A survey of Energy Efficient Network
Protocols for Wireless Networks
Christine E. Jones
Krishna M. Sivalingam
Prathima Agrawal
Jyh-Cheng Chen
Issue 1/2


Rapid expansion of wireless services,
mobile data and wireless LANs
Greatest limitation: finite power
supplies
Issue 2/2

Typical example of power consumption from a
mobile computer (Toshiba 410 CDT):
o
o
o
o

36%
21%
18%
18%
Display
CPU/memory
Wireless interface
Hard drive
Goal
o
Incorporate energy conservation at all layers of
protocol stack
Energy Efficiency Research in Protocol Stack
Physical Layer


Two different perspectives
Increase battery capacity
o
o

Increase capacity while restricting weight
However battery technology hasn’t experienced
significant advancement in the past 30 years
Decrease of energy consumed
o
o
o
Variable clock speed CPUs
Flash memory
Disk spindown
Sources of Power Consumption

Two types
o
o

Communication related
Computation related
Tradeoff between them
Communication related

Three modes:
o
o
o

Example:
o


Transmit
Receive
Standby
Proxim RangeLAN2 2.4 GHz 1.6 Mbps PCMCIA
card 1.5W transmit, 0.75W receive, 0.01W
standby
Turnaround between transmit and receive
typically takes 6 to 30 microseconds
Optimize the transceiver usage
Computation related


Usage of CPU, main memory and disk
Data compression techniques for
reduction of packet length increase
power consumption
General Guidelines and
Mechanisms 1/5

Reduce collisions in MAC
o
o

Retransmissions lead to power
consumption and delays
Cannot be completely eliminated due to
user mobility and varying set of mobiles
Change typical broadcast mechanism
o
802.11: Receiver keeps track of channel
status through constant monitoring
General Guidelines and
Mechanisms 2/5

Turnaround between transmit and
receive mode spends time and power
o
o

Allocate contiguous slots for transmission
or reception
Request multiple transmission slots with a
single reservation packet
Computation of transmission schedule
should be relegated to base station
General Guidelines and
Mechanisms 3/5



Scheduling algorithm may additionally
consider battery power level
Allow mobile to re-arrange allocated
slots under low-power conditions
At link layer:
o
o
Avoid transmissions when channel
conditions are poor
Combine ARQ and FEC mechanisms
General Guidelines and
Mechanisms 4/5

Energy efficient routing protocols
o
o
Ensure all nodes equally deplete their power level
Avoid routing through nodes with lower battery
power

o
Requires mechanism for dissemination of node battery
power
Periodicity of routing updates can be reduced

May result in inefficient routes
General Guidelines and
Mechanisms 5/5

OS level
o
Suspend of specific sub-unit (disk, memory,
display etc.) when detect prolonged
inactivity
MAC Sublayer

Three specific MAC protocols
o
o
o
IEEE 802.11
EC-MAC
PAMAS
IEEE 802.11 Standard 1/2


A mobile that wishes to conserve power may
switch to sleep mode and inform the base
station
The base station
o
o

Buffers packets that are destined for the sleeping
mobile
Periodically transmits a beacon that contains
information about such buffered packets
When the mobile wakes up, it listens for this
beacon, and responds to the base station
which then forwards the packets
IEEE 802.11 Standard 2/2


Conserves power but results in additional
delays and may affect the QoS
Experimental measurements of per packet
energy consumption
o
o
Same incremental costs for both unicast and
broadcast traffic
Higher fixed costs for unicast transmission
because of MAC coordination and CTS and ACK
messages
EC-MAC Protocol 1/7




Energy Conserving-Medium Access Control
Developed with the issue of energy efficiency
as a primary goal
Defined for infrastructure network but can be
extended to ad-hoc by allowing mobiles to
elect a coordinator
It is based on reservation and scheduling and
supports QoS
EC-MAC Protocol 2/7
EC-MAC Protocol 3/7

FSM:
o
o

transmitted at the start of each frame by the base
station
contains synchronization information and uplink
transmission order for subsequent reservation
phase
Request/Update Phase:
o
o
Each registered mobile transmits new connection
requests and status of established queues
Collisions avoided
EC-MAC Protocol 4/7

New User Phase (Aloha):
o
o
o

Registration of new users
Collisions occur
Provides time for BS to compute the data phase
transmission schedule
Schedule Message:
o
o
Broadcasted by the base station
Contains the slot permissions for the subsequent
data phase
EC-MAC Protocol 5/7

Data phase (Downlink):
o
o

Transmission from base station to mobiles
Scheduled considering QoS requirements
Data phase (Uplink):
o
Slots allocated using a suitable scheduling
algorithm
EC-MAC Protocol 6/7



Collisions are avoided and this reduces
the number of retransmissions
Mobile receivers are not required to
monitor the channel because of
schedules
Centralized scheduler can optimize
schedule so that mobiles transmit and
receive within contiguous slots
EC-MAC Protocol 7/7


Scheduling algorithms may consider
also battery power level in addition to
packet priority
Frames may be fixed or variable length
o
o
Fixed are desirable from energy efficient
perspective since a mobile will know when
to wake up to receive FSM
Variable are better for meeting the
demands of bursty traffic
PAMAS Protocol 1/3


Designed for ad hoc network, with
energy efficiency as primary goal
Provides separate channels for RTS/CTS
control packets and data packets
PAMAS Protocol 2/3




A mobile with a packet to transmit sends a
RTS over the control channel, and awaits the
CTS
If no CTS arrives the mobile enters a backoff
state
However, if CTS is received, then the mobile
transmits the packet over the data channel
The receiving mobile transmits a “busy tone”
over the control channel for the others to
determine that the data channel is busy
PAMAS Protocol 3/3


The use of control channel allows mobiles to
determine when and for how long to power
off
A mobile can power off when:
o
o

It has no packets to transmit and a neighbor
begins transmitting a packet not destined for it
It does have packets to transmit but at least one
neighbor-pair is communicating
The length of power off time is determined
through the use of a probe protocol (Singh
and Raghavendra, 1998)
LLC Sublayer



Is responsible for the error control
The two most common techniques for
the error control are Automatic Repeat
Request (ARQ) and Forward Error
Correction (FEC)
Both waste network bandwidth and
power resources due to retransmissions
and greater overhead
LLC Sublayer

Recent research has addressed lowpower error control and several energy
efficient link layer protocols have been
proposed:
o
o
o
Adaptive Error Control with ARQ
Adaptive Error Control with ARQ/FEC
Combination
Adaptive Power Control and Coding
Scheme
Adaptive Error Control with ARQ 1/3

Three guidelines:
o
o
o
Avoid persistence in retransmitting data
Trade off number of retransmission
attempts for probability of successful
transmission
Inhibit transmission when channel
conditions are poor
Adaptive Error Control with ARQ 2/3



Works as normal until the transmitter detects
an error due to the lack of a received ACK.
Then the protocol enters a probing mode in
which a probing packet is transmitted every t
slots. Probe packet contains only header
This mode continues until an ACK is received.
Then the protocol returns to normal mode
and continues transmission from where it was
interrupted
Adaptive Error Control with ARQ 3/3


Analysis results show that under slow fading
channel conditions it is superior to standard
ARQ in terms of energy efficiency
There is an optimal transmission power in
respect to energy efficiency
o
Decreasing the transmission power results in an
increased number of transmission attempts but
may be more efficient than attempting to
maximize the throughput
Adaptive Error Control with
ARQ/FEC Combination

Each packet stream
o
o

is associated with service quality parameters
(packet size, QoS requirements)
maintains its own time-adaptive customized error
control scheme
Error control scheme
o
is a combination of


o
an ARQ scheme (Go-Back-N, CACK, SACK, etc.) and
a FEC scheme
modifies as channel conditions change over time
Adaptive Power Control and
Coding Scheme

Each transmitter operates at a power-code
pair
o
o

At each iteration (timeframe):
o
o

Power level lies between a specified minimum and
maximum
The error code is chosen from a finite set
Receiver checks the word error rate (WER)
If the WER lies within an acceptable range, powercode is retained, otherwise a new power-code pair
is computed by the transmitter
Variations of algorithm include average WER
Network Layer



Energy efficient routing algorithms for ad hoc
networks
Does not apply to infrastructure networks
because all traffic is routed through BS
Two different approaches:
o
Frequent topology updates


o
Improved routing
Consumes bandwidth
Infrequent topology updates


Decreased update messages
Inefficient routing and occasional missed packets
Network Layer

Typical metrics for ad hoc routing protocols
o
o
o

Shortest-hop
Shortest-delay
Locality-stability
However they may result in the overuse of
energy resources of a small set of mobiles
decreasing mobile and network life
Network Layer example



Using shortest-hop routing,
traffic from A to D will
always be routed through
E
E’s energy reserves will be
drained faster and then F
will be disconnected from
network
A to D traffic should also
use the B-C path extending
networks life
Network Layer: Unicast Traffic 1/6
Five different metrics

o
o
Energy consumed per packet
Time to network partition


Given a network topology, a minimal set of
mobiles exist such that their removal will
cause the network to partition
The traffic in that mobiles should be divided
in such a way that they drain their power at
equal rates
Network Layer: Unicast Traffic 2/6
o
Variance in power level across mobiles

o
Cost per packet

o
All mobiles are equal and remain powered-on together
for as long as possible
Routes should be created such that mobiles with
depleted energy reserves do not lie on many routes
Maximum mobile cost

By minimizing the cost experienced by a mobile when
routing a packet through it significant reductions in the
maximum mobile cost result
Network Layer: Unicast Traffic 3/6




The goal is to minimize all the metrics except
for the second which should be maximized
Shortest-cost routing protocol is more
appropriate instead of shortest-hop
So although packets may be routed through
longer paths, the paths contain mobiles that
have greater amounts of energy reserves
Also routing traffic through lightly loaded
mobiles conserves energy because it
minimizes contention and retransmission
Network Layer: Unicast Traffic 4/6




Simulation results showed no extra delay over
the traditional shortest-hop metric
This is true because congested paths are
often avoided
However this approach requires that every
mobile have knowledge of every other mobile
and the links between them
This creates significant communication
overhead and increased delay
Network Layer: Unicast Traffic 5/6



Stojmenovic and Lin proposed localized
routing algorithms
These algorithms depend only on information
about the source location, the location of
neighbors and location of the destination
This information is collected through GPS
receivers which are included in every mobile
Network Layer: Unicast Traffic 6/6

They proposed a new power-cost metric
o

Incorporates both a mobile’s lifetime and distance
based power metrics
Three power-aware localized routing
algorithms were developed
o
Power

o
Cost

o
Minimize total amount of power utilized when
transmitting a packet
Avoid mobiles with low battery reserves
Power-cost

Combination of the other two
Network Layer: Broadcast Traffic 1/4



Each mobile needs to receive a packet
only once
Intermediate mobiles are required to
retransmit the packet
Key idea: allow each mobile’s radio to
turn off after receiving a packet if its
neighbors have already received a copy
of the packet
Network Layer: Broadcast Traffic 2/4

In traditional networks broadcast technique is
a simple flooding algorithm
o
o
o


No global information topology gathered
Requires little control overhead
Completes with minimum number of hops
Not suitable for wireless networks because
many intermediate nodes must retransmit
packets needlessly
It is more beneficial to spend some energy in
gathering topology information in order to
determine the most efficient broadcast tree
Network Layer: Broadcast Traffic 3/4




A broadcast approach is presented in (Singh
et al., 1999)
The tree is constructed starting from the
source and expanding to the neighbor that
has the lowest cost per outgoing degree
Mobile costs continuously change so
broadcast transmissions may traverse
different trees
Simulations showed very little difference in
delay but 20% or better in energy
consumption
Network Layer: Broadcast Traffic 4/4


In (Wieselthier et al., 2000) is presented an
algorithm for determining the minimumenergy tree
There exists an optimal point in the trade-off
between reaching greater number of mobiles
in a single hop by using higher transmission
power versus reaching fewer mobiles but
using lower power levels
Transport Layer

TCP was designed initially for wired networks
o
o

Over a wireless link it degrades significantly
o

Physical links are fairly reliable
Packet loss is random in nature
It resorts to a larger number of retransmissions
and frequently invoke congestion control
measures because it confuses link errors and loss
as channel congestion
The increased retransmissions consume
battery energy and bandwidth
Transport Layer

Various schemes have been proposed
o
o
o
Split connection protocols
Link-layer protocols
End-to-end protocols
Split connection protocols 1/2
Split connection protocols 2/2


Completely hide the wireless link from
the wired network by splitting each TCP
connection into two separate
connections at the BS
The second one may use modified
versions of TCP that enhance
performance over the wireless channel
Link-layer protocols 1/2
Link-layer protocols 2/2



Hides link related losses from the TCP
source
Uses a combination of local
retransmissions and FEC over the
wireless link
Local retransmissions use techniques
that are tuned to the characteristics of
the wireless channel
End-to-end protocols


Include modified versions of TCP that are
more sensitive to the wireless environment
Uses mechanisms such as
o
SACK

o
allow the TCP source to recover from multiple packet
losses
ELN

Aid the TCP source to distinguish between congestion
and other forms of loss
Energy Consumption Analysis of TCP
1/4



The energy consumption of Tahoe, Reno and
New Reno is analyzed in (Zorzi and Rao,
2000)
Efficiency is defined as the average number
of successful transmissions per energy unit
Results demonstrate that
o
o
o
error correlation affects the energy performance
congestion control algorithms of TCP allow for
greater energy savings by backing off and wait
during error bursts
energy efficiency is sensitive to the version of TCP
Energy Consumption Analysis of TCP
2/4


The same versions of TCP were studied in
(Tsaoussidis et al., 2000a) in terms of
energy/throughput tradeoffs
Results showed that
o
o

no single version is most appropriate within
wired/wireless heterogeneous networks
the key to balancing energy and throughput is
through the error control mechanism
They proposed a modified version of TCP,
referred to as TCP-Probing in (Tsaoussidis
and Badr, 2000)
Energy Consumption Analysis of TCP
3/4


In TCP-Probing when a segment is delayed or
lost, instead of invoking congestion control,
transmission is suspended and a probe cycle
is initiated
Probe cycle:
o
o
exchange of probe segments (TCP header with no
payload) between sender and receiver
terminates when two consecutive RTT are
successfully measured
Energy Consumption Analysis of TCP
4/4


The sender invokes standard TCP
congestion control if persistent error
conditions are detected
However, if conditions indicate transient
random error, then the sender resume
transmissions according to available
network bandwidth
OS/Middleware




The main functions of an operating system is
to manage access to physical resources like
CPU, memory and disk space
CPUs can be operated at lower speeds by
scaling down the supply voltage (quadratic
relationship between power and supply
voltage)
Predictive shutdown
Different page placement algorithms exploit
the new power management features of
memory technology
Application Layer 1/2

Load Partitioning
o
o

Applications may be selectively partitioned
between the mobile and base station
Most of the power intensive computations of an
application are executed at the BS
Proxies
o
o
Middleware that automatically adapt the
applications to changes in battery power and
bandwidth
Either on the mobile or BS side of wireless link
Application Layer 2/2

Databases
o

Minimize power consumed per transaction
through embedded indexing
Video Processing
o
o
Reduce effective bit rate of video
Carefully discard video frames