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. 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 SLEEP 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 SLEEP 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.