Sensor node

Sensor Node Architecture

Sensor Node, is similar to a computer node that is capable of doing little processing, sense and communicate with other sensor nodes in the sensor network. A mote is a common term given to a wireless sensor node that represents a single node in a wireless sensor network. Motes combine processing, sensing and wireless networking into a single tiny package. Typical architecture of the sensor node is shown in figure.

History of Sensor Node

File:MICAz.jpg

MicaZ Commerical Sensor node from CrossBow Technologies

History of development of sensor nodes dates back to 1998 in Smartdust project ^[1]. 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 few more research projects. They are Berkely NEST ^[2] and CENS ^[3]. 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. Microcontrollers 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 an 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 software.

Transceiver

Main Article: 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 433MHz and 2.4GHz. 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 ^[4]. 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 1Kb a distance of 100m 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

Main article: 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 0.5Ah 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 c) 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 gatewat node that interfaces sensor network to the external world. More details can be found at ^[6] and ^[7].

List of Sensor Nodes

The following table lists out the prototype and commerical motes/sensor nodes available.

List of Sensor Nodes or motes available
Sensor Node Name	Microcontroller	Tranceiver	Program Memory	External Memory	Programming	Remarks
COTS
weC	Atmel AVR AT90S2313	RFM TR1000 RF
Rene	ATMEL8535	916 Mhz radio with bandwidth of 10 Kbps	512MB RAM	8K Flash		TinyOS Support
Dot	ATMEGA163		1K RAM	8-16K Flash	weC
Mica
Mica2	ATMEGA 128L	Chipcon 868/916 MHz	4K RAM	128K Flash
Mica2Dot	ATMEGA 128		4K RAM	128K Flash
MicaZ	ATMEGA 128	802.15.4/ZigBee compliant RF transceiver	4K RAM	128K Flash	nesC	TinyOS Support
Telos	Motorola HCS08		4K RAM
T-Mote Sky	Texas Instruments MSP430 microcontroller	250kbps 2.4GHz IEEE 802.15.4 Chipcon Wireless Transceiver	10k RAM	48k Flash		TinyOS Support
XYZ	ML67 series ARM/THUMB microcontroller	CC2420 Zigbee compliant radio from Chipcon	32K RAM	256K Flash	C Programming	SOS Operating System Support
Nymph
BTNode	Atmel ATmega 128L (8 MHz @ 8 MIPS)	Chipcon CC1000 (433-915 MHz)	64+180 K RAM	128K FLASH ROM, 4K EEPROM	C Programming	TinyOS compatible
WINS
IMote	ARM core 12MHZ	Bluetooth with the range of 30 m	64K SRAM	512K Flash		TinyOS Support
IMote 1.0	ARM 7TDMI 12-48 MHZ	Bluetooth with the range of 30 m	64K SRAM	512K Flash		TinyOS Support

List of Gateway Sensor Nodes

List of Gateway Nodes
	Microcontroller	Tranceiver	Program Memory	External Memory
Stargate	IntelPXA255	802.11 and serial connection to WSN	64MB SDRAM	32MB Flash

References

^ http://robotics.eecs.berkeley.edu/~pister/SmartDust/
^ http://webs.cs.berkeley.edu/
^ http://research.cens.ucla.edu/
^ Y. Xu, J. Heidemann, and D. Estrin, Geography-informed energy conservation for ad-hoc routing, in Proc. Mobicom, 2001, pp. 70-84
^ Dynamic Power Management in Wireless Sensor Networks, Amit Sinha and Anantha Chandrakasan, IEEE Design & Test of Computers, Vol. 18, No. 2, March-April 2001
^ http://www.btnode.ethz.ch/Projects/SensorNetworkMuseum
^ http://www.cse.unsw.edu.au/~sensar/hardware/hardware_survey.html

Topics in Wireless Sensor Networks
Software				Hardware	Conferences/Journals	Standards
Operating Systems	Programming Languages	Middleware	Simulators	Hardware	Conferences/Journals	Standards
TinyOS, SOS, Contiki	nesC	TinyDB	TOSSIM,NS-2	Sun SPOT	Sensys, IPSN, EWSN,SECON, INSS	ZigBee

[1] ttp://robotics.eecs.berkeley.edu/~pister/SmartDust/

[2] ttp://webs.cs.berkeley.edu/

[3] ttp://research.cens.ucla.edu/

[4] Y. Xu, J. Heidemann, and D. Estrin, Geography-informed energy conservation for ad-hoc routing, in Proc. Mobicom, 2001, pp. 70-84

[5] Dynamic Power Management in Wireless Sensor Networks, Amit Sinha and Anantha Chandrakasan, IEEE Design & Test of Computers, Vol. 18, No. 2, March-April 2001

[6] ttp://www.btnode.ethz.ch/Projects/SensorNetworkMuseum

[7] ttp://www.cse.unsw.edu.au/~sensar/hardware/hardware_survey.html

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