Wireless Sensor Networks Mixalis Ombashis ECE654 Advanced Networks
Wireless Sensor Networks Mixalis Ombashis ECE-654 Advanced Networks Instructor: Dr. Christos Panayiotou
Outline • Introduction • Design Factors Ø Ø Ø Fault Tolerance Scalability Production Cost Hardware Constrains … • Protocol Stack Ø Physical Layer Ø Data link Layer Ø … • 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.
• Area Monitoring • Environmental Sensing • Military Applications • Health • Fire Detection • Home Automation Applications
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 routing 2. Sensor value aggregation 3. Ignore values from faulty nodes – Passive replication 1. Node selection a) Self-election : Probabilistic Algorithms b) Group election: Clusters With Cluster Heads c) Hierarchical election 2. Service Distribution a) Pre-Copy: Make The Code of All nodes available on all nodes before deployment b) Code distribution c) 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 •
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 infrastructure 2. 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 Qo. S 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
SMACS protocol • Major components of SMAC – Periodic listen and sleep – Collision avoidance – Overhearing avoidance • Neighboring nodes are synchronized together – Periodic updating using a SYNC packet Sender Node ID Next-Sleep Time • 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.
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 (e. Xtensible Cross-layer design Platform) • 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 (e. Xtensible Cross-layer design Platform) • Procedures of process of the XCP 1. 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) b) c) d) RTS signals requires that the received signal to noise ratio (SNR) of an RTS packet, , is above some threshold Prevents congestion by limiting the traffic a node can relay Ensures that the node does not experience any buffer overflow 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 Mobi. Com ’ 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 3 D 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
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