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Bluetooth Based Smart Sensor Networks
"Dragging the world towards wireless galaxy"
Various sensors are already in a broad use today as part of different devices or as standalone devices connected to a network usually to monitor industrial processes, equipments or installations.
The advancements in technology, wireless communications have enhanced development of small, low power and low cost devices. Such devices when organized into a network, present a powerful platform that can be used in many interesting applications.
Bluetooth is a low cost, short-range, wireless technology with small footprint, low power consumption and reasonable throughput. Bluetooth wireless technology has become global technology specification for "always on" wireless communication not just as a point-to-point but was a network technology as well.
The kernel of this paper, deals about an implementation of bluetooth based sensor networks.
The communications capability of devices and continuous transparent information routes are indispensable components of future oriented automation concepts. Communication is increasing rapidly in industrial environment even at field level.
In any industry the process can be realized through sensors and can be controlled through actuators. The process is monitored on the central control room by getting signals through a pair of wires from each field device in Distributed Control Systems (DCS). With advent in networking concept, the cost of wiring is saved by networking the field devices. But the latest trend is elimination of wires i.e., wireless networks.
Wireless sensor networks - networks of small devices equipped with sensors, microprocessor and wireless communication interfaces.
In 1994, Ericsson Mobile communications, the global telecommunication company based in Sweden, initiated a study to investigate, the feasibility of a low power, low cost ratio interface, and to find a way to eliminate cables between devices. Finally, the engineers at the Ericsson named the new wireless technology as "Blue tooth" to honour the 10th century king if Denmark, Harald Blue tooth (940 to 985 A.D).
The goals of blue tooth are unification and harmony as well, specifically enabling different devices to communicate through a commonly accepted standard for wire less connectivity.
Blue tooth operates in the unlicensed ISM band at 2.4 GHZ frequency band and use frequency hopping spread spectrum technique. A typical Blue tooth device has a range of about 10 meters and can be extended to 100meters. Communication channels supports total bandwidth of 1 Mb / sec. A single connection supports a maximum asymmetric data transfer rate of 721 KBPS maximum of three channels.
In bluetooth, a Piconet is a collection of up to 8 devices that frequency hop together. Each Piconet has one master usually a device that initiated establishment of the Piconet, and up to 7 slave devices. Master's Blue tooth address is used for definition of the frequency hopping sequence. Slave devices use the master's clock to synchronize their clocks to be able to hop simultaneously.
When a device wants to establish a Piconet it has to perform inquiry to discover other Blue tooth devices in the range. Inquiry procedure is defined in such a way to ensure that two devices will after some time, visit the same frequency same time when that happens, required information is exchanged and devices can use paging procedure to establish connection.
When more than 7 devices needs to communicate, there are two options. The first one is to put one or more devices into the park state. Blue tooth defines three low power modes sniff, hold and park. When a device is in the park mode then it disassociates from and Piconet, but still maintains timing synchronization with it. The master of the Piconet periodically broadcasts beacons (Warning) to invite the slave to rejoin the Piconet or to allow the slave to request to rejoin. The slave can rejoin the Piconet only if there are less than seven slaves already in the Piconet. If not so, the master has to 'park' one of the active slaves first. All these actions cause delay and for some applications it can be unacceptable for eg: process control applications, that requires immediate response from the command centre (central control room).
Scatternet consists of several Piconets connected by devices participating in multiple Piconet. These devices can be slaves in all Piconets or master in one Piconet and slave in other Piconets. Using scatternets higher throughput is available and multi-hop connections between devices in different Piconets are possible. i.e., The unit can communicate in one Piconet at time so they jump from pioneer to another depending upon the channel parameter.
BLUE TOOTH BASED SENSOR NETWORK
The main challenge in front of Blue tooth developers now is to prove interoperability between different manufactures' devices and to provide numerous interesting applications. One of such applications is wireless sensor networks.
Wireless sensor networks comprise number of small devices equipped with a sensing unit, microprocessors, and wireless communication interface and power
1. An important feature of wireless sensor networks is collaboration of network nodes during the task execution.
2. Another specific characteristics of wireless sensor network is Data-centric
As deployment of smart sensor nodes is not planned in advance and positions of nodes in the field are not determined, it could happen that some sensor nodes end in such positions that they either cannot perform required measurement or the error probability is high. For that a redundant number of smart nodes is deployed in this field. These nodes then communicate, collaborate and share data, thus ensuring better results.
Smart sensor nodes scattered in the field, collect data and send it to users via "gateway" using multiple hop routes.
A Wireless sensor network
The main functions of a gateway are
â€“ Communication with sensor Networks
2 Shortage wireless communication is used.
2 It provides functions like discovery of smart sensor nodes, generic methods of sending and receiving data to and from sensors, routing .
â€“ Gateway logic
2 It controls gateway interfaces and data flow to and from sensor network.
2 It provides an abstraction level that describes the existing sensors
and their characteristics. 2 It provides functions for uniform access to sensors regardless of
their type, location or N/W topology, inject queries and tasks and
â€“ Communication With Users
2 Gateway communications with users or other sensor networks over the Internet, WAN, Satellite or some shortage communication technology.
From the user point of view, quering and tasking are two main services provided by wireless sensor networks. Queries are used when user requires only the current value of the observed phenomenon. Tasking is a more complex operation and is used when a phenomenon has to be observed over a large period of time.Both queries and tasks of time to the network by the gateway which also collects replies and forwards them to users.
SENSOR NETWORK IMPLEMENTATION The main goal of our implementation was to build a hardware platform and generic software solutions that can serve as the basis and a test bed for the research of wireless sensor network protocols.
Implemented sensor network consists of several smart sensor nodes and a gateway. Each smart node can have several sensors and is equipped with a microÃ‚Â¬controller and a bluetooth radio module.
Gate way and smart nodes are members of the Piconet and hence maximum seven smart nodes can exist simultaneously in the network.
For example, a pressure sensor is implemented, as bluetooth node in a following way.
The sensor is connected to the bluetooth node and consists of the pressure sensing element, smart signal-conditioning circuitry including calibration and temperature compensation, and the Transducer Electronic Data Sheet (TEDS). These features are built directly into the sensor microcontroller used for node communication control plus memory for TEDS configuration information.
Smart Sensor Node Architecture
The architecture shown in figure can easily be developed for specific sensor configurations such as thermocouples, strain gauges, and other sensor technologies and can include sensor signal conditioning as well as communications functions.
Conditioned along sensor signal is digitized and digital data is then processed using stored TEDS data. The pressure sensor node collects data from multiple sensors and transmits the data via bluetooth wireless communications in the 2.4 GHZ base band to a network hub or other internet appliance such as a computer.
The node can supply excitation to each sensor, or external sensor power can be supplied. Up to eight channels are available on each node for analog inputs as well as digital output. The sensor signal is digitized with 16-bit A/D resolution for transmission along with the TEDS for each sensor. This allows each channel to identify itself to the host system. The node can operate from either an external power supply or an attached battery. The maximum transmission distance is 10 meters with an optional capability to 100 meters.
The IEEE 1451 family of standards are used for definition of functional boundaries and interfaces that are necessary to enable smart transducer to be easily connected to a variety of networks. The standards define the protocol and functions that give the transducer interchangeability in networked system, with this information a host microcomputer recognized a pressure sensor, a temperature sensor, or another sensor type along with the measurement range and scaling information based on the information contained in the TEDS data.
With blue tooth technology, small transceiver modules can be built into a wide range of products including sensor systems, allowing fast and secure transmission of data within a given radius (Usually up to 10m).
A blue tooth module consists primarily of three functional blocks - an analog 2.4 GHz., Blue tooth RF transceiver unit, and a support unit for link management and host controller interface functions.
The host controller has a hardware digital signal processing part- the Link Controller (LC), a CPU core, and it interfaces to the host environment. The link controller consists of hardware and software parts that perform blue tooth based band processing, and physical layer protocols. The link controller performs low-level digital-signal processing to establish connections, assemble or disassemble, packets, control frequency hopping, correct errors and encrypt data.
Bluetooth module Hardware Architecture
The CPU core allows the blue tooth module to handle inquiries and filter page request without involving the host device. The host controller can be programmed to answer certain page messages and authenticate remote links. The link manager(LM) software runs on the CPU core. The LM discovers other remote LMs and communicates with them via the link manager protocol (LMP) to perform its service provider role using the services of the underlying LC. The link manager is a software function that uses the services of the link controller to perform link setup, authentication, link configuration, and other protocols. Depending on the implementation, the link controller and link manager functions may not reside in the same processor.
Another function component is of course, the antenna, which may be integrated on the PCB or come as a standalone item. A fully implemented blue tooth module also incorporates higher-level software protocols, which govern the functionality and interoperability with other modules.
Gate way plays the role of the Piconet's master in the sensor network. It controls establishments of the network, gathers information about the existing smart sensor nodes and sensor attached to them and provides access to them. Discovery Of The Smart Sensor Nodes
Smart sensor node discovery is the first procedure that is executed upon the gateway installation. It goals to discover all sensor nodes in the area and to build a list of sensor's characteristics and network topology. Afterwards, it is executed periodically to facilitate addition of new or removal of the existing sensors. The following algorithm is proposed.
When the gateway is initialized, it performs bluetooth inquiry procedure. When the blue tooth device is discovered, the major and minor device classes are checked. These parameters are set by each smart node to define type of the device and type of the attached sensors. Service class field can be used to give some additional description of offered services. if discovered device is not smart node it is discarded. Otherwise service database of the discovered smart node is searched for sensor services. As currently there is no specific sensor profile, then database is searched for the serial port profile connection parameters. Once connection strings is obtained from the device. Blue tooth link is established and data exchange with smart mode can start.
Blue tooth represents a great chance for sensor-networked architecture. This architecture heralds wireless future for home and also for industrial implementation. With a blue tooth RF link, users only need to bring the devices with in range, and the devices will automatically link up and exchange information.
Thus implementation of blue tooth technology for sensor networks not only cuts wiring cost but also integrates the industrial environment to smarter environment.
Today, with a broader specifications and a renewed concentration on interoperability, manufacturers are ready to forge ahead and take blue tooth products to the market place. Embedded design can incorporate the blue tooth wireless technology into a range of new products to meet the growing demand for connected information appliances.
â€“ Future work is aimed to develop and design a blue tooth-enabled data concentrator for data acquisition and analysis.
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â€“ C.Shen, C.Srisathapomphat "sensor networking architecture and application", IEEC personal communication. Aug,2001.
â€“ C.Chellappan, RTCBPA, June 2003.
â€“ Pappa,Transducer networks, RTCBPA, June 2003.