secure systems research group - fau a pattern for a sensor node 11/12/2009 anupama sahu

Secure Systems Research Group - FAU

A Pattern for A Sensor Node

11/12/2009

Anupama Sahu


Introduction

Sensors are widely used in specific and day to day applications. A Wireless Sensor Network is a deployment of massive number of sensors. In order to design a Wireless Sensor Network, it is important to know the architecture of a sensor node or a mote as it is also called.

A sensor node is made up of various components that communicate and interact with each other. A sensor node also communicates with other nodes in order to gather information or forward information about the physical environment.


Outline

Intent Example Context Problem Forces Solution Block Diagram of Sensor

node Sensor Node Class Diagram Sensing Sequence Diagram

Forwarding Sequence Diagram

Example Resolved Consequences Known uses Variants Related patterns References


Intent.

This pattern describes how to sense, monitor and gather local information about a physical environment and thus make global decisions about it.


Example

We have an oil refinery where crude oil is processed and refined into more useful petroleum products.

We have accidents wherein our pipes have burst due to excessive pressure and temperature.

We are unable to monitor constantly the temperature in our chemical processing units which has led to small fires in the past.

If we continue like this we would incur huge losses due to accidents and would also endanger the lives of our employees.


Context

The oil refinery is a huge complex with large no. of pipes running throughout and carrying streams of fluids between large chemical processing units.

The fluids flowing in the pipes are at high pressure and temperature and should be maintained at a suitable pressure and temperature in order to avoid accidents.

The chemical processing units operate at different temperatures

depending on the type of product being extracted from crude oil. Most of the operations in the refinery are automated since it could be hazardous for the employees to work in an environment of high temperature and explosive fluids.


Problem

We need to stop or minimize the bursting of oil pipes and fire accidents in the refinery.

In order to do so we should be able to know instantly if there is an abnormal increase of pressure and temperature in the pipes or the chemical processing units so that we can take immediate steps to prevent accidents.

We should have some mechanism which can automatically activate preventive measures without human intervention

This is only possible if we have a device that can sense, detect and monitor some attributes of the physical environment like pressure, temperature, light and smoke.


Forces

The possible solution is constrained by the following forces:

We should have a device that can sense and collect local information about its surrounding environment like temperature, pressure, light, smoke, humidity and acoustic.The device should be able to communicate with other devices in order to gather or forward information about the physical environment. The device should have sufficient memory in order to store the collected data. It should also be able to process the collected data, perform simple computations and transmit only the required data and routing information. The device should be robust and be able to operate in harsh conditions.


Forces The device should be able to operate autonomously and

unattended. The device should be small in size so that it can be easily

deployed in large numbers. The device should be energy efficient and capable of operating

on battery power for a reasonably long time in spite of its routing and computational capabilities.

The device should be able to support actuators such as switches, servos, and motors that would allow it to control various actions.

The architecture of the device should be extensible so that any new component can be added if required.


Solution

A sensor is such a device that can sense, compute and communicate with other devices for the purpose of gathering local information to make global decisions about a physical environment.

A sensor is mounted on a sensor board or a mote. The sensor board has other components like the processing unit, the transceiver unit and the power unit. All these components together make a sensor node.

A sensor node acts both as a data collector and a data router. So it can monitor a physical environment and broadcast information either periodically or in case of an event.

A sensor node can have various other components depending on the type of sensor it has like light or pressure or humidity.


Block Diagram of a Sensor Node


Components of the Sensor Node

The block diagram[Aky02] shows a single sensor and ADC in the sensing unit but it can also have multiple and different kinds of sensors sharing the same ADC. The main function of the ADC is to convert the analog data to digital format so that it can be further processed and forwarded to other nodes.

The Processing unit consists of the processor which is responsible for managing and coordinating the various activities of the sensor node. It generates control messages that direct the sensor to start or stop collecting information about the environment as and when required. It also directs the transceiver to be either in the receiving mode or transmitting mode depending upon the scenario.


Components of the Sensor Node

Processing unit keeps information about its neighboring nodes, decides the routing path and communicates the routing information to the other nodes. The processor also stores the collected data in the main memory or the flash memory until it is forwarded to the next node.

The Power unit supplies power to the sensing unit, processing unit and the transceiver.

Apart from the four basic components the sensor node might also have some additional application specific components. It can have a Location finding system or GPS to keep a track of its location and of its neighbors. It can have a mobilizer which can help the sensor to move short distances if required, or it can have a power generator which supplies continuous power to a node having some additional functionality.


Structure

The class diagram shows the PowerUnit supplying power to the SensingUnit, ProcessingUnit and the Transceiver.

A sensor can sense various attributes like temperature, humidity, light, pressure and many others. It can either measure a single component or a combination of them.

These measurements are in analog format and the ADC converts them to the digital format. The Processor runs various data gathering and routing protocols. It partially processes the gathered data and then either stores it in the storage memory for later use or forwards it to the Transceiver.

The Transceiver transmits and receives control and data messages. It routes the data from the source to the destination either in a single hop or in multiple hops.


Class Diagram for Sensor Node


Dynamics

We describe the dynamic aspects of the sensor node using a sequence diagram for its sensing use case.

The Processor sends control message to enable the sensor The Sensor starts collecting information about the physical

world. The Sensor reads the information in the analog form. The

information which is in the analog format is sent to the ADC The ADC converts the analog information into the digital format

and sends it to the processor The Processor processes the data locally and sends it to the

transceiver and/or storage


Sensing Sequence Diagram



The Processor stores routing tables and runs various routing protocols in order to route the data from one node to the other.

The Processor sends message to the Transceiver which then routes the data to the Transceiver of the neighboring node.

The Transceiver of the neighboring node forwards the data to its Processor.

The Processor processes together the received data as well as its own data and sends it back to the Transceiver.

The Transceiver might go ahead and send this data to the Transceiver of another sensor node.

Sometimes the Processor might locally store small amount of data until it is ready to be routed


Implementation

The components in a sensor node should have some required specifications.

To build a sensor node, some of the sensor boards that could be used are MTS310, MTS420, MTS400,MDA300, MDA100 and various others.

The transceiver used should be IEEE 802.15.4 compliant and/or Zigbee compliant.

The operating systems that could be used are TinyOS, SOS, LiteOS


Example Resolved

We are able to minimize the bursting of oil pipes and fire accidents in the refinery because of the deployment of sensors in the pipes and chemical processing units. The sensors constantly monitor and report instantly if there is an abnormal increase of pressure and temperature in the pipes or the chemical processing units. The sensors also start or activate water sprinklers in case of a fire.


Consequences

The sensor node pattern has the following advantages:

The design of a sensor node is simple and easy to extend or modify.Sensor nodes are easy to deploy and can be deployed in hard to reach places. They can operate in harsh conditions. They are autonomous in nature.Low maintenanceCan monitor and control the physical location remotely

The sensor node pattern has the following disadvantages:

They have limited resources in terms of processor, radio, memory and battery life.


Known Uses

The architecture of the sensor node described in this pattern corresponds to the architecture of the Berkeley Motes manufactured by Crossbow Technologies.

It also corresponds to the architecture of IPsensor Node manufactured by Arch Rock.

Squidbee Mote manufactured by Libelium also has the same architecture .


Conclusion

A Sensor node consists of four basic components but these components could be of different types as long as they fall within the specifications range and are compatible with the other supporting devices.

Thus knowing the architecture of the sensor node helps us in choosing the type of node we require for our application.


References

• [Aky02] I. A. Akyildiz et. al., “A Survey on Sensor Networks”, IEEE Communications Magazine, Aug. 2002.

• [Bos09] Raja Bose, “Sensor Networks- Motes, Smart Spaces, and Beyond” Proc. IEEE, 2009

• [Bus96] F. Buschmann, R. Meunier, H. Rohnert, P. Sommerland, and M. Stal., Pattern- oriented software architecture, Wiley 1996.

• [Cho03] C. -Y. Chong and S.P. Kumar, "Sensor networks: Evolution, opportunities, and challenges," Proc. IEEE, Aug. 2003.

• [Est99] D. Estrin, R. Govindan, J. Heidermann, S. Humar, “Next Century Challenges: Scalable Coordination in Sensor Networks”, Mobicom 1999.

• http://en.wikipedia.org/wiki/Micaz


References

• http://www.dustnetworks.com/technology• http://www.scatterweb.de/content/products/node_en.html• http://www.automation.com/content/moteiv-launches-tmote-

mini-wireless-sensor-network-node-mote


Questions?


Querying Sequence Diagram

secure systems research group - fau a pattern for a sensor node 11/12/2009 anupama sahu

Documents

physical environment

oil refinery

forward information

high pressure

local information

wireless sensor network

bursting of oil pipes

surrounding environment