Wireless Sensor Networks

1. Wireless Sensor Networks Mixalis Ombashis ECE-654 Advanced Networks Instructor: Dr. Christos Panayiotou

2. 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

3. 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.

4. Area Monitoring • Environmental Sensing • Military Applications • Health • Fire Detection • Home Automation Applications

5. Introduction • Sensor Node Components

6. 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

7. Centralized Approach where all sensors readings are gathered at a sink (Directed Diffusion) • Stationary Sink – Pre determined Position

8. Implementation of SensorField - Sink - User

9. 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

10. Design Factors • Fault Tolerance • Nodes May Fail, Blocked or Physical Damaged • Ability to sustain functionalities without any interruption due to sensor node failures

11. Source of Faults in WSN Applications • Node Faults • Network Faults • Sink Faults • Failure Classification • Crash or Omission • Timing • Value • Arbitrary

12. Design Factors • Fault detection techniques • Self-Diagnosis • Group Detection: Only if a reference value is available • Hierarchical Detection: Trees • Fault recovery techniques • Active replication • Multipath routing • Sensor value aggregation • Ignore values from faulty nodes • Passive replication • Node selection • Self-election : Probabilistic Algorithms • Group election: Clusters With Cluster Heads • Hierarchical election • Service Distribution • Pre-Copy: Make The Code of All nodes available on all nodes before deployment • Code distribution • Remote Execution

13. 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

14. 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

15. 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

16. Protocol Stack Management Planes

17. 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

18. Protocol Stack • The Physical Layer • Responsible for • Frequency selection • Carrier frequency generation • Signal detection • Modulation • Data encryption

19. 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

20. 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

21. Protocol Stack • The Data Link Layer • Responsible for • Multiplexing of data streams • Data frame detection • Medium Access Control • Error Control

22. Medium Access Control (MAC) • Two Goals: • Creation of the network infrastructure • 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 recoverystrategies

23. Medium Access Control (MAC) Need To Turn Off The RADIO!!

24. Medium Access Control (MAC) • Major sources of energy waste • Long idle time when no sensing event happens • Collisions • Overhearing • Control overhead

25. 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

26. 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 • 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. Sender Node ID Next-Sleep Time

27. 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

28. 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

29. 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

30. 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

31. 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>

34. 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.

35. 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

36. XCP (eXtensible Cross-layer designPlatform) • Enables the exchange of information between different layers for performance optimization

37. CPL (Communication Protocol Layer), MRL (Mutual Reference across Layer) PO (Performance Optimization) component

38. XCP (eXtensible Cross-layer designPlatform) • Procedures of process of the XCP • In initialization, each cross-layer module in the PO component requests the interesting information to the MRL component using REQUEST_INFORMATION() • If a cross-layer module need not more any information, it can release the requested information using RELEASE_INFORMATION() • The bus arbiter thread pops a data from information queues and informs it to requested cross layer modules • When the requested information is stored at information base in the each cross-layer module, it performs optimization • Then the results of optimization by each cross-layer module are applied to information set using APPLY_INFORMATION()

39. Cross-layer module (XLM) • Complete unified cross-layering • Incorporates • Initiative determination • Received based contention • Local congestion control • Distributed duty cycle operation

40. 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

41. 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: 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

42. 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

43. 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 SungwooTak, “A Communication Framework Supporting Cross-Layer Design for Wireless Networks”, IEEE Int’l Symposium On Ubiquitous Multimedia Computing, Hobart, Australia, Oct. 2008