A Discrete-Event Network Simulator
Models

Energy Framework

Energy consumption is a key issue for wireless devices, and wireless network researchers often need to investigate the energy consumption at a node or in the overall network while running network simulations in ns-3. This requires ns-3 to support energy consumption modeling. Further, as concepts such as fuel cells and energy scavenging are becoming viable for low power wireless devices, incorporating the effect of these emerging technologies into simulations requires support for modeling diverse energy sources in ns-3. The ns-3 Energy Framework provides the basis for energy consumption, energy source and energy harvesting modeling.

Model Description

The source code for the Energy Framework is currently at: src/energy.

Design

The ns-3 Energy Framework is composed of 3 parts: Energy Source, Device Energy Model and Energy Harvester. The framework is implemented into the src/energy/models folder.

Energy Source

The Energy Source represents the power supply on each node. A node can have one or more energy sources, and each energy source can be connected to multiple device energy models. Connecting an energy source to a device energy model implies that the corresponding device draws power from the source. The basic functionality of the Energy Source is to provide energy for devices on the node. When energy is completely drained from the Energy Source, it notifies the devices on node such that each device can react to this event. Further, each node can access the Energy Source Objects for information such as remaining energy or energy fraction (battery level). This enables the implementation of energy aware protocols in ns-3.

In order to model a wide range of power supplies such as batteries, the Energy Source must be able to capture characteristics of these supplies. There are 2 important characteristics or effects related to practical batteries:

Rate Capacity Effect
Decrease of battery lifetime when the current draw is higher than the rated value of the battery.
Recovery Effect
Increase of battery lifetime when the battery is alternating between discharge and idle states.

In order to incorporate the Rate Capacity Effect, the Energy Source uses current draw from all the devices on the same node to calculate energy consumption. Moreover, multiple Energy Harvesters can be connected to the Energy Source in order to replenish its energy. The Energy Source periodically polls all the devices and energy harvesters on the same node to calculate the total current drain and hence the energy consumption. When a device changes state, its corresponding Device Energy Model will notify the Energy Source of this change and new total current draw will be calculated. Similarly, every Energy Harvester update triggers an update to the connected Energy Source.

The Energy Source base class keeps a list of devices (Device Energy Model objects) and energy harvesters (Energy Harvester objects) that are using the particular Energy Source as power supply. When energy is completely drained, the Energy Source will notify all devices on this list. Each device can then handle this event independently, based on the desired behavior that should be followed in case of power outage.

Device Energy Model

The Device Energy Model is the energy consumption model of a device installed on the node. It is designed to be a state based model where each device is assumed to have a number of states, and each state is associated with a power consumption value. Whenever the state of the device changes, the corresponding Device Energy Model will notify the Energy Source of the new current draw of the device. The Energy Source will then calculate the new total current draw and update the remaining energy.

The Device Energy Model can also be used for devices that do not have finite number of states. For example, in an electric vehicle, the current draw of the motor is determined by its speed. Since the vehicle’s speed can take continuous values within a certain range, it is infeasible to define a set of discrete states of operation. However, by converting the speed value into current directly, the same set of Device Energy Model APIs can still be used.

Energy Harvester

The energy harvester represents the elements that harvest energy from the environment and recharge the Energy Source to which it is connected. The energy harvester includes the complete implementation of the actual energy harvesting device (e.g., a solar panel) and the environment (e.g., the solar radiation). This means that in implementing an energy harvester, the energy contribution of the environment and the additional energy requirements of the energy harvesting device such as the conversion efficiency and the internal power consumption of the device needs to be jointly modeled.

WiFi Radio Energy Model

The WiFi Radio Energy Model is the energy consumption model of a Wifi net device. It provides a state for each of the available states of the PHY layer: Idle, CcaBusy, Tx, Rx, ChannelSwitch, Sleep, Off. Each of such states is associated with a value (in Ampere) of the current draw (see below for the corresponding attribute names). A Wifi Radio Energy Model PHY Listener is registered to the Wifi PHY in order to be notified of every Wifi PHY state transition. At every transition, the energy consumed in the previous state is computed and the energy source is notified in order to update its remaining energy.

The Wifi Tx Current Model gives the possibility to compute the current draw in the transmit state as a function of the nominal tx power (in dBm), as observed in several experimental measurements. To this purpose, the Wifi Radio Energy Model PHY Listener is notified of the nominal tx power used to transmit the current frame and passes such a value to the Wifi Tx Current Model which takes care of updating the current draw in the Tx state. Hence, the energy consumption is correctly computed even if the Wifi Remote Station Manager performs per-frame power control. Currently, a Linear Wifi Tx Current Model is implemented which computes the tx current as a linear function (according to parameters that can be specified by the user) of the nominal tx power in dBm.

The Wifi Radio Energy Model offers the possibility to specify a callback that is invoked when the energy source is depleted. If such a callback is not specified when the Wifi Radio Energy Model Helper is used to install the model on a device, a callback is implicitly made so that the Wifi PHY is put in the OFF mode (hence no frame is transmitted nor received afterwards) when the energy source is depleted. Likewise, it is possible to specify a callback that is invoked when the energy source is recharged (which might occur in case an energy harvester is connected to the energy source). If such a callback is not specified when the Wifi Radio Energy Model Helper is used to install the model on a device, a callback is implicitly made so that the Wifi PHY is resumed from the OFF mode when the energy source is recharged.

Future Work

For Device Energy Models, we are planning to include support for other PHY layer models provided in ns-3 such as WiMAX, and to model the energy consumptions of other non communicating devices, like a generic sensor and a CPU. For Energy Sources, we are planning to included new types of Energy Sources such as Supercapacitor and Nickel-Metal Hydride (Ni-MH) battery. For the Energy Harvesters, we are planning to implement an energy harvester that recharges the energy sources according to the power levels defined in a user customizable dataset of real measurements.

References

[1]ns-2 Energy model: http://www.cubinlab.ee.unimelb.edu.au/~jrid/Docs/Manuel-NS2/node204.html
[2]H. Wu, S. Nabar and R. Poovendran. An Energy Framework for the Network Simulator 3 (ns-3).
[3]M. Handy and D. Timmermann. Simulation of mobile wireless networks with accurate modelling of non-linear battery effects. In Proc. of Applied simulation and Modeling (ASM), 2003.
[4]D. N. Rakhmatov and S. B. Vrudhula. An analytical high-level battery model for use in energy management of portable electronic systems. In Proc. of IEEE/ACM International Conference on Computer Aided Design (ICCAD‘01), pages 488-493, November 2001.
[5]D. N. Rakhmatov, S. B. Vrudhula, and D. A. Wallach. Battery lifetime prediction for energy-aware computing. In Proc. of the 2002 International Symposium on Low Power Electronics and Design (ISLPED‘02), pages 154-159, 2002.
[6]C. Tapparello, H. Ayatollahi and W. Heinzelman. Extending the Energy Framework for Network Simulator 3 (ns-3). Workshop on ns-3 (WNS3), Poster Session, Atlanta, GA, USA. May, 2014.
[7]C. Tapparello, H. Ayatollahi and W. Heinzelman. Energy Harvesting Framework for Network Simulator 3 (ns-3). 2nd International Workshop on Energy Neutral Sensing Systems (ENSsys), Memphis, TN, USA. November 6, 2014.

Usage

The main way that ns-3 users will typically interact with the Energy Framework is through the helper API and through the publicly visible attributes of the framework. The helper API is defined in src/energy/helper/*.h.

In order to use the energy framework, the user must install an Energy Source for the node of interest, the corresponding Device Energy Model for the network devices and, if necessary, the one or more Energy Harvester. Energy Source (objects) are aggregated onto each node by the Energy Source Helper. In order to allow multiple energy sources per node, we aggregate an Energy Source Container rather than directly aggregating a source object.

The Energy Source object keeps a list of Device Energy Model and Energy Harvester objects using the source as power supply. Device Energy Model objects are installed onto the Energy Source by the Device Energy Model Helper, while Energy Harvester object are installed by the Energy Harvester Helper. User can access the Device Energy Model objects through the Energy Source object to obtain energy consumption information of individual devices. Moreover, the user can access to the Energy Harvester objects in order to gather information regarding the current harvestable power and the total energy harvested by the harvester.

Examples

The example directories, src/examples/energy and examples/energy, contain some basic code that shows how to set up the framework.

Helpers

Energy Source Helper

Base helper class for Energy Source objects, this helper Aggregates Energy Source object onto a node. Child implementation of this class creates the actual Energy Source object.

Device Energy Model Helper

Base helper class for Device Energy Model objects, this helper attaches Device Energy Model objects onto Energy Source objects. Child implementation of this class creates the actual Device Energy Model object.

Energy Harvesting Helper

Base helper class for Energy Harvester objects, this helper attaches Energy Harvester objects onto Energy Source objects. Child implementation of this class creates the actual Energy Harvester object.

Attributes

Attributes differ between Energy Sources, Devices Energy Models and Energy Harvesters implementations, please look at the specific child class for details.

Basic Energy Source

  • BasicEnergySourceInitialEnergyJ: Initial energy stored in basic energy source.
  • BasicEnergySupplyVoltageV: Initial supply voltage for basic energy source.
  • PeriodicEnergyUpdateInterval: Time between two consecutive periodic energy updates.

RV Battery Model

  • RvBatteryModelPeriodicEnergyUpdateInterval: RV battery model sampling interval.
  • RvBatteryModelOpenCircuitVoltage: RV battery model open circuit voltage.
  • RvBatteryModelCutoffVoltage: RV battery model cutoff voltage.
  • RvBatteryModelAlphaValue: RV battery model alpha value.
  • RvBatteryModelBetaValue: RV battery model beta value.
  • RvBatteryModelNumOfTerms: The number of terms of the infinite sum for estimating battery level.

WiFi Radio Energy Model

  • IdleCurrentA: The default radio Idle current in Ampere.
  • CcaBusyCurrentA: The default radio CCA Busy State current in Ampere.
  • TxCurrentA: The radio Tx current in Ampere.
  • RxCurrentA: The radio Rx current in Ampere.
  • SwitchingCurrentA: The default radio Channel Switch current in Ampere.
  • SleepCurrentA: The radio Sleep current in Ampere.
  • TxCurrentModel: A pointer to the attached tx current model.

Basic Energy Harvester

  • PeriodicHarvestedPowerUpdateInterval: Time between two consecutive periodic updates of the harvested power.
  • HarvestablePower: Random variables that represents the amount of power that is provided by the energy harvester.

Tracing

Traced values differ between Energy Sources, Devices Energy Models and Energy Harvesters implementations, please look at the specific child class for details.

Basic Energy Source

  • RemainingEnergy: Remaining energy at BasicEnergySource.

RV Battery Model

  • RvBatteryModelBatteryLevel: RV battery model battery level.
  • RvBatteryModelBatteryLifetime: RV battery model battery lifetime.

WiFi Radio Energy Model

  • TotalEnergyConsumption: Total energy consumption of the radio device.

Basic Energy Harvester

  • HarvestedPower: Current power provided by the BasicEnergyHarvester.
  • TotalEnergyHarvested: Total energy harvested by the BasicEnergyHarvester.

Validation

Comparison of the Energy Framework against actual devices have not been performed. Current implementation of the Energy Framework is checked numerically for computation errors. The RV battery model is validated by comparing results with what was presented in the original RV battery model paper.