A Survey of Energy Efficient Network Protocols for Wireless Networks.
Wireless networking has witnessed an explosion of interest from consumers in recent years for its applications in mobile and personal communications. As wireless networks become an integral component of the modern communication infrastructure, energy efficiency will be an important design consideration due to the limited battery life of mobile terminals. Power conservation techniques are commonly used in the hardware design of such systems. Since the network interface is a significant consumer of power, considerable research has been devoted to low-power design of the entire network protocol stack of wireless networks in an effort to enhance energy efficiency. This paper presents a comprehensive summary of recent work addressing energy efficient and low-power design within all layers of the wireless network protocol stack.
Introduction
The rapid expansion of wireless services such as cellular voice, PCS
(Personal Communications Services), mobile data and wireless LANs
in recent years is an indication that signicant value is placed on accessibility
and portability as key features of telecommunication (Salkintzis and Mathiopoulos (Guest Ed.), 2000).
devices have maximum utility when they can be used \any-
where at anytime". One of the greatest limitations to that goal, how-
ever, is nite power supplies. Since batteries provide limited power, a
general constraint of wireless communication is the short continuous
operation time of mobile terminals. Therefore, power management is
y Corresponding Author: Dr. Krishna Sivalingam. Part of the research was
supported by Air Force Oce of Scientic Research grants F-49620-97-1-
0471 and F-49620-99-1-0125; by Telcordia Technologies and by Intel. Part of
the work was done while the rst author was at Washington State Univer-
sity. The authors' can be reached at cej@bbn.com, krishna@eecs.wsu.edu,
pagrawal@research.telcordia.com, jcchen@research.telcordia.com
c
2001 Kluwer Academic Publishers. Printed in the Netherlands.
Jones, Sivalingam, Agrawal and Chen
one of the most challenging problems in wireless communication, and
recent research has addressed this topic (Bambos, 1998). Examples include
a collection of papers available in (Zorzi (Guest Ed.), 1998) and
a recent conference tutorial (Srivastava, 2000), both devoted to energy
ecient design of wireless networks.
Studies show that the signicant consumers of power in a typical
laptop are the microprocessor (CPU), liquid crystal display (LCD),
hard disk, system memory (DRAM), keyboard/mouse, CDROM drive,
oppy drive, I/O subsystem, and the wireless network interface card
(Udani and Smith, 1996, Stemm and Katz, 1997). A typical example
from a Toshiba 410 CDT mobile computer demonstrates that nearly
36% of power consumed is by the display, 21% by the CPU/memory,
18% by the wireless interface, and 18% by the hard drive. Consequently,
energy conservation has been largely considered in the hardware design
of the mobile terminal (Chandrakasan and Brodersen, 1995) and in
components such as CPU, disks, displays, etc. Signicant additional
power savings may result by incorporating low-power strategies into
the design of network protocols used for data communication. This
paper addresses the incorporation of energy conservation at all layers
of the protocol stack for wireless networks.
The remainder of this paper is organized as follows. Section 2 introduces
the network architectures and wireless protocol stack considered
in this paper. Low-power design within the physical layer is brie
y
discussed in Section 2.3. Sources of power consumption within mobile
terminals and general guidelines for reducing the power consumed are
presented in Section 3. Section 4 describes work dealing with energy
ecient protocols within the MAC layer of wireless networks, and
power conserving protocols within the LLC layer are addressed in Section
5. Section 6 discusses power aware protocols within the network
layer. Opportunities for saving battery power within the transport
layer are discussed in Section 7. Section 8 presents techniques at the
OS/middleware and application layers for energy ecient operation.
Finally, Section 9 summarizes and concludes the paper.
2. Background
This section describes the wireless network architectures considered in
this paper. Also, a discussion of the wireless protocol stack is included
along with a brief description of each individual protocol layer. The
physical layer is further discussed.
Survey of Energy Ecient Network Protocols for Wireless 3
2.1. Wireless Network Architectures
Two dierent wireless network architectures are considered in this pa-
per: infrastructure and ad hoc networks. Below, a description of each
system architecture is presented.
2.1.0.1. Infrastructure: Wireless networks often extend, rather than
replace, wired networks, and are referred to as infrastructure networks.
The infrastructure network architecture is depicted in Figure 1. A
hierarchy of wide area and local area wired networks is used as the
backbone network. The wired backbone connects to special switching
nodes called base stations. Base stations are often conventional PCs
and workstations equipped with custom wireless adapter cards. They
are responsible for coordinating access to one or more transmission
for mobiles located within the coverage cell. Transmission
channels may be individual frequencies in FDMA (Frequency Division
Multiple Access), time slots in TDMA (Time Division Multiple Access),
or orthogonal codes or hopping patterns in the case of CDMA (Code
Division Multiple Access). Therefore, within infrastructure networks,
wireless access to and from the wired host occurs in the last hop between
base stations and mobile hosts that share the bandwidth of the
wireless channel.
2.1.0.2. Ad hoc: Ad hoc networks, on the other hand, are multi-hop
wireless networks in which a set of mobiles cooperatively maintain
network connectivity (Macker and Corson, 1998). This on-demand net-work
architecture is completely un-tethered from physical wires. An
example of an ad hoc topology is pictured in Figure 2. Ad hoc networks
are characterized by dynamic, unpredictable, random, multi-hop
topologies with typically no infrastructure support. The mobiles must
periodically exchange topology information which is used for routing
updates. Ad hoc networks are helpful in situations in which temporary
network connectivity is needed, and are often used for military envi-
ronments, disaster relief, and so on. Mobile ad hoc networks have attracted
considerable attention as evidenced by the IETF working group
MANET (Mobile Ad hoc Networks). This has produced various Internet
drafts, RFCs, and other publications (Macker and Corson, 1998,
Macker and Corson, ). Also, a recent conference tutorial presents a
good introduction to ad hoc networks (Vaidya, 2000).
4 Jones, Sivalingam, Agrawal and Chen
2.2. Protocol Layers
This section provides an introduction to the software used in wireless
data network systems. Application programs using the network
do not interact directly with the network hardware. Instead, an application
interacts with the protocol software. The notion of protocol
layering provides a conceptual basis for understanding how a complex
set of protocols work together with the hardware to provide a powerful
communication system. Recently, communication protocol stacks
such as the Infrared Data Association (IrDa) protocol stack for point-
to-point wireless infrared communication and the Wireless Application
Protocol (WAP) Forum protocol stack for enabling developers
to build advanced services across diering wireless network technologies
(Infrared Data Association, 2000, WAP Forum, 2000) have been
developed specically for wireless networks. This paper focuses on the
traditional OSI protocol stack, depicted in Figure 3, for a generic wireless
communication system. The application and services layer occupies
the top of the stack followed by the operating system/middleware,
transport, network, data link, and physical layers. The problems inherent
to the wireless channel and issues related to mobility challenge
the design of the protocol stack adopted for wireless networks. In ad-
dition, networking protocols need to be designed with energy eciency
in mind.
Physical: The physical layer consists of Radio Frequency (RF) circuits,
modulation, and channel coding systems. From an energy ecient per-
spective, considerable attention has already been given to the design of
this layer (Chandrakasan and Brodersen, 1995).
Data Link: The data link layer is responsible for establishing a reliable
and secure logical link over the unreliable wireless link. The data link
layer is thus responsible for wireless link error control, security (en-
cryption/decryption), mapping network layer packets into frames, and
packet retransmission.
A sublayer of the data link layer, the media access control (MAC)
protocol layer is responsible for allocating the time-frequency or code
space among mobiles sharing wireless channels in a region.
Network: The network layer is responsible for routing packets, establishing
the network service type (connection-less versus connection-
oriented), and transferring packets between the transport and link
layers. In a mobile environment this layer has the added responsibility
of rerouting packets and mobility management.
Survey of Energy Ecient Network Protocols for Wireless 5
Transport: The transport layer is responsible for providing ecient and
reliable data transport between network endpoints independent of the
physical network(s) in use.
OS/Middleware: The operating system and middleware layer handles
disconnection, adaptivity support, and power and quality of service
(QoS) management within wireless devices. This is in addition to the
conventional tasks such as process scheduling and le system management

Application: The application and services layer deals with partitioning
of tasks between xed and mobile hosts, source coding, digital signal
processing, and context adaptation in a mobile environment. Services
provided at this layer are varied and application specic.
The next section further examines the low-power research completed
at the physical layer.
2.3. Physical Layer
In the past, energy ecient and low-power design research has centered
around the physical layer due to the fact that the consumption of
power in a mobile computer is a direct result of the system hardware.
Research addresses two dierent perspectives of the energy problem:
(i) an increase in battery capacity and (ii) a decrease in the amount of
energy consumed at the wireless terminal.
The primary problem concerning energy in wireless computing is
that battery capacity is extremely limited. The focus of battery technology
research has been to increase battery power capacity while
restricting the weight of the battery. However, unlike other areas of
computer technology such as micro-chip design, battery technology has
not experienced signicant advancement in the past
fore, unless a breakthrough occurs in battery technology, an attainable
goal of research would be a decrease in the energy consumed in the
wireless terminal (Lettieri and Srivastava, 1999).
Low-power design at the hardware layer uses dierent techniques
including variable clock speed CPUs (Govil et al., 1995),
ash memory
(Marsh et al., 1994), and disk spindown (Douglis et al., 1994). Numerous
energy ecient techniques for the physical layer are discussed in
(Chandrakasan and Brodersen, 1995). Although the above techniques
have resulted in considerable energy savings, other venues should also
be explored to improve energy eciency. One way to achieve this for
future wireless networks is to design the higher layers of the protocol
stack with energy eciency as an important goal.
6 Jones, Sivalingam, Agrawal and Chen
3. Power Consumption Sources and Conservation
Mechanisms
This section rst presents the chief sources of power consumption with
respect to the protocol stack. Then, it presents an overview of the main
mechanisms and principles that may be used to develop energy ecient
network protocols.
3.1. Sources of Power Consumption
The sources of power consumption, with regard to network opera-
tions, can be classied into two types: communication related and
computation related.
Communication involves usage of the transceiver at the source, intermediate
(in the case of ad hoc networks), and destination nodes.
The transmitter is used for sending control, route request and re-
sponse, as well as data packets originating at or routed through the
transmitting node. The receiver is used to receive data and control
packets { some of which are destined for the receiving node and some
of which are forwarded. Understanding the power characteristics of
the mobile radio used in wireless devices is important for the ecient
design of communication protocols. A typical mobile radio may exist
in three modes: transmit, receive, and standby. Maximum power is
consumed in the transmit mode, and the least in the standby mode.
For example, the Proxim RangeLAN2 2.4 GHz 1.6 Mbps PCMCIA card
requires 1.5W in transmit, 0.75W in receive, and 0.01W in standby
mode. In addition, turnaround between transmit and receive modes
(and vice-versa) typically takes between 6 to microseconds. Power
consumption for Lucent's 15 dBm 2.4 GHz 2 Mbps Wavelan PCMCIA
card is 1.82W in transmit mode, 1.80W in receive mode, and 0.18W in
standby mode. Thus, the goal of protocol development for environments
with limited power resources is to optimize the transceiver usage for a
given communication task.
The computation considered in this paper is chie
y concerned with
protocol processing aspects. It mainly involves usage of the CPU and
main memory and, to a very small extent, the disk or other components.
Also, data compression techniques, which reduce packet length (and
hence energy usage), may result in increased power consumption due
to increased computation.
There exists a potential tradeo between computation and communication
costs. Techniques that strive to achieve lower communication
costs may result in higher computation needs, and vice-versa.
Survey of Energy Ecient Network Protocols for Wireless 7
Hence, protocols that are developed with energy eciency goals should
attempt to strike a balance between the two costs.
3.2. General Conservation Guidelines and Mechanisms
The following discussion presents some general guidelines that may
be adopted for an energy ecient protocol design, and Figure 3 lists
areas in which conservation mechanisms are benecial. Examples are
provided in which these guidelines have been adopted. Some mechanisms
are better suited for infrastructure networks and others for ad
hoc networks.
Collisions should be eliminated as much as possible within the MAC
layer since they result in retransmissions. Retransmissions lead to un-necessary
power consumption and to possibly unbounded delays. Re-transmissions
cannot be completely avoided in a wireless network due
to the high error-rates. Similarly, it may not be possible to fully eliminate
collisions in a wireless mobile network. This is partly due to user
mobility and a constantly varying set of mobiles in a cell. For example,
new users registering with the base station may have to use some form
of random access protocol. In this case, using a small packet size for
registration and bandwidth request may reduce energy consumption.
The EC-MAC protocol (Sivalingam et al., 2000) is one example that
avoids collisions during reservation and data packet transmission.
In a typical broadcast environment, the receiver remains on at all
times which results in signicant power consumption. The mobile radio
receives all packets, and forwards only the packets destined for the
receiving mobile. This is the default mechanism used in the IEEE 802.11
wireless protocol in which the receiver is expected to keep track of channel
status through constant monitoring. One solution is to broadcast a
schedule that contains data transmission starting times for each mobile
as in (Sivalingam et al., 2000). This enables the mobiles to switch to
standby mode until the receive start time. Another solution is to turn
the transceiver whenever the node determines that it will not be receiving
data for a period of time. The PAMAS protocol (Singh and Raghavendra, 1998)
uses such a method.
Furthermore, signicant time and power is spent by the mobile radio
in switching from transmit to receive modes, and vice versa. A
protocol that allocates permission on a slot-by-slot basis suers substantial
overhead. Therefore, this turnaround is a crucial factor in the
performance of a protocol. If possible, the mobile should be allocated
contiguous slots for transmission or reception to reduce turnaround,
resulting in lower power consumption. This is similar to buering writes
to the hard disk in order to minimize seek latency and head move-
8 Jones, Sivalingam, Agrawal and Chen
ment. Also, it is benecial for mobiles to request multiple transmission
slots with a single reservation packet when requesting bandwidth in
order to reduce the reservation overhead. This leads to improved band-width
usage and energy eciency. The scheduling algorithms studied
in (Chen et al., 1999b) consider contiguous allocation and aggregate
packet requests.
Assuming that mobiles transmit data transmission requests to the
base station, a centralized scheduling mechanism that computes the
system transmission schedule at the base station is more energy ef-
cient. A distributed algorithm in which each mobile computes the
schedule independently may not be desirable because mobiles may
not receive all reservation requests due to radio and error constraints,
and schedule computation consumes energy. Thus, computation of the
transmission schedule ought to be relegated to the base station, which
in turn broadcasts the schedule to each mobile. Most reservation and
scheduling based protocols require the base station to compute the
schedule.
The scheduling algorithm at the base station may consider the node's
battery power level in addition to the connection priority. This allows
trac from low-power mobiles that may be dropped due to depletion
of power reserves to be transmitted sooner. Such a mechanism has
been studied in (Price, 2000, Price et al., 2001). Also, under low-power
conditions, it may be useful to allow a mobile to re-arrange allocated
slots among its own
ows. This may allow certain high-priority trac to
be transmitted sooner rather than waiting for the originally scheduled
time. Such mobile-based adaptive algorithms have been considered in
(Damodaran and Sivalingam, 1999) and (Chen, 2000) in the context of
energy eciency and channel error compensation.
At the link layer, transmissions may be avoided when channel conditions
are poor, as studied in (Zorzi and Rao, 1997a). Also, error control
schemes that combine Automatic Repeat Request (ARQ) and Forward
mechanisms may be used to conserve power
(i.e. tradeo retransmissions with ARQ versus longer packets with
FEC) as in (Lettieri et al., 1997).
Energy ecient routing protocols may be achieved by establishing
routes that ensure that all nodes equally deplete their battery power,
as studied in (Woo et al., 1998, Chang and Tassiulas, 2000). This helps
balance the amount of trac carried by each node. A related mechanism
is to avoid routing through nodes with lower battery power, but this
requires a mechanism for dissemination of node battery power. Also,
the periodicity of routing updates can be reduced to conserve energy,
but may result in inecient routes when user mobility is high. Another
method to improve energy performance is to take advantage of the
Survey of Energy Ecient Network Protocols for Wireless 9
broadcast nature of the network for broadcast and multicast trac as in
(Singh et al., 1999, Wieselthier et al., 2000). In (Ramanathan and Rosales-Hain, 2000),
the topology of the network is controlled by varying the transmit power
of the nodes, and the topology is generated to satisfy certain network
properties.
At the OS level, the common factor to all the dierent techniques
proposed is suspension of a specic sub-unit such as disk, memory, dis-
play, etc. based upon detection of prolonged inactivity. Several methods
of extending battery lifetime within the operating system and middle-ware
layer are discussed in (Tiwari et al., 1994, Chandrakasan and Brodersen, 1995,
Mehta et al., 1997). Other techniques studied include power-aware CPU
scheduling (Weiser et al., 1994, Lorch and Smith, 1997) and page allocation
(Lebeck et al., 2000). Within the application layer, the power
conserving mechanisms tend to be application specic { such as database
access (Imielinski et al., 1994, Alonso and Ganguly, 1993) and video processing
(Chandrakasan and Brodersen, 1995, Gordon et al., 1996, Agrawal et al., 1998).
A summary of software strategies for energy eciency is presented in
(Lorch and Smith, 1998).
4. MAC Sublayer
The MAC (Media Access Control) layer is a sublayer of the data link
layer which is responsible for providing reliability to upper layers for
the point-to-point connections established by the physical layer. The
MAC sublayer interfaces with the physical layer and is represented
by protocols that dene how the shared wireless channels are to be
allocated among a number of mobiles. This section presents the details
of three specic MAC protocols: IEEE 802.11 (IEEE, 1998), EC-MAC
(Sivalingam et al., 2000), and PAMAS (Singh and Raghavendra, 1998).
4.1. IEEE 802.11 Standard
The IEEE 802.11 (IEEE, 1998) protocol for wireless LANs is a multiple
access technique based on CSMA/CA (Collision Sense Multiple
Access/Collision Avoidance), and is derived from the MACA protocol
described in (Karn, 1990). The basic protocol is dened as follows. A
mobile with a packet to transmit senses the transmission channel for
activity. The mobile captures the channel and transmits all pending
data packets if the channel is not busy. Otherwise, the mobile defers
transmission and enters the backo state. The time period that follows
is called the contention window and consists of a pre-determined
number of transmission slots. The mobile randomly selects a slot in
Jones, Sivalingam, Agrawal and Chen
the contention window, and continuously senses the medium until its
selected contention slot. The mobile enters the backo state again if
it detects transmission from some other mobile during that period.
However, if no transmission is detected, the mobile transmits the access
packet and captures the channel. Extensions to the basic protocol include
provisions for MAC-level acknowledgements and request-to-send
(RTS)/clear-to-send (CTS) mechanisms.
The IEEE 802.11 (IEEE, 1998) standard recommends the following
technique for power conservation. A mobile that wishes to conserve
power may switch to sleep mode and inform the base station of this
decision. The base station buers packets received from the network
that are destined for the sleeping mobile. The base station periodically
transmits a beacon that contains information about such buered pack-
ets. When the mobile wakes up, it listens for this beacon, and responds
to the base station which then forwards the packets. This approach
conserves power but results in additional delays at the mobile that
may aect the quality of service (QoS). A comparison of power-saving
mechanisms in the IEEE 802.11 and HIPERLAN standards is presented
in (Woesner et al., 1998). Presented in (Dhaou, 1999) is a load-sharing
method for saving energy in an IEEE 802.11 network. Simulation results
indicate total power savings of 5 { 15 %.
Experimental measurements of per-packet energy consumption for
an IEEE 802.11 wireless network interface is reported in (Feeney, 1999b).
This work uses the Lucent WaveLAN card for its experiments. The cost
of sending and receiving a packet is measured for a network using UDP
point-to-point (or unicast) and broadcast trac with varying packet
sizes. The energy cost is studied in terms of xed cost per packet
which re
ects MAC operation and incremental cost that depends on
packet size. The results show that both point-to-point and broadcast
trac transmission incur the same incremental costs, but point-to-point
transmission incurs higher xed costs because of the MAC coordina-
tion. The reception of point-to-point trac maintains higher xed costs
since the receiver must respond with CTS and ACK messages. However,
incremental costs of packet reception were identical for both trac
types. The study also measures power consumption for non-destination
mobiles that are in range of the sender and receiver. These experiments
are a valuable source of information and represent an important step
in expanding the knowledge of energy ecient protocol development.
4.2. EC-MAC Protocol
Although the IEEE 802.11 standard addresses energy eciency, it was
not one of the central design issues in developing the protocol. The EC-
Survey of Energy Ecient Network Protocols for Wireless 11
MAC (Energy Conserving-Medium Access Control) protocol (Sivalingam et al., 2000,
Chen et al., 1999a), on the other hand, was developed with the issue
of energy eciency as a primary design goal. The EC-MAC protocol is
dened for an infrastructure network with a single base station serving
mobiles in its coverage area. This denition can be extended to an ad
hoc network by allowing the mobiles to elect a coordinator to perform
the functions of the base station. The general guidelines outlined in the
previous section and the need to support QoS led to a protocol that
is based on reservation and scheduling strategies. Transmission in EC-
MAC is organized by the base station into frames as shown in Figure 4,
and each slot equals the basic unit of wireless data transmission.
At the start of each frame, the base station transmits the frame
synchronization message (FSM) which contains synchronization information
and the uplink transmission order for the subsequent reservation
phase. During the request/update phase, each registered mobile
transmits new connection requests and status of established queues
according to the transmission order received in the FSM. In this phase,
collisions are avoided by having the BS send the explicit order of reservation
transmission. New mobiles that have entered the cell coverage
area register with the base station during the new-user phase. Here,
collisions are not easily avoided and hence this may be operated using a
variant of Aloha. This phase also provides time for the BS to compute
the data phase transmission schedule. The base station broadcasts a
schedule message that contains the slot permissions for the subsequent
data phase. Downlink transmission from the base station to the mobile
is scheduled considering the QoS requirements. Likewise, the uplink
slots are allocated using a suitable scheduling algorithm.
Energy consumption is reduced in EC-MAC because of the use of a
centralized scheduler. Therefore, collisions over the wireless channel are
avoided and this reduces the number of retransmissions. Additionally,
mobile receivers are not required to monitor the transmission channel
as a result of communication schedules. The centralized scheduler
may also optimize the transmission schedule so that individual mobiles
transmit and receive within contiguous transmission slots. The priority
round robin with dynamic reservation update and error compensation
scheduling algorithm described in (Chen et al., 1999b) provides
for contiguous slot allocation in order to reduce transceiver turnaround.
Also, scheduling algorithms that consider mobile battery power level
in addition to packet priority may improve performance for low-power
mobiles. A family of algorithms based on this idea is presented in
(Kishore et al., 1998, Price et al., 2001).
The frames may be designed to be xed or variable length. Fixed
length frames are desirable from the energy eciency perspective, since
Jones, Sivalingam, Agrawal and Chen
a mobile that goes to sleep mode will know when to wake up to receive
the FSM. However, variable length frames are better for meeting
the demands of bursty trac. The EC-MAC studies used xed length
frames.
The energy eciency of EC-MAC is compared with that of IEEE
802.11 and other MAC protocols in (Chen et al., 1999a). This comparative
study demonstrates how careful reservation and scheduling of
transmissions avoids collisions that are expensive in energy consumption

4.3. PAMAS Protocol
While the EC-MAC protocol described above was designed primarily
for infrastructure networks, the PAMAS (Power Aware Multi-Access)
protocol (Singh and Raghavendra, 1998) was designed for the ad hoc
network, with energy eciency as the primary design goal.
The PAMAS protocol modies the MACA protocol described in
(Karn, 1990) by providing separate channels for RTS/CTS control packets
and data packets. In PAMAS, a mobile with a packet to transmit
sends a RTS (request-to-send) message over the control channel, and
awaits the CTS (clear-to-send) reply message from the receiving mobile.
The mobile enters a backo state if no CTS arrives. However, if a CTS
is received, then the mobile transmits the packet over the data channel.
The receiving mobile transmits a \busy tone" over the control channel
enabling users tuned to the control channel to determine that the data
channel is busy.
Power conservation is achieved by requiring mobiles that are not
able to receive and send packets to turn o the wireless interface.
The idea is that a data transmission between two mobiles need not
be overheard by all the neighbors of the transmitter. The use of a
separate control channel allows for mobiles to determine when and for
how long to power o. A mobile should power itself
no packets to transmit and a neighbor begins transmitting a packet not
destined for it, and (ii) it does have packets to transmit but at least one
neighbor-pair is communicating. Each mobile determines the length of
time that it should be powered through the use of a probe protocol,
the details of which are available in (Singh and Raghavendra, 1998).
Theoretical bounds on power savings for random, line, and fully connected
topologies are also presented. The results from simulation and
analysis show that between 10% to 70% power savings can be achieved
for fully connected topologies.
Survey of Energy Ecient Network Protocols for Wireless 13
5. LLC Sublayer
In this section, we focus on the error control functionality of the logical
link control (LLC) sublayer. The two most common techniques used for
error control are Automatic Repeat Request (ARQ) and Forward Error
Correction (FEC). Both ARQ and FEC error control methods waste
network bandwidth and consume power resources due to retransmission
of data packets and greater overhead necessary in error correction. Care
must be exercised while adopting these techniques over a wireless link
where the error rates are high due to noise, fading signals, and disconnections
caused from mobility. A balance needs to be maintained within
this layer between competing measures for enhancing throughput, reli-
ability, security, and energy eciency. For example, channel encoding
schemes for enhancing channel quality tend to reduce the throughput
as more redundancy is added to the transmitted information. Also,
increasing transmitted power to improve the channel to interference
ratio depletes battery energy.
Recent research has addressed low-power error control and several
energy ecient link layer protocols have been proposed. Three such
protocols are described below.
5.1. Adaptive Error Control with ARQ
An ARQ strategy that includes an adaptive error control protocol is
presented and studied in (Zorzi and Rao, 1997a, Zorzi and Rao, 1997b).
First, though, the authors propose a new design metric for protocols
developed specically for the wireless environment and three guidelines
in designing link layer protocols to be more power conserving. The new
design metric introduced in (Zorzi and Rao, 1997a) is the energy e-
ciency of a protocol which is dened as the ratio between total amount
of data delivered and total energy consumed. Therefore, as more data
is successfully transmitted for a given amount of energy consumption,
the energy eciency of the protocol increases.
The following guidelines in developing a protocol should be considered
in order to maximize the energy eciency of the protocol.
1. Avoid persistence in retransmitting data.
2. Trade o number of retransmission attempts for probability of
successful transmission.
3. Inhibit transmission when channel conditions are poor.
The energy ecient protocol proposed in (Zorzi and Rao, 1997a,
Zorzi and Rao, 1997b) incorporates a probing protocol that slows down
14 Jones, Sivalingam, Agrawal and Chen
data transmission when degraded channel conditions are encountered.
The ARQ protocol works as normal until the transmitter detects an
error in either the data or control channel due to the lack of a received
acknowledgement (ACK). At this time the protocol enters a probing
mode in which a probing packet is transmitted every t slots. The probe
packet contains only a header with little or no payload and therefore
consumes a smaller amount of energy. This mode is continued until
a properly received ACK is encountered, indicating the recovered status
of both channels. The protocol then returns to normal mode and
continues data transmission from the point at which it was interrupted.
Using a Markov model based analysis and a recursive technique, the
probing protocol is compared to traditional ARQ schemes, and
the tradeo between performance and energy eciency is investigated.
The results show that under slow fading channel conditions the proposed
protocol is superior to that of standard ARQ in terms of energy
eciency, increasing the total number of packets that can be trans-
mitted. The analysis also demonstrates that an optimal transmission
power in respect to energy eciency exists. Using a high transmission
power to maximize the probability of a successful transmission may
not be the best strategy. Although decreasing the transmission power
results in an increased number of transmission attempts, it may be
more ecient than attempting to maximize the throughput per slot.
The conclusion reached is that although throughput is not necessarily
maximized, the energy eciency of a protocol may be maximized by
decreasing the number of transmission attempts and/or transmission
power in the wireless environment.
5.2. Adaptive Error Control with ARQ/FEC Combination
The above error control scheme included only ARQ strategies. However,
the energy ecient error control scheme proposed in (Lettieri et al., 1997)
combines ARQ and FEC strategies. The authors describe an error
control architecture for the wireless link in which each packet stream
maintains its own time-adaptive customized error control scheme based
on certain set up parameters and a channel model estimated at run-
time. The idea behind this protocol is that there exists no energy
ecient \one-size-ts-all" error control scheme for all trac types and
channel conditions. Therefore, error control schemes should be customized
to trac requirements and channel conditions in order to
obtain more optimal energy savings for each wireless connection.
The dynamic error control protocol described in (Lettieri et al., 1997)
operates as follows. Service quality parameters, such as packet size and
QoS requirements, used by the MAC sublayer and packet scheduler
Survey of Energy Ecient Network Protocols for Wireless 15
are associated with each data stream. These parameters are further
used to select an appropriately customized combination of an ARQ
scheme (Go-Back-N, Cumulative Acknowledgement (CACK), Selective

Acknowledgement

etc.) and FEC scheme. In order to keep
energy consumption at a minimum, the error control scheme associated
with each stream may need to be modied as channel conditions change
over time. Studies based on analysis and simulation under dierent scenarios
were presented as a guideline in choosing an error control scheme
to achieve low energy consumption while trading
channel conditions, trac types, and packet sizes. The authors extend
their research in (Lettieri and Srivastava, 1998) to include a protocol
for dynamically sizing the MAC layer frame, depending upon wireless
channel conditions.
5.3. Adaptive Power Control and Coding Scheme
Finally, a dynamic power control and coding protocol for optimizing
throughput, channel quality, and battery life is studied in (Agrawal et al., 1996,
Narendran et al., 1997). This distributed algorithm, in which each mobile
determines its own operating point with respect to power and error
control parameters, maintains the goal of minimizing power utilization
and maximizing capacity in terms of the number of simultaneous con-
nections. Power control, as dened by the authors, is the technique
of controlling the transmit power so as to aect receiver power, and
ultimately the carrier-to-interference ratio (CIR).
The energy ecient power control and coding protocol operates in
the following manner. Each transmitter operates at a power-code pair
in which the power level lies between a specied minimum and maximum
and the error code is chosen from a nite set. The algorithm is
iterative in nature with the transmitter and receiver, at each iteration,
cooperatively evaluating channel performance and determining if an
adjustment in the power-code pair is necessary. The time between each
iteration is referred to as a timeframe. After each timeframe the receiver
involved in the data transmission evaluates the channel performance by
checking the word error rate (WER). If the WER lies within an acceptable
range, the power-code pair is retained; otherwise a new power-code
pair is computed by the transmitter. The basic frame of the algorithm
can be modied such that optimal levels of control overhead and channel
quality are traded o. Also, variations of the base algorithm include
the evaluation of the average WER, rather than the instantaneous
WER in each timeframe, in determining channel quality and in the
evaluation of anticipated channel performance. The latter adaptation of
the algorithm attempts to predict changes in error rates due to mobility
Jones, Sivalingam, Agrawal and Chen
by sampling the received powers and extrapolating the values to the
next timeframe. If the predicted WER is not within acceptable ranges,
then the power-code pair is adapted to avoid unsatisfactory channel
conditions.
A study of the dynamic power control and coding protocol was performed
through simulation of a cellular system with roaming mobiles.
Simulation results indicate that the proposed dynamic power control
and coding protocol supports better quality channels as compared to
schemes that use xed codes; therefore power-control alone does not
perform as well as an adaptive power-control/FEC protocol.
The next section discusses energy ecient routing protocols within
the network layer.
6. Network Layer
The main functions of the network layer are routing packets and congestion
control. In wireless mobile networks, the network layer has the
added functionality of routing under mobility constraints and mobility
management including user location, update, etc. In this section, we
present energy ecient routing algorithms developed for wireless ad
hoc networks. Energy ecient routing does not apply to infrastructure
networks because all trac is routed through the base station.
As mentioned earlier, in ad hoc networks the mobiles cooperate to
maintain topology information and use multi-hop packet routing. The
problem of routing is complicated due to user mobility resulting in
frequently changed network topologies. The rate of topology change
depends on many factors including user mobility speeds and terrain
characteristics. Typical routing algorithms for ad hoc networks consider
two dierent approaches:
1. Use frequent topology updates resulting in improved routing, but
increased update messages consume precious bandwidth.
2. Use infrequent topology updates resulting in decreased update mes-
sages, but inecient routing and occasionally missed packets results

Typical metrics used to evaluate ad hoc routing protocols are shortest-
hop, shortest-delay, and locality stability (Woo et al., 1998). However,
these metrics may have a negative eect in wireless networks because
they result in the overuse of energy resources of a small set of mobiles,
decreasing mobile and network life. For example, consider the wireless
network in Figure 5. Using shortest-hop routing, trac from mobile A
Survey of Energy Ecient Network Protocols for Wireless 17
to mobile D will always be routed through mobile E, which will drain
the energy reserves of E faster. If mobile E's battery becomes fully
drained, then mobile F is disconnected from the network and communication
to and from F is no longer possible. By using a routing algorithm
that takes into account such issues, trac from A to D may not always
be routed through E, but through mobiles B and C, extending network
life. Consequently, it is essential to consider routing algorithms from
an energy ecient perspective, in addition to traditional metrics. Such
research is described in the following paragraphs.
6.1. Unicast Traffic
Unicast trac is dened as trac in which packets are destined for
a single receiver. In (Woo et al., 1998), routing of unicast trac is
addressed with respect to battery power consumption. The authors'
research focuses on designing protocols to reduce energy consumption
and to increase the life of each mobile, increasing network life as well.
To achieve this, ve dierent metrics are dened from which to study
the performance of power-aware routing protocols.
Energy consumed per packet: It is easy to observe that if energy consumed
per packet is minimized then the total energy consumed is also
minimized. Under light loads, this metric will most likely result in the
shortest-hop path. As network load increases, this is not necessarily
the case because the metric will tend to route packets around areas of
congestion in the network.
Time to network partition: Given a network topology, a minimal set
of mobiles exist such that their removal will cause the network to
partition. Routes between the two partitions must go through one of
the \critical" mobiles; therefore a routing algorithm should divide the
work among these mobiles in such a way that the mobiles drain their
power at equal rates.
Variance in power levels across mobiles: The idea behind this metric is
that all mobiles in a network operate at the same priority level. In this
way, all mobiles are equal and no one mobile is penalized or privileged
over any other. This metric ensures that all mobiles in the network
remain powered-on together for as long as possible.
Cost per packet: In order to maximize the life of all mobiles in the
network, metrics other than energy consumed per packet need to be
used. When using these metrics, routes should be created such that
mobiles with depleted energy reserves do not lie on many routes. To-
gether, these metrics become the \cost" of a packet, which needs to be
minimized.
Jones, Sivalingam, Agrawal and Chen
Maximum mobile cost: This metric attempts to minimize the cost experienced
by a mobile when routing a packet through it. By minimizing
the cost per mobile, signicant reductions in the maximum mobile cost
result. Also, mobile failure is delayed and variance in mobile power
levels is reduced due to this metric.
In order to conserve energy, the goal is to minimize all the metrics
except for the second which should be maximized. As a result,
a shortest-hop routing protocol may no longer be applicable; rather,
a shortest-cost routing protocol with respect to the ve energy e-
ciency metrics would be pertinent. For example, a cost function may
be adapted to accurately re
ect a battery's remaining lifetime. The
premise behind this approach is that although packets may be routed
through longer paths, the paths contain mobiles that have greater
amounts of energy reserves. Also, energy can be conserved by routing
trac through lightly loaded mobiles because the energy expended in
contention and retransmission is minimized.
The properties of the power-aware metrics and the eect of the
metrics on end-to-end delay are studied in (Woo et al., 1998) using
simulation. A comparison of shortest-hop routing and the power-aware
shortest-cost routing schemes was conducted. The performance measures
were delay, average cost per packet, and average maximum node
cost. Results show that usage of power-aware metrics result in no extra
delay over the traditional shortest-hop metric. This is true because
congested paths are often avoided. However, there was signicant improvement
in average cost per packet and average maximum mobile
cost in which the cost is in terms of the energy ecient metrics dened
above. The improvements were substantial for large networks and
heavily-loaded networks. Therefore, by adjusting routing parameters
a more energy ecient routing scheme may be utilized for wireless
networks.
The above approach to routing in wireless ad hoc networks requires,
at the least, that every mobile have knowledge of the locations of every
other mobile and the links between them. This creates signicant
communication overhead and increased delay. Research completed in
(Stojmenovic and Lin, 2000) addresses this issue by proposing localized
routing algorithms which 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
within each mobile. Therefore, excessive network communication is not
required which, the authors report, more than makes up for the extra
energy consumed by the GPS units.
Survey of Energy Ecient Network Protocols for Wireless 19
A new power-cost metric incorporating both a mobile's lifetime and
distance based power metrics is proposed, and using the newly dened
metric, three power-aware localized routing algorithms are developed:
power, cost, and power-cost. The power algorithm attempts to minimize
the total amount of power utilized when transmitting a packet,
whereas the cost algorithm avoids mobiles that maintain low battery
reserves in order to extend the network lifetime. Finally, the power-cost
routing algorithm is a combination of the two algorithms. Experiments
validated the performance of these routing algorithms.
6.2. Broadcast Traffic
In this section broadcast trac, which is dened as trac in which
packets are destined for all mobiles in the system, is considered. With
a single transmission, a mobile is able to broadcast a packet to all
immediate neighbors. However, each mobile needs to receive a packet
only once. Intermediate mobiles are required to retransmit the packet.
The key idea in conserving energy is to allow each mobile's radio to
turn after receiving a packet if its neighbors have already received a
copy of the packet. Addressed in (Singh et al., 1999) is the routing of
broadcast trac in terms of power consumption.
The broadcast technique used in traditional networks is a simple
ooding algorithm. This algorithm gathers no global topology infor-
mation, requiring little control overhead and completes the broadcast
with minimum number of hops. However, the
ooding algorithm is not
suitable for wireless networks because many intermediate nodes must
retransmit packets needlessly which leads to excessive power consump-
tion. Therefore, the authors of (Singh et al., 1999) propose that it is
more benecial to spend some energy in gathering topology information
in order to determine the most ecient broadcast tree.
In order to increase mobile and network life, any broadcast algorithm
used in the wireless environment should focus on conserving energy
and sharing the cost of routing among all mobiles in the system. One
way to conserve power is by ensuring that a transmission reaches as
many new nodes as possible. A broadcast tree approach is presented
in (Singh et al., 1999), in which the tree is constructed starting from
a source and expanding to the neighbor that has the lowest cost per
outgoing degree, where the cost associated with each mobile increases
as the mobile consumes more power. Therefore, priority for routing
packets through the broadcast tree is given to nodes that have consumed
lower amounts of power and nodes that have more neighbors
which have not already received the data transmission. Since mobile
costs continuously change, broadcast transmissions originating from
Jones, Sivalingam, Agrawal and Chen
the same source may traverse dierent trees, as they are determined
based on current costs of nodes.
Simulations were conducted in order to study the performance of the
proposed power-aware broadcast protocol as compared to traditional
ooding in terms of energy savings as well as delay. Averaged over
a period of time, the power-aware protocol demonstrates very little
dierence in broadcast delay. However, results indicate that savings
in energy consumption of 20% or better are possible using the power-aware
broadcast algorithm, with greater savings in larger networks and
networks with increased trac loads.
The construction of energy ecient broadcast and multicast trees for
the wireless environment is also studied in (Wieselthier et al., 2000).
The authors state that mobiles may experience greater energy conservation
if routing decisions are combined with decisions concerning
transmission power levels. An algorithm is presented for determining
the minimum-energy source-based tree for each broadcast/multicast
session request. The algorithm is based on the concept that there exists
an optimal point in the trade-o between reaching greater number of
mobiles in a single hop by using higher transmission power versus reaching
fewer mobiles but using lower power levels. Performance results
demonstrate that the combination of routing and transmission power
decisions provide greater energy conservation. A similar idea concerning
the incorporation of transmission power levels into routing algorithms
is also presented for unicast trac in (Chang and Tassiulas, 2000).
In (Feeney, 1999a), a simulation based comparison of energy consumption
for two ad hoc routing protocols { Dynamic Source Routing
(DSR) (Johnson et al., 2000) and Ad hoc On Demand Vector routing
(AODV) (Perkins et al., 2000) protocols is presented. The analysis
considers the cost for sending and receiving trac, for dropped pack-
ets, and for routing overhead packets. User mobility is modeled in the
analysis. The observations indicate that energy spent on receiving and
discarding packets can be signicant. Also, the costs of
ooding-based
broadcast trac and MAC control were seen to be signicant. For DSR,
results show that the cost of source routing headers was not very high,
but operating the receiver in promiscuous mode for caching and route
response purposes resulted in high power consumption. Results also
indicate that since AODV generates broadcast trac more often, the
energy cost is high given that broadcast trac consumes more energy.
Refer to (Feeney, 1999a) for more detailed results.
The next section presents work related to improving transport protocol
performance in the wireless environment.
Survey of Energy Ecient Network Protocols for Wireless 21
7. Transport Layer
The transport layer provides a reliable end-to-end data delivery service
to applications running at the end points of a network. The most
commonly used transport protocol for wired networks, where underlying
physical links are fairly reliable and packet loss is random in
nature, is the Transmission Control Protocol (TCP) (Postel, 1981).
However, due to inherent wireless link properties, the performance
of traditional transport protocols such as TCP degrades signicantly
over a wireless link. TCP and similar transport protocols resort to
a larger number of retransmissions and frequently invoke congestion
control measures, confusing wireless link errors and loss due to hando
as channel congestion. This can signicantly reduce throughput and
introduce unacceptable delays (Caceres and Iftode, 1995). As stated
increased retransmissions unnecessarily consume battery energy
and limited bandwidth.
Recently, various schemes have been proposed to alleviate the eects
of non congestion-related losses on TCP performance over networks
with wireless links. These schemes, which attempt to reduce retrans-
missions, are classied into three basic groups: (i) split connection
protocols, (ii) link-layer protocols, and (iii) end-to-end protocols.
connection protocols completely hide the wireless link from
the wired network by terminating the TCP connections at the base
station as shown in Figure 6. This is accomplished by splitting each
connection between the source and destination into two separate
connections at the base station. The result is one TCP connection
between the wired network and the base station and a second TCP connection
between the base station and the mobile. The second connection
over the wireless link may use modied versions of TCP that enhance
performance over the wireless channel. Examples of split connection
protocols include Indirect-TCP (Bakre and Badrinath, 1995), Berkeley
Snoop Module (Balakrishnan et al., 1995), and M-TCP (Brown and Singh, 1997).

Figure

7 depicts the link layer approach which attempts to hide
link related losses from the TCP source by using a combination of
local retransmissions and forward error correction over the wireless
link. Local retransmissions use techniques that are tuned to the characteristics
of the wireless channel to provide signicant increase in
performance. One example of a link layer protocol is the AIRMAIL
protocol (Ayanoglu et al., 1995), which employs a combination of both
FEC and ARQ techniques for loss recovery.
Finally, end-to-end protocols include modied versions of TCP that
are more sensitive to the wireless environment. End-to-end protocols
require that a TCP source handle losses through the use of such mecha-
22 Jones, Sivalingam, Agrawal and Chen
nisms as selective acknowledgements and explicit loss notication (ELN).
Selective acknowledgements allow the TCP source to recover from multiple
packet losses, and ELN mechanisms aid the TCP source in distinguishing
between congestion and other forms of loss.
7.1. Energy Consumption Analysis of TCP
The procotols described previously generally achieve higher through-put
rates over the wireless channel than standard TCP because the
protocols are better able to adapt to the dynamic mobile environ-
ment. However, the performance of a particular protocol is largely
dependent upon various factors such as mobility handling, amount of
overhead costs incurred, frequency and handling of disconnections, etc.
Therefore, performance and energy conservation may range widely for
these protocols depending upon both internal algorithm and external
environmental factors. Although the above protocols, along with many
others proposed in research, have addressed the unique needs of designing
transport protocols in the wireless environment which may or may
not lead to greater energy eciency, they have not directly addressed
the idea of a low-power transport protocol.
The energy consumption of Tahoe, Reno, and New Reno versions
of TCP is analyzed in (Zorzi and Rao, 2000). Energy consumption is
the main parameter studied with the objective of measuring the effect
of TCP transmission policies on energy performance. The energy
eciency of a protocol is dened as the average number of successful
transmissions per energy unit, which can be computed as the average
number of successes per transmission attempt. A two-state Markov
packet error process is used in the performance evaluation of a single
transceiver running the various versions of TCP on a dedicated
wireless link. Results of the study demonstrate that error correlation
signicantly aects the energy performance of TCP and that congestion
control algorithms of TCP actually allow for greater energy savings by
backing waiting during error bursts. It is also seen that energy
eciency may be quite sensitive to the version of TCP implemented
and the choice of protocol parameters.
The same versions of TCP were studied in (Tsaoussidis et al., 2000a)
in terms of energy/throughput tradeos. Simulation results show that
no single TCP version is most appropriate within wired/wireless heterogenous
networks, and that the key to balancing energy and through-put
performance is through the error control mechanism. Using these
results, the authors propose a modied version of TCP, referred to as
TCP-Probing, in (Tsaoussidis and Badr, 2000).
Survey of Energy Ecient Network Protocols for Wireless 23
In TCP-Probing, data transmission is suspended and a probe cycle
is initiated when a data segment is delayed or lost, rather than immediately
invoking congestion control. A probe cycle consists of an exchange
of probe segments between sender and receiver. Probe segments are implemented
as extensions to the TCP header and carry no payload. The
sender monitors the network through the probe cycle which terminates
when two consecutive round-trip-times (RTT) are successfully
measured. The sender invokes standard TCP congestion control if persistent
error conditions are detected. However, if monitored conditions
indicate transient random error, then the sender resumes transmission
according to available network bandwidth. Simulation results provided
in (Tsaoussidis and Badr, 2000) indicate that TCP-Probing achieves
higher throughput rates while consuming less energy. Therefore, the
authors believe that TCP-Probing provides a universal error control
mechanism for heterogenous wired/wireless networks. The authors also
present in (Tsaoussidis et al., 2000b) an experimental transport proto-
col, called Wave and Wait Protocol (WWP), developed specically for
a wireless environment with limited power.
8. OS/Middleware and Application Layers
This section addresses research completed at the OS/middleware and
application layers with respect to energy eciency.
8.1. OS/Middleware
One important advantage of integrating wireless communication with
computing is that it facilitates user mobility and connectivity to the
network. Mobility, directly or indirectly, impacts the design of operating
systems, middleware, le systems, and databases. It also presents a
new set of challenges that result from power constraints and voluntary
disconnections. To be consistent with xed counterparts like PCs and
workstations, mobile computers need to process multimedia informa-
tion. However, such processing is expensive in terms of both bandwidth
and battery power. In general, the majority of the techniques used in
the design of today's applications to conserve bandwidth also conserve
battery life.
The main function of an operating system is to manage access to
physical resources like CPU, memory, and disk space from the applications
running on the host. To reduce power dissipation, CPUs used
in the design of portable devices can be operated at lower speeds by
scaling down the supply voltage (Chandrakasan and Brodersen, 1995).
Jones, Sivalingam, Agrawal and Chen
Due to the quadratic relationship between power and supply voltage,
halving the supply voltage results in one fourth of the power being
consumed. To maintain the same throughput, the reduction in circuit
speed can be compensated by architectural techniques like pipelining
and parallelism. These techniques increase throughput resulting in an
energy ecient system operating at a lower voltage but with the same
throughput. The operating system is active in relating scheduling and
delay to speed changes.
Another technique of power management at this layer is predictive
shutdown (Chandrakasan and Brodersen, 1995). This method exploits
the event driven nature of computing in that sporadic computation
activity is triggered by external events and separated by periods of
inactivity. A straightforward means of reducing average energy consumption
is to shut down the system during periods of inactivity. How-
ever, preserving the latency and throughput of applications requires
intelligent activity-based predictive shutdown strategies.
In (Lebeck et al., 2000), a study of dierent page placement algorithms
that exploit the new power management features of memory
technology is presented. The study considers DRAM chips that support
dierent power modes: active, standby, nap and powerdown. Trace-driven
and execution-driven simulations show that improvement of 6%
to 55% in the Energy  Delay metric are obtained using power-aware
page allocation mechanisms that operate in conjunction with hardware
policies.
CPU scheduling techniques that attempt to minimize power consumption
are presented in (Weiser et al., 1994, Lorch and Smith, 1997).
The impact of software architecture on power consumption is studied
in (Tiwari et al., 1994, Mehta et al., 1997).
8.2. Application Layer
The application layer in a wireless system is responsible for such things
as partitioning of tasks between the xed and mobile hosts, audio and
video source encoding/decoding, and context adaptation in a mobile
environment. Energy eciency at the application layer is becoming an
important area of research as is indicated by industry. APIs such as Advanced
Conguration and Power Interface (Intel Corporation, Microsoft and Toshiba Corporation, 2000)
and power management analysis tools such as Power Monitor (Intel Corporation, 2000)
are being developed to assist software developers in creating programs
that are more power conserving. Another power management tool developed
at Carnegie Mellon University is PowerScope (Flinn and Satyanarayanan, 1999).
PowerScope maps energy consumption to program structure, producing
a prole of energy usage by process and procedure. The authors
Survey of Energy Ecient Network Protocols for Wireless 25
report a 46% reduction in energy consumption of an adaptive video
playing application by taking advantage of the information provided
by PowerScope. This section summarizes some of the research being
conducted at the application layer with respect to power conservation.
Load Partitioning: Challenged by power and bandwidth constraints,
applications may be selectively partitioned between the mobile and base
station (Weiser et al., 1994, Narayanaswamy et al., 1996). Thus, most
of the power intensive computations of an application are executed at
the base station, and the mobile host plays the role of an intelligent terminal
for displaying and acquiring multimedia data (Narayanaswamy et al., 1996).
Proxies: Another means of managing energy and bandwidth for applications
on mobile clients is to use proxies. Proxies are middleware that
automatically adapt the applications to changes in battery power and
bandwidth. A simple example of proxy usage during multimedia transmissions
in a low-power or low bandwidth environment is to suppress
video and permit only audio streams. Another example is to direct a
le to be printed at the nearest printer when the host is mobile. Proxies
are either on the mobile or base station side of the wireless link.
Databases: Impact of power eciency on database systems is considered
by some researchers. For example, energy eciency in database design
by minimizing power consumed per transaction through embedded indexing
has been addressed in (Imielinski et al., 1994). By embedding
the directory in the form of an index, the mobile only needs to become
active when data of interest is being broadcast (the system architecture
consists of a single broadcast channel). When a mobile needs a piece of
information an initial probe is made into the broadcast channel. The
mobile is then able to determine the next occurrence of the required
index and enters probe wait mode while it waits for the index to be
broadcast. After receiving the index information relevant to the required
data, the mobile enters bcast wait mode while it waits for the
information to be broadcast. Access time is dened as the sum of the
two waiting periods, probe wait and bcast wait. The goal of the authors
is to provide methods to combine index information together with data
on the single broadcast channel in order to minimize access time. The
authors propose two such strategies which are further described in
(Imielinski et al., 1994). Also, energy ecient query optimization for
database systems is described in (Alonso and Ganguly, 1993).
Video Processing: Multimedia processing and transmission require considerable
battery power as well as network bandwidth. This is especially
true for video processing and transmission. However, reducing the eec-
tive bit rate of video transmissions allows lightweight video encoding
26 Jones, Sivalingam, Agrawal and Chen
and decoding techniques to be utilized thereby reducing power con-
sumption. Under severe bandwidth constraints or low-power situations,
video frames can even be carefully discarded before transmission while
maintaining tolerable video quality.
In (Agrawal et al., 1998), research on processing encoded video for
transmission under low battery power conditions is presented. The basic
idea of this work is to decrease the number of bits transmitted over the
wireless link in response to low-power situations. The challenge is to
accomplish this goal while preserving or minimally degrading the video
quality. Decreasing the number of transmitted bits reduces the energy
consumption due to reduced transmitter usage. In fact, several studies
have shown that transmission accounts for more than a third of the
energy consumption in video processing and exchange in a portable
device. The reduction in the number of bits can be achieved in one
of two ways: (i) reducing the number of bits in the compressed video
stream generated by the video encoder, and (ii) discarding selected
packets at the wireless network interface card (WNIC).
The rst approach is possible only if two conditions are satised.
The portable device must be encoding a video stream as opposed to
transmitting a stored video, and the application must be able to modify
parameters in the video encoder. The second approach is possible if the
WNIC is
exible and sensitive to battery power conditions. Further
details on how the dierent encoding schemes aect the choice of discarding
may be found in (Agrawal et al., 1998). Also, a testbed implementing
this research was developed, and preliminary results reported
in (Mahadevan, 1999).
Power-aware video processing is an important and exciting topic.
There are several approaches for developing ecient encoding schemes
that will impact performance and energy consumption as in (Liu and Zarki, 1998,
Swann and Kingsbury, 1998). However, a complete discussion is not
presented due to space constraints.
9.

Summary

As wireless services continue to add more capabilities such as multi-media
and QoS, low-power design remains one of the most important
research areas within wireless communication. Research must focus on
decreasing the amount of energy consumed by the wireless terminal.
Power conservation has typically been considered at the physical layer.
However, most of the energy savings at the physical layer have already
been achieved. Therefore, the key to energy conservation in wireless
Survey of Energy Ecient Network Protocols for Wireless 27
communications lies within the higher levels of the wireless protocol
stack. This paper describes research completed at the data link, net-
work, transport, OS/middleware, and application protocol layers that
have addressed energy eciency for wireless networks. However, power
conservation within the wireless protocol stack remains a very crucial
research area for the viability of wireless services in the future.

Acknowledgements

The authors wish to thank the reviewers and the Editor for their
valuable suggestions and comments that helped improve the paper;
and Stephanie Lindsey and Harini Krishnamurthy who assisted with
editing the paper.
28 Jones, Sivalingam, Agrawal and Chen
Correspondence Information
Prof. Krishna Sivalingam
Boeing Associate Professor of Computer Science
School of Elect. Engg. & Computer Science
Building
Washington State University
Pullman,
Phone: 509 335 3220
Fax: 253-295-9458 (Please email to addr below after sending Fax)
Survey of Energy Ecient Network Protocols for Wireless 29
Author Biographies:
Christine Jones received her M.S. in Computer Science from Washington
State University, Pullman in 2000 and her B.S. in Computer
Science from Whitworth College in 1998. She is presently with BBN
Technologies in Cambridge, MA, USA.
Photo for Jones is included as a JPEG le.
Krishna M. Sivalingam (ACM '93, IEEE SM '00 M '95) received
his Ph.D. and M.S. degrees in Computer Science from State University
of New York at Bualo in 1994 and 1990 respectively. While at SUNY
Bualo, he was a Presidential Fellow from 1988 to 1991. Prior to that,
he received the B.E. degree in Computer Science and Engineering in
1988 from Anna University, Madras, India. He is Boeing Associate Professor
of Computer Science in the School of Electrical Engineering and
Computer Science, at Washington State University, Pullman, where
he was an Assistant Professor from 1997 to 2000. Earlier, he was an
Assistant Professor at University of North Carolina Greensboro from
1994 until 1997. He has conducted research at Lucent Technologies'
Bell Labs in Murray Hill, NJ, and at AT&T Labs in Whippany, NJ.
His research interests include wireless networks, optical wavelength
division multiplexed networks, and performance evaluation. He has
served as a Guest Co-Editor for a special issue of the IEEE Journal
on Selected Areas in Communications on optical WDM networks. He
is co-recipient of the Best Paper Award at the IEEE International
Conference on Networks 2000 held in Singapore. He has published an
edited book on optical WDM networks in 2000. His work is supported
by AFOSR, Laboratory for Telecommunication Sciences, NSF, Cisco,
Bellcore, Alcatel, Intel, and Washington Technology Center. He holds
three patents in wireless networks and has published several papers
including journal publications. He has served on several conference
committees including ACM Mobicom 2001, Opticom 2000, ACM Mobicom
99, MASCOTS 99, and IEEE INFOCOM 1997. He is a Senior
Member of IEEE and a member of ACM. Email: krishna@eecs.wsu.edu
Photo of Krishna Sivalingam is enclosed in hard-copy.
Prathima Agrawal is Assistant Vice President of the Internet Architecture
Research Laboratory and Executive Director of the Computer
Networking Research Department at Telcordia Technologies (formerly
Bellcore), Morristown, NJ. She is also an adjunct Professor of Electrical
and Computer Engineering, Rutgers University, NJ. She worked
for 20 years in AT&T/Lucent Bell Laboratories in Murray Hill, NJ,
where she was Head of the Networked Computing Research Depart-
ment. Presently, she leads the ITSUMO joint research project between
Telcordia and Toshiba Corp. ITSUMO is a third generation wireless
Jones, Sivalingam, Agrawal and Chen
access system for multimedia communication over end-to-end packet
networks. The ITSUMO project received the Telcordia CEO Award for
2000. Dr. Agrawal received her Ph.D. degree in Electrical Engineering
from the University of Southern California. Her research interests are
computer networks, mobile and wireless computing and communication
systems and parallel processing. She has published over 150 papers and
has received or applied for more than 50 U.S. patents. Dr. Agrawal is
a Fellow of the IEEE and a member of the ACM. She is the recipient
of an IEEE Third Millennium Medal in 2000. She chaired the IEEE
Fellow Selection Committee during 1998-2000.
Photo of Agrawal is enclosed as a JPEG le.
Jyh-Cheng Chen received the B.S. degree in information science
from Tunghai University, Taichung, Taiwan, in 1990, the M.S. degree in
computer engineering from Syracuse University, Syracuse, NY, in 1992,
and the Ph.D. degree in electrical engineering from the State University
of New York, Bualo, in 1998.
Since August 1998, he has been a Research Scientist in Applied
Research at Telcordia Technologies, Morristown, NJ. At Telcordia, he
is one of the key architects and implementers of the ITSUMO (Inter-
net Technologies Supporting Universal Mobile Operation) project, in
which he has been working on QoS for mobile and wireless IP net-
works, IP-based base station design, SIP-based mobility management,
and multimedia applications, etc. He received the 2000 Telcordia CEO
Award that was intended to recognize and honor the Company's most
exceptional teams and individuals. While working toward his Ph.D.,
he also worked on energy ecient MAC protocols for wireless ATM
networks at AT&T Labs, Whippany, NJ, and non-destructive coating
thickness measurement at ASOMA-TCI Inc., N. Tonawanda, NY.
Dr. Chen is a member of IEEE and ACM.
Email: jcchen@research.telcordia.com
Photo of Chen is enclosed as a JPEG le.
Survey of Energy Ecient Network Protocols for Wireless 31



--R









Computer Communication Review 27(5)

Low Power Digital CMOS Design.
Kluwer Academic Publishers.



IEEE Transactions on Multimedia 1(2)

Master's thesis







http://www.




Infrared Data Association:
http://www.

http://developer.
Microsoft and Toshiba Corporation:


In: ARRL/CRRL Amateur Radio 9 th Computer Networking Conference.



















Master's thesis






ACM/Baltzer









http://www.


http://www.



IEEE Personal Communications 5(3)




(Guest









--TR
Energy efficient indexing on air
Power analysis of embedded software
AIRMAIL: a link-layer protocol for wireless networks
Comparing algorithm for dynamic speed-setting of a low-power CPU
Design of a low power video decompression chip set for portable applications
Control and Energy Consumption in Communications for Nomadic Computing
Low power error control for wireless links
Techniques for low energy software
Scheduling techniques for reducing processor energy use in MacOS
Improving reliable transport and handoff performance in cellular wireless networks
Adaptive source rate control for real-time wireless video transmission
Power-aware routing in mobile ad hoc networks
PAMASMYAMPERSANDmdash;power aware multi-access protocol with signalling for ad hoc networks
Design and analysis of low-power access protocols for wireless and mobile ATM networks
Performance comparison of battery power consumption in wireless multiple access protocols
Power aware page allocation
Energy efficiency of TCP in a local wireless environment
Low Power Digital CMOS Design
Mobile ad hoc networking and the IETF
Adaptive Scheduling at Mobiles for Wireless Networks with Multiple Priority Traffic and Multiple Transmission Channels
PowerScope
Energy/Throughput Tradeoffs of TCP Error Control Strategies
TCP-probing
Architecture and algorithms for quality of service support and energy-efficient protocols for wireless/mobile networks

--CTR
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mobile computing;network protocols;low-power design;energy efficient design;wireless networks;power aware protocols