Combined tuning of RF power and medium access control for WLANS.
Mobile communications, such as handhelds and laptops, still suffer from short operation time due to limited battery capacity. We exploit the approach of protocol harmonization to extend the time between battery charges in mobile devices using an IEEE 802.11 network interface. Many known energy saving mechanisms only concentrate on a single protocol layer while others only optimize the receiving phase by on/off switching. We show, that energy saving is also possible during the sending process. This is achieved by a combined tuning of the data link control and physical layer. In particular, harmonized operation of power control and medium access control will lead to reduction of energy consumption. We show a RF power and medium access control trade-off. Furthermore we discuss applications of the results in IEEE 802.11 networks.
Introduction
Reduction of energy consumption for mobile devices
is an emerging eld of research and engineering. The
driving factors are the weight and time in operation of
mobile devices, which should be small and should allow
for a long operation time, respectively. The weight
is determined to a large extent by the batteries. Besides
the display, CPU and hard disk, one of the main
power sinks is the wireless network interface card, which
requires power for transmitting radio signals and protocol
processing (see [1]). In this paper, we concentrate
on the wireless network interface of a mobile device.
In particular we investigate the dependencies between
MAC protocol processing and the physical layer of an
IEEE 802.11 network interface.
This work has been supported partially by a grant from the
BMBF (German Ministry of Science and Technology) within
the Priority Program ATMmobil.
Various options of power saving on the protocol level
have been published in literature. In [2] it is reported,
that contention protocols result in high energy con-
sumption, while reservation and polling may reduce it.
Furthermore in [3] it is shown that solving the hidden
terminal problem by means of a busy tone channel the
energy consumption substantially reduces in ad hoc net-
works. In [4] it is shown, that powering o the mobile's
network interface during idle times is an important option
to save energy.
The aforementioned mechanisms try to minimize energy
consumption on the MAC/DLC level (Medium Access
Control/Data Link Control). There are also several
options on the physical layer for instance by choosing
appropriate modulation and coding schemes with
respect to the assumed channel characteristics as well
as the use of low power ICs and algorithms with low
computational complexity. Another important option
in the physical layer is power control. In [5] and [6]
it is stated that not only cochannel interference is reduced
but also the system capacity and the time interval
s
between battery charges are increased. The main parameter
for power control is the required level of link
reliability, which is often expressed in terms of the bit
error rate (BER). Power control mechanisms adapt the
radio transmit power to a minimum level required to
achieve a certain link reliability. In this paper we show,
that minimizing the transmit energy does not necessarily
lead to energy savings.
We exploit a novel approach to reduce energy con-
sumption: Protocol Harmonization. In contrast to the
methods mentioned above, which try to optimize a certain
protocol or layer with respect to energy consump-
tion, protocol harmonization strives to balance the protocols
and mechanisms of dierent layers. The need for
protocol harmonization was realized at the start of the
nineties, where the poor Transmission Control Protocol
(TCP) performance over wireless received agreat deal of
attention. For instance, in [7,8] it is reported that link
level retransmissions competing with transport protocol
retransmissions are not only redundant but can degrade
the performance, especially in the case of a higher bit
error rate. This approach was rst used for the reduction
of energy consumption in [9] and [10], where error
control schemes are proposed, which perform optimally
with respect to the channel characteristics. We adopt
this approach for the reduction of power drain of an
IEEE 802.11 (see [11]) 2Mbit/s DSSS network interface
using the Distributed Coordination Function. In par-
ticular, we propose a combined tuning of the physical
and MAC layer. The system under study is shown in

Figure

1.
The idea is to reduce energy consumption by reducing
the RF transmission power. But reduction of
RF transmission power causes a higher bit error rate
and results in a higher packet error rate. The IEEE
802.11 MAC reacts with retransmissions of corrupted
packets leading to a higher power drain because of multiple
transmissions of the same packet. By reversing
this idea, it is possible to increase RF power and decrease
the bit error rate and therefore the probability
of retransmissions. But increasing RF power increases
energy consumption. These two ideas lead to a MAC re-transmission
and RF transmission power trade-o. We
analyze this trade-o and investigate the optimal operating
points to minimize energy consumption. The
next three sections, Link Budget Analysis, Gilbert-Elliot
Channel Model, and IEEE 802.11 present the basics
necessary to analyze the trade-o. In the sections Energy
Consumption and Investigation of the RF transmit
power in
uence we show that there is an optimal
value of RF transmission power minimizing the negative
eects of retransmission and in turn energy con-
sumption. We conclude the paper in Protocol Design
Recommendations with a possible application of the results
to IEEE 802.11 and Summarize the paper in the
last section.
2. IEEE 802.11 Link Budget Analysis
We present shortly the basics of top level link budget
analysis (LBA, see [12,13]). As one of the main results
RF power can be calculated for a given set of parameters
and requirements (e.g. level of link reliability).
In our case we assume the IEEE 802.11 2 MBit/s Direct
Sequence Spread Spectrum (DSSS) physical layer,
which uses a DQPSK modulation scheme, and a single
ad hoc network.
Shannons capacity theorem gives the system capacity
in an ideal environment. The real world system capacity
can approach very closely the theoretical value by
means of modulation. As we can see from equation (2.1)
the channel capacity depends on bandwidth, noise, and
signal strength. The channel capacity C is dened by
strength (watt), and (watt). The
thermal channel noise N is dened by
(Hz). An important LBA factor is the range. In
space the power of the radio signal decreases with
the square of range. The path loss L (dB) for line of
site (LOS) wave propagation is dened by
distance between transmitter and receiver
length (meter).  is dened
by c=f , where c is the speed of light (3
and f is the frequency (Hz). The formula has to be
modied for indoor scenarios, since the path loss is usually
higher and location dependent. As a rule of thumb,
LOS path loss is valid for the rst 7 meters. Beyond 7
meter, the degradation is up to 30 dB every meter
(see [13]).
RF indoor propagation very likely results in multi-path
fading. Multi-path causes signal cancellation.
Fading due to multi-path can result in signal reduction
of more than 30 dB. However, signal cancellation
is never complete. Therefore one can add a priori a certain
amount of power to the sender signal, referred to
as fade margin (L fade ), to minimize the eects of signal
cancellation.
Another important factor of LBA is the Signal-to-
Noise-Ratio (SNR in dB) dened by
required per information bit
(watts), thermal noise in 1Hz of bandwidth
data rate (bit/s) and B
bandwidth (Hz). E b =N 0 is the required energy per
bit relative to the noise power to achieve a given BER.
It depends on the modulation scheme. In Figure 2 we
show the in
uence of E b =N 0 on the bit error rate for
the DQPSK modulation. The SNR gives the required
dierence between the radio signal and noise power to
achieve a certain level of link reliability.
Given the equation described above we can compute
the required signal strength at the receiver. In addition
to the channel noise we assume some noise of the

Table
Assumed parameter in Figure 3
Parameter Value
Frequency 2.4GHz
Channel Noise -111dBm
Fade Margin 30dB
Receiver noise gure 7dB
Antenna gain
Range loss
Modulation DQPSK
Data rate 2Mbps
Bandwidth (de-spread) 2MHz
receiver circuits (N rx in dB). The receiver sensitivity
(P rx in dBm) is dened by
Given P rx we can further compute the required RF
power P tx (dBm) at the sender
where G tx and G rx are transmitter and receiver antenna
gain, respectively. In Figure 3 we show for the
IEEE 802.11 2Mbit/s DSSS physical layer the computed
radio transmission power required to achieve a given bit
error rate. The assumed parameters are given in Table
1. It is important to note, that we can control the bit
error rate by controlling the transmission power. The
bit error rate has a strong impact on the medium access
control protocol performance.
3. Gilbert-Elliot Channel Model
The link budget analysis provides for a given transmission
power a certain bit error rate and vice versa.
The bit errors are assumed to occur independently,
which is far from reality where error bursts are seen.
For instance, in [14] it is shown, that the throughput of
a WLAN with parameters similarly chosen is dependent
on position and time. The varying throughput is caused
by varying bit error rates during the measurements. To
s
consider dynamic changes in the bit error rate we use a
Gilbert-Elliot channel model (see [15]).
The Gilbert-Elliot channel model is basically a two
state discrete time Markov chain (see Figure 4). One
state of the chain represents the Good-State, the other
state represents the Bad-State. In every state errors
occur with a certain bit error probability. In [16] an
analytical solution is proposed, which parameterizes
the Markov chain for DQPSK modulation assuming a
Rayleigh-fading channel and movements of mobile ter-
minals. To improve the accuracy of the model more
than two states in a Markov chain can be used. We
follow this approach in computing the channel model
parameter (see [17]). In the following investigations we
use the two state model. The state sojourn times (be-
tween 1 and 200 milliseconds) and the bit error probability
depend on the bit error rate provided by the link
budget analysis. The Gilbert-Elliot model gives periods
with higher bit error and lower bit error probabilities,
which represents the bursty nature of the bit errors sufciently
4. IEEE 802.11 Medium Access Control
The responsibility of a Medium Access Control
(MAC) protocol is the arbitration of accesses to a
shared medium among several terminals. In IEEE
802.11 this is done via an Ethernet-like stochastic and
distributed mechanism - Carrier Sense Multiple Access
Avoidance (CSMA/CA). Since wireless
LANs lack the capability of collision detection, the collision
avoidance mechanism tries to minimize access conicts
a priori. In general, a MAC packet will be transmitted
immediately after a small sensing interval called
DIFS (Distributed Inter-Frame Space) as long as the
radio channel remains free. If the channel is busy or
becomes busy during sensing the MAC packet transmission
has to be postponed until the channel becomes
free and an additional waiting time has elapsed during
which the radio channel must remain free. This additional
waiting time consists now of a DIFS followed by
a Backo interval. The Backo interval is a uniformly
chosen random number of the interval [0,CW] times a
Backo slot time. CW represents the physical layer dependent
Contention Window parameter. The current
CW value is doubled after every packet transmission
error which can be caused by bit errors or collisions. In
the following we concentrate on the error control mechanism
of the IEEE 802.11 MAC protocol. For further
details on this MAC protocol the reader is referred to
[18] or [11].
The IEEE 802.11 MAC protocol uses an immediate
acknowledgment (ACK) to recover from transmission
errors. Transmission errors are caused either by bit errors
or by simultaneous channel access by two or more
mobiles (collisions). Figure 5 shows the ACK process-
ing. After a successful data packet reception, an ACK
transmission has to be started after a short interframe
space (SIFS) to indicate the correct reception. If the
reception of a packet was not successful no ACK will
be sent by the receiver. In case the sender received no
ACK, the packet will be retransmitted. The retransmission
is performed either until the data packet was received
correctly and conrmed by an ACK or the maximum
number of retransmissions is reached according
to the MAC rules. These retransmissions increase the
overall energy needed to transmit the packet. Energy
consumption can be reduced by reducing the number of
retransmissions. This in turn can be achieved by improving
the signal quality due to a higher transmission
power. But an increase of the transmit power also leads
to an increase in energy consumption which is counter-productive
to the goal of reducing the consumed energy.
Therefore the number of retransmissions and transmission
power have to be carefully balanced to reduce energy
consumption.
5. Energy consumption
Our goal is to achieve an optimal operating point
with respect to energy consumption of a IEEE 802.11
DSSS LAN. Therefore we look for a certain RF transmission
power level where the retransmission eects of
the MAC protocol is traded best. In an ideal case,
where no bit errors, no collisions, and no protocol overhead
occur, the energy E ideal (Ws) required to transmit
data equals the duration of the data transmission
T times the mean transmitted power
1 .
The transmission time for the ideal case can be computed
from the bit time (T bit ) and the number of transmitted
data bits (B succ ). Hence, from equation (5.1)
we get
for the required energy, whereas
is the energy required to transmit one bit in the ideal
case.
In reality, the energy to transmit data will be higher
due to protocol overheads and retransmissions, taking
errors and collisions into account. Therefore we introduce
the coe-cient  pr , which we call protocol e-
ciency
is the number of successful transmitted
data bits and B all is the number of overall transmitted
bits. The latter includes MAC control packets, successful
and retransmitted data bits and MAC
packet header and trailer.  pr indicates how e-cient
the protocol works during the transmission phase. In
other words,  pr indicates in a long run how much payload
data is contained in every transmitted bit . The
range of  pr is between 0 and 1, whereas the value 1 will
never be achieved because of physical and MAC layer
overheads. By rewriting Eqn. (5.2) and taking (5.4)
into consideration we get
1 Note that we only consider
Ptx . Additional power is required
to keep the entire or parts of the network interface card active
for transmission or reception.
pr
pr
the resulting energy, which considers now the total
number of transmitted bits (B all ) to get the data bits
the radio link. The following equation
pr
represents the resulting bit energy, which is eventually
needed to transmit one data bit successfully.
res incorporates the fact, that one has to send several
overhead bits before getting one data bit successfully
over the radio link.
6. Investigation of the RF transmit power
in
uence
To investigate the RF transmission power and MAC
retransmission trade-o we performed discrete event
simulations (DES). The simulation model for the system
under investigation (see Figure 1) is composed of
three parts as described above: the link budget analysis,
the Gilbert-Elliot channel model, and the IEEE 802.11
DCF model. The simulation parameters are shown in

Table

(see also Table 1).
We used a relatively static simulation setup to investigate
the power control and MAC trade-o. The simulated
WLAN network operates in ad-hoc mode, that
is, there is no access point which arbitrates the channel
access. Further we consider a single ad-hoc radio
cell. Implications of other radio cells (e.g. interference)
are not taken into account. Each mobile is in transmission
range of all other mobiles. The (mean) distance
between a sending and a receiving mobile is assumed
to be meters. Mobility is covered by the bit error
model, which allows changes in bit error rate (good ,
bad state) over the time. It is further assumed, that for
each sender/receiver pair a independent radio channel
s
exists, i.e. while one station receives a packet correctly
other stations might receive the same packet incorrectly.
Every mobile has a packet ready to send at every point
in time. Therefore all mobiles are involved in every
channel access cycle. A mobile always sends a packet
to its successor, which is determined by the mobile's
identier. A packet will be sent at a constant transmission
power to another mobile.
In the following we present the protocol e-ciency
pr and the energy used to successfully transmit one
res from the simulation results we obtained. To
rate these results we also present the channel access de-
lay. We dene the channel access delay as the interval
between the time there the MAC takes a packet from
the MAC queue to transmit it and the start time of
the successful transmission attempt. Figure 6 shows
the protocol e-ciency dependence on the transmission
power 2 used. The parameter of the curves is the number
of mobiles in an ad-hoc network. The graph shows,
that the protocol e-ciency is very small for a relatively
low transmission power of 14 dBm ( BER of 10 4 , see

Figure

3). The primary reason are corrupted packets,
which have to be retransmitted by the MAC protocol.
As a result the protocol e-ciency is low. By increasing
the transmission power, the protocol e-ciency increases
fast up to a certain level, which depends on the number
of stations in the ad-hoc network. An increased transmission
power is equivalent to a smaller BER, which
results in a better protocol e-ciency. The reason for
the better protocol e-ciency for a smaller number of
mobiles can be explained as follows: A large number
2 The transmission power is a (nonlinear) equivalent for the bit
error rate (see section 2).

Table
Simulation parameter
Parameter Value
Number of Mobiles 2,4,8,16
Packet sizes 64 . 2312 Byte
Tra-c Load > 100%
of mobiles results in more collisions during the access
phase since all mobiles have packets to transmit, which
leads to a smaller protocol e-ciency. Furthermore, it is
important to note that if the transmission power reaches
a certain level, only a marginal increase of protocol efciency
can be reported. That indicates that the optimal
operating point is in the region where the curves
start to
atten out (approximately 16 dBm for 512 Byte
packets). This behavior is independent of the number
of mobiles. Figure 14 and 17 (see Appendix) show the
same behavior for very small (64 Byte) and very large
(2312 Byte) MAC packets. We observe that the protocol
e-ciency remains smaller for 64 Byte packets and a
little bit higher using 2312 Byte packets.

Figure

res vs. the transmission power
for 512 Byte. The curve parameter is the number of mo-
biles. The graph clearly indicates that there is an optimal
transmission power providing the smallest E bit res
value, that is, when energy consumption for the transmission
phase is at its lowest level. This optimal transmission
power is nearly independent of the number of
stations. Figure 15 and show the results for 64 and
2312Byte packets, respectively. The graphs show the
same behavior as for 512 Byte packets. There is only
one important dierence. With increasing packet size
the optimal transmission power leading to the smallest
res value is increasing. In other words, for smaller
packets a smaller
should be chosen. The shape of
the curve is aected by the protocol e-ciency. Before
reaching the optimal transmission power (around
dBm for 512 Byte packets) a large amount of energy is
wasted for retransmissions resulting in a low protocol ef-
ciency. After the optimal point of transmission power,
a large amount of energy is unnecessarily sent out because
the protocol e-ciency only increases marginal in
this range.
The access delay curves (see Figure 8) reveal, that
at the optimum transmission power the lowest achievable
channel access delay is nearly achieved. Very small
transmission power levels for a certain packet size are
very harmful since the access delay grows fast while for
higher power levels the access delay does not improve
signicantly. In particular for very large packets it is
important, that the power level is at its optimum or
higher since the channel access delay goes in the region
of seconds if the used transmission power is too low (see

Figure

19).
The gures clearly indicate that there is an optimal
transmission power for a certain packet size and that
this power is nearly independent of the number of sta-
tions. Therefore we investigate the in
uence of packet
size in further detail. In Figures 9 and 10,  pr and
res are shown for dierent packet sizes. The curve
parameter is the bit error rate, which is a (nonlinear)
equivalent to the transmitted power (see Figure 3). The
number of stations is xed to 4. In Figures 20 and
(see Appendix) the similiar curves for mobiles
are shown. The protocol e-ciency graph indicates for
low bit error rates larger packets have
the best performance. For bit error rates higher than
5 an optimal packet size is visible. This is around
500 Bytes. The reasons are twofold. At rst, for small
packets the protocol e-ciency is mainly in
uenced by
the MAC. The collision and protocol overheads take the
main share of bandwidth. For long packets the MAC
plays a minor role, but long packets will be corrupted
with a higher probability, resulting in retransmissions.
The graphs for E bit res (see Figure 10 and 21) re
ect
this behavior. 500 Byte packets show the best performance
for high error conditions
packets should be as large as possible.
7. Protocol Design Recommendations
Our results clearly indicate a strong correlation between
the MAC and the physical layer. A poorly selected
transmission power may result in a waste of en-
ergy. In other words, MAC protocols need ne tuning
according to the underlying physical layer and channel
characteristics and vice versa. Therefore we will
elaborate on how we can achieve a reduction of energy
consumption in WLANs using our results.
7.1. Fixed high RF power and large sized MAC packets
Todays Internet tra-c carries packets of dierent
size. Assuming that a WLAN network interface experiences
this kind of tra-c one way to reduce energy is to
adapt the packet size according to the used RF tranmis-
sion power level. The highest power saving gain could
be achieved if only large packets (e.g. > 1000Bytes)
with the appropriate high transmission power would be
transmitted by the WLAN interface. This can be concluded
from the fact, that the optimal energy per successfully
transmitted bit value (E bit res ) is lowest for the
largest possible MAC packet size (2312 Byte, see Figure
7, 15 and 18). But having internet tra-c in mind,
where a large portion of the packets are smaller or equal
than 512 Bytes, a MAC packet assembly mechanism is
required to build up large packets. MAC packet assembly
is not an easy task and might be counterproductive
with respect to energy consumption. For instance, it is
not easy to resolve which packets should be assembled
in one large packet and how long should be waited to ll
up a large packet. Furthermore, an assembly of packets
which are directed to dierented receivers into one
large packet would require that every mobile stations
is awake to receive the big packet and check whether
there is a packet for itself in the large packet. That
might lead to unnecessary awake times of mobile stations
and result in a waste of energy. Last but not least,
the IEEE 802.11 standard does not specify an assembly
mechanism, which makes this method unpractical for
application. Despite that we believe, that the energy
saving potential of a carefully designed MAC assembly
mechanism will outweight the drawbacks.
7.2. Fixed medium RF power and medium sized MAC
packets
The following proposed opportunity to reduce energy
consumption of IEEE 802.11 network interfaces appears
to be the simplest and most realizable since it does not
require any changes to the existing IEEE 802.11 stan-
dard. The idea is to use medium sized MAC packets of
about 512 Byte and transmit them with the xed optimal
sRF power. This is based on the observation, that
for our assumed conditions 512 Byte packets seem to
have a good performance except at very low bit error
rates. The E bit res value for 512 Byte packets is relatively
close to the E bit res for large packets (see Figure
To achieve this large packets have to be
fragmented to 512 Byte chunks. Small packets should
be left as they are, since MAC packet assembly is a difcult
task as we explained above. They are transmitted
with the same RF power as the 512 Byte packets. MAC
packet fragmentation is specied in the IEEE 802.11
standard and supported in nearly every commercially
available WLAN product.
We simulated this approach. For that purpose we
analyzed a half hour tra-c trace le of an 10MBit/s
Ethernet segment connecting the main campus of Harvard
University (USA) with the Internet in the year
1997 (see [19]). We extracted a packet size distribution
of the TCP (Transmission Control Protocol) tra-c from
the trace le as shown in Figure 11 and incorporated the
distribution in our tra-c generation model. As stated
in [20], the TCP tra-c makes up a great share (up to
90%) of the overall network tra-c 3 . We did not sample
the inter-arrival times of the packets from the trace le,
since it is not an easy task to scale from 10MBit/s to
2Mbit/s, where the latter is the transmission speed of
IEEE 802.11. Network tra-c, especially internet and
LAN tra-c is in general very bursty (see e.g. [20] and
[21]). We accomplish the burst characteristic of the
tra-c by means of the Pareto distribution, which exhibits
a heavy tail characteristic. The  parameter of
the Pareto distribution was set to the value 1.5. The k
parameter was used to control the tra-c intensity.
In

Figure

12 and Figure 13 we show the E bit res over
normalized network load for 4 and 16 mobiles, where
all mobiles use a source model as described above. We
simulated with a bit error rate of 10 4 and 10 5 , respec-
3 Tra-c shares of protocols are recently changing mainly due to
the availability of multimedia software and services, which rely
on UDP (User Datagram Protocol).
tively 4 . Furthermre, we used the ability of the MAC to
fragment packets into smaller packets. On one hand
MAC level fragmention adds overhead due to protocol
header and necessarily more channel accesses. On the
other hand smaller packets are less likely to be erroneous
due to bit errors. In our simulations we fragmented
packets whereas the fragment size was set to
500 Byte according to the previously achieved results.
The gures show, that MAC level fragmentation has
its advantages when the bit error rate is higher than
. The improvement is relatively high, taking into
account that mobiles very rarely send large packets (i.e.
backbone access tra-c) which can be fragmented. Assuming
networks with more local tra-c (department,
o-ce LANs) where the mean packet size is larger, an
even higher improvement can be anticipated when using
MAC level fragmentation. The curves also show, that
fragmentation should not be used if the radio channel
quality is good (BER < Fragmentation adds un-necessary
overhead in that case . The small ascend in
the graph is a result of increased collision probability
due to increased load. In addition, the more mobiles
are located in a radio cell, the higher E bit res . Again,
this is a result of higher collision probability.
7.3. Variation of transmit power
In contrast to the two proposals we made above, it
also possible to adapt the RF tranmission power according
to the packet size assuming that a WLAN experience
some kind of internet tra-c with varying packet
sizes. This can be done by power control. From the
simulation results (see Figure 7, 15 and 18) we can conclude
that small packets should be sent with a lower
RF transmit power while larger packets should be sent
with a higher RF transmit power 5 . Of course the ac-
4 The packets are sent at the current optimal transmit power for
500 Byte packets regardless of the actual packet size.
5 So far, the main objectives of power control are minimizing the
interferences in multi-radio cell congurations and maximizing
the system capacity. The algorithms used for these goals should
also be taken into account when choosing a power level to minimize
the energy consumption.
tual values of transmit power depend on the WLAN
setup like range, transmission speed and environmental
circumstances.
Although a power control algorithm is not specied
in the standard, IEEE 802.11 provides two means of
power control support. First, it denes dierent power
levels, whereby up to 4 and 8 power levels are allowed
for DSSS and FHSS, respectively. The values for these
power levels are not dened and therefore implementation
dependent. The approach presented here might
be used for a meaningful setting of the power levels
with respect to energy consumption. For instance, the
power level range should be set from about 15 to 17
dBm for the assumptions we have made. The choice of
a transmit power for sending a packet with a certain
packet size should in general tend to a higher transmission
power since this is less harmful with respect to
energy consumption and channel access delays. Second,
the IEEE 802.11 denes a Received Signal Strength Indicator
(RSSI). This indicates the received energy of
a signal and can have a value from 0 up to 256 and
for DSSS and FHSS, respectively. A power control
mechanism should exploit this value to achieve information
about the current channel state. By means of this
information additonal or less RF transmission power,
according to the value of transmission power which depends
on the packet size, can be chosen. That of course
requires, that the receiver passes this information to the
sender. This information could be obtained by means
of the immediate acknowledgment which follows a successful
packet reception. Even if the packet or the acknowledgment
gets lost, the packet sender can assess
the channel state and might in turn stop transmission
for a while or resend the packet with more energy. Such
an approach and the quantication of the gain is subject
of our current research.
8.

Summary

In this paper we study the mutual in
uences of the
medium access protocol and the physical layer with respect
to energy consumption for an IEEE 802.11 LAN.
We showed that by harmonizing these dierent protocol
levels, that is to say sending MAC packets with its
optimal transmit power and exploiting various MAC
level mechanisms, a substantial reduction in energy consumption
is achievable. For the upcoming ultra low
powered micro radios the power saving gain will be even
higher since the share of power consumed by signal and
protocol processing will be smaller. The approach used
here is general and may be used for any wireless sys-
tem. This approach might also be extended to higher
protocol layers such as link error control, transport or
application layer.

Acknowledgements

We would like to thank Andreas Kopsel for his contribution
to the simulation model. We would also like
to thank the reviewers for their detailed and fruitful
comments.



--R

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Daji Qiao , Sunghyun Choi , Amit Jain , Kang G. Shin, MiSer: an optimal low-energy transmission strategy for IEEE 802.11a/h, Proceedings of the 9th annual international conference on Mobile computing and networking, September 14-19, 2003, San Diego, CA, USA
power saving;MAC;IEEE 80211;retransmission;protocol harmonization;energy saving;power control;vertical optimization;WLAN