sensor network

A (WSN) is a wireless network consisting of spatially distributed autonomous devices using sensors to cooperatively monitor physical or environmental conditions, such as temperature, sound, vibration, pressure, motion or pollutants, at different locations. The development of wireless sensor networks was originally motivated by military applications such as battlefield surveillance. However, wireless sensor networks are now used in many civilian application areas, including environment and habitat monitoring, healthcare applications, , and traffic control. In addition to one or more sensors, each node in a sensor network is typically equipped with a radio transceiver or other wireless communications device, a small , and an energy source, usually a battery. The size of a single can vary from shoebox- sized nodes down to devices the size of grain of dust. The cost of sensor nodes is similarly variable, ranging from hundreds of dollars to a few cents, depending on the size of the sensor network and the complexity required of individual sensor nodes. Size and cost constraints on sensor nodes result in corresponding constraints on resources such as energy, memory, computational speed and bandwidth. In computer science and telecommunications, wireless sensor networks are an active research area with numerous workshops and conferences arranged each year. Sensor node

Sensor Node Architecture A sensor node, also known as a mote (chiefly in North America), is a node in a wireless sensor network that is capable of performing some processing, gathering sensory information and communicating with other connected nodes in the network. The typical architecture of the sensor node is shown in figure. History of Sensor Node History of development of sensor nodes dates back to 1998 in Smartdust project. One of the objectives of this project is to create autonomous sensing and communication in a cubic millimeter. Though this project ended early on, it has given birth to a few more research projects. They are Berkeley NEST and CENS. The researchers involved in these projects coined the term 'mote' to refer to a sensor node. Components of a Sensor Node The main components of a sensor node as seen from the figure are microcontroller, transceiver, external memory, power source and one or more sensors. Microcontroller Main article: Microcontroller. Microcontroller performs tasks, processes data and controls the functionality of other components in the sensor node. Other alternatives that can be used as a controller are: General purpose desktop microprocessor, Digital signal processors, Field Programmable Gate Array and Application-specific integrated circuit. are most suitable choice for sensor node. Each of the four choices has their own advantages and disadvantages. Microcontrollers are the best choices for embedded systems. Because of their flexibility to connect to other devices, programmable, power consumption is less, as these devices can go to sleep state and part of controller can be active. In general purpose microprocessor the power consumption is more than the microcontroller, therefore it is not a suitable choice for sensor node. Digital Signal Processors are appropriate for broadband wireless communication. But in Wireless Sensor Networks, the wireless communication should be modest i.e., simpler, easier to process modulation and signal processing tasks of actual sensing of data is less complicated. Therefore the advantages of DSP's is not that much of importance to wireless sensor node. Field Programmable Gate Arrays can be reprogrammed and reconfigured according to requirements, but it takes time and energy. Therefore FPGA's is not advisable. Application Specific Integrated Circuits are specialized processors designed for a given application. ASIC's provide the functionality in the form of hardware, but microcontrollers provide it through . Transceiver Sensor nodes make use of ISM band which gives free radio, huge spectrum allocation and global availability. The various choices of wireless transmission media are Radio frequency, Optical communication (Laser) and Infrared. Laser requires less energy, but needs line of sight for communication and also sensitive to atmospheric conditions. Infrared like laser, needs no antenna but is limited in its broadcasting capacity. Radio Frequency (RF) based communication is the most relevant that fits to most of the WSN applications. WSN’s use the communication frequencies between about 433 MHz and 2.4 GHz. The functionality of both transmitter and receiver are combined into a single device know as transceivers are used in sensor nodes. Transceivers lack unique identifier. The operational states are Transmit, Receive, Idle and Sleep. Current generation radios have a built-in state machines that perform this operation automatically. Radios used in transceivers operate in four different modes: Transmit, Receive, Idle, and Sleep. Radios operating in Idle mode results in power consumption, almost equal to power consumed in Receive mode. Thus it is better to completely shutdown the radios rather than in the Idle mode when it is not Transmitting or Receiving. And also significant amount of power is consumed when switching from Sleep mode to Transmit mode to transmit a packet. External Memory From an energy perspective, the most relevant kinds of memory are on-chip memory of a microcontroller and FLASH memory - off-chip RAM is rarely if ever used. Flash memories are used due to its cost and storage capacity. Memory requirements are very much application dependent. Two categories of memory based on the purpose of storage a) User memory used for storing application related or personal data. b) Program memory used for programming the device. This memory also contains identification data of the device if any. Power Source Power consumption in the sensor node is for the Sensing, Communication and Data Processing. More energy is required for data communication in sensor node. Energy expenditure is less for sensing and data processing. The energy cost of transmitting 1 Kibit a distance of 100 m is approximately the same as that for the executing 3 million instructions by 100 million instructions per second/W processor. Power is stored either in Batteries or Capacitors. Batteries are the main source of power supply for sensor nodes. Namely two types of batteries used are chargeable and non-rechargeable. They are also classified according to electrochemical material used for electrode such as NiCd(nickel-cadmium), NiZn(nickel- zinc), Nimh (nickel metal hydride), and Lithium-Ion. Current sensors are developed which are able to renew their energy from solar or vibration energy. Two major power saving policies used are Dynamic Power Management (DPM) and Dynamic Voltage Scaling (DVS)[5]. DPM takes care of shutting down parts of sensor node which are not currently used or active. DVS scheme varies the power levels depending on the non- deterministic workload. By varying the voltage along with the frequency, it is possible to obtain quadratic reduction in power consumption.

Sensors Sensors are hardware devices that produce measurable response to a change in a physical condition like temperature and pressure. Sensors sense or measure physical data of the area to be monitored. The continual analog signal sensed by the sensors is digitized by Analog-to-Digital converter and sent to controllers for further processing. Characteristics and requirements of Sensor node should be small size, consume extremely low energy, operate in high volumetric densities, be autonomous and operate unattended, and be adaptive to the environment. As wireless sensor nodes are micro- electronic sensor device, can only be equipped with a limited power source of less than 0.5 Ah and 1.2 V. Sensors are classified into three categories.

• Passive, Omni Directional Sensors: Passive sensors sense the data without actually manipulating the environment by active probing. They are self powered i.e energy is needed only to amplify their analog signal. There is no notion of “direction” involved in these measurements. • Passive, narrow-beam sensors: These sensors are passive but they have well-defined notion of direction of measurement. Typical example is ‘camera’. • Active Sensors: These group of sensors actively probe the environment, for example, a sonar or radar sensor or some type of seismic sensor, which generate shock waves by small explosions. The overall theoretical work on WSN’s considers Passive, Omni directional sensors. Each sensor node has a certain area of coverage for which it can reliably and accurately report the particular quantity that it is observing. Several sources of power consumption in sensors are a) Signal sampling and conversion of physical signals to electrical ones, b) signal conditioning, and ) analog-to-digital conversion. Spatial density of sensor nodes in the field may be as high as 20 nodes/ m3 . List of Commercial Sensor Nodes/Motes There are two kinds of sensor nodes used in the sensor network. One is the normal sensor node deployed to sense the phenomena and the other is gateway node that interfaces sensor network to the external world.

An overview of commonly used sensor network platforms, components, technology and related topics is available in the SNM - Sensor Network tm Museum Applications The applications for WSNs are many and varied. They are used in commercial and industrial applications to monitor data that would be difficult or expensive to monitor using wired sensors. They could be deployed in wilderness areas, where they would remain for many years (monitoring some environmental variables) without the need to recharge/replace their power supplies. They could form a perimeter about a property and monitor the progression of intruders (passing information from one node to the next). There are many uses for WSNs. Typical applications of WSNs include monitoring, tracking, and controlling. Some of the specific applications are habitat monitoring, object tracking, nuclear reactor controlling, fire detection, traffic monitoring, etc. In a typical application, a WSN is scattered in a region where it is meant to collect data through its sensor nodes.

• Environmental monitoring[4] • Habitat monitoring • Acoustic detection • Seismic Detection • Military surveillance • Inventory tracking • Medical monitoring • Smart spaces • Process Monitoring • Structural health monitoring • Health Monitoring Area monitoring Area monitoring is a typical application of WSNs. In area monitoring, the WSN is deployed over a region where some phenomenon is to be monitored. As an example, a large quantity of sensor nodes could be deployed over a battlefield to detect enemy intrusion instead of using landmines. When the sensors detect the event being monitored (heat, pressure, sound, light, electro-magnetic field, vibration, etc), the event needs to be reported to one of the base stations, which can take appropriate action (e.g., send a message on the internet or to a satellite). Depending on the exact application, different objective functions will require different data- propagation strategies, depending on things such as need for real-time response, redundancy of the data (which can be tackled via data aggregation techniques), need for security, etc. Environmental monitoring Our environment is undergoing frightening changes at a global scale, and many dramatic climatic accidents have recently drawn a lot of attention on ecology and environmental monitoring. However, the current lack of appropriate observations prevents environmental researchers from addressing crucial questions about these changes and their actual consequences. SensorScope is a new generation of measurement system based on a multi-hop wireless sensor network with built-in capacity to produce high temporal and spatial density measures. By gathering the most recent results from a large range of research domains, we aim at providing the fundamental tools that will help in taking care of our environment. Unique characteristics of a WSN are:

• Small-scale sensor nodes • Limited power they can harvest or store • Harsh environmental conditions • Node failures • Mobility of nodes • Dynamic network topology • Communication failures • Heterogeneity of nodes • Large scale of deployment • Unattended operation Sensor nodes can be imagined as small computers, extremely basic in terms of their interfaces and their components. They usually consist of a processing unit with limited computational power and limited memory, sensors (including specific conditioning circuitry), a communication device (usually radio transceivers or alternatively optical), and a power source usually in the form of a battery. Other possible inclusions are energy harvesting modules, secondary ASICs, and possibly secondary communication devices (e.g. RS232 or USB). The base stations are one or more distinguished components of the WSN with much more computational, energy and communication resources. They act as a gateway between sensor nodes and the end user.

Hardware The main challenge is to produce low cost and tiny sensor nodes. With respect to these objectives, current sensor nodes are mainly prototypes. Miniaturization and low cost are understood to follow from recent and future progress in the fields of MEMS and NEMS. Some of the existing sensor nodes are given below. Some of the nodes are still in research stage. An overview of commonly used sensor network platforms, components, technology and related topics is available in the SNM - Sensor Network Museum tm . Standards While mainstream computers have an abundance of standards, the only official standards that have been adopted for wireless sensor networks are [5] and WirelessHART . Below are some other standards being investigated for use by researchers in the field:

• ZigBee • Wibree ZigBee ZigBee is the name of a specification for a suite of high level communication protocols using small, low-power digital radios based on the IEEE 802.15.4 standard for wireless personal area networks (WPANs). ZigBee is targeted at RF applications that require a low data rate, long battery life, and secure networking. Uses ZigBee protocols are intended for use in embedded applications requiring low data rates and low power consumption. ZigBee's current focus is to define a general-purpose, inexpensive, self-organizing, mesh network that can be used for industrial control, embedded sensing, medical data collection, smoke and intruder warning, building automation, home automation, etc. The resulting network will use very small amounts of power so individual devices might run for a year or two using the originally installed battery.

Device types There are three different types of ZigBee device:

• ZigBee coordinator(ZC): The most capable device, the coordinator forms the root of the network tree and might bridge to other networks. There is exactly one ZigBee coordinator in each network since it is the device that started the network originally. It is able to store information about the network, including acting as the Trust Centre & repository for security keys. • ZigBee Router (ZR): As well as running an application function a router can act as an intermediate router, passing data from other devices. • ZigBee End Device (ZED): Contains just enough functionality to talk to its parent node (either the coordinator or a router); it cannot relay data from other devices. This relationship allows the node to be asleep a significant amount of the time thereby giving you the much quoted long battery life. A ZED requires the least amount of memory, and therefore can be less expensive to manufacture than a ZR or ZC. Software and hardware The software is designed to be easy to develop on small, cheap microprocessors. The radio design used by ZigBee has been carefully optimized for low cost in large scale production. It has few analog stages and uses digital circuits wherever possible. Even though the radios themselves are cheap, the ZigBee Qualification Process involves a full validation of the requirements of the physical layer. This amount of concern about the Physical Layer has multiple benefits, since all radios derived from that semiconductor mask set would enjoy the same RF characteristics. On the other hand, an uncertified physical layer that malfunctions could cripple the battery lifespan of other devices on a ZigBee network. Where other protocols can mask poor sensitivity or other esoteric problems in a fade compensation response, ZigBee radios have very tight engineering constraints: they are both power and bandwidth constrained. Thus, radios are tested to the ISO 17025 standard with guidance given by Clause 6 of the 802.15.4-2003 Standard. Most vendors plan to integrate the radio and microcontroller onto a single chip. (Some interesting links: look for the USB Dongle)

Wibree

Wibree is a digital radio technology (intended to become an open standard of wireless communications) designed for ultra low power consumption (button cell batteries) within a short range (10 meters / 30 ft) based around low-cost transceiver microchips in each device

Technical information Wibree is designed to work side-by-side with and complement . It operates in 2.4 GHz ISM band with physical layer bit rate of 1 Mbit/s. Main applications include devices such as wrist , wireless keyboards, toys and sports sensors where low power consumption is a key design requirement. The technology was announced on 3 October 2006 by Nokia . Partners that currently license the technology and cooperate in defining the specification are Nordic Semiconductor, Broadcom Corporation, CSR and Epson. Other contributors are Suunto and Taiyo Yuden. Wibree is not designed to replace Bluetooth, but rather to complement the technology in supported devices. Wibree-enabled devices will be smaller and more energy-efficient than their Bluetooth counterparts. This is especially important in devices such as wristwatches, where Bluetooth models may be too large and heavy to be comfortable. Replacing Bluetooth with Wibree will make the devices closer in dimensions and weight to current standard wristwatches. Bob Iannucci, head of Nokia's Research Centre, claims the technology is up to ten times more efficient than Bluetooth. Reportedly, it will have an output power around -6 dBm . Nordic Semiconductor is aiming to sample Wibree chips during the second half of 2007. Ultra-wideband Ultra-wideband (UWB, ultra-wide band, ultraband, etc.) is a radio technology that can be used for short- range high-bandwidth communications by using a large portion of the radio spectrum in a way that doesn't interfere with other more traditional 'narrow band' uses. It also has applications in radar imaging, precision positioning and tracking technology. Ultra-Wideband (UWB) may be used to refer to any radio technology having bandwidth exceeding the lesser of 500 MHz or 20% of the arithmetic center frequency, according to Federal Communications Commission (FCC). This article discusses the meaning of Ultra- wideband in the field of radio communications. Applications Due to the extremely low emission levels currently allowed by regulatory agencies, UWB systems tend to be short-range and indoors. However, due to the short duration of the UWB pulses, it is easier to engineer extremely high data rates, and data rate can be readily traded for range by simply aggregating pulse energy per data bit using either simple integration or by coding techniques. Conventional OFDM technology can also be used subject to the minimum bandwidth requirement of the regulations. High data rate UWB can enable wireless monitors, the efficient transfer of data from digital camcorders, wireless printing of digital pictures from a camera without the need for an intervening , and the transfer of files among cell phone handsets and other handheld devices like personal digital audio and video players. UWB is used as a part of location systems and real time location systems. The precision capabilities combined with the very low power makes it ideal for certain radio frequency sensitive environments such as hospitals and healthcare. Another benefit of UWB is the short broadcast time which enables implementers of the technology to install orders of magnitude more transmitter tags in an environment relative to competitive technologies. USA based Parco Merged Media Corporation was the first systems developer to deploy a commercial version of this system in a Washington, DC hospital. UWB is also used in "see-through-the-wall" precision radar imaging technology, precision positioning and tracking (using distance measurements between radios), and precision time-of-arrival-based localization approaches. It exhibits excellent efficiency with a spatial capacity of approximately 10,000,000,000,000 bit/s/m². UWB is a possible technology for use in personal area networks and appears in IEEE 802.15.3a draft PAN standard.

Bluetooth Bluetooth is an industrial specification for wireless personal area networks (PANs). Bluetooth provides a way to connect and exchange information between devices such as mobile phones, , PCs, printers, digital cameras, and video game consoles over a secure, globally unlicensed short-range radio frequency. The Bluetooth specifications are developed and licensed by the Bluetooth Special Interest Group. Uses

A typical Bluetooth headset Bluetooth is a standard and communications protocol primarily designed for low power consumption, with a short range (power-class-dependent: 1 meter, 10 meters, 100 meters) based on low-cost transceiver microchips in each device. Bluetooth enables these devices to communicate with each other when they are in range. The devices use a radio communications system, so they do not have to be in line of sight of each other, and can even be in other rooms, as long as the received transmission is powerful enough.

List of applications More prevalent applications of Bluetooth include:

• Wireless control of and communication between a mobile phone and a hands-free headset or car kit. This was one of the earliest applications to become popular. • Wireless networking between PCs in a confined space and where little bandwidth is required. • Wireless communications with PC input and output devices, the most common being the mouse, keyboard and printer. • Transfer of files between devices with OBEX. • Transfer of contact details, calendar appointments, and reminders between devices with OBEX. • Replacement of traditional wired serial communications in test equipment, GPS receivers, medical equipment, bar code scanners, and traffic control devices. • For controls where infrared was traditionally used. • Sending small advertisements from Bluetooth enabled advertising hoardings to other, discoverable, Bluetooth devices. • Seventh-generation game consoles—Nintendo Wii [3] , Sony PlayStation 3—use Bluetooth for their respective wireless controllers. • Receiving commercial advertisements ("spam") via a kiosk, e.g. at a movie theatre or lobby Wireless USB Wireless USB (WuzzBee) is a short-range, high- bandwidth wireless radio communication protocol created by the Wireless USB Promoter Group. Wireless USB is sometimes abbreviated as "WUSB", although the USB Implementers Forum discourages this practice and instead prefers to call the technology "Certified Wireless USB" to differentiate it from competitors (see below, "Competitors"). Wireless USB is based on the WiMedia Alliance's Ultra-WideBand (UWB) common radio platform, which is capable of sending 480 Mbit/s at distances up to 3 meters and 110 Mbit/s at up to 10 meters. It was designed to operate in the 3.1 to 10.6 GHz frequency range, although local regulatory policies may restrict the legal operating range for any given country.

Uses Wireless USB is used in game controllers, printers, scanners, digital cameras, MP3 players, hard disks and flash drives. It is also suitable for transferring parallel video streams Software Energy is the scarcest resource of WSN nodes, and it determines the lifetime of WSNs. WSNs are meant to be deployed in large numbers in various environments, including remote and hostile regions, with ad-hoc communications as key. For this reason, algorithms and protocols need to address the following issues:

• Lifetime maximization • Robustness and fault tolerance • Self-configuration Some of the "hot" topics in WSN software research are:

• Security • Mobility (when sensor nodes or base stations are moving) • Middleware: the design of middle-level primitives between the software and the hardware

Operating systems Operating systems for wireless sensor network nodes are typically less complex than general-purpose operating systems both because of the special requirements of sensor network applications and because of the resource constraints in sensor network hardware platforms. For example, sensor network applications are usually not interactive in the same way as applications for PCs. Because of this, the operating system does not need to include support for user interfaces. Furthermore, the resource constraints in terms of memory and memory mapping hardware support make mechanisms such as virtual memory either unnecessary or impossible to implement. Wireless sensor network hardware is not different from traditional embedded systems and it is therefore possible to use embedded operating systems such as eCos or uC/OS for sensor networks. However, such operating systems are often designed with real-time properties. Unlike traditional embedded operating systems, however, operating systems specifically targeting sensor networks often do not have real-time support. TinyOS [7] is perhaps the first[citation needed] operating system specifically designed for wireless sensor networks. Unlike most other operating systems, TinyOS is based on an event-driven programming model instead of multithreading. TinyOS programs are composed into event handlers and tasks with run to completion- semantics. When an external event occurs, such as an incoming data packet or a sensor reading, TinyOS calls the appropriate event handler to handle the event. Event handlers can post tasks that are scheduled by the TinyOS kernel some time later. Both the TinyOS system and programs written for TinyOS are written in a special programming language called nesC which is an extension to the C programming language. NesC is designed to detect race conditions between tasks and event handlers. There are also operating systems that allow programming in C. Examples of such operating systems include , MANTIS, BTnut, SOS and Nano-RK. Contiki is designed to support loading modules over the network and supports run-time loading of standard ELF files[. The Contiki kernel is event-driven, like TinyOS, but the system supports multithreading on a per-application basis. Furthermore, Contiki includes protothreads that provide a thread-like programming abstraction but with a very small memory overhead. Unlike the event-driven Contiki kernel, the MANTIS and Nano-RK kernels are based on preemptive multithreading. With preemptive multithreading, applications do not need to explicitly yield the microprocessor to other processes. Instead, the kernel divides the time between the active processes and decides which process that currently can be run which makes application programming easier. Nano-RK is a real-time resource kernel that allows fine grained control of the way tasks get access to CPU time, networking and sensors. Like TinyOS and Contiki, SOS is an event- driven operating system. The prime feature of SOS is its support for loadable modules. A complete system is built from smaller modules, possibly at run-time. To support the inherent dynamism in its module interface, SOS also focuses on support for dynamic memory management. BTnut is based on cooperative multi-threading and plain C code, and is packaged with a developer kit and tutorial Middleware There is considerable research effort currently invested in the design of middleware for WSN's.[3] In general approaches can be classified into distributed database, mobile agents, and event-based.[22] Programming languages Programming the sensor nodes is difficult when compared to normal computer systems. The resource constrained nature of these nodes gives rise to new programming models. Although most nodes are currently programmed in C. • c@t (Computation at a point in space (@) Time ) • DCL (Distributed Compositional Language) • galsC • nesC • Protothreads • SNACK • SQTL

Algorithms WSNs are composed of a large number of sensor nodes, therefore, an algorithm for a WSN is implicitly a distributed algorithm. In WSNs the scarcest resource is energy, and one of the most energy-expensive operations is data transmission. For this reason, algorithmic research in WSN mostly focuses on the study and design of energy aware algorithms for data transmission from the sensor nodes to the base stations. Data transmission is usually multi-hop (from node to node, towards the base stations), due to the polynomial growth in the energy- cost of radio transmission with respect to the transmission distance. The algorithmic approach to WSN differentiates itself from the protocol approach by the fact that the mathematical models used are more abstract, more general, but sometimes less realistic than the models used for protocol design. Simulators There are platforms specifically designed to simulate Wireless Sensor Networks, like TOSSIM, which is a part of TinyOS. Traditional network simulators like ns-2 have also been used. An extensive list of simulation tools for Wireless Sensor Networks can be found at the CRUISE WSN Simulation Tool Knowledgebase Data visualization

The data gathered from wireless sensor networks is usually saved in the form of numerical data in a central base station. There are many programs, like TosGUI , SenSor and MonSense,GSN that facilitate the viewing of these large amounts of data. Additionally, the Open Geospatial Consortium (OGC) is specifying standards for interoperability interfaces and metadata encodings that enable real time integration of heterogeneous sensor webs into the Internet, allowing any individual to monitor or control Wireless Sensor Networks through a Web Browser.