Wireless Sensor NetworksMixalis Ombashis

ECE-654Advanced Networks

Instructor: Dr. Christos Panayiotou

Outline• Introduction

• Design Factors Fault Tolerance Scalability Production Cost Hardware Constrains …

• Protocol Stack Physical Layer Data link Layer …

• Cross layer Protocols For WSN– XCP– XLM

What Is A Sensor ?

• A sensor (also called detector) is a converter that measures a physical quantity and converts it into a signal which can be read by an observer or by an (today mostly electronic) instrument.

Applications• Area

Monitoring

• Environmental Sensing

• Military Applications

• Health

• Fire Detection

• Home Automation

Introduction

• Sensor Node Components

Introduction

• Sensor Position– Need to be engineered or predetermined– Random Deployment in inaccessible terrains– Disaster Relief Operations

• Self organizing Capabilities – Protocols– Algorithms

• Local Computation– Transmit Only Required Partially Processed Data

• Centralized Approach where all sensors readings are gathered at a sink (Directed Diffusion)

• Stationary Sink – Pre determined Position

Implementation of Sensor Field - Sink - User

Two-Tier Data Dissemination Model For Large Scale WSN

• Locations are known through the use of GPS and localization algorithms

• Homogeneous Sensor nodes

• Short Range Radio

• Multiple Hops for long distances

• Sinks query the network

• Two level Flooding

Design Factors

• Fault Tolerance– Nodes May Fail, Blocked or Physical Damaged

– Ability to sustain functionalities without any interruption due to sensor node failures

• Source of Faults in WSN Applications• Node Faults• Network Faults• Sink Faults

• Failure Classification• Crash or Omission• Timing• Value • Arbitrary

Design Factors• Fault detection techniques

– Self-Diagnosis– Group Detection: Only if a reference value is available– Hierarchical Detection: Trees

• Fault recovery techniques– Active replication

1. Multipath routing2. Sensor value aggregation3. Ignore values from faulty nodes

– Passive replication1. Node selection

a) Self-election : Probabilistic Algorithmsb) Group election: Clusters With Cluster Headsc) Hierarchical election

2. Service Distributiona) Pre-Copy: Make The Code of All nodes available on all nodes before deploymentb) Code distributionc) Remote Execution

Design Factors

• Scalability– Number of Deployed nodes vary from hundreds to thousands or

millions depending on the applications– Density has to be utilized:

• N is the number of scattered nodes• R is the ratio transmission range• μ(R) gives the number of nodes within the transmission radius of each node in

region A

• Production Cost– Obviously has to be low

Design Factors• Hardware Constrains– May need to fit into a matchbox-sized module– Consume Extremely Low Power

• Environment– Unattended in Remote geographic areas– Bottom of an ocean– Battlefield

Design Factors

• Transmission Media– Wireless Medium: Radio, Infrared

• Power Consumption– Limited Power Source– May be Impossible to Replenish Power Source– The malfunctioning of few nodes can cause

significant topological changes and might require rerouting of packets and reorganization of the network

Protocol Stack

Management Planes

Protocol Stack

• Management Planes– Power Management Plane:

• Manage how a sensor node uses its power

– Mobility Management Plane:• Detects and registers the movement of sensor nodes, so a

route back to the user is always maintained and the sensor nodes can keep track of who their neighbour sensors are

– Task Management Plane:• Sensor can work together in a power efficient way, route

data in a mobile sensor network, and share resources between sensor nodes

Protocol Stack

• The Physical Layer– Responsible for • Frequency selection • Carrier frequency generation • Signal detection • Modulation• Data encryption

The Physical Layer

• Requirements– The radio must be containable in a small device,

since the sensor nodes are small

– The radios must be cheap, since the sensors will be used in large numbers in redundant fashion

– The radio technology must work with higher layers in the protocol stack to consume very low power levels

The Physical Layer

• Signal propagation effects– Power required to transmit a signal is Proportional

to dn , – n closer to 4 for low-lying antennas and near

ground channels, due to signal cancellation by a ground-reflected ray.

– Multihop communication in a sensor network can effectively overcome shadowing and path loss effects, if the node density is high enough

Protocol Stack

• The Data Link Layer– Responsible for• Multiplexing of data streams• Data frame detection• Medium Access Control• Error Control

Medium Access Control (MAC)• Two Goals:

1. Creation of the network infrastructure2. Share communication resources between sensor nodes

• Collision avoidance• Energy efficiency• Scalability in node density

• Why existing MAC protocols can’t be used?– The primary goal of the existing MAC protocol is the provision of high QoS

and bandwidth efficiency– Energy is not taken into account

• MAC protocols for sensor network must have– Built-in power conservation– Mobility management– Failure recovery strategies

Medium Access Control (MAC)

Need To Turn Off The RADIO!!

Medium Access Control (MAC)

• Major sources of energy waste– Long idle time when no sensing event happens– Collisions– Overhearing– Control overhead

MAC Protocols Proposed For Sensor Networks

• The SMACS protocol - Self-Organizing Medium Access Control For Sensor Networks– Achieves network start-up and link-layer

organization

• CSMA - Carrier Sense Multiple Access based MAC

• Hybrid TDMA/FDMA based

• Major components of SMAC– Periodic listen and sleep– Collision avoidance– Overhearing avoidance

• Neighboring nodes are synchronized together– Periodic updating using a SYNC packet

• Listen interval divided into two parts– Each part further divided into time slots

• RTS/CTS Similar to IEEE 802.11– Interfering nodes go to sleep after they hear the RTS or CTS packet

• Power conservation is achieved by using a random wake-up schedule during the connection phase and by turning the radio off during idle time slots.

SMACS protocol

Sender Node ID

Next-Sleep Time

CSMA Based Mac Protocol

• Two important components– The listening mechanism– The back off scheme.

• As reported and based on simulations– Constant listen periods are energy efficient – The introduction of random delay provides

robustness against repeated collisions

CSMA Based Mac Protocol

• Adaptive Transmission Rate Control Scheme - ARC– Achieves medium access fairness by balancing the rates of

originating and route-through traffic

– The ARC controls the data origination rate of a node in order to allow the route-through traffic to propagate.

– Route-through traffic is preferred over the originating traffic• Since dropping route-through traffic is costlier ,the associated

penalty is lesser

Hybrid TDMA/FDMA based Protocol

• Centrally controlled MAC scheme

• The system is made up of energy constrained sensor nodes that communicate to a single, nearby, high powered base station (<10 m).

• While a pure TDMA scheme dedicates the full bandwidth to a single sensor node, a pure FDMA scheme allocates minimum signal bandwidth per node.

• Optimum number of channels found to depend on the ratio of power consumption between transmitter and receiver– If transmitter consumes more power TDMA scheme is preferred– If receiver consumes more power FDMA scheme is preferred

The Data Link Layer

• Power saving modes of operation• Turn the transceiver off when it is not required.

– Not exactly– Dominance of Start-up Energy

Power saving modes of operation

• Dynamic Power Management Scheme– An event occurs when a sensor node picks up a

signal with power above a predetermined threshold.

– Probability assumed to be Exponential <e-λt>

The Data Link Layer

• Error Control–Two important modes of error control• Forward error correction (FEC)– Higher Decoding Complexity– If the associated processing power is greater than

the coding gain, then the whole process in energy inefficiency and the system is better off without coding.

• Automatic repeat request (ARQ)– Limited by the additional retransmission energy

cost and overhead.

Cross layer Protocols For WSN

• Performance limitations in the layered architecture– It doesn’t consider dependencies between different layers.

• Two kinds of cross-layer architecture– Packet-based interaction scheme

• Each layer puts all information that used for cross-layer approaches into packet header and other layers catch interesting information by inspecting the each packet.

– Direct interaction scheme• Allows any two layers to communicate directly with one another via new

APIs

• Both schemes, existing system software may need to be modified to support new packet structures or APIs

XCP (eXtensible Cross-layer designPlatform)

• Enables the exchange of information between different layers for performance optimization

CPL (Communication Protocol Layer),

MRL (Mutual Reference across Layer)

PO (Performance Optimization) component

XCP (eXtensible Cross-layer designPlatform)

• Procedures of process of the XCP1. In initialization, each cross-layer module in the PO

component requests the interesting information to the MRL component using REQUEST_INFORMATION()

2. If a cross-layer module need not more any information, it can release the requested information using RELEASE_INFORMATION()

3. The bus arbiter thread pops a data from information queues and informs it to requested cross layer modules

4. When the requested information is stored at information base in the each cross-layer module, it performs optimization

5. Then the results of optimization by each cross-layer module are applied to information set using APPLY_INFORMATION()

Cross-layer module (XLM)

• Complete unified cross-layering

• Incorporates – Initiative determination – Received based contention– Local congestion control– Distributed duty cycle operation

Cross-layer module (XLM)

• Communication in XLM is built on initiative concept– Provides freedom for each node to decide on

participating in communication– The next-hop in each communication is not

determined in advance

Cross-layer module (XLM)

• Initiative determination procedure– A node initiates transmission by broadcasting an RTS packet to

indicate its neighbors that it has a packet to send– Upon receiving an RTS packet, each neighbor of node i decides to

participate in the communication or not– This decision is given through initiative determination– The initiative determination is a binary operation where a node

decides to participate in communication if its initiative is 1. – Denoting the initiative as I, it is determined as follows:

a) RTS signals requires that the received signal to noise ratio (SNR) of an RTS packet,, is above some threshold

b) Prevents congestion by limiting the traffic a node can relayc) Ensures that the node does not experience any buffer overflowd) Ensures that the remaining energy of a node stays above a minimum

value

Cross-layer module (XLM)• Distributed duty cycle operation

– Each node is implemented with a sleep frame with length TS sec. As a result, a node is active for δ × TS sec and sleeps for (1 − δ) × TS sec.

• Transmission Initiation– Listens to the channel for a specific period of time– Checks if its information is correlated with the transmitting source nodes– If the channel is occupied, the node performs back off based on its contention

window– When the channel is idle, the node broadcasts an RTS packet, which contains the

location of the sensor node i and the location of the sink– When a node receives an RTS packet, it first checks the source and destination

locations

• Receiver Contention– After an RTS packet is received, if a node has initiative to participate in the

communication, it performs receiver contention to forward the packet

References • G.Hoblos, M. Staroswiecki, and A. Aitouche, “Optimal Design of Fault Tolerantt Sensor Networks”, IEEE Int’l.

Conf. Cont. Apps., Anchorage, AK, Sept. 2000, pp. 467-72

• Bulusu et al., “Scalable Coordination for Wireless Sensor Networks: Self-Configuring Localization Systems”, ISCTA 2001, Ambleside, U.K., July 2001

• E.Shih et al., “Physical Layer Driven Protocol aand Algorithm Design for Energy-Efficient Wireless Sensor Networks”, Proc. ACM MobiCom ’01, Rome, Italy, July 2001, pp 272-86

• A.Sinha and A. Chandrakasan, “Dynamic Power Management in Wireless Sensor Networks”, IEEE Design Test Comp., Mar./April. 2001

• M.-S. Pan, C.-H. Tsai, and Y.-C. Tseng, Implementation of an Emergency Guiding and Monitoring System in Indoor 3D Environments by Wireless Sensor Networks, Technical Report of CS/NCTU 2006.

• T. Melodia, M. C. Vuran, D. Pompili, “The State of the Art in Cross layer Design for Wireless Sensor Networks,” to appear in Springer Lecture Notes in Computer Science (LNCS), 2006.

• Byounghoon Kim and Sungwoo Tak, “A Communication Framework Supporting Cross-Layer Design for Wireless Networks”, IEEE Int’l Symposium On Ubiquitous Multimedia Computing, Hobart, Australia, Oct. 2008