TechPoint

Physical Layer for Wireless Sensor Networks

2011-08-07T06:20:00.000-07:00

The main concern of the physical layer is modulation and demodulation of digital data, i.e. transmission and reception of the data. This is done by the transceivers in the sensor nodes. The main functions of physical layer are carrier frequency selection and generation, encryption and decryption, modulation and demodulation, transmission and reception of data.

Wireless sensor network generally work in ISM bands. But many other standards like 802.11b and Bluetooth also use the same band, so all systems, in this band have to be robust against interference form other systems.
The most important parameters which are to be considered while designing Physical layer in wireless sensor networks are
1. Low Power Consumption.
2. Low Transmission and Reception range.
3. Interference from other systems, working in the same band.
4. Low complexity.
5. Low duty cycle, i.e. most of the time sensor nodes are switched off.
6. Low data rates most of the time and high data rate only for a short period of time.
The most challenging aspect in physical layer design for sensor networks is to find, low cost transceivers which consume less power, simple modulation schemes which are robust enough to provide required service.

Generally the transceivers used in sensor network are only 10% efficient. To radiate a power of 1 mw the transceivers will consume at least 10 mw of power. The power consumed for reception is some what similar to power consumed for transmission; some times one of them may be more and one less depending upon design of the transceivers. For Mica motes 21 mw is consumed in transmit mode and 15 mw is consumed in receive mode. The transmission and reception of data are the most power consuming activity in the sensor node. So transmission and reception of data should be kept as less possible. For the WIN nodes, 1500 to 2700 instructions can be executed per transmitted bit, for the MEDUSA II nodes this ratio ranges from 220:1 up to 2900:1, and for the WINS NG nodes, it is around 1400:1. The summary is computation is cheaper than communication.

The power consumed by the transceivers in idle mode will not be significantly less than the power consumed by transceivers in receive or transmit mode. So it is always preferable to put the transceiver in sleep mode rather than in idle mode when not required. But care has to be taken to see to that, the power consumed during start up and time taken to startup the transceiver does not over come the advantage of putting the transceiver in sleep mode. The most commonly used transceiver is CC2420.
The choice of modulation scheme is another important aspect to be considered here. The choice depends upon the complexity that can be supported by the node, as one cannot go for very complex processes in sensor networks. The data rate is another factor to be considered, as this tells about what amount of data can be transmitted or received when the node is on. To save more and more energy the sensor node should be in sleep as much as possible and more the data rate more amount of data can be transmitted and received in small active period. The symbol rate is yet another important factor, as it has be found that more is the symbol rate more is the power consumption. The BER has also to be considered, because to keep BER low the power radiated has to be more.

As an example we shall consider the most commonly used protocol for WSN, IEEE 802.15.4. IEEE 802.15.4 defines wireless personal area network standard for low rate devices, this specifies the standards for Physical and MAC layer. 802.15.4 Works at a data rate of less than 250kbps. It works up to a range of 75m and it supports up to 254 nodes. This protocol is designed specially for networks whose data rate is less, which have energy constraints, and which require good QoS. The network using this protocol can survive from 6 months to 2 years with two AA batteries.

802.15.4 Works in three frequency bands they are 868 MHz (20 kbps data rate), 915MHz (40 kbps data rate) and 2.4GHz (250 kbps data rate). It uses Direct Sequence Spread Spectrum for modulation and uses BPSK for first two bands and 16 bit array QPSK for the last band. The first band has 1 channel in it the second band has 10 channels in it and third band has 16 channels in it. This can be seen from the diagram below

Channels in 802.15.4 physical layer

Communication Protocols for Wireless Sensor Networks

2011-08-07T06:16:00.000-07:00

The protocol stack used in wireless sensor networks is as shown below. It consists of physical layer, data link layer, network layer and application layer. 802.15.4 is an IEEE standard for physical and data link layers. The ZigBee protocol specifies standards above MAC and physical layer. However other than these standards, there are many other protocols and papers defined for MAC, Network, Application layers. We shall brief them in this section.

Protocol Stack of Wireless Sensor Network.

QoS in Sensor Networks

2011-08-07T06:14:00.000-07:00

Quality of service required for different applications in sensor networks can be different. Some may require more precise and accurate information some may require less. Some applications might not tolerate any delay in notification after the detection of event and some may tolerate the delay. Some applications may monitor a large area with more number of active sensors and some applications might monitor only small area with less number of active sensors.

In sensor network the importance is on collective data delivery rather than packet form a single node, because in sensor network many sensor nodes produce similar data. So in sensor network terms such as collective latency, collective bandwidth usage, collective packet loss, collective information throughput come into picture.
Collective latency is the time between, the first packet related to an event is received by the sink and last packet related to that event is received at the sink, no matter which node produced the data. Collective bandwidth is the bandwidth required to report an event to the sink by sensor nodes al together. Collective packet loss is the number of packets lost al together during reporting of an event.
Any QoS mechanism introduced for WSN should be very simple, it should not be, computationally complex, time consuming, requiring large amount of memory. Because the sensor nodes are energy, memory constrained and sensor nodes do not use very high end processors also.

QoS support in sensor network should consider the fact that, event driven sensor networks are characterized by long periods of silence, when there are no events detected and small periods where large amount of traffic is produced, when any event is detected. And all the data produced are generally destined to sink, so traffic form nodes to sink will very large. So the QoS has to support this unbalanced traffic.

The probability of having redundant data is quite large in sensor networks, so the redundant data should be reduced as much as possible may be data aggregation. But one has to consider the delay involved and complexities involved in data aggregation and take a decision.

Sensor networks are very dynamic; there are high chances of, node failures, link failures, node mobility and node state transitions. So QoS support for sensor network should consider all the above factors.

The number of sensor nodes in sensor network is very large, probability of addition of nodes is also quite high, and so any sensor network application should be scalable.There are different types of data supported by the sensor networks QoS provided should support all different types of data.

Connection of Sensor Network with TCP/IP Network , Article -2

2011-08-07T06:04:00.000-07:00

Continuing from the previous article ........

Unifying Micro Sensor Networks with the Internet via Overlay Networking is another paper, which proposes to use sensor network overlay over TCP/IP networks. By using such an overlay sensor network can use data centric approach, i.e. assigning unique address to each and every senor node is not essential. The sensor network overlay is formed by extending address less sensor network routing into internet as application overlay. The figure below shows the Sensor network overlay on TCP/IP networks.

Sensor Network Overlay on TCP/IP network.

The nodes in internet with Sensor network overlay are called as virtual sensor nodes, and these help in forming virtual sensor network.
TCP/IP overlay over Sensor Network paper proposes to use TCP/IP stack in the sensor nodes.
The key advantage is that it makes WSN highly compatible with internet as both them will have almost similar protocols running. However this TCP/IP stack can be loaded into sensor nodes which have higher memory and higher processing capabilities. The authors have developed something called as u-IP stack. This is similar to normal TCP/IP stack but with lesser features. The memory required to load the stack is also less compared to full TCP/IP stack. The figure below shows the TCP/IP overlay on WSN.

TCP/IP Overlay on WSN

In yet another paper called “VIP Bridge Method” authors propose to use node centric or location centric WSN. It is not applicable for data centric WSN. This paper uses a VIP Bridge it does mapping between IP address and location address or local address of the sensor node. The sensor nodes will not have IP address configured, but the IP address to each node exists in VIP Bridge. The sensors will be identified in the WSN by the local identifiers the VIP Bridge does the conversion require. The VIP Bridge does translation form sensor network to TCP/IP and vice versa. This paper explains the method to do the conversion. The advantage of this VIP Bridge method is that communication is done between TCP/IP network and WSN using IP address itself and the local sensor identifier is used for communication within the senor networks. The internet users need not know the protocols used in WSN they can use WSN as if it is a normal TCP/IP network. In other words WSN is transparent to internet users. The architecture of the VIP Bridge method is as shown below.

VIP Bridge Architecture.

Connection of Sensor Network with TCP/IP Network, Article - 1

2011-08-05T22:26:00.001-07:00

There are many situations where sensor networks need to be connected via TCP/IP network. In other words it might be necessary to integrate many sensor networks into one virtual sensor network using TCP/IP network.
To allow users all over the world to use a WSN deployed in some remote place it is necessary to connect WSN to TCP/IP network. The users may issue a query to get information form WSN or it might be necessary to issue a command or some information to a particular node in WSN. The particular node might be one of several higher capability nodes in the WSN.

So connecting WSN with TCP/IP network is very important but due to sensor node’s limitations it is not possible to install full existing TCP/IP stack into a sensor node. So to connect WSN and TCP/IP network many new methods are proposed.

Different authors propose different methods to connect WSN and TCP/IP networks. In “Integrating Future Large-scale Wireless Sensor Networks with the Internet” application level gateway is used to connect WSN and TCP/IP network. This paper classifies WSN as homogeneous and heterogeneous WSN. It provides solutions to connect both homogeneous and heterogeneous networks to TCP/IP networks. Homogeneous sensor network is one in which all sensor nodes have similar capabilities and heterogeneous sensor network is one in which some node have more capabilities than other nodes and outside user wants to interact with the more capable nodes directly. To connect homogeneous sensor network to TCP/IP networks this paper proposes to use a gateway as shown in the figure below. Gateway Based Approach for connecting Sensor Networks with TCP/IP Networks

The gateway is viewed as front end for the distributed database. The user who wants information form WSN may issue query to the gateway. Query optimization is provided through data centric in network processing. The response is obtained from WSN by the gateway and reply is sent to the user. The gateway acts as the interface between WSN and internet.
The advantage of this approach is both WSN and internet can be completely independent; it’s the headache of gateway to do conversions required. And it is most easy approach as very less amount of changes are required to the existing architecture; a gateway need’s to be added in between internet and WSN. The drawback of this approach is that ever gateway has to be designed according to the WSN. And as we see this has single point of failure, i.e. if gateway goes down WSN becomes inaccessible. To overcome the above said problem we can use two or more gateways also.

To connect heterogeneous WSN to TCP/IP network the paper proposes to use an IP overlay over the WSN. In the heterogeneous network some nodes have more capability or some nodes may be acting as cluster heads. These nodes can be given IP address, and these nodes may be directly accessed by the user using internet. In this type of network an IP overlay network is created over the underlying WSN to connect user directly to higher capable nodes. To create an IP overlay network it is necessary to create a tunneling mechanism to allow the more capable nodes to communicate among themselves using address centric approach. To create tunnels between the more capable nodes the paper proposes to use either Directed Diffusion or ACQUIRE protocols. Directed Diffusion can be used for high IP traffic applications and ACQUIRE can be used for low IP traffic applications. The figure below makes the concept clearer.

Heterogeneous network, the red line shows the tunneling formation using Data Diffusion or ACQUIRE protocol.

The red color line in the figure above shows the tunnel between the higher capable nodes using normal sensor nodes. This tunnel makes IP overlay possible over existing WSN architecture. Once the tunnel is established it will be possible to the users to interact with the higher capable nodes directly using existing WSN architecture itself.

A paper called “A Delay-Tolerant Network Architecture for Challenged Internets” proposes to use a method to integrate the challenged networks. Challenged networks are those which have a very long end-end delay, low bandwidth, high rate of network portioning, high error rates. Sensor network can be considered as a challenged network so this paper is applicable to the sensor network also. This paper uses an application layer above transport layer in TCP/IP stack as well as in WSN stack called the bundle layer. This layer is responsible to store and forward packet. This is shown in the figure below.

Connecting WSN and Internet using DTN gateway

It looks similar to gateway approach described before, but it has several dissimilarities with the application level gateway approach. The DTN gateway is universal to connect any network, it is not application based. It takes into account many aspects of different challenged network but application level gateway fails to do so. One region itself can have two or more DTN gateways, so even within a region if partition takes place DTN gateway based approach works where as the earlier one fails.

The network is divided into different regions. One region itself has many DTN gateways. The DTN gateways communicate with each other. Initially routing is done based on the region address using the bundle layer in the DTN gateways. Once the packet reaches the destined region the address of the host is used to reach the host in the destination. The address format will be (region address, host address) host address can be of any type. So this architecture works for any network. This paper speaks about integration of all challenged networks and TCP/IP network. The figure in the next page shows DTN based architecture.

DTN based Architecture

The advantage of this architecture is that it can be used to connect any type of network, and it supports more than one DTN gateway in only one network patch. The DTN gateway takes care of interface between different networks. So every network can be designed independently. The disadvantage of this architecture is the extra bundle layer that needs to be installed.

This Article will be continued in the next post ...

Fault Tolerance in Sensor Networks

2011-08-05T22:22:00.000-07:00

The probability of failure of sensor nodes in sensor networks can be very high in some applications. So it very necessary to design the sensor networks such that, the failure of sensor nodes do not affect the proper functioning of network. Much importance has been given to make sensor network fault tolerant and reliable.

In a paper called “Fault tolerant clustering of WSN” fault tolerance of WSN is increased using detection of failed gateways, and re associating the senor nodes attached to the failed gateway with another existing gateway. Periodic checking for failure of gateways is done. If a failure is found, all sensor nodes attached to the failed gateway are reattached to a new gateway. This method prevents re election of the gateway, and increases reliability of wireless sensor networks.

In “Prolonging Sensor Network Lifetime with Energy Provisioning and Relay Node Placement” authors propose to provide extra energy to the AFN (Aggregating and Forwarding Node) similar to gateway. As a result of providing extra energy, failure of gateways can be prevented hence sensor network become more fault tolerant. The approach also proposes to use extra relay nodes after AFN and provide extra energy to the relay nodes also. The SPINDS algorithm proposed in this paper tries to solve the joint problem of relay node placement and energy provisioning.

In another approach called “WSN with Guaranteed Capacity and Fault Tolerance” authors propose deployment of extra nodes form a robot helicopter. These extra nodes are deployed to make wireless sensor network fault tolerant. All the nodes will not be awake simultaneously. Redundant nodes will be sleeping and wake up only when an active node dies.

In a patent called “Fault Tolerance in WSN” authors try to make WSN fault tolerant by the usage of secondary and tertiary gateways along with primary gateway. The primary gateway synchronizes the secondary and tertiary gateway with itself. If the primary gateway fails, secondary gateway takes up its place and if the secondary gateway fails tertiary gateway take up its place. This increases fault tolerance of WSN.

Energy Usage in Sensor Nodes

2011-05-28T23:19:00.000-07:00

Data transmission and reception, and sensing are the prime cosumers of power in a sensor node . In other words Transceiver, Controller, Sensors and to some extent the memory are the components that feed on the avaialable energy in a sensor node.

Transceiver is costliest in terms energy usage. A bit transmitted or received is equal to 1500-2700 instructions being executed. Hence it is adviceable to put sensor nodes(transceivers) to sleep when not is use. There are three states, sleep state, operational state and idle sate. Operational state consumes the max, Idle state being next and sleep state consumes the least.. The energy required to change state from sleep to active is considerable, so one has decide on when a sensor node should sleep and when should it be awake.

The energy cosumption of each component is described below
Microcontroller: The MSP430 family features a wider range of operation modes: One fully operational mode, which consumes about 1.2 mW (all power values given at 1 MHz and 3 V). There are four sleep modes in total. The deepest sleep mode, LPM4, only consumes 0.3 μW, but the controller is only woken up by external interrupts in this mode. In the next higher mode, LPM3, a clock is also still running, which can be used for scheduled wake ups, and still consumes only about 6 μW.
Memory: The main consumer in memory is the Flash Memory. Reading, Writing and Erasing flash memory all consume energy. Among them writing consumes the most energy.
Transreceiver: Energy is consumed in transreceiver during transmission, reception, start up (switch off to switch on). The following example explains the energy consumption pattern. µAMPS -1 consumes 279mW for reception, 151 mW for transmission and 58.7mW for start up.
Sensors: Predicting power consumption of sensors is not easy task, as variety of sensors consume different amount of energy. Passive sensors consume very less energy and the energy consumed by them is negligible compared to other energy consumption however active sensors consume considerable amount of energy.

Software Architecture of a Sensor Node

2011-03-14T08:29:00.000-07:00

An Operating System, Sensor Module, Antenna Module, Communication Module, Applications, Mobilizer Module, and Location and navigation module make the software of a senor node. Some sensor nodes may have all of modules and some may have only few modules according to the necessity. The naming and classification of these modules may be different in other literature's but let us stick to the above module names in this article and get to know about them. The following figure gives the overall software architecture of sensor node.

Overall Software Architecture of a Sensor Node

Operating System: This is hardware common micro code which will be used by all other high level node resident modules. As is generally the case the purpose of operating system is to shield the higher level modules from machine level functionality of the microprocessor. We can have Tiny-OS, which is open source software designed particularly targeting WSNs. It is written using Nesc. Nesc is made up of modules and configuration files, these are called as components. Tiny-OS is made up of components, components provide and use interfaces. The modules contain the actual implementation and configuration files wire the modules. There are tasks and hardware events for execution, tasks can be postponed but hardware events cannot be postponed.

Sensor Module: This module is responsible, to provide drivers to sensors present in the sensor node, to collect data from sensor when they are generated, to switch on and off the sensors, configure the sensors, and any other function that is related to sensors is done by this module.

Mobilizer Module: This module is responsible for the mobilizer’s driver, controlling of the mobilizer, this module comes into picture only if the sensor has a mobilizer in it.

Location and Navigation Module: This module is responsible for finding location of the sensor node and providing drivers for the location finding instruments if they are present on the sensor node.

Antenna Module: This module is responsible for the driver of the transceivers like CC2420. Any driver’s main function is to shield the software modules from the machine level functionality of the hardware.

Communication Module: This module is responsible for the route set up, channel allocation, new node detection, route table maintenance, encryption, decryption of the data , providing other security features required, providing good QoS, error checking and correction, data buffering, data forwarding many others.

Applications: This module is responsible for data aggregation, in node processing, and other application specific functions.

Schematic Digaram of a Sensor Node

2011-02-24T09:05:00.000-08:00

Sensor Network is made up of many sensor nodes. These nodes are tiny in size. They have limited, power supply, memory and processing capability. Each sensor consists of, Transceiver, Power supply, Processor, Memory and Sensors. The overall architecture of a sensor node is shown in the diagram below.

Overall Architecture of a Sensor Node(click on the pic to see enlarged view)

Every sensor node should try to meet various design requirement of any sensor network, i.e. they should be inexpensive, small, less power consuming, robust, and equipped with right sensors to sense the environment. The node should be as small as 1 cc in size , weigh less than 100 g , be cheaper than 1$ and dissipate power less than 100 micro Watt.However it may not be possible in to meet all the above said requirements all cases or in many cases .

Power Supply: The power supply unit of the sensor node is responsible to provide power to all other components in the sensor node. The power supply can be rechargeable or non rechargeable in nature, the later one being most common. The supply is generally from 3 to 5 V, in most cases it is DC battery. Having a power source which generates energy by some means (Using Solar energy), so that node is provided power for a longer time is more desirable, however it will be expensive.

Micro controller: It is the CPU of the sensor node. It is responsible for all processing and decision making. Many chip makers have produced micro controller for the purpose of sensor node. Intel is one of the leaders in the production. These are some of the processor that can be used ARM7, Atmel, AVR Intel, Xscale , Intel 8051, PIC, and TI MSP430 . Micro controller will be interfaced to the transceiver and sensors.

Sensors: These are responsible to sense the surrounding environment and inform the controller about the phenomenon being observed. There are different sensors available they are Light Sensors, Pressure Sensors, Humidity Sensors, Gas Sensors, Temperature Sensors and many more. These sensors are further divided as active and passive sensors. Instead of sensors actuators also can be used. Sensors sense the physical quantity convert it to electrical signals process the same and obtain digital data and give it to the controller.

Transceiver: Transceivers are responsible to transmit and receive data to and from the communicating media. Communication media in most cases is air. Laser, Infrared and RF based Transceivers can be used according to what is used for communication. RF based communication is most preferred form, as laser and infrared require direct sight of communication. Generally transceivers work in ISM band, which is license free band. Transceivers are interfaced to micro controller using transceiver chip. The most commonly used transceivers chips are CC1000, CC1020, and CC2420.

Memory: Sensor nodes will have programmable Flash memory and RAM in most cases. The flash memory used in MSP430 is 48KB and it has 10KB RAM. So the protocols that are designed for the sensor network should be simple enough to be loaded into the available memory.

Some sensor node may also have Location Finding system such as GPS and mobilizers (which assist the sensor node to move form one place to another if the application requires it).

Coding Guidelines

2010-10-22T09:12:00.000-07:00

I happened to read a book called "Software Development Guidelines", I will list out the points from this book which I liked.

Let each routine designed perform an unique task. Divide the program into routines such that each of them performs a separate task. Let all routines be clearly distinguishable.
Do not make any routine too long. Also do not simply divide the routines without any logical need. If a routine is more than 100 lines one may try to divide it if it can be divided into routines which can perform separate tasks.
Let the routines be loosely coupled. (pass less parameters, make the routine interfaces flexible, make them more visible )
Let all the files related to a module be in one directory.
Let all modules be divided logically. The interfaces should be provided for interaction between modules
All other internal data within module should be private
Declare int, float and all as typedef e.g typedef int int16 (on 16 bit machine) so that one need not go and the variable type everywhere. This makes the program more platforms independent.
Do explicit declarations of all variables before it is used.
Declare all the variables , constants at the beginning of the routine.
Every variable declared should have a comment describing the need, function and limitation of the variable.
Better to declare each variable in a separate line with its description
Always use meaningful names for the variables , this increases the readability of the program like anything (have high quality identifiers )
Do not create two different variables in the program which only differ in their cases (upper case or lower case) because not all compilers are case sensitive. To make program portable use case neutral variables.
Capitalize the first letter of interior words in all multi-word identifiers.
Variable names should be meaningful, concise, and non-ambiguous to an average programmer fluent in the English language.
Try to avoid abbreviated variable names .If used give description of the abbreviated variable names.
Never use a numeric suffix to differentiate two names.
Try to make most identifiers unique in the first few character positions of the identifier. This makes the program easier to read.
If your code contains a chain of if..elseif..elseif.......elseif..... statements, do not use the final else clause to handle a remaining case. Only use the final else to catch an error condition. If you need to test for some value in an if..elseif..elseif.... chain, always test the value in an if or elseif statement.
Always use the most appropriate type of loop (categorized by termination test position). Never force one type of loop to behave like another.
Avoid empty loops. If testing the loop termination condition produces some side effect that is the whole purpose of the loop, move that side effect into the body of the loop. If a loop truly has an empty body, place a comment like "/* nothing */" or "{null statement}" within your code.
Loops with a single exit point are more easily understood
All loops should have one entry point. The program should enter the loop with the instruction at the top of the loop.
Make each loop perform only one function.
Code, as much as possible, should read from top to bottom( do not use lot of goto statements , deeply nested if else with lot of possibility of code jumping to some other location )
An expression should not produce any side effects.
Do not user lesser used operators, porting to different language may be difficult.
Use parentheses where ever necessary. We can do without parentheses for mul, division, addition, and subtraction but for others different languages, different compliers may have different precedence hence better user brackets.
Indentation should be three to four spaces in an indented control structure with four spaces probably being the optimal value.
If you use tabs to indent your code, insert a comment at the very beginning of the program that states the number of positions for each tab stop. E.g., "/* this program is formatted using four character position tab stops. */"
All related statements should be together, there should not be blank lines in between them.
Blank lines should be present in between of declaration and start of code.
Blank should be present in between comment block and code
Whenever there is different logical block blank line should be present.
Source code lines will not exceed 80 characters in length
Always put a blank line between any block statement and the statement(s) it encloses
Write comments that describe blocks of statements rather than individual statements. Comments covering single statements tend to discuss the mechanics of that statement rather than discussing what the program is doing.
Focus paragraph comments on the why rather than the how. Code should explain what the program is doing and why the programmer chose to do it that way rather than explain what each individual statement is doing.
Use comments to prepare the reader for what is to follow. Someone reading the comments should be able to have a good idea of what the following code does without actually looking at the code. Note that this rule also suggests that comments should always precede the code to which they apply.
All comments will be high-quality comments that describe the actions of the surrounding code in a concise manner.
All comments will be up to date. If a programmer makes changes to the code, that programmer is responsible for updating the internal comments and any external documentation affected by those changes.
All inter module communication in C/C++ programs must take place through header files (“.h” files). All extern directives, public class definitions, public type definitions, and public constants must appear in the header file that all interested modules will include.
For functions like malloc which is used at many places we can used a customized safe_malloc for our C application so that the necessary check is also included in the safe_malloc function.

For detailed explanation on guidelines one can refer to the above mentioned book itself , it is available freely in the internet.

Sensor Network for Better, Efficient and Automated (BEA) Farming

2010-08-09T07:03:00.000-07:00

Diagram representing overall architecture for BEA farming

Wireless sensor networks have wide variety of applications . I would like to brief on how WSN can be used for better,efficient and automated farming. I and my friends got this idea during a casual discussion. So this articles belongs to them also. [:)].

Sensor network can be used in agriculture to monitor, the moisture content in soil , the fertility of soil, the surrounding weather conditions of the farm. Various sensor nodes can be deployed in the farm which have the capability to monitor the moisture,fertility and temperature. These sensor nodes can in turn give data to a gateway or a farmers mobile. Mobile is one of the very common devices it can act as gateway to collect data from all the senors nodes deployed in the farm. The data can be obtained whenever the farmer goes to his farm along with his mobile or there can be gateway, which aggregates the data from all the senor nodes in the farm, and sends it to the farmers mobile.

Once the data is available at the farmer's mobile he can send this data to experts or his friends or use this data himself to get any advice on farming if necessary. The data obtained on moisture level can be used to water the whole farm appropriately , such that none of the part of the farm will be over watered neither will less watered. Actuators can be used to take appropriate decisions based on sensor readings . There can be server to which all farmers can connect using their mobile and get necessary data. The server can also be used to store the data provided by the farmers. In this way all the agriculture lands can be connected , the data provided by all farmers can be used for various research purposes like to check what fertilizer level and moisture level has resulted in best yield. The data collected from many farmers at a place will help in building a detailed picture of farming at that respective place.

The shared server can also be used to share information such as , crops that can be grown on particular type of soil, best agriculture practices , present price for a particular type of crop etc. These information can be sent to farmers mobile so that farmers are updated with latest happenings.

The sensor nodes can be connected to the gateway or mobile using 802.15.4 (Low rate Wireless Personal Area network ) and the mobile and server can be connected using Internet or using other mobile communication system such as GSM or CDMA. The common sensors that can be used are soil moisture sensor, soil nutrients sensor , temperature sensor and rain sensors.

The diagram below makes the explanation given above more clearer.

The diagram given above is taken from the link given below.

http://www.ubicomp.in/uagri/

Design Issues in WSN

2010-07-23T20:44:00.000-07:00

Wireless sensor networks are made of large number of tiny sensor nodes, which have limited power and less processing capability. The life time of the individual sensor node is not easily predictable and also the network needs to be formed autonomously as it is not possible to manually set up the sensor network for all applications. The sensor network also consists of several different kind of nodes hence heterogeneity needs to be supported. The number of sensor nodes in the network is not constant through out the life time of the network it may vary because of addition of senor nodes or reduction of nodes due to their death . The major factors that need to considered while designing sensor network are listed below.

Fault Tolerance: Possibility of node failure and change of topology of network is quite high in case of WSN. Hence the designer of network should make the network robust and reliable even in case of node failures and topology changes. The network should function smoothly and normally irrespective of node failures and topology changes.

Life Time: WSN are supposed to work for a quite long time with low power consumption. They are supposed to last at-least for 6 months to 1 year. We need to keep in mind that every node in WSN may be powered using just a 3 V battery and this should be sufficient for the entire life time of the node . The design of protocols of WSN should be such that the node consumes as less energy as possible. This will help in making the WSN last longer.

Scalability: The design of WSN should support addition of new nodes any time and also the design should support large number of nodes because some applications in WSN may require quite a huge number of sensor nodes.

Date Aggregation: The sensor nodes in WSN are located close to each other hence the possibility of similar data being generated by the nodes next to each other is quite high. So the data needs to be aggregated and the duplicate data needs to be avoided because the transmission and reception data is the most costly affair in WSN.The data needs to be aggregated at different levels in WSN so that only the necessary data is transmitted and recevied and the redundant data is not communicated.

Cost: The cost of each sensor node is supposed to be 1$ , as WSN can have large number of sensor nodes the total cost of the network can become a quite expensive affair. So the designer of WSN needs to decide on the optimal number of nodes necessary for the application.

Environment: The environment in which the WSN is deployed can be very demanding , so the design of WSN should be such that WSN should be able to survive regardless of the conditions in which WSN s deployed.

Heterogeneity Support: The protocols designed for WSN should support different kinds of sensor nodes and also be able to support variety of applications.

Autonomous Operations: The WSN should be able to organize , reorganize and operate autonomously because sometimes WSN deployed in places where human habitation is not possible.

Limited Memory and Processing Capability : The sensor nodes have very limited memory, power and processing capabilities , so all designs of WSN should not be demanding in terms of processing requirements or memory requirements .

Applications of Sensor Networks

2010-05-20T09:52:00.000-07:00

Wireless Sensor Networks are used to build wide variety of applications . The apps range from agriculture apps to defense apps. All applications involve many sensor nodes, which collect data from the deployed site and this collected data is used for some specific purpose. I will give some examples of applications which are built using sensor networks.

Military or Defense Applications
Agriculture Applications
Environment and Weather Monitoring Applications
Habitat Monitoring Applications
Applications for Home Automation
Applications for Industries
Traffic Surveillance and Road Monitoring Applications
Applications for Health Monitoring , which is very broad area called as Body Area Network
Pollution Monitoring Applications
Applications to predict Natural Calamities
Applications for Surveillance and Intrusion Prevention
Structure Monitoring Applications like bridge monitoring apps

Addressing in WSN

2010-05-07T08:46:00.000-07:00

Several approaches have been proposed for addressing in WSN(Wireless Sensor Network) . I will brief about some approaches which I have read.

"Lease based addressing for event driven wireless sensor networks" proposes to use a network level unique address to use a node temporarily. Three phases are proposed by this paper, they are boot up phase , address request phase and address release phase. Initially when the node boots up, it assigns itself a link local address and waits for the broadcast message from sink. Once the node receives the broadcast message from sink it send address request message to sink. The sink maintains a table of address assigned and table of address unassigned. If the sink has any unassigned address then the sink assigns an unique address to the requesting node. The node utilizes this address for required duration and then releases the address , the node uses release address message for this purpose.

"Distributed address assignment" specified in ZigBee standard provides a method to distribute addresses available at the coordinator among all nodes. It gives a mathematical formula, which is utilized to distibute address among the nodes. The coordinator distributes the available addresses to its immediate children, every router child is given equal number of addresses and non router child is given a single address. Every router node in turn assigns available addresses to its children (differentiating between router child and non router child). This method may not be very relevant if the network does not grow uniformly .

"A novel approach for dynamic address assignment in wireless sensor networks" proposes to use only a part of available addresses for initial distribution . This initial distribution is done as specified in ZigBee. Later if any other node requires more addresses then it requests for more addresses from its parent, which is allocated if the parent has required number of addresses else the request is relayed till the initial server. Th parent also may ask some of its children to return unused addresses if the parent is in need of addresses. This method is applicable even if the network grows non uniformly.

"Dynamic Address Allocation for Management and Control in WSN" proposes a novel method. The address server sends a broadcasts message once the network starts up and in turn all nodes select an address dynamically(randomly selected) and send the selected address to the server. The server checks if the address is already in use and if so the server asks the respective node (which ever is newer) to select another address using redo message. If the address is unique then the node is confirmed about the address using assign message. This process continues until the node gets an unique address.

"An energy efficient node address naming scheme for WSN" provides procedures for cluster based sensor networks. Along with the above specified methods there are several other approaches for addressing in WSN, one can refer to the references in the above papers for information on other approaches.

Save Energy - The Buzz Word in Sensor Network

2010-03-05T06:57:00.000-08:00

WSN(Wireless Sensor Network)applications use sensor nodes which are mostly powered using a 3 V battery.Sensor networks are supposed to last at least for six months ,hence the available energy needs to be used very prudently. This is similar to usage of available financial sources during recession,every penny needs to be spent after much thought. Lot of research has been done and is being done in the field of optimal usage of energy in sensor networks.

I will be briefing about some of these energy saving methods in WSN. All different procedures and methods given here are from papers or patents which I have come across.

Dynamic Power Management(DPM) specifies five modes of operations for each sensor node.Each mode consumes different amount of power. The mode of operation of sensor node varies from being fully active to deep sleep state. Maximum energy is consumed in fully active state and least is consumed in deep sleep state. A method called as Dynamic Voltage Scaling (DVS) is also proposed , the operating frequency and operating voltage is varied according to the sensor node's mode. The components in sensor node such as transceiver , processor, sensors are dynamically switched off or put to sleep in different modes. By these dynamic changes in the sensor nodes energy conservation achieved in the sensor nodes. The table below summarizes the different sates of the sensor node.

State Processor Memory Sensor Radio
S0 Active Active On Tx, Rx
S1 Idle Sleep On Rx
S2 Sleep Sleep On Rx
S3 Sleep Sleep On Off
S4 Sleep Sleep Off Off

After DPM lets see how a protocol called "LEACH" to uses energy optimally. LEACH is one of the well known protocol in WSN. In this protocol data aggregation is done at Cluster heads before further forwarding data , this in turn reduces the amount of data being transferred and saves lot of energy, which would be otherwise consumed for transmission and reception of the un-aggregated data. In WSN data transmission or data reception is the most energy consuming activity compared any other activity so data aggregation at various levels helps in conservation of energy. LEACH also saves energy by making all nodes(other than cluster head) go to sleep when they do not having anything to communicate. All nodes are allocated different time slots to communicate by the cluster head hence the nodes are awake only during their time slot. The figure below shows Cluster Heads, normal nodes and Sink in LEACH protocol.

In paper titled "Power Efficient Organization of Wireless Sensor Network" authors try to use only the optimal number of sensor nodes from all available sensor nodes to cover a given area. By this unnecessary sensor nodes are made to sleep and unnecessary redundant data is avoided and also one can rotate between sleeping and active sensor nodes.

In paper tilted " Power efficient Topologies for Wireless Sensor Networks" authors propose a method to find a sensor network topology which consumes least power to cover given area, in other words the sensor nodes are placed such that they cover the given area by utilizing minimum number of sensor nodes.

There are some more protocols such as S-MAC, STEM, SMACS which try use to available energy in best possible way , if interested one can find them in Google. As I told earlier the efficient usage of energy in one of the most important and one of most researched topics in Wireless Sensor Network, hence one can find many articles, papers and patents on the same.

Wireless Sensor Networks - A New Network Paradigm

2009-12-23T05:18:00.000-08:00

Welcome to the world of sensor networks , where small is big and unity is strength. In this fascinating domain of sensors one needs to think at micro level rather than at the macro. The designer of this network needs to think one hundred times before he decides to introduce communication of few bytes of data. Even a three volt battery is considered to be considerably huge source of energy to a sensor node., and the sensor node itself is as big as a dollar. Wireless sensor network collects data utilizing a group of tiny sensor nodes and here the data as a whole is more important than the individual sensor node collecting the . Hence it is appropriate to say that "In sensor network small is big and unity is strength ".

Wireless sensor network consists of a group of tiny sensor nodes, distributed in a wide geographic area, forming an ad-hoc network, collecting and conveying information regarding the area under surveillance. The data collected by these sensor nodes is aggregated and analyzed at a more capable node called as sink or gateway or base station. Wireless sensor network combine simple wireless communication,minimal computation facilities and some sort of sensing of the physical environment and this leads to a new paradigm of networks that can be deeply embedded in our physical environment,fueled by the low cost and wireless communication facilities.

Wireless sensor networks have many wide variety of applications . Some of the examples being

Military Applications : In battlefield sensor network can be used to monitor enemy movements, to monitor various defense equipments. to detect biological and chemical attacks and to evaluate damage caused by the war.

Environmental Applications : Sensor networks can be used, to monitor habitats, to monitor forest fire, to monitor the pollution caused, to monitor weather conditions etc...

Traffic Surveillance: WSN can be used to monitor traffic on land or water or in air.

Industrial Applications: WSN can be used to monitor equipments, processes, conditions inside a factory and also can be used to detect leakage of gas/chemicals etc..

Home Applications: The applications at home that can be built using WSN range form home automation to intruder detection.

Health Applications: Another branch called Body Area Networks is carved specifically to concentrate on health applications using senor network.

Precision Agriculture: Examples for this include monitoring of fertility and humidity in soil using sensor network.

Each wireless sensor network needs to be designed according to the applications for which it will be used. The number of nodes present in the network, the redundancy required, the precision necessary, the periodicity of collection of data all depends on the application to which the sensor network is used.Wireless sensor networks are self organizing, autonomous, fault tolerant,scalable, long lasting and are able to survive with less energy consumption and are generally low cost.

A typical sensor node in a WSN is made up of, a micro controller such as MSP430FG4618 which is specially designed for low power consumption , a transceiver such as CC2420 which works in ISM band, sufficient memory,power supply and sensors such as temperature sensor, photo sensor ,IR sensor .

If we see from software perspective then a sensor node has necessary drivers, a tiny operating system such as TinyOS, MAC layer modules such as 802.15.4 or S-MAC or STEM , Network Layer modules such as Zigbee Specification or LEACH or gossiping or SPIN , Security modules for data confidentiality, authentication , data integrity and Applications such as data aggregation apps, in node processing apps, senor collaboration module .

Sensor network was considered among top ten technologies in the world. Sensor network is also one of researched topic in many leading universities and companies. Research mainly concentrates on reducing power consumption, lighter protocols, self organization , autonomous operation , lighter OS. Time is not far when many of our day today applications will be built and powered using sensor networks.

Note: I have refered several books, articles , IEEE papers to write all the artciles on sensor networks . I have tried to explain my understanding of sensor networks through these articles in my blog. In some cases I have used the orginal author's examples and diagrams. I am thankful to all the orginal authors.