Lecture notes for Wireless Sensor networks

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Sokwoo Rhee, Ph.D. Sheng Liu, Ph.D.Wireless Sensor Networking Source Book 1. Introduction ..................................................................................1 Purpose of This Source Book ....................................................................................1 Symbols Used in this Book .....................................................................................2 Defining Wireless Sensor Networks ...........................................................................3 Opportunities ........................................................................................................5 Replacing Traditional Wired Networks .................................................................................5 New Opportunities ...........................................................................................................5 2. Wireless Sensor Networking Overview ..........................................6 System Modules ....................................................................................................6 Application Platform .........................................................................................................7 Gateway ........................................................................................................................7 Mesh Node Module ..........................................................................................................8 End Node Module ............................................................................................................8 Sensor/Actuator ..............................................................................................................8 System Software ...................................................................................................9 Module Firmware .............................................................................................................9 API ................................................................................................................................9 Network Monitoring System ............................................................................................ 10 3. Network Design Considerations ..................................................11 Design Drivers ..................................................................................................... 11 Range ................................................................................................................12 Shout Versus Whisper .................................................................................................... 13 Environmental Concerns ................................................................................................. 14 Radio Frequency ........................................................................................................... 16 Radio Transmission Techniques ....................................................................................... 16 Power ................................................................................................................. 18 Data Rate ........................................................................................................... 18 Raw Data Rate .............................................................................................................. 18 Network Throughput ...................................................................................................... 19 Duty Cycle .......................................................................................................... 19 Scalability ........................................................................................................... 20 Mobility .............................................................................................................. 21 Mobile Sensors .............................................................................................................. 21 Mobile Gateways ........................................................................................................... 22 4. Topologies and Data Models ........................................................23 Network Topologies .............................................................................................. 23 Star ............................................................................................................................. 23 Mesh ...........................................................................................................................24 Star-Mesh Hybrid .......................................................................................................... 25 Data Models ........................................................................................................ 26 Data Collection Models ................................................................................................... 27 Bi-Directional Dialogue Data Models ................................................................................. 28 5. Routing Techniques .....................................................................30 Efficient Protocol .................................................................................................. 30 Proactive Protocols ........................................................................................................ 31 Reactive Protocols ......................................................................................................... 32 Routing Protocol Design ........................................................................................ 32 6. The Millennial Net System ...........................................................34 Persistent Dynamic Routing™ Protocol .................................................................... 34 Highly Responsive ......................................................................................................... 34 Reliable ........................................................................................................................ 35 Extremely Power Efficient ............................................................................................... 35 Scalable .......................................................................................................................35 Build vs. Buy ....................................................................................................... 35 A Complete System .............................................................................................. 36 System Software ........................................................................................................... 36 Hardware Modules ......................................................................................................... 36 Development and Management Tools ............................................................................... 36 Evaluation Kits .................................................................................................... 37 Glossary ..........................................................................................38Wireless Sensor Networking Source Book 1. Introduction The Wireless Sensor Your company’s project requires integrating a wire- Networking Source less sensor network between a network of sensors Book provides a meth- odology for selecting and the application used to monitor and control them. and implementing a You have been put in charge of selecting the wireless wireless sensor net- work. sensor network to use. You’re familiar with some con- cepts of wireless networks, but don’t feel comfortable enough to make an informed decision on this particu- lar type of wireless system. What do you need to know? What questions need to be asked? Where do you start? You start here with the Wireless Sensor Networking Source Book. This guide is for engineers and decision makers that will be designing, specify- ing, selecting, or implementing a wireless sensor net- work. Purpose of This Source Book The source book provides a broad understanding of the technology fundamentals and design consider- ations that affect function and performance of a wire- less sensor network. As opposed to a high-data-rate wireless systems used in LAN applications (WLAN), a low-power wireless sensor network is specifically designed for low-data-rate applications. This guide is designed to provide you with the information neces- sary to make an informed decision when selecting and integrating such a network system. Table 1-1 provides a quick reference to the information you will find in this guide. 1Wireless Sensor Networking Source Book Table 1-1: Information provided in this guide Guide Section Information Provided Chapter 2: Wireless This chapter provides a basic understanding of the Sensor Networking wireless sensor network building blocks as a prereq- Overview uisite to a discussion of fundamental network design considerations outlined in Chapter 3. Chapter 3: Network The information presented in this chapter will help Design Consider- you assess the feasibility of a wireless sensor net- ations work in your application, to make important scoping and sizing decisions, and to establish a framework to assess different options in specifying and selecting a wireless sensor network system for your application. Chapter 4: Topolo- This chapter provides a look at three “textbook” gies and Data Models topologies and discusses the different data models used by wireless sensor networks to collect and manage data. Chapter 5: Routing In this chapter, you’ll learn the advantages and dis- Techniques advantages associated with the different routing techniques developed specifically for wireless sensor networks. Chapter 6: The Mil- This chapter provides a brief overview of Millennial lennial Net System Net’s wireless sensor networking platform with Per- sistent Dynamic Routing™. Symbols Used in this Book The symbols shown in table 1-2 are used in this book to illustrate sensor networking concepts: Table 1-2: Symbols used in this guide Symbol Description Sensor/actuator Application End node 2Wireless Sensor Networking Source Book Symbol Description Mesh node Gateway Defining Wireless Sensor Networks Typical wireless sensor Until recently, networks designed for monitoring and network applications controlling sensors or actuators on a network were share three common requirements: small limited in application and scope due to a major net- form factor, long bat- work design consideration—the cables required to tery life, and dynamic operating environment. connect the various sensors and actuators to a cen- tralized collection point. In addition to the costs asso- ciated with installing and maintaining communication cables (fiber optic or copper), this type of network infrastructure prevents sensor mobility and severely limits the feasible applications of such a network. Thanks to significant advances in low-power radio and digital circuit design, self-organizing wireless sensor networks are now a reality. Sensors of all types (temperature, motion, occupancy, vibration, etc.) can now be wirelessly enabled and deployed inexpensively and quickly. Wireless sensor networks fundamentally change the economics of deploying and operating a sensor net- work, unlocking opportunities to achieve new effi- ciencies in production processes, building control, or monitoring, to name just a few. Wireless sensor net- works also enable the development of a brand new 3Wireless Sensor Networking Source Book class of applications and services not previously pos- sible with wired sensor networks. There are no adminis- As illustrated in Figure 1-1, wireless sensor networks trative duties associ- form what is called a wireless ad hoc network, which ated with establishing and maintaining an ad refers to a network’s ability to self-organize and self- hoc network. heal. This means there are no administrative duties associated with establishing and maintaining a wire- less sensor network. By comparison, a wired infra- structure network, such as the LAN found in most office environments, requires a significant amount of overhead to install and maintain in terms of cabling and administrative time. In an ad hoc network, sensor nodes consisting of a sensor attached to a wireless module can be ran- domly placed and moved as needed. If the network needs to scale up, additional sensor nodes are easily added. The new sensor nodes and surrounding net- work will do the work of discovering each other and establishing communication paths through single- and multi-hop paths. All this is made possible through the use of robust, efficient network protocols developed specifically for wireless sensor networks. Figure 1-1: Untethered, mobile ad hoc network nodes 4Wireless Sensor Networking Source Book Opportunities Looking forward, wire- Today, wireless sensor networks are being used in a less sensor networks number of low-power, low-data-rate applications aid- will unlock new and exciting applications ing digital precision instruments on the factory floor, and services. collecting water and gas meter readings, monitoring shipments through the supply chain, and reporting on the vital signs of individual wearers. Looking forward, wireless sensor networks will unlock new and exciting applications and services. Replacing Traditional Wired Networks Sensors and actuators can now be monitored and controlled wirelessly, obviating the expensive instal- lation and maintenance of copper or fiber optic cables. For instance, wireless sensor networks are now being installed in building maintenance systems, replacing the traditional RS-485 cables used to con- nect the building controller with the various thermo- stats located throughout a building. The wireless sensor network is transparent to the controller and thermostats, that up until now used the RS-485 cables to communicate with each other. New Opportunities The emerging technology behind wireless sensor net- works is opening the door to a new world of opportu- nities in data collection and system monitoring applications—opportunities where traditional wired networks made them economically or physically impossible to consider. Today, applications as varied as monitoring water usage in large apartment com- plexes to unobtrusively monitoring a patient’s blood sugar level are now possible. Wireless sensor net- works will allow companies to develop new sources of revenue and cut or eliminate waste. 5Wireless Sensor Networking Source Book 2. Wireless Sensor Networking Overview This chapter provides you with a basic understanding of the wireless sensor network building blocks as a prerequisite to a discussion of fundamental network design considerations outlined in Chapter 3. System Modules The modules of a wireless sensor network enable wireless connectivity within the network, connecting an application platform at one end of the network with one or more sensor or actuator devices at the other end. As shown in Figure 2-1, the gateway and end node modules create a transparent, wireless data path between the application platform and sensor. Figure 2-1: Basic wireless sensor network components Exchange of analog or digital information between an application platform and one or more sensor nodes takes place in a wireless fashion. In this example, the data path between the gateway and end node is referred to as a single-hop network link. To extend the range of a network or circumvent an obstacle, a wireless mesh node module can be added between a gateway and an end node as shown in Fig- ure 2-2. 6Wireless Sensor Networking Source Book Figure 2-2: Adding a mesh node module This particular example represents a multi-hop data path, in which data packets are handed off from one module to the next before reaching their destination (gateway-to-mesh node-to-end node and vice versa). More elaborate network layouts are discussed later in “Network Topologies,” but for now, we’ll take a closer look at each of the network components shown in Figure 2-2. Application Platform This is the network device (PC, handheld, etc.) used to monitor and control the actions of the various sen- sors and actuators that are connected to the wireless sensor network. The application platform is capable of making decisions based on the information it gath- ers from the network. Typically, the wireless sensor network will come with an API (application program- ming interface) and/or a GUI (graphical user inter- face) used to interface with the wireless modules. Gateway The gateway is the interface between the application platform and the wireless nodes on the network. The gateway can be a discrete module, or it can be inte- grated onto a Flash card form factor for a handheld device, for example. All information received from the various network nodes is aggregated by the gateway and forwarded on to the application platform. In the 7Wireless Sensor Networking Source Book reverse direction, when a command is issued by the application program to a network node, the gateway relays the information to the wireless sensor network. The gateway can also perform protocol conversion to enable the wireless network to work with other indus- try-standard network protocols. Mesh Node Module Hardware design will Considered full-function devices (FFD), mesh node affect a module’s modules (sometimes called routers) are used to power-efficiency. extend network coverage area, route around obsta- cles, and provide back-up routes in case of network congestion or device failure. In some cases, mesh nodes may also be connected via analog and digital interfaces to sensors and actuators, providing the same I/O functionality of an end node module. Mesh nodes can be battery powered or line powered. End Node Module Considered reduced-function devices (RFD), end nodes (sometimes called endpoints) provide the physical interface between the wireless sensor net- work and the sensor or actuator that it is wired to. End nodes will usually have one or more I/O connec- tions for connecting to and communicating with ana- log or digital sensor or actuator devices. End nodes are typically battery powered. Sensor/Actuator These are the devices you ultimately wish to monitor and/or control. An example is a sensor monitoring the pressure in an oil pipeline. 8Wireless Sensor Networking Source Book System Software The software required to integrate and operate a wireless sensor network resides as firmware in the system modules and in the application platform as a set of API functions or network monitoring system (NMS). Module Firmware Firmware design will Module firmware is a small, efficient piece of code affect a module’s that incorporates the module into a larger ad hoc net- power-efficiency. work. It “drives” the module's operation as part of the larger ad hoc network. The firmware is also responsible for packaging the analog and digital sensor data into digital packets and delivering them across the wireless sensor network. API An API, or application programming interface, is a set of commonly used functions for streamlining applica- tion development. Used by application developers, an API provides hooks to integrate the application plat- forms with the modules on the wireless sensor net- work. API functions are grouped into “libraries.” In wireless sensor networks, there two different API libraries: •High-level Library: These functions are used to integrate the application with the gateway mod- ule. •Low-level library: These functions are used to integrate the sensor/actuator with the end node module. 9Wireless Sensor Networking Source Book Network Monitoring System A network monitoring system (NMS) is software used to interface with a particular wireless sensor network, eliminating the need for any programming. Through the NMS’s graphical user interface (GUI), network operators are able to see the various nodes of their wireless sensor network. Depending on the type of network, control commands can also be issued through the NMS. For example, a pin on a digital interface between an end node and an actuator can be set to high to change the state of the actuator. 10Wireless Sensor Networking Source Book 3. Network Design Considerations The information presented in this chapter will help you assess the feasibility of a wireless sensor network in your application, make important scoping and siz- ing decisions, and establish a framework to assess different options in specifying and selecting a wireless sensor network system. In this chapter, the major design drivers associated with wireless sensor networks are described. Most importantly, you’ll learn about the performance trade-offs that may need to be considered during the network design process. Understanding how the design drivers are inter-related will help if and when a trade-off decision needs to be made. Ultimately, this will provide you with the tools needed to design a wireless sensor network that will operate at its opti- mal level of performance. Design Drivers Table 3-1 contains a matrix used to develop a profile of your particular wireless sensor network applica- tion. The profile matrix lists the important network design drivers and will help you determine how important each driver is in the overall design and operation of your network. This exercise will also help determine what design trade-offs may need to be made with each wireless sensor networking system you are investigating. 11Wireless Sensor Networking Source Book Table 3-1: Application profile Level of Importance Design Driver Minimal Moderate Critical Range ‡‡‡ Short distance between Long distance between Maximum distance modules modules to minimize between modules hops desired Power ‡‡‡ External power source Long battery life Single battery must available desired to minimize provide power for mul- battery replacement tiple years Data rate ‡‡‡ Very low data rate Moderate data rate High data rate Duty cycle ‡‡‡ Low duty cycles Moderate duty cycles High duty cycles Scalability ‡‡‡ Small network size Moderate network size Large network size Mobility ‡‡‡ Modules stationary and Modules mobile and/or Modules extremely data paths stable data paths changeable mobile and data paths highly changeable Range The term “range” can be used to describe either of the following: •Network Range: The total physical area covered by a wireless sensor network. •Module Range: The distance that data can be transmitted between two modules on a network. Two communicating modules represent the most basic building block in designing a wireless sen- sor network. Factors that affect the range of a network or network module include: •Number of supported network nodes as deter- mined by the manufacturer. 12Wireless Sensor Networking Source Book •Power associated with the radio frequency used. •Environmental issues, such as walls, electrical interference, etc. By understanding some of the range-related concepts and issues associated with module range, you’ll understand how to efficiently attain the desired net- work range. Shout Versus Whisper Even with the addi- When transmitting data between two distant points tional modules, the on a network, more power is not always the best multi-hop whisper method consumes answer to bridging the distance between them. Fig- much less power to ure 3-1 illustrates two different methods for transmit- move data between the two points on a net- ting data between two points on a network. work. Figure 3-1: Shout versus whisper links With the “shout” method, the modules use high out- put power to transmit data packets between them. While the two modules are able to communicate effectively, they are not doing it in a very power-effi- 13Wireless Sensor Networking Source Book cient manner. The “whisper” method illustrates how multiple modules using low output power are used to bridge the same distance. Even with the additional modules, the multi-hop whisper method consumes much less total RF transmit power to move data between the two points on a network. Figure 3-2 illustrates the relationship between power and dis- tance for the two methods. Figure 3-2: Power/distance relationship Multi-hopping is a technique also used in wireless sensor networks to extend the range of a network far beyond the limits of the radio frequency used. If for example, the frequency being used restricted the dis- tance between network modules to no more than 50- 100 feet, this distance could be extended by inserting one or more mesh node modules. The data would then “hop” from source to destination modules using the mesh nodes as stepping stones. Environmental Concerns The maximum range at which two modules on a net- work can communicate is affected by a number of environmental and network characteristics. 14Wireless Sensor Networking Source Book Items blocking the line of sight between network modules, such as walls and floors, will limit wireless communications. The type of building material used in such obstacles will affect how well a radio fre- quency can penetrate the object. Figure 3-3: Penetrating line-of-sight obstacles In cases where obstacles must be circumvented to provide radio connectivity between modules, one or more mesh node modules can be inserted for this purpose as shown in Figure 3-4. Figure 3-4: Circumventing radio-frequency obstacles 15Wireless Sensor Networking Source Book Radio Frequency Each module on the network contains a radio trans- mitter used to communicate with the other wireless modules on the network. Wireless sensor networks generally use one of the license-free ISM frequency bands. Lower radio frequen- Typically, the radio or RF components consume more cies, such as 916 MHz than 70% of the total power in full-operation mode, which is license-free in North America, require sometimes consuming even more while receiving less power and are bet- (RX) than transmitting (TX) data. The RF components ter at penetrating objects such as walls or also burn significant amounts of power during TX/RX doors. switching or waking up. So, many different scenarios must be considered in the power budget. Radio Transmission Techniques For applications where environmental noise is an issue, the modulation scheme of the radio should also be considered as a way of working around such prob- lems. There are typically two modulation schemes or techniques used for transmitting radio signals over a wireless sensor network—one uses narrowband sig- nals while the other transmits wideband or spread- spectrum signals. Narrowband Signals These radio signals use a very narrow portion of the radio frequency bandwidth as shown in Figure 3-4. Spread-Spectrum Signals Spread-spectrum sig- A spread-spectrum transmitter takes a narrowband nals are resistant to signal and spreads it across a broad portion of the interference and hard to intercept. radio frequency in a predefined method. Destination devices receiving the signal understand the pre- 16Wireless Sensor Networking Source Book defined method and de-spread the signal before the data can be interpreted. Spread-spectrum signals are usually created using the direct sequence spread spectrum (DSSS) method or the frequency hopping spread spectrum (FHSS) method shown in Figure 3-4. The DSSS method spreads the narrowband signal out over a broad portion of the frequency band. The FHSS method spreads its signal by “hopping” the nar- rowband signal across a broad frequency range as a function of time. Figure 3-4: Narrowband, DSSS, and FHSS signals RF circuitry power consumption is highly dependent on the modulation scheme. Spread-spectrum RF chips consume much more power than typical nar- rowband radios because of the complex base-band processing. Although spread-spectrum radios offer better immunity to interference, for many sensor net- work applications, narrowband radios remain a prac- tical and more power-efficient choice. 17Wireless Sensor Networking Source Book Power Power efficiency is a The importance of how efficiently the modules in a critical design factor for wireless sensor network manage their power wireless sensor net- work components. resources can vary with each application. In some applications, power consumption efficiency is not an issue as access to local power resources is readily available to each module. Modules integrated with the thermostats of a building automation system, for example, can draw their power from the same 24 VAC source used by the thermostats. In other appli- cations, wireless sensor network modules are located in areas where access to local power is not possible, either because of module location or mobility issues. In such instances, the modules typically draw their power from small, coin-cell batteries, making efficient use of power critical. Being able to operate efficiently for long periods of time using battery power is a major advantage of wireless sensor networks over wired networks and a critical design factor. Data Rate Data rate refers to the amount of data the wireless sensor network is capable of carrying or supporting. Expressed in bits per seconds (bps), data rate is eval- uated in two ways: the raw data rate and the actual network throughput. Raw Data Rate This value is determined by the radio used in the wireless sensor network modules and is less relevant to wireless sensor network design than network throughput described below. 18

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