Unit 4 -2 energy management in adhoc wireless network

ENERGY MANAGEMENT IN AD HOC
WIRELESS NETWORKS
• The nodes in an ad hoc wireless network are constrained by limited
battery power for their operation.
• energy management is an important issue in such networks.
• The use of multi-hop radio relaying requires a sufficient number of
relaying nodes to maintain the network connectivity
• Energy management deals with the process of managing energy
resources by means of controlling the battery discharge, adjusting the
transmission power, and scheduling of power sources so as to
increase the lifetime of the nodes of an ad hoc wireless network

NEED FOR ENERGY MANAGEMENT IN AD HOC
WIRELESS NETWORKS
• The main reasons for energy management in adhoc wireless networks are
• Limited energy reserve:Ad hoc wireless networks have very limited energy
resources.
• Advances in battery technologies have been negligible as compared to the
recent advances that have taken place in the field of mobile computing and
communication
• Difficulties in replacing the batteries: Sometimes it becomes very difficult
to replace or recharge the batteries.
• In situations such as battlefields, this is almost impossible.
• Hence, energy conservation is essential in such scenarios

WIRELESS NETWORKS
• Lack of central coordination
• Constraints on the battery source : Batteries tend to increase the size and
weight of a mobile node. Reducing the size of the battery results in less
capacity which, in turn, decreases the active lifespan of the node
• Selection of optimal transmission power: The transmission power selected
• determines the reachability of the nodes.
• The consumption of battery charge increases with an increase in the
transmission power.
• An optimal value for the transmission power decreases the interference
among nodes, which, in turn, increases the number of simultaneous
transmissions

WIRELESS NETWORKS
• Channel utilization: A reduction in the transmission power increases
frequency reuse, which leads to better channel reuse.
• Power control becomes very important for CDMA-based systems in
which the available bandwidth is shared among all the users
• power control is essential to maintain the required signal to
interference ratio (SIR) at the receiver and to increase the channel
reusability.

CLASSIFICATION OF ENERGY MANAGEMENT
SCHEMES
• Energy conservationcan be implemented using the
• Battery management schemes
• Transmission power management schemes
• System power management schemes
• Maximizing the life of an ad hoc wireless network requires an
understanding of the capabilities and the limitations of energy sources of
the nodes
• Increasing the capacity of the batteries can be achieved by taking into
consideration either the internal characteristics of the battery (battery
management) or by minimizing the activities that utilize the battery
capacity (power management)

SCHEMES
• The system power management approach can be further divided into
the following categories:
Device management schemes
Processor power management schemes

SCHEMES

SCHEMES
• BATTERY MANAGEMENT SCHEMES:
• Battery-driven systems are those systems which are designed taking into
consideration mainly the battery and its internal characteristics.
Overview of Battery Characteristics:
• A battery mainly consists of an anode, a cathode, an electrolyte medium,
and a case
• The anode is often a metal and the cathode a metallic oxide.
• The electrolyte is a salt solution that promotes the ion flow.
• The porous separator is used to prevent a short circuit between anode and
cathode by keeping them from touching one another.

Overview of Battery Characteristics:
• The battery is contained in a structural support (case) that provides
dimensional stability and a positive and a negative electrode for
discharging (or recharging) the cell
• The positive ions move from the anode toward the cathode through
the electrolyte medium and the electrons flow through the external
circuit.
• A number of separate electrochemical cells can also be combined
within the same case to create a battery.

Unit 4 -2 energy management in adhoc wireless network

• Battery technologies:
• The most popular rechargeable battery technologies developed over
the last two decades are comprised of nickel-cadmium, lithium ion,
nickel metal-hydride, reusable alkaline, and lithium polymer
• The main factors considered while designing a battery technology are
the energy density (the amount of energy stored per unit weight of
the battery), cycle life [the number of (re)charge cycles prior to
battery disposal], environmental impact, safety, cost, available supply
voltage, and charge/discharge characteristics

• Principles of battery discharge: A battery typically consists of an array
of one or more cells.
• the terms "battery" and "cell" are used interchangeably.
• The three main voltages that characterize a cell are:
(1) the open circuit voltage (Voc), that is, the initial voltage under a no-
load condition of a fully charged cell,
(2) the operating voltage (Vi), that is, the voltage under loaded
conditions
(3) the cut-off voltage (Vcut ) at which the cell is said to be discharged.
All the cells are defined by three main capacities

Theoretical capacity: The amount of active materials (the materials
that react chemically to produce electrical energy when the cell is
discharged and restored when the cell is charged) contained in the cell
refers to its theoretical capacity.
• A cell cannot exceed its theoretical capacity.
Nominal (standard) capacity: This corresponds to the capacity actually
available when discharged at a specific constant current. It is expressed
in ampere-hours.
Actual capacity: The energy delivered under a given load is said to be
the actual capacity of the cell. A cell may exceed the actual capacity but
not the theoretical capacity.

• The performance of a cell's discharge is measured using the following
parameters
Discharge time: The time elapsed when a fully charged cell reaches its
cut-off voltage and has to be replaced or recharged is called the
discharge time of the cell.
Specific power (energy): This is the power (energy) delivered by a fully
charged cell under a specified discharge current.
• It is expressed in watt-perkilogram (watt-hour-per-kilogram).
Discharge current: There are mainly two models of battery discharge:
constant current discharge and pulsed current discharge.
• In pulsed current discharge, the battery switches between short
discharge periods and idle periods (rest periods)

• the performance of the bipolar
lead-acid battery subjected to
a pulsed discharge current of
six current pulses.
• After each discharge, which
lasts for 3 ms, the cell was idled
for 22 ms during which no
recharging was allowed to take
place

Impact of discharge characteristics on battery
capacity:
• The important chemical processes that affect the battery characteristics
are
Diffusion process: When the battery is actively involved in discharging, that
is, at a non-zero current, the active materials move from the electrolyte
solution to the electrodes and are consumed at the electrode.
• If this current is above a threshold value called the limiting current, the
active materials get depleted very quickly
• as the current decreases, the concentration of the active materials around
the electrode drops.
• By increasing the rest time periods of the battery, longer lifetimes can be
achieved due to the recovery capacity effect

Impact of discharge characteristics on battery
capacity:
• Passivation process: The cell discharge is limited not only by the
diffusion process but also by a process called passivation, which
induces in the cell the precipitation of crystals which are produced by
the discharge due to the chemical reactions on the electrode.
• This phenomenon increases during higher current densities.
• Two important effects for battery's discharge properties
1 Rate capacity effect
2 Recovery capacity effect

Rate capacity effect: As the intensity of the discharge current
increases, an insoluble component develops between the inner and
outer surfaces of the cathode.
• The inner layer becomes inaccessible as a result of this phenomenon,
rendering the cell unusable even while a sizable amount of active
materials still exists.
• This effect depends on the actual capacity of the cell and the
discharge current.
Recovery capacity effect: This effect is concerned with the recovery of
charges under idle conditions.
• By increasing the idle time, one may be able to completely utilize the
theoretical capacity of the cell.

Battery models
• Battery models depict the characteristics of the batteries used in real
life .
• Supply voltage scaling
• Battery-aware task scheduling
• Dynamic power management

Battery models
Supply voltage scaling: An optimal value of supply voltage (vdd ) is
maintained, by means of scaling, that provides a balance between
battery charge consumption and performance
Battery-aware task scheduling: a battery-aware static scheduling
scheme that optimizes the discharge power of the batteries
• This is done as a two-step process. In the first step, the initial
schedule obtained is adjusted in order to reduce peak current
requirements.
• The second step consists of a local transformation which changes the
position of the scheduled events so as to minimize the delay and also
the energy drawn off the cell.

Battery models
Dynamic power management: Energy conservation can be achieved at
the nodes carrying multimedia traffic by a graceful degradation of the
quality of audio output when the battery is about to reach the
completely discharged state.

Device-Dependent Schemes
• some of the device dependent approaches that increase the battery
lifetime by exploiting its internal characteristics are
1. Modeling and Shaping of Battery Discharge Patterns
2. Effect of Battery Pulsed Discharge
3. Binary Pulsed Discharge
4. Generalized Pulsed Discharge
5. Battery-Scheduling Techniques

Modeling and Shaping of Battery Discharge Patterns
The stochastic model of the discharge pattern of batteries
introduced in employs the following two key aspects affecting the
battery life: the rate capacity effect and the recovery effect.
Effect of Battery Pulsed Discharge
The model proposed consists of a battery with a theoretical
capacity of C charge units and an initial battery capacity of N charge
units
• Battery behavior is considered as a discrete time Markov process with
the initial state equal to N and the fully discharged state 0.

• Time is divided into slots (frames). Each packet for the node is
transmitted in one time slot and the battery enters the previous state
by losing a charge unit.
• If the battery remains idle, it recovers one charge unit and enters the
next state.
• The results suggest that at the most C (theoretical capacity) packets
can be transmitted if the battery is given enough time to recover

Binary Pulsed Discharge
• In this mode, if there are packets in the queue, transmission of a
packet occurs in one time slot; one charge unit is recovered if the
queue is empty.
• The current required for transmission is drained during the entire
time frame
• An additional dummy state is added to the Markov chain representing
the cell behavior, which represents the start of the discharge.
• The cell is modeled as a transient process and the packet arrival
follows a Bernoulli process.

• If the probability that a packet arrives in one time frame is stated as
a1 = q and the probability for transmitting a packet in a time slot is
given by a1 , then the probability of recovery is given by a0 = (1 - q).
• The cell can never cross the charge state of N.
• The gain obtained in this scheme is given by G=mp/n, where mp is the
total expected number of packets transmitted and N is the amount of
charge in a fully charged battery.
• The gain, however, cannot exceed C/N where C is the theoretical
capacity.

Generalized Pulsed Discharge
• In a particular time frame, either one or more packets are transmitted
or the cell is allowed to recover a charge unit.
• The quantity of the impulse is equal to the current required to
transmit all the packets, and the duration of the impulse is equal to a
fraction of the time frame.
• In the remaining fraction of the time frame, the cell is allowed to
recover one unit

• Using this model, the battery lifetime estimation can be made as
follows:
• The manufacturer specifies the rated capacity, the time constant, and
the discharge plot of the battery.
• The discharge current ratio, which is the ratio between the specified
rated current (irated ) and the calculated average current (iavg), is
computed.
• Efficiency is calculated by the interpolation of points in the discharge
plot as the variation of efficiency with the current ratio.

Battery scheduling techniques
• battery-scheduling techniques that improve the battery lifetime. In a
battery package of L cells, a subset of batteries can be scheduled for
transmitting a given packet, leaving other cells to recover their
charge.
• The following approaches are applied to select the subset of cells
• Delay-free approaches
• No delay-free approaches
• Using heterogeneous batteries

Device-Dependent Schemes -Battery
scheduling techniques
• Delay-free approachesjob is defined as a demand for battery
discharge which can be satisfied by the subset of cells.
• As soon as a job arrives, the battery charge for processing the job will
be provided from the cells without any delay.
• The scheduling scheme for batteries can be any one of the following:
• Joint technique (JN)
• Round robin technique (RR)

Joint technique (JN): As soon as a job arrives, the same amount of current is
drawn equally from all the cells, which are connected in parallel.
• If there are L cells, the current discharged from each of them is times the
required supply.
Round robin technique (RR): This scheme selects the battery in round robin
• fashion and the jobs are directed to the cells by switching from one to the
next One
• The job from job queue gets energy from the battery selected by the
transmission module based on round robin technique

• Figure 11.8. Battery-scheduling techniques: (a) round robin
technique (b) random technique.

• In these kinds of approaches, the batteries coordinate among
themselves based on their remaining charge.
• In one such technique, a threshold is defined for the remaining
charge of the cell
• Delay-free approaches such as round robin scheduling can be applied
to these eligible cells. The cells which are not eligible stay in the
recovery state.
• This enables the cells to maximize their capacity.
• The general battery discharge policy employed in portable devices
completely drains battery packs one after the other.

• Using heterogeneous batteries:
• a new model suggested for a battery-powered electronic system,
which is based on the continuous time Markovian decision process
(CTMDP).
• It attempts to exploit the two main characteristics of the
rechargeable batteries, that is,
• The recharging capability under no load condition
• The rate capacity effects

• Figure 11.9. Heterogeneous battery-scheduling technique

3. Data Link Layer Solutions
• The data link layer solutions take into consideration battery
characteristics while designing the protocols.
• Designing a battery-aware scheduling technique and maximizing the
number of packets being transmitted are conflicting objectives.
• The following schemes attempt to find a trade-off between them.
Subsequent sections deal with:
• Lazy packet scheduling scheme
• BAMAC protocol

Lazy Packet Scheduling Scheme:
• The basic principle behind the development of this scheme is that in
many of the channel coding schemes for wireless transmission, the
energy required to transmit a packet can be reduced significantly by
minimizing the transmission power and increasing the duration of
transmission
• a transmission schedule is designed taking into account the delay
constraints of the packets

Optimal Offline Schedule
• Assuming the arrival times of all the packets (ti, {i = 1, ..., M}) are
known a priori and t1 = 0, the problem is to find optimal values for τi
, 1 ≤ i ≤ M, so as to minimize .
• A necessary condition for optimality is (time window) and satisfies
the
• necessary condition for optimality stated above. mj denotes the
maximum
• packet inter-arrival time among all the packets that arrive after the
arrival of packet j.

Battery -aware MAC (BAMAC)
The battery-aware MAC (BAMAC) protocol is an energy-efficient
contention-based node scheduling protocol, which tries to increase the
lifetime of the nodes by exploiting the recovery capacity effect of battery
• If the battery remains idle for a specified time interval, it becomes possible
to extend the lifetime of the battery due to the recovery capacity effect. By
increasing the idle time of the battery, the whole of its theoretical capacity
can be completely utilized.
• In the BAMAC protocol, each node maintains a battery table which
contains information about the remaining battery charge of each of its
one-hop neighbor nodes

• The entries in the table are arranged in the non-increasing order of
the remaining battery charges.
• The RTS, CTS, Data, and ACK packets carry the remaining battery
charge of the node from which they originated

BAMAC(K) protocol
• In the BAMAC(K) protocol, whenever the node attempts to gain
access to the channel, it waits for DIFStime duration before
transmitting the first packet.
• If no other neighbor transmits in this duration, the active node (the
node that gains access to the channel) initiates its transmission

4 Network Layer Solutions
• The lifetime of a network is defined as the time from which the
network starts operating until the time when the first node runs out
of battery charge
• The major solutions provided focus primarily on developing routing
protocols that use routing metrics such as low energy cost and
remaining battery charge

Traffic-Shaping Schemes:
• The scheme is based on the fact that most of the network traffic is bursty.
• Introducing some acceptable delays in the battery discharge requests paves the way for
the battery to be idle for a few time slots.
• This allows charge recovery to a certain extent
Shaping Algorithm
• The main goal of the algorithm is to introduce delay slots in the battery discharge
process.
• This is done by defining a threshold which is expressed in terms of the amount of charge.
• The model used in this algorithm consists of a
• battery with a nominal capacity of N, a charge request rate of αN, a theoretical capacity
of T, and a threshold (expressed as a state of charge) of B.

Strategies for Blocking Relay Traffic
• each node deals with two kinds of traffic: relay traffic and its own
traffic.
• A trade-off is reached between the blocking probability of the relay
traffic and the battery efficiency.
• The intermediate nodes may not wish to transmit the whole of the
neighbors' traffic.
• A method that calculates the optimal fraction of relay traffic
• The amount of selfishness is defined using a quantity called sympathy

• The value of sympathy lies between 0 and 1, which reflects the willingness of the
node to accept the relay traffic.
• trade-off which is based on the sympathy level:
• Random strategy: Assuming a session between source s and destination d,
available routes are stored in R(s, d) in the increasing order of the sympathy level
• Pay-for-it strategy: According to this strategy, each node keeps an account of the
help that it had received from other nodes relaying its messages , termed credit,
• the amount of help it has given to others by allowing the relay traffic, that is,
debit.
• The node tries to help if it has received more help in the recent past and rejects if
its own traffic has been rejected often, that is, the node tries to find a balance
between these two parameters.

Unit 4 -2 energy management in adhoc wireless network

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Unit 4 -2 energy management in adhoc wireless network