Outline
Wireless Ad Hoc and Sensor Networks
(Energy Management)
• Energy Management Issue in ad hoc
networks
WS 2009/2010
• Main Reasons for Energy Management
in ad hoc networks
• Classification of Energy Management
S h
Schemes
• Summary
Prof. Dr. Dieter Hogrefe / Prof. Dr. Xiaoming Fu
Dr. Omar Alfandi
2
Energy Management Issue in Ad Hoc Networks
Outline
• In ad hoc networks the devices are battery powered
• So the computation and communication capacity of each
device is constrained
• Devices that expend their whole energy can be
recharged when they leave the network
• Energy resources and computation workloads have
diff
different
t distributions
di t ib ti
within
ithi th
the network
t
k
• Therefore it is beneficial to redistribute spare energy
resources to satisfy varying workloads in the network
• Energy Management Issue in ad hoc
networks
• Main Reasons for Energy Management
in ad hoc networks
3
• Classification of Energy Management
S h
Schemes
• Summary
4
Main Reasons for Energy Management in Ad Hoc
Networks (1/2)
Main Reasons for Energy Management in Ad Hoc
Networks (2/2)
• Limited Energy Reserve: The improvement in battery
• Constraints on the battery source: The batteries should
be small and not heavy; So low power is available at
each node
• Selection of optimal transmission power: Higher
transmission power results in higher energy consumption
and higher interference between nodes
technologies is very slow
• Difficulties in replacing the batteries: E.g. in battlefields or
emergency applications
• Lack of central coordination: In ad hoc networks as distributed
networks
et o s some
so e nodes
odes may
ay work
o as relay
e ay nodes;
odes; when
e relay
e ay ttraffic
a c is
s
heavy the power consumption is high
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6
In General
Outline
• Energy management deals with the process of managing
energy resources by means of :
– Controlling the battery discharge
– Adjusting
Adj ti th
the ttransmission
i i power
– Scheduling of power sources
• Energy Management Issue in Ad Hoc
Networks
• Main Reasons for Energy Management
in ad hoc networks
• Classification of Energy Management
Schemes
• Summary
To increase the lifetime of the nodes of an ad hoc wireless network
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8
Classification of Energy Management Schemes (1/2)
Classification of Energy Management Schemes (2/2)
• Battery management :
Energy Management
Schemes
Battery management
schemes
Transmission power
management schemes
Device dependent
schemes: e.g. battery
scheduling
Data link layer: e.g.
Dynamic power
adjustment
Data link layer: e.g. Lazy
packet scheduling
Network layer: Power
aware routings
Network layer: e.g.
Routing based on battery
status
– Concerned with problems that lie in the selection of battery
t h l i
technologies,
fi
finding
di the
th optimal
ti l capacity
it off th
the b
battery
tt
System power
management schemes
• Transmission power management:
– Attempt to find an optimum power level for the nodes in the ad
hoc wireless network
Processor power management
schemes:
e g Power saving modes
e.g.
Device management schems:
e.g. Low power design of
hardware
• System power management:
– Deals mainly with minimizing the power required by hardware
peripherals of a node (such as CPU, DRAM and LCD display)
Higher layers: Congestion
control, Transmission
policies at TCP/IP
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9
Classification of Energy Management Schemes
• The lifetime of a node is determined by the capacity of its
energy source and the energy required by the node.
• There
Th
are some device
d i d
dependent
d t approaches
h th
thatt iincrease
the battery lifetime by exploiting its internal characteristics.
Energy Management
Schemes
Battery management
schemes
Device dependent
schemes: e.g. battery
scheduling
Data link layer: e.g. Lazy
packet scheduling
Network layer: e.g.
Routing based on battery
status
Transmission power
management schemes
Data link layer: e.g.
Dynamic power
adjustment
Network layer: Power
aware routings
1. Battery Management Schemes
System power
management schemes
•
Processor power management
schemes:
e g Power saving modes
e.g.
Key Fact: Batteries recover their charge when idle
֜ Use some batteries and leave others to idle/recover
• Device depending schemes:
I. Battery scheduling techniques:
• 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. There are some approaches to select
the subset of cells, e.g.:
Device management schems:
e.g. Low power design of
hardware
Higher layers: Congestion
control, Transmission
policies at TCP/IP
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12
Battery Management Schemes
Battery Management Schemes
1. Joint technique: The same amount of current is drawn equally
from all the cells which are connected in parallel.
2. Round robin technique: The current is drawn from the batteries in
turn by switching from one to the next one.
3. Random technique: any one of the cells is chosen at random with
a uniform probability
probability.
• Data link Layer Battery Management
I. Lazy Packet Scheduling:
– Reduce the power ֜ Increase the transmission time (lower bit rate)
– But this may not suit practical wireless environment packets
֜ a transmission schedule is designed taking into account the delay
constraints of the packets
II. Battery-Aware MAC Protocol:
– to provide uniform discharge of the batteries of the nodes that
contend for the common channel
– Lower back off interval for nodes with higher charge
•
Network Layer Battery Management
– Goal: Increase the lifetime of the network
I. Shaping algorithm:
g delay
y slots in the battery
y discharge
g p
process
• introducing
• If battery charge becomes below threshold, stop next transmission
allowing battery to recover through idling
• The remaining requests arriving at the system are queued up at a
buffer
• As soon as the battery recovers its charge and enters state higher
than the threshold,
threshold it starts servicing the queued-up requests
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Classification of Energy Management Schemes
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2. TRANSMISSION POWER MANAGEMENT
SCHEMES
• Link layer solutions:
Energy Management
Schemes
Battery management
schemes
Transmission power
management schemes
Device dependent
schemes: e.g. battery
scheduling
Data link layer: e.g.
Dynamic power
adjustment
Data link layer: e.g. Lazy
packet scheduling
Network layer: Power
aware routings
Network layer: e.g.
Routing based on battery
status
I. Power Save in IEEE 802.11 Ad Hoc Mode
– Time is divided into beacon intervals
– Each beacon interval begins with an ATIM (ad hoc traffic
indication message) window
System power
management schemes
Processor power management
schemes:
e g Power saving modes
e.g.
Device management schems:
e.g. Low power design of
hardware
Higher layers: Congestion
control, Transmission
policies at TCP/IP
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16
TRANSMISSION POWER MANAGEMENT SCHEMES
TRANSMISSION POWER MANAGEMENT SCHEMES
• If host A has a packet to transmit to B, A must send an ATIM
Request to B during an ATIM Window
• On
O receipt
i t off ATIM Request
R
t from
f
A,
A B will
ill send
d an ATIM Ack,
A k
and stay up during the rest of the beacon interval
• If a host does not receive an ATIM Request during an ATIM
window, and has no pending packets to transmit, it may sleep
during rest of the beacon interval
• Size of ATIM window and beacon interval affects
performance:
– If ATIM window
i d
is
i too
t large,
l
energy saving
i iis reduced
d
d and
d
may not have enough time to transmit buffered data
– If ATIM window is too small,
small not enough time to send ATIM
request
• E.g. A has some data packets to send to B; C is idle
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TRANSMISSION POWER MANAGEMENT SCHEMES
TRANSMISSION POWER MANAGEMENT SCHEMES
II. Power Control in IEEE 802.11 Ad Hoc Mode
• A power control MAC protocol allows nodes to vary transmit power
level on a per
per-packet
packet basis
• When C transmits to D at a high power level, B cannot receive A’s
transmission due to interference from C
• But difference in transmit power can lead to increased
collisions
• In following example suppose nodes A and B use lower
power level than nodes C and D
• When A is transmitting to B, C and D may not sense the
transmission
• When
Wh C and
d D ttransmit
it tto each
h other
th using
i hi
higher
h
power, their transmission may collide with the on-going
transmission from A to B
•
If C reduces transmit power, it can still communicate to D
– Reduces energy consumption at node C
– Allows B to receive A’s transmission (spatial reuse)
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TRANSMISSION POWER MANAGEMENT SCHEMES
TRANSMISSION POWER MANAGEMENT SCHEMES
• As a solution to this problem, RTS-CTS are transmitted
at the highest possible power level but DATA and ACK
att the
th minimum
i i
power llevell necessary tto communicate
i t
• In figure nodes A and B send RTS and CTS respectively
with highest power level such that node C receives the
CTS and defers its transmission
• By using a lower power level for DATA and ACK
packets, nodes can save energy
• In the previous scheme, RTS-CTS handshake is used to
decide the transmission power for subsequent DATA
and
d ACK which
hi h can b
be achieved
hi
d iin ttwo diff
differentt ways
1 Suppose node A wants to send a packet to node B
1.
B. Node
A transmit RTS at power level pmax (maximum possible).
When B receives the RTS from A with signal level pr, B
calculates the minimum necessary transmission power
level, pdesired. For the DATA packet based on received
power level,
level pr, transmitted power level,
level pmax, and noise
level at the receiver B. Node B specifies pdesired in its CTS
to node A. After receiving CTS, node A sends DATA
using power level pdesired.
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TRANSMISSION POWER MANAGEMENT SCHEMES
2.
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TRANSMISSION POWER MANAGEMENT SCHEMES
• Network layer solutions:
When a destination node receives an RTS, it responds
by sending a CTS (at power level pmax). When source
node receives CTS
CTS, it calculates pdesired based on
received power level, pr, and transmitted power level
(pmax
a ) as
I. Common Power Control:
– Given reachability of each node as a function of power
power, find the
min power level that provides network connectivity
– If the common power level is selected too high, this may lead to
interference (fig. a); if too low, the reachability of nodes may
become weak (fig. b)
֜ choosing an optimum value is a difficult task
task.
Pdesired = (pmax / pr) x Rxthresh x c
g
where Rxthresh is minimum necessaryy received signal
strength and c is a constant
III. Centralized Topology Control:
The power of each node is reduced until it has single
connectivity, i.e. there is one path between each pair of nodes
or bi-connectivity
bi
ti it
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TRANSMISSION POWER MANAGEMENT SCHEMES
TRANSMISSION POWER MANAGEMENT SCHEMES
II. Min Variance in Node Power Levels:
IV. Conditional Min-Max Battery Cost Routing:
– The motivation is to ensure that all the nodes are given equal
importance and no node is drained at a faster rate compared to other
nodes in the network
– For transmitting a packet, a node selects the next-hop node so that it
has the least amount of traffic among all neighbours of the node
– Using only nodes that have battery charge over a threshold, Find the
min total power path
path.
V. Minimum Energy Disjoint Path Routing:
– The important need for having disjoint paths in ad hoc networks is
b
because
off the
th need
d ffor reliable
li bl packet
k t ttransmission
i i and
d energy
efficiency.
– Ad hoc networks are highly unreliable due to the mobility of nodes and
h
hence
the
h probability
b bili off lilink
k ffailure
il
iis quite
i hi
high
h iin such
h networks.
k
– This problem can be overcome by means of link disjoint routing.
– Also, since the ad hoc nodes have stringent
g
battery
y constraints, node
disjoint routing considerably increases the lifetime of the network by
choosing different routes for transmitting packets at different points of
time.
III. Min Battery Cost Routing:
– Minimize sum of battery cost along a path
֜ Does not ensure that lower charge nodes are not used
֜
»
The lower path is used
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Classification of Energy Management Schemes
26
3. SYSTEM Power Management Schemes
•
Energy Management
Schemes
•
Battery management
schemes
Transmission power
management schemes
Device dependent
schemes: e.g. battery
scheduling
Data link layer: e.g.
Dynamic power
adjustment
Data link layer: e.g. Lazy
packet scheduling
Network layer: Power
aware routings
Network layer: e.g.
Routing based on battery
status
System power
management schemes
•
Processor power management
schemes:
e g Power saving modes
e.g.
Efficient design of the hardware brings about significant reduction in
the power consumed.
This can be effected by operating some of the peripheral devices in
power-saving mode by turning them off under idle conditions.
System power consists of the power used by all hardware units of
the node. This power can be conserved significantly by applying the
following schemes:
– Processor power management schemes
– Device power management schemes
Device management schems:
e.g. Low power design of
hardware
Higher layers: Congestion
control, Transmission
policies at TCP/IP
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28
SYSTEM Power Management Schemes
•
SYSTEM Power Management Schemes
•
Processor Power Management Schemes:
– Deals with techniques that try to reduce the power consumed by the processor,
g reducing
g the number of calculations p
performed.
e.g.
I.
Power-Saving Modes:
– The nodes consume a substantial amount of power even when they are
in an idle state since they keep listening to the channel
channel, awaiting
request packets from the neighbours.
– As a solution, the nodes are switched off during idle conditions and
switched on only when there is an arrival of a request packet
packet.
– Since the arrival of request packets is not known a priori, it becomes
difficult to calculate the time duration for which the node has to be
switched off
off.
– One solution to this problem calculates the node's switch-off time based
on the quality of service (QoS) requirements. Hard QoS requirements
make
k the
th node
d stay
t active
ti mostt off the
th time.
ti
As soon as the node enters the idle state, it is switched off by the
remote activated switch RAS. The receiver of the RAS switch still listens
to the channel. The remote neighbours send the wake-up signal and a
sequence. The receiver, on receiving the wake-up signal, detects the
sequence (the waking-up signal). The logic circuit compares it with the
standard sequence for the node. It switches on the node only if both the
sequences match.
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30
SYSTEM Power Management Schemes
SYSTEM Power Management Schemes
II. Power-Aware Multi-Access Signaling (PAMAS):
• Power-Aware Multi-Access Signaling is another approach for
determining the time duration for which the node should be turned
off.
– Conditions under which the node enters the power-off mode:
•
•
Device Power Management Schemes:
Some of the major consumers of power in ad hoc wireless networks
are the
th h
hardware
d
d
devices
i
presentt iin th
the nodes.
d
D i power
Device
management schemes minimize the power consumption.
• Condition 1: The node has no packets for transmission.
• Condition 2: A neighbour node is transmitting or receiving packets,
that is,, the channel is busy.
y
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SYSTEM Power Management Schemes
Outline
I.
Low-Power Design of Hardware: varying clock speed CPUs, disk
spin down, and flash memory
II CPU Power Consumption: by changing the clock frequency,
II.
frequency etc.
etc
III. Power-Aware CPU Scheduling: a small reduction in the value of
the voltage produces a quadratic in the power consumed, so clock
rate has to be also reduced.
IV. Hard Disk Drive (HDD) Power Consumption: by bring down the
speed of spinning on the disc drives
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Summary
• Three major divisions in energy management
• Battery Management:
When idling increases the capacity of the battery
• Transmission Power Management:
Distance vs. Power tradeoff
• System Power Management:
Put system/components
y
p
to sleep
p whenever p
possible
35
• Energy Management Issue in Ad Hoc
Networks
• Main Reasons for Energy Management
in ad hoc networks
• Classification of Energy Management
Schemes
• Summary
S
34