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
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.
Blue tooth networks
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.
A Piconet
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 a wireless
sensor network.
Wireless sensor networks comprise number of small devices equipped with a
sensing unit, microprocessors, and wireless communication interface and power
source.
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 nature.
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
- Communication with sensor Networks
- Shortage wireless communication is used.
- It provides functions like discovery of smart sensor nodes, generic methods of sending and receiving data to and from sensors, routing.
- Gateway logic.
- It controls gateway interfaces and data flow to and from sensor network.
- It provides an abstraction level that describes the existing sensors and their characteristics.
- It provides functions for uniform access to sensors regardless of their type, location or N/W topology, inject queries and tasks and collect replies.
- Communication With Users.
- Gateway communications with users or other sensor networks over the Internet, WAN, Satellite or some shortage communication technology.
From the user point of view, querying 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.
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