL)

A node is in HCL only if it recently receivedan Explicit Contention Notification (ECN)

from a two-hop neighbor Nodes in HCL are not allowed to contend

for the channel on their two-hopneighbors time slots

A node decides to send an ECN if it islosing too many messages (applicationACKs) or based on noise measuredthrough CCA


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Z-MAC Receiving Schedule

B-MAC based

Time slots should be large enoughfor contention, CCA and one B-MACpacket transmission

Slot size choice, like in B-MAC, left toapplication


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Z-MAC Results

Z-MAC performs better than B-MAC whenload is high

As expected, fairness increases with Z-MAC

Complexity of the protocol can be aproblem


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Conclusions

Between the protocols studied, B-MAC still seems to be the best onefor applications in general

Application developers seem not touse B-MACs control interface

Middleware service could make such

optimizations according to networkstatus

mac wireless sensor networks

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