1. Fall 2006 Introduction to Wireless Sensor Network Part 2 Choong Seon Hong Kyung Hee University cshong@khu.ac.kr cshong@khu.ac.kr

2. Fall 2006 2  Sensor Networks Architecture Sensor node  Made up of four basic components Sensing unit, Processing unit, Transceiver unit, and Power unit  Additional application-dependent components Location finding system, power generator, and mobilizer  Scattered in a sensor field  Collect data and route data back to the sink Sink  Communicate with the task manager node (user) via Internet or satellite

3. Fall 2006 3  Hardware Constraints Location finding systemMobilizer   Power Unit   Sensor   ADC   Processor   Storage Transceiver Sensing Unit Processing Unit Power generator Optional part Components of a Sensor Node

4. Fall 2006 4  Energy Consumption in Each Sensor Node Each sensor node has limited energy supply Nodes may not be rechargeable Energy consumption in  Sensing  Data processing  Communication (most energy intensive)

5. Fall 2006 5  Sensor Network Protocol Stack Transport Data Link Physical Network Power Management Application Mobility Management Task Management Power Management – How the sensor uses its power, e.g. turns off its circuitry after receiving a message. Mobility Management – Detects and register the movements of the sensor nodes Task Management – Balances and schedules the sensing tasks given to a specific region

6. Fall 2006 6  Physical Layer Physical Data Link Network Transport Application Frequency selection – The use of the industrial, scientific, and medical (ISM) bands has been often proposed Carrier frequency generation and Signal detection – Depend on the transceiver and hardware design constraints which aim for simplicity, low power consumption, and low cost per unit Modulation   Binary and M-ary modulation schemes can transmit multiple bits per symbol at the expense of complex circuitry   Binary modulation schemes are simpler to implement and thus deemed to be more energy-efficient for WSN applications Low transmission power and simple transceiver circuitry make Ultra Wideband (UWB) an attractive candidate   Baseband transmission, i.e. no intermediate or carrier frequencies   Generally uses pulse position modulation   Resilient to multipath   Low transmission power and simple transceiver circuitry

7. Fall 2006 7  Physical Layer Radio Model – Energy Consumption Energy consumption minimization is of paramount importance when designing the physical layer for WSN in addition to the usual effects such as scattering, shadowing, reflection, diffraction, multipath, and fading. E TC =energy used by the transmitter circuitry E TA =energy required by the transmitter amplifier to achieve an acceptable signal to noise ratio or at the receiver

8. Fall 2006 8  Physical Layer Assuming a linear relationship for the energy spent per bit by the transmitter and receiver circuitry e TC, e TA, and e RC are hardware dependent parameters An explicit expression for energy consumption in the AMP can be derived as,

9. Fall 2006 9  Physical Layer (S/N) r = minimum required signal to noise ratio at the receiver’s demodulator for an acceptable Eb/N0 NF rx = receiver noise figure N 0 = thermal noise floor in a 1 Hertz bandwidth (Watts/Hz) BW = channel noise bandwidth λ= wavelength in meters α = path loss exponent whose value varies from 2 (for free space) to 4 (for multipath channel models) G ant = antenna gain η amp = transmitter power efficiency R bit = raw bit rate in bits per second

10. Fall 2006 10  Data Link Layer Medium Access Control (MAC) – Let multiple radios share the same communication media Functions: Local Topology Discovery and Management Media Partition by Allocation or Contention Provide Logical Channels to Upper Layers Physical Data Link Network Transport Application The data link layer is responsible for Multiplexing of data stream Data frame detection Medium access and error control Ensures reliable point-to-point and point-to-multipoint connections in a communication network MAC protocol for sensor network must have built-in power conservation mechanisms, and strategies for the proper management of node mobility or failure

11. Fall 2006 11  Wireless MAC Protocols Wireless MAC protocols can be classified into two categories, distributed and centralized, according of the type of network architecture for which they have been designed. Protocols can be further classified, based on the mode of operation, into random access protocols, guaranteed access protocols, and hybrid access protocols Wireless MAC protocols Distributed MAC protocols Centralized MAC protocols Random access Guaranteed access Hybrid access Since it is desirable to turn off the radio as much as possible in order to conserve energy some type of TDMA mechanism is often suggested for WSN applications. Constant listening times and adaptive rate control schemes have also been proposed.

12. Fall 2006 12  Network Layer Physical Data Link Network Transport Application Basic issues to take into account when designing the network layer for a WSN are: Power efficiency Data centric – The nature of the data (interest requests and advertisement of sensed data) determines the traffic flow Data aggregation is useful to manage the potential implosion of traffic because of the data centric routing Rather than conventional node addresses an ideal sensor network uses attribute-based addressing, e.g. “region where humidity is below 5%” Locationing systems, i.e. ability for the nodes to establish position information Internetworking with external networks via gateway or proxy nodes

13. Fall 2006 13  Routing Multihop routing common due to limited transmission range Phenomenon being sensed Sink Low node mobility Power aware Irregular topology MAC aware Limited buffer space Some routing issues in WSNs Data aggregation takes place here

14. Fall 2006 14  Data Aggregation It is a technique used to solve the problem of implosion in WSNs. This problem arises when packets carrying the same information arrive a node. This situation can happen when more than one node sense the same phenomenon. This is different than the problem of “duplicate packets” in conventional ad hoc networks. Here it is the high level interpretation of the data in the packets is what determines if the packets are the “same.” Even for the case when the packets are deemed to be different they could still be aggregated into a single packet before the relaying process continues. Data coming from multiple sensor nodes are aggregated, if they have about the same attributes of the phenomenon being sensed, when they reach a common routing or relaying node on their way to the sink. In this view the routing mechanism in a sensor network can be considered as a form of reverse multicast tree. Phenomenon being sensed

15. Fall 2006 15  Data Centrality In data-centric routing, an “interest ” dissemination is performed in order to assign the sensing tasks to the sensor nodes. This dissemination can take different forms such as: The sink or controlling nodes broadcast the nature of the interest, e.g. “four legged animals of at least 50 Kg in weight” Sink Four-legged animal of at least 50 Kg Flow of the request

16. Fall 2006 16  Data Centrality   Sensor nodes broadcast an advertisement of available sensed data and wait for a request from the interested sinks Sink Flow of the advertisement Tiger, tiger, burning bright, In the forest of the night, What immortal hand or eye Could frame thy fearful symmetry?

17. Fall 2006 17  Transport Layer Physical Data Link Network Transport Application TCP variants developed for the traditional wireless networks are not suitable for WSNs where the notion of end-to-end reliability has to be reinterpreted due to the “sensor” nature of the network which comes with features such as: Multiple senders, the sensors, and one destination, the sink, which creates a reverse multicast type of data flow For the same event there is high level of the redundancy or correlation in the data collected by the sensors and thus there is no need for end-to-end reliability between individual sensors and the sink but instead between the event and the sink On the other hand there is need of end-to-end reliability between the sink and individual nodes for situations such as re-tasking or reprogramming The protocols developed should be energy aware and simple enough to be implemented in the low-end type of hardware and software of many WSN applications

18. Fall 2006 18  Application Layer Physical Data Link Network Transport Application There has not been a lot of development on this layer for WSNs. Some potential applications have been suggested as listed below but little work of substance has been reported on any particular area.   Authentication, key distribution, and other security tasks   Sensor movement management Sensor Management Protocol (SMP) – Carries out tasks such as:   Turning sensors on and off   Exchanging data related to the location finding algorithms Interest Dissemination – Interest is send to a sensor or a group of sensors. The interest is expressed in terms of an attribute or a triggering event. Advertisement of Sensed Data – Sensor nodes advertise sensed data in a concise and descriptive way and users reply with requests of data they are interested in receiving

19. Fall 2006 19  Technical Challenges Performance metrics  Energy efficiency/system lifetime: The sensors are battery operated, sensor energy must be wisely managed in order to extend the lifetime of the network  Latency: Many sensor applications require delay-guaranteed service. Protocols must ensure that sensed data will be delivered to the users within a certain delay.  Accuracy: Obtaining accurate information is the primary objectives; accuracy can be improved though joint detection and estimation  Fault tolerance: Robustness to sensor and link failures must be achieved through redundancy and collaborative processing and communication

20. Fall 2006 20  Technical Challenges (cont’d) Scalability: Because a sensor network may contain thousand of nodes, scalability is a critical factor that guarantees that the network performance does not significantly degrade as the network size (or node density) increases Transport capacity/throughput: Because most sensor data must be delivered to a single base station or fusion center, a critical area in the sensor network exists, whose sensor nodes must relay the data generated by virtually all nodes in the network BS Critical Region

21. Fall 2006 21  Technical Challenges (Cont’d) Power Supply  The most difficult constraints in the design of WSN  When miniaturizing the node, the energy density of the power supply is the primary issue  Current technology yields batteries with approximately 1 J/mm 3, while capacitors can achieve 1mJ/mm 3  Sensor acquisition can be achieved at 1nJ/sample and modern processors can perform computations as low as 1nJ/instruction  In WSN, where sensor sampling, processing, data transmission and possibly actuation are involved, the trade- off between these tasks plays important role in power usage. Balancing these parameters will be the focus of the design process of WSN

22. Fall 2006 22  Technical Challenges (cont’d) Design of energy-efficient protocols:  Clustering is an efficient way to save energy for static sensor networks Data compression can be applied to reduce the number of packet Data-centric property makes and identity Randomize rotation of cluster heads helps ensure a balanced energy consumption  Broadcast and multicast trees can be used to increase energy efficiency Communication pattern many to one (when data from sources to sink) and one to many (when query from sink to sources). So, multicast advantages offers less benefit  The exploration of sleep modes

23. Fall 2006 23  Technical Challenges (cont’d) Capacity/Throughput  Throughput is a traditional measure of how much traffic can be delivered by the network  In packet network, the throughput is defined as the expected number of successful packet transmissions of a given node per time slot  Throughput is related to transmission rate of each transmitter, which in turn, is upper bounded by the channel capacity  Considering node density, congestion, channel condition achieving good throughput is necessary for energy constrained sensor network

24. Fall 2006 24  Technical Challenges (Cont’d) Channel Accessing and Scheduling  Scheduling is studied at two levels System level: At system level, a scheme determines which nodes will be transmitting. –System level scheduling essentially a medium access (MAC) problem, with the goal of minimum collisions and maximum spatial reuse Node level: At the node level, a scheduler determines which flow among all multiplexing flows will be eligible to transmit next  Current scheduling algorithms aim at improved fairness, delay, robustness (with respect to network topology changes) and energy efficiency  The scheduling algorithm and routing protocols must aim at energy and delay balancing, ensuring Packets originating close and far away from the base station experience a comparable delay and Critical nodes do not die prematurely due to the heavy traffic

25. Fall 2006 25  Technical Challenges (Cont’d) Energy Consumption  Considering energy consumption, four basic states can be defined: Transmission, reception, listening and sleeping. They consists of the following tasks: Acquisition: A/D conversion and preprocessing Transmission: Address determination, packetization, encoding, framing and queuing Reception: similar activities like transmission Listening: same as reception except that the signal processing chain stops at the detection Sleeping: power supply to stay alive

26. Fall 2006 26  Technical Challenges (cont’d) Connectivity:  Network connectivity is an important issue because it is crucial for most applications that the network is not partitioned into disjoint parts  Node redundancy should be in such a rate that frequent node failure must keep the network connected  Protocols practicing periodic sleep must ensure connectivity among active node for reliable data transfer from sources to sink

27. Fall 2006 27  Technical Challenges (Cont’d) Node Distribution and Mobility Security Distributed Signal Processing Synchronization and Localization Wireless Programming Wireless Link Modeling

28. Fall 2006 28  Cross Layer Design Avoid Conflicting Behavior – For example a routing protocol that favors smaller hops to save transmission energy consumption does require a proper MAC protocol to coordinate the transmissions along the data flow that minimizes contention and keeps the transceivers off as much as possible Remove Unnecessary Layers – Some applications do not require all layers New Paradigm – WSNs does not have many of the feature of the conventional networks for which the OSI protocol layer stack model has proven to be successful. Therefore it is quite possible that a different mix of layers might prove to be more efficient for many WSN applications Motivations:

29. Fall 2006 29  Sensor Network Management Issues WSN is a tool for distributed sensing of one or more phenomenon that reports the sensed data to one or more observers Wireless network services for observers as well as for itself Sensor nodes execute a common goal in a collaborative way A managed WSN is responsible for configuring and reconfiguring under varying conditions Energy is a critical resource in WSNs. Thus all operations performed in the network should be energy efficient Topology is dynamic because sensor nodes become out of service temporarily or permanently. In this scenario failure in sensor network is a common fact and needed to be managed efficiently WSN must detect, identify and protect itself against various types of attacks to maintain overall systems security and integrity

30. Fall 2006 30  Sensor Network Management Issues (cont’d) An well managed sensor network must know its environment and the context surrounding its activities and act accordingly WSN must be autonomic, i.e., self managed and robust to changes in network states while maintaining the quality of service It must be capable of self-configuration, self- organization, self-healing and self-optimized Finally management of WSN is concerned with how the management can promote plant and resource productivity, and How it integrates functions of configuration, operation, administration and maintenance of all elements and services in an efficient way

31. Fall 2006 31  Dimensions for WSN Management WSN Functionalities  Configuration  Maintenance  Sensing  Processing  Communication Functional Areas   Configuration Management   Fault Management   Performance Management   Security Management   Accounting Management Management Levels   Business Management   Service Management   Network Management   Network Element Management   Network Element

32. Fall 2006 32  Management Levels Business Management  Development and determination of cost functions  Cost function is associated with network setup, sensing, processing, communication and maintenance  WSN applications have enormous potential benefits for society as a whole and represent new business opportunities  In the future, it is expected to have Internet end- points equipped with a variety of sensors to monitor the network and their own state

33. Fall 2006 33  Management Levels (cont’d) Service Management  Quality of service: QoS support to WSN includes QoS model: It specifies the architecture in which some of the services can be provided in WSN QoS sensing: QoS sensing considers the sensor device calibration, environment interference monitoring and exposure (time, distance, and angle between sensor device and phenomenon) QoS dissemination: Handled in two layers –QoS routing –QoS medium access control QoS processing: Processing quality depends on the robustness and complexity of the algorithms used, as well as processor and memory capacities

34. Fall 2006 34  Management Levels (Cont’d) Network Management  Network Element Management Power Management Mobility Management (in case of mobile sink) State Management (States: operational, administrative and usage) Task Management (Schedules the sensing, processing and dissemination)  Network Element Power Supply –Linear Model –Dependent Model –Relaxation Model Computational Model Sensor Element Transceiver Software

35. Fall 2006 35  WSN Functionalities Configuration: The configuration functionality is related to the following:  Definition of WSN application requirements  Determination of the monitoring area (shape and dimension)  Characteristics of environment  Choice of nodes  Definition of the WSN type  Service provided

36. Fall 2006 36  WSN Functionalities (Cont’d) Sensing:  Lowest level of sensing application is provided by the autonomous sensor nodes  Sensing functionality depends on the type of the phenomenon  Sensing can be classified into Continuous (when sensors collect data continuously along the time) Reactive (when they answer to an observer’s query) Periodic (when nodes collects data according to conditions defined by the application)  Redundancy (overlapping sensing coverage) should be utilized in such a way that fault tolerance in the communication network is avoided and better accuracy can be found

37. Fall 2006 37  WSN Functionalities (Cont’d) Processing  Memory and processor of a sensor node form the computational module  It is a programmable unit that provides computation and storage for other nodes in the system  The computational module performs Basic signal processing Dispatches the data according to the application  Processing also involves data aggregation  Other tasks are security processing and data compression

38. Fall 2006 38  WSN Functionalities (Cont’d) Communication  Two types of communication cost Infrastructure: refers to the communication needed to configure, maintain and optimize operation Application: relates to the transfer of sensed data  The communication approach can be classified as: Flooding Gossiping Bargaining Unicast Multicast

39. Fall 2006 39  MANNA – An Integrating Architecture MANNA architecture was proposed to provide a management solution to different WSN application It provides a separation between both sets of functionalities, i.e.  Application and management  Making integration of organizational, administrative and maintenance activities possible for this kind of network The architecture also provides some automatic services, which feature  Self-managing  self-sustaining  self-diagnostic, with a minimum of human interference

40. Fall 2006 40  MANNA (Cont’d) Figure represents scheme to construct management To determine management service the scheme uses different WSN model A management service can use one or more management functions Different services can use common functions that use models to retrieve a network state concerning a given aspect

41. Fall 2006 41  MANNA (Cont’d) Partial List of Management Functions  Environmental monitoring functions  Monitored area definition function  Coverage area supervision function  Node deployment definition function  Environmental requirement acquisition function  Network operating parameters configuration function  Topology map discovery function  Network connectivity discovery function  Aggregation function  Data fusion function  Node density control function  Management operation schedule function  Energy map generation function  Energy level discovery function  Node localization discovery function  Node mobile function

42. Fall 2006 42  MANNA (Cont’d) Functional architecture  WSN manager Depends on type or network In centrally managed network, a single manager collects information from all agents and control the entire network Distributed managed network has several managers, each responsible for a sub-network and communicating with other managers In hierarchically managed network, intermediate managers distribute the management tasks  WSN Agent

43. Fall 2006 43  MANNA (Cont’d) Manager and agent location in flat WSN  Agents inside the network and external manager  Agent in the sink node  Agents and manager in the network Hierarchical organization Distributed organization

44. Fall 2006 44  MANNA (Cont’d) Manager and agent location in hierarchical WSN  Agents in the cluster head and external manager  Agent in the base station

45. Fall 2006 45  MANNA (Cont’d) Manager and agent location in hierarchical WSN  Agents in the network and intermediate manager  Agents and distributed managers in the network

46. Fall 2006 46  Concluding Words Wireless Sensor Networks provide a fundamentally new set of research and application challenges WSNs are a rich source of problems in communication protocols, sensor tasking and control, sensor fusion, distributed data bases, probabilistic reasoning, and algorithmic design Few more issues in sensor network are:  Deployment strategies  Node localization  Network capacity  Fault tolerance  Security

47. Fall 2006 47  Example of Some Sensor Nodes

48. Fall 2006 48  References “Handbook of Sensor Networks: Compact Wireless and Wired Sensing Systems, CRC PRESS, 2005 “I. F. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Cayirci, “A survey on sensor networks,” IEEE Communications Magazine, 2002. Tilak, S., Abu-Ghazaleh, N.B., Heinzelman, W.: A taxonomy of wireless micro-sensor network models. ACM Mobile Computing and Communications Review (MC2R) 6 (2002) 28–36 L.B. Ruiz, J.M.S. Nogueria and A.A.F Loureiro, “MANNA: a management architecture for wireless sensor network”, IEEE communications magazine, 41(2) 116-125, February 2003 www.google.com

49. Fall 2006 49  Thanks !