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DESCRIPTIONwireless sensor networks using different simulators
IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 7, NO. 4, NOVEMBER 2011 601
Power-Aware System Design of WirelessSensor Networks: Power Estimation and
Power Profiling StrategiesJan Haase, Senior Member, IEEE, Javier Moreno Molina, Member, IEEE, and Dietmar Dietrich, Senior Member, IEEE
AbstractModern design of wireless devices requires the de-signers to have a special focus on power consumption to prolongthe battery life of the final system. The designer therefore needspower consumption information very early in the process to beable to decide on system parameters, design methods, communica-tion protocols, functionality restrictions. Typically, this is done byrunning simulations of the system to be developed and performingdesign space exploration. However, there is a tradeoff betweenspeed and accuracy of simulation, therefore the designer has tobe aware of available tools and simulation methods he can choosefrom to achieve the best possible solution for his case.
This paper gives an overview over the currently existing simu-lators and simulation methods for fast system simulation with re-spect to power consumption and concludes with a vision of futuresimulator opportunities.
Index TermsPower estimation, power profiling, power-awaresystems, simulation, simulator, wireless sensor network (WSN).
T HE TYPICAL wireless device of today is from one oftwo classes. There are portable consumer products, small,easy to use, fun to use, smart helpers in daily life. Examples aremobile phones, MP3 players, portable electronic games, GPSdevices, etc. The second class are wireless sensors in cars, air-planes, building automation or even at every single item found ina building or private homethe vision of the internet of things.
As the device is wireless, it has to run on battery power. Thegrowing number of features of todays consumer products needmore and more power, so that battery lifetimes shrink consider-ablye.g., many mobile phones have to be recharged every day. In the area of sensor networks, sensors are part of a wire-less network using up much energy for sending and receivingmessages . Typical examples are wireless light switches inmodern buildings, or pressure monitors in car tires, which haveto be able to send information about suddenly changing tirepressures. A low battery may render a safety device like thisinoperable and thus be dangerous. Some of these devices are in-stalled at remote or hard to reach places and hence cannot be
Manuscript received April 13, 2011; revised May 13, 2011; accepted July18, 2011. Date of publication September 06, 2011; date of current versionNovember 09, 2011. Paper no. TII-11-220.
The authors are with the Institute of Computer Technology, Vienna Universityof Technology, Vienna 1040, Austria (e-mail: email@example.com).
Digital Object Identifier 10.1109/TII.2011.2166793
recharged easily. Energy efficiency is therefore very importantin Wireless Sensor Networks (WSNs) .
To tackle the problem of short battery lifetimes, system de-signers take the power consumption of the devices into accountalready at the time when they are developed. One approach is toinclude power saving features at run time like automatic shut-down of displays or clock gating to switch off unused parts of thedevice . Another way is the estimation of power consumptionfor different sets of system parameters (e.g., duty cycles, com-munication protocols, bus widths, or processor types) in order tochoose the design solution having the lowest energy consump-tion. This is mainly done by means of simulation, which has tobe performed anyway to assure the correct function of the de-vice. However, simulation is in most cases slow, i.e., in order tosimulate all details of a system, the run time of the simulationis often higher by orders of magnitude than the run time of thefinal device itself . This slowdown is further increased by in-cluding power consumption analyses into the simulation.
This paper gives an overview over the existing solutionsfor fast system simulation with respect to power consumption.Section II explains the new dimension acquired by poweraware and low-power approaches in the case of WSNs designcompared to those in other embedded systems. Section IIIportrays the different motivations for simulation in WSNs,which are the foundations for WSN simulators presentedin Sections IVVI. Section VII focuses in power simula-tion, describing the required tasks. Section VIII introducesstate-of-the-art approaches and tools for power simulation andprofiling. This paper concludes in Section IX.
II. POWER CONSUMPTION IN WIRELESS SENSOR NETWORKS(WSNS)
The first concept that shall be clarified, in order to understandpower concerned design is the difference between low-powerand power-aware design. The former consists on consuming asless power as possible in every moment, no matter the long-term efficiency of the decision made. The latter tries to reach thehighest efficiency, in terms of the metric the designer is tryingto optimize, for a specific power budget. This distinction wasalready defined by Pedram et al. . Even though both strategieshave a lot in common, sometimes decisions to make may differfrom one to the other.
The essential starting point on the way to power optimiza-tion, is gathering knowledge about power consumption of the
1551-3203/$26.00 2011 IEEE
602 IEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 7, NO. 4, NOVEMBER 2011
system. In traditional embedded systems, this can be done bytracking energy consumption values of different subsystems.There is already, however, a semantic gap between hardwarepower consumption and software power consumption. Manysoftware tasks involve several subsystems, and therefore profilesare required in order to sum all the related energy consumption.These profiles enable identifying the contribution of softwareparts to power consumption. Hence, the designer can focus onoptimization of those tasks whose impact in power consumptionis more significant.
Software profiles are an old concept, that emerged due toprogram complexity. Complex programs involve many rou-tines, processes and threads and therefore, information aboutresources utilization is difficult to obtain . Rialto softwareprofiler  classified different threads into so called activi-ties. By grouping threads into coherent semantic categories,they could be fairly and efficiently scheduled in the CPU.
In WSNs, the same concept is needed, although oriented topower consumption. This kind of profiling has already beenported to WSN in AEON , Quanto , and SNOPS .
Nonetheless, WSNs cannot be just evaluated as traditionalembedded systems, even those with network connectivity. Inthose cases, the power efficiency which concerned the designerwas always within the boundaries of the embedded system.However, WSN must usually be considered as a whole singledistributed system.
One of the main characteristics of most WSNs is redundancyand fault tolerance. Maximizing a node lifetime can be com-pletely different from maximizing the network lifetime, as a net-work may be completely functional in spite of having nodes withdepleted batteries. Therefore, not only the node local knowledgeis required for power optimization, but a global network-wideknowledge as well.
Thus, in addition to hardware and software profiles, commu-nication and network-wide profiles are crucial in order to beable to optimize the network as a whole . Network profilespermit evaluating and improving protocols, topologies, appli-cations and even hardware architectures, as some requirementscan be tuned finer after evaluating the network behavior.
III. WSNS SIMULATION
The need of a whole network model to evaluate applicationsand algorithms was one of the first motivations for simulation.
Simulation is a useful technique that assists the designer atmany different stages and levels of the system development.
A preliminary simulation is one of the most valuable sourcesof information to make the best decisions when designing yoursystem architecture. High-level simulation, through behavioralor functional models, provides a good overview of the systemeven at the very beginning of the project.
Virtual prototyping, provides reliable data about the systemoperation and can be used by the designer to test differenthardware architectures before producing the real prototype.Emulation permits executing nodes source code or instructionsin a different machine and is usually combined with simulators.Although accurate models are preferred, even the notion of thesystem behavior provided by rough simulation models with
coarse granularity can assist the designer to get an idea aboutwhat components have the most significant impact in overallpower consumption.
Besides, in systems with many nodes, testing the networkbehavior of different protocols implementation in the protocolstack would be very expensive. Furthermore, even when propa-gation conditions are difficult to simulate, special conditions areeasier to model on simulation rather than recreating them on atest platform.
Simulation is also a very powerful tool for software optimiza-tion. Although software can easily be tested on a real device, andthere are profiling add-ons in operating systems , doing iton simulation permits gathering network-wide knowledge aboutresources utilization. Operating system emulators execute realcode applications, making possible the development of softwareat the same time as hardware and independent from it, once theoperating system is fixed.
Hence, simulation helps with all hardware, software and net-work design. In WSNs, all these levels are strongly coupled, andeven when focusing in one of them, the others sh