AN EVALUATION OF A CONSERVATIVE TRANSMIT

POWER CONTROL MECHANISM ON AN INDOOR 802.11

WIRELESS MESH TESTBED

Karol Kowalik, Marek Bykowski, Brian Keegan and Mark Davis

Communications Network Research Institute, Dublin Institute of Technology, Ireland

Keywords:

Transmit Power Control, Wireless Mesh, 802.11.

Abstract:

Power control techniques for IEEE 802.11 wireless networks have already gained considerable attention. Such

techniques are particularly attractive because they can improve various aspects of wireless network operation

such as interference mitigation, spatial reuse in dense wireless deployments, topology control, and link quality

enhancement. In this paper we propose a novel delivery ratio based Conservative Transmit Power Control

(ConTPC) mechanism. Our implementation is conservative when it comes to deciding if the transmit power

should be reduced for a given link. This is because we do not want poor quality wireless links to further

reduce their quality and be overwhelmed by other links transmitting at maximum power. We have experi-

mentally evaluated the beneﬁt of the proposed power control scheme when compared with ﬁxed power level

systems. We show that our ConTPC mechanism can increase the throughput, however the magnitude of this

enhancement largely depends on the topology of the wireless network.

1 INTRODUCTION

Interference has been identiﬁed as a key cause of

performance degradation in Wireless Mesh Networks

(WMNs) (Jain et al., 2003; Padhye et al., 2005).

Transmit power control is one of the methods which

allows for interference mitigation and spatial reuse

through per-link power control.

Per-link power control allows a network node to

transmit packets to its neighbours at different power

levels. Data transfers to close neighbours may be

performed at low transmission powers, thereby min-

imising the interference with remote nodes. Con-

versely, the communication with remote neighbours

may be improved by using a higher transmission

power, i.e. by providing a stronger signal at the re-

ceiver. It has been shown by Muqattash et al (Muqat-

tash and Krunz, 2005) that a power controlled MAC

(POWMAC) protocol can signiﬁcantly improve net-

work throughput. Furthermore, power control be-

comes a necessity in dense deployments which are

now becoming common as demonstrated by Akella

et al. (Akella et al., 2005).

However, increasing the transmission power for

weak links also has a negative effect of producing in-

creased interference. There are other factors which

can impact on the beneﬁt of employing transmit

power control. For example, Broustis et al. (Broustis

et al., 2007) have observed that when power control is

combined with virtual carrier sensing (RTS/CTS mes-

sages) the performance is often degraded.

Transmit Power Control (TPC) mechanisms when

implemented in WMNs can be used to:

• minimise interference with other nodes (and thus

increasing spatial reuse) as implemented in POW-

MAC (Muqattash and Krunz, 2005) or in (Navda

et al., 2007);

• improve the quality of wireless links (as imple-

mented in (Son et al., 2004));

• reduce the energy consumption (Gomez et al.,

2003);

• control the network topology (Ramanathan and

Hain, 2000);

• reduce interference with satellites and radar oper-

ating in the 5 GHz frequency band (as required by

the IEEE 802.11h Standard (802.11h IEEE Stan-

dard for Information technology - Telecommuni-

cations and information exchange between sys-

tems – Local and metropolitan area networks -

Speciﬁc requirements, ));

Kowalik K., Bykowski M., Keegan B. and Davis M. (2008).

AN EVALUATION OF A CONSERVATIVE TRANSMIT POWER CONTROL MECHANISM ON AN INDOOR 802.11 WIRELESS MESH TESTBED.

In Proceedings of the International Conference on Wireless Information Networks and Systems, pages 5-14

DOI: 10.5220/0002022400050014

 SciTePress

• ensure good coverage (as implemented by some

access point manufacturers).

Some of these objectives can be realised by mod-

ifying a single layer, such as the POWMAC (Muqat-

tash and Krunz, 2005). However, in general power

control needs to be aware of the operation at multiple

layers and is more accurately a cross-layer optimisa-

tion problem. Kawadia and Kumar present an com-

prehensive discussion about the principles of such a

cross layer design of power control in (Kawadia and

Kumar, 2005) .

The majority of the power control techniques pro-

posed require power control with per-packet level

granularity and with low latency. In (Kowalik et al.,

2008) we have shown that WLAN cards with Atheros

chipsets allow for such a ﬁne power control.

In this paper we present a novel delivery ratio

based Conservative Transmit Power Control (Con-

TPC) mechanism. We evaluate it experimentally on

an eleven node static indoor 802.11 wireless mesh

testbed as well as on a four node topology which pro-

vides an easy to analyse example of a disjoint com-

munication case.

2 RELATED WORK

Reports in (Monks et al., 2001; Muqattash and Krunz,

2003) present 802.11 MAC modiﬁcations which aim

to utilise power control. However, these solutions re-

quire the use of a separate control channel for the

exchange of collision avoidance information. Such

schemes would require two wireless interfaces at each

mesh node. Thus this approach is not feasible for the

single radio solution considered in this work.

The POWMAC (Muqattash and Krunz, 2005) pro-

posed by Muqattash and Krunz is an enhancement

of the 802.11 MAC enabling power control and re-

sembles the ConTPC scheme proposed in this paper.

However, POWMAC requires detailed information

about the power of a receivedcontrol signal, as well as

the interference power. This may not be a problem if

one uses simulation tools to evaluate the approach as

the authors did. However, this is unrealistic because

we and other researchers have observed that SNR

measurements exhibit a high variability. For exam-

ple Akella et al. (Akella et al., 2005) have shown that

4 dB of variance in RSSI values and noise estimates

is typical. Moreover, under the POWMAC scheme

the transmission of each data packet is preceded by

an access window (AW) during which several pairs

of neighboring terminals exchange their RTS/CTS

control messages. Consequently this can generate a

signiﬁcant overhead. There is a similarity between

POWMAC and LPERF (Akella et al., 2005) (Load-

sensitive, Power-controlled Estimated Rate Fallback)

because both combine the load information with sig-

nal strength measurements. However, it was stated

by Akella et al. (Akella et al., 2005) that in practice

they found that “achieving good performance and in-

terference reduction using the LPERF technique can

be challenging” owing to the poor accuracy of signal

strength measurements and the difﬁculties in estimat-

ing the load.

Broustis et al. (Broustis et al., 2007) performed

physical experiments and have analysed three scenar-

ios in which power control may improve network per-

formance:

• “overlapping case” – where power control can-

not improve the performance of two overlapping

links;

• “hidden terminal case” – where power control can

improve fairness;

• “potentially disjoint case” – where power control

can improve throughput signiﬁcantly.

Their experiments show that power control may im-

prove overall throughput, however virtual carrier

sensing (RTS/CTS messages) needs to be disabled.

Also, their results demonstrate that the beneﬁt from

using power control is topology dependent. In Sec-

tion 6.2 we also present similar ﬁndings.

Akella et al. (Akella et al., 2005) have pro-

posed a number of combined power and rate con-

trol mechanisms. The one which resulted in the

best performance was PERF (Power-controlled Esti-

mated Rate Fallback) (Akella et al., 2005) which im-

plemented conservative power control. PERF per-

forms its decisions regarding the rate and transmit

power based on the delivery rate of previous pack-

ets and SNR estimates obtained from the WLAN

adapter. Moreover, each packet is extended to in-

clude information about the transmit power, path

loss, noise estimate of the last packet, and deliv-

ery rate information. ConTPC adopts a similar

approach. However, PERF reduces the transmit

power until estimatedSNR = decisionThreshold +

powerMargin. The powerMargin is used to con-

trol aggressiveness of the algorithm. Our ConTPC

scheme is also conservative, but it takes a different

approach in trying to reduce power provided that it

does not result in an increased packet loss.

WINSYS 2008 - International Conference on Wireless Information Networks and Systems

3 CONSERVATIVE TRANSMIT

POWER CONTROL (CONTPC)

We believe that many of the proposed power control

schemes are too opportunistic. The rationale behind

these schemes is that by tolerating a degree of inter-

ference at each node, one can trade-off an increased

BER (Bit Error Rate) for multiple simultaneous trans-

missions. For example, POWMAC (Muqattash and

Krunz, 2005) follows this approach.

We have discovered that in real indoor deploy-

ments this approach may be overly optimistic and

can degrade the network performance. In our wire-

less eleven node static indoor testbed wireless links

often exhibit high loss rates. Moreover, their deliv-

ery rate vs TX power characteristics often resembles

that shown in Figure 1. This sort of characteristic

has the distinct feature that even a small reduction in

the transmit power from its maximum value can re-

sult in a signiﬁcant drop in the delivery rate. More-

over, when multiple simultaneous transmissions oc-

cur, the noise ﬂoor increases. Thus for a link with

such a characteristic its delivery rate would drop even

further. Experimentally we have found that for links

with such a characteristic even small decreases in the

transmit power can signiﬁcantly degrade their perfor-

mance.

0.2

0.4

0.6

0.8

0 2 4 6 8 10 12 14 16 18

Delivery Rate

TX Power [dBm]

Figure 1: Delivery ratio vs. TX power characteristics for a

low quality link.

In our ConTPC mechanism we do not allow

the power control mechanism to reduce the transmit

power for all the network links. Instead the Con-

TPC mechanism ﬁrst checks if the delivery rate vs

TX power curve is ﬂat around its maximum power

level. For example, the characteristic could look like

the one shown in Figure 2. On such a link any reduc-

tion in the TX power does not adversely impact on its

delivery rate. Therefore, reducing the transmit power

will typically not degrade the performance of the link,

but can potentially reduce the interference on nearby

links.

0.2

0.4

0.6

0.8

0 2 4 6 8 10 12 14 16 18

Delivery Rate

TX Power [dBm]

Selected power level = 13 dBm

Figure 2: Delivery ratio vs. TX power characteristics for a

high quality link.

However, even for high quality links we do not

advocate large reductions in the transmit power as

this may expose the node to the risk of being over-

whelmed by neighboring nodes transmitting at maxi-

mum power. Thus, our powercontrol scheme adopts a

quite conservative approach when it comes to reduc-

ing the transmit power on any given link. ConTPC

will only reduce the transmit power while it does not

adversely impact of the delivery rate. The exact de-

tails of our scheme are provided in the following sec-

tion.

4 CONTPC IMPLEMENTATION

DETAILS

Our ConTPC mechanism controls the output power

on a per-link basis. Therefore, each node periodi-

cally broadcasts probe frames at all allowed power

levels which includes information about the power

level used to sent a given frame. This information

allows each node to create delivery rate vs TX power

characteristics for all its incoming links. In our ex-

periments we have used custom probe messages, but

HELLO messages which are usually exchanged be-

tween routers could also serve this purpose.

Our conservative approach requires the delivery

rate vs TX power characteristic to be almost ﬂat

AN EVALUATION OF A CONSERVATIVE TRANSMIT POWER CONTROL MECHANISM ON AN INDOOR 802.11

WIRELESS MESH TESTBED

around the maximum power level. In order to de-

scribe the details of ConTPC power selection algo-

rithm we denote:

max

to be the maximum TX power level (for our

WLAN adapters it was 18dBm);

selected

to be the power level selected by our Con-

TPC mechanism;

step

to be the granularity of our power mecha-

nism;

safety

to be the safety margin, which deﬁnes the

minimum range of the ﬂat area around the maxi-

mum power;

thr to be the threshold value which deﬁnes the end

of the ﬂat area around the maximum power;

dlr to be the delivery rate vs TX power character-

istic.

In Figure 3 we present a diagram showing the de-

tails of ConTPC. Essentially, it selects the power level

at the leftmost part of the ﬂat region around the maxi-

mum TX power, provided that the size of this region is

larger than the safety margin. ConTPC uses three con-

ﬁgurable parameters: P

step

, P

safety

and thr. The P

step

together with the frequency of the power broadcasts

can be used to tune the overhead and the accuracy of

the estimation of the delivery rate characteristics. The

safety

and thr parameters are used to control the ag-

gressiveness of ConTPC. An evaluation of the relative

importance of these parameters will be given in Sec-

tions 6.5 and 6.6.

if dlr(P) < thr * dlr(P_max) ?

P = P_max

P = P - P_step

if P_max - P > P_safety ?

YES

P_selected = P + P_step

P_selected = P_max

NO YES

Figure 3: ConTPC – power selection details.

The ConTPC mechanism allows a node to deter-

mine the transmit power levels for its incoming links.

Once this has been done, each node needs to inform

its neighbours of the transmit power level that they

need to use when communicating with the node. In

static wireless mesh networks the communication pat-

tern usually resembles a tree whose main root is lo-

cated at the gateway node. To reduce the overhead in-

volved in informing neighbours about the power lev-

els that they need to use, we propose to distribute this

information along this tree-like communication pat-

tern only. Consequently, power control is only per-

formed on links within this communication pattern.

5 EXPERIMENTAL SETUP

We have implemented ConTPC as a Click (Kohler

et al., 2000) module. This module allows us to broad-

cast custom made power probes, generate delivery

rate vs TX power characteristics and inform neigh-

bouring nodes about the power levels that they need

to use.

To perform the experimental tests we used eleven

WLAN nodes distributed across two ﬂoors of our re-

search institute building as shown in Figure 4. The

nodes were based around the Soekris net4521 plat-

form. The experiments were performed indoors, and

unfortunately we could not completely isolate the

tested network from external sources of interference.

All nodes were running Linux operating systems and

were operating in the monitor mode to allow for

packet injection by our Click module.

In order to enable the TPC feature of Mad-

Wiﬁ drivers one needs to compile it with

COPTS+= -DATH_CAP_TPC=1

and pass the

tpc=1

parameter to

insmod ath_pci

Moreover, the transmission rate was ﬁxed for all

the experiments at 11Mbps and the antenna diversity

was also switched off.

6 RESULTS

6.1 Delivery Ratio Vs SNR

In the ConTPC scheme each node measures the deliv-

ery ratio vs TX power characteristics of its incoming

wireless links. In Figure 5 we have presented these

characteristics for all links of our test bed. One might

think that instead of the delivery ratio it would be eas-

ier to use Signal–to–Noise Ratio (SNR), since it di-

rectly relates to the power level used for the transmis-

sion. Moreover, SNR value can be easily obtained

from the wireless card, for example Atheros chipsets

report RSSI value which can by simply converted to

SNR by subtracting 95dB.

However, we do not advocate the use of SNR be-

cause we have observed a poor accuracy on the RSSI

measurements especially for weak signals as illus-

trated in Figure 6. Akella et al. (Akella et al., 2005)

WINSYS 2008 - International Conference on Wireless Information Networks and Systems

(a) Floor 1 (b) Floor 2

Figure 4: Locations of wireless nodes.

have shown that that the variance in RSSI estimates is

high and is typically around 4dB. Moreover, in Fig-

ure 6 one may observe that the SNR vs TX power

characteristics are much ﬂatter than the delivery rate

vs TX power curves.

0.2

0.4

0.6

0.8

0 2 4 6 8 10 12 14 16 18

Delivery Rate

TX Power [dBm]

A-D

A-O

A-Q

A-B

A-M

A-C

D-A

D-K

D-B

D-C

D-R

O-A

O-D

O-B

K-A

K-D

K-Q

K-B

K-C

K-R

Q-A

Q-D

Q-K

Q-B

Q-C

B-A

B-D

B-O

B-K

B-Q

B-M

B-C

M-A

M-B

C-A

C-D

C-K

C-Q

C-B

C-M

C-R

R-D

R-K

R-C

Figure 5: Delivery rate vs TX power.

In Figure 7 we show how the delivery rate is re-

lated to the RSSI values. Although, it can seen that

there is a correlation between the SNR and the deliv-

ery rate, the large variance associated with the SNR

measurements make it difﬁcult to use. Consequently,

in ConTPC we have decided to use the delivery rate

instead of the SNR.

0 2 4 6 8 10 12 14 16 18

RSSI

TX Power [dBm]

A-D

A-O

A-Q

A-B

A-M

A-C

D-A

D-K

D-B

D-C

D-R

O-A

O-D

O-B

K-A

K-D

K-Q

K-B

K-C

K-R

Q-A

Q-D

Q-K

Q-B

Q-C

B-A

B-D

B-O

B-K

B-Q

B-M

B-C

M-A

M-B

C-A

C-D

C-K

C-Q

C-B

C-M

C-R

R-D

R-K

R-C

Figure 6: SNR vs TX power.

6.2 Disjoint Case

In order to validate the operation of the ConTPC

scheme, we ﬁrst decided to investigate how it per-

forms in the disjoint case where the expected beneﬁt

is the greatest according to Broustis et al. (Broustis

et al., 2007). In this case transmit power control can

allow for the simultaneous use of two links which re-

sults in a twofold increase in throughput compared to

the case where the default maximum transmit power

is used. In general, if we were to have n such node

pairs all interfering with each other under maximum

transmission power conditions, then we could expect

a n-fold increase in throughput through the use of

power control. However, such topologies based on

pairs of nodes located close to each other and traf-

AN EVALUATION OF A CONSERVATIVE TRANSMIT POWER CONTROL MECHANISM ON AN INDOOR 802.11

WIRELESS MESH TESTBED

0.2

0.4

0.6

0.8

0 10 20 30 40 50 60

Delivery rate

RSSI

A-D

D-A

O-A

O-D

K-A

Q-A

Q-D

B-A

B-D

M-A

C-A

C-D

R-D

Figure 7: Delivery rate vs TX power.

ﬁc patterns based on communication between pairs of

nodes only are quite rare.

In our testbed shown in Figure 4 we have moved

node B towards A and D towards B, to create a sce-

nario with two disjoint pairs A–B and C–D. Then we

performed an experiment in which each node sent a

512kbps UDP ﬂow to its neighbouring node. Thus we

had four ﬂows in the network and the experiment du-

ration was 6 hours. During this period we have com-

pared the performance of ConTPC with ﬁxed power

levels at 0dBm and at 18dBm. To ensure that each

of these power schemes is tested under similar con-

ditions, they were dealt with in a round robin fashion

over a duration of 1 min each.

As can be observed in Figure 8 the delivery rate

characteristics were excellent for all power levels be-

tween pairs of nodes. Even though the node pairs

were located on different ﬂoors the delivery rate on

link B–D was still good, and also on links A–D and

C–A. Therefore, these pairs were not completely dis-

joint, so contention on the link B–D was still high

even though the minimum power of 0dBm was used.

Thus we did not observe doubling of the throughput,

as one might expect.

In Figure 9 we show that ConTPC resulted in an

increase of 35% when compared with transmission at

the maximum power of 18dBm. What is also interest-

ing here is that ConTPC slightly outperformed trans-

mission at maximum power of 0dBm. This is because

it emerged that for links B–A and C–D it was more

beneﬁtial to use a transmit power of around2 or 3dBm

instead of the minimum value.

The nodes in our test bed are usually not placed in

pairs so close to each other, as presented in the above

experiment. Therefore, for the next step in the inves-

tigation we moved the nodes back to their normal po-

sitions and repeated the tests. Thus, it was no longer

0.2

0.4

0.6

0.8

0 2 4 6 8 10 12 14 16 18

Delivery Rate

TX Power [dBm]

A-D

A-B

A-C

D-A

D-B

D-C

B-A

B-D

B-C

C-A

C-D

C-B

Figure 8: Delivery rate characteristics observed in disjoint

case.

100

120

140

160

180

A-B B-A C-D D-C

Throughput [kbps]

Nodes pair

ConTPC

18dBm

0dBm

avg ConTPC

avg 18dBm

avg 0dBm

Figure 9: Throughput observed in disjoint case.

disjoint case. As it can be seen in Figure 10 only link

B–A continued to exhibit an excellent delivery rate

characteristic. Thus, it achieved the highest through-

put when a ﬁxed power of 0dBm was used (as it is

shown in Figure 11).

In Figure 11 it may be observed that in this more

realistic 4 nodes case our ConTPC also outperforms

ﬁxed power levels at 18dBm and at 0dBm. We also

demonstrate that in such a case where nodes are dis-

tributed evenly around the area (usually to maximise

coverage) the use of maximum power is close to op-

timal. Thus if the nodes were to be spread more uni-

formly to maximise coverage then we would observe

that ConTPC would provide the same overallthrough-

put as when the maximum power of 18dBm is used.

This experiment demonstrates that the beneﬁt of

using transmit power control is topology dependent.

It also shows that our ConTPC outperforms ﬁxed

WINSYS 2008 - International Conference on Wireless Information Networks and Systems

0.2

0.4

0.6

0.8

0 2 4 6 8 10 12 14 16 18

Delivery Rate

TX Power [dBm]

A-B

D-C

B-A

C-A

C-D

C-B

Figure 10: Delivery rate characteristics observed in 4 nodes

case.

100

150

200

A-B B-A C-D D-C

Throughput [kbps]

Nodes pair

ConTPC

18dBm

0dBm

avg ConTPC

avg 18dBm

avg 0dBm

Figure 11: Throughput observed in 4 nodes case.

power levels if there is some nonuniformity in the

node distribution. Thus it can reduce the transmit

power for strong links, thereby reducing the interfer-

ence.

6.3 Eleven Nodes Testbed Case

To evaluate ConTPC over our eleven node test bed we

have used a similar procedure. However, this time the

communication pattern was not performed in disjoint

pairs. Instead each node was sending a 512kbps UDP

ﬂow to the node which was the next hop towards the

gateway node. Such communication pattern is similar

to the typical case when nodes want to connect to the

Internet through a gateway node. We have decided not

to forward the data packets beyond the gateway node

as we wished to observe the effect of transmit power

control on individual links instead of observing the

effect of hop penalty. The duration of this experiment

was 24 hours and all the power schemes were tested

in a round robin fashion.

In Figure 12 we have compared the throughput

achieved by the individual links on our eleven node

testbed. It can be observed that the ConTPC mech-

anism resulted in approximately a 50% increase in

throughputwhen compared with a ﬁxed power levelat

0dBm and a 15% increase in throughput when com-

pared with a maximum power of 18dBm. One can

100

150

200

250

A-BA-DA-OB-AB-DC-AC-BC-DD-CK-CK-DN-JN-RO-AO-BQ-KR-CR-N

Throughput [kbps]

Nodes pair

ConTPC

18dBm

0dBm

avg ConTPC

avg 18dBm

avg 0dBm

Figure 12: Throughput observed on 11 node testbed.

also observe in Figure 12 that ConTPC offered an im-

provement mostly for links which achieved low or

average throughput when the maximum power was

used. This is because when the transmission was oc-

curring at maximum power, these links were suffering

due to increased contention and noise level. Reduc-

ing the transmit power of high quality links resulted

in improved performance of weak links. So ConTPC

has not only increased the average throughput, but has

also increased the fairness.

6.4 Stability of ConTPC

In Figure 13 we present the probability distribution

functions (PDFs) of the power level selection per-

formed by ConTPC during the experiment performed

in Section 6.3. This diagram allows one to determine

how often ConTPC changes the power levels or if

the power level remains ﬁxed for extended periods of

time. As can be observed on link A–B, ConTPC se-

lected a wide range of power levels. This is because

AN EVALUATION OF A CONSERVATIVE TRANSMIT POWER CONTROL MECHANISM ON AN INDOOR 802.11

WIRELESS MESH TESTBED

0.2

0.4

0.6

0.8

0 5 10 15 20 25 30 35 40

Probability

Power [dBm]

A-B

A-D

A-O

B-A

B-D

C-A

C-B

C-D

D-C

K-C

K-D

N-J

N-R

O-A

O-B

Q-K

R-C

R-N

Figure 13: Power level selection PDF for links of 11 node

testbed.

node A was located in a corridor where there was a

high variability in the propagation conditions due to

the movement of people. However, most of the time

this link exhibited a high delivery rate, thus the se-

lected power tended to switch between 8 and 9dBm.

For the majority of the links, the transmit power

level was selected around a small range of values or

remained at one value for the duration of the experi-

ment.

6.5 Overhead of ConTPC

ConTPC obtains the delivery rate vs TX power char-

acteristics by broadcasting power probes at all speci-

ﬁed power levels. In this section we consider the di-

rect overhead which is the amount of time consumed

by the transmission of these broadcast probe packets.

The overhead can be controlled by the frequency

of these broadcasts. In our experiment we have mea-

sured the overhead on node D which was the gateway

node and featured the highest number of neighbours,

and thus was inﬂuenced by the highest overhead. In

Figure 14 we have presented the overhead as a per-

centage for three different intervals between consecu-

tive broadcasts, i.e. every 50ms, 100ms, and 200ms.

As expected reducing the frequency of these broad-

casts resulted in a drop in the overhead from 2.5% to

1.3%, and ﬁnally to 0.6%. Although the 200ms in-

terval resulted in the smallest overhead we have ob-

served that ConTPC has signiﬁcantly slowed the pro-

cess of generation of the delivery rate vs TX power

characteristics. Therefore, if one would wished to

keep the overhead low at around 0.6%, we would rec-

ommend to reduce the number of power levels being

tested. In (Kowalik et al., 2008) we have shown that

WLAN cards with Atheros chipsets allow for the con-

0.5

1.5

2.5

0 50 100 150 200 250 300 350

Overhead

Time [sec]

50ms

100ms

200ms

Figure 14: ConTPC overhead.

trol of the transmit power with 0.5dBm granularity.

Not all the power levels need to be used by the power

control mechanism, especially if we wish to limit the

probing overhead. Thus, for a 200ms broadcast inter-

val we have increased the value of P

step

form 0.5dBm

to 1dBm. The increase of P

step

with the broadcast in-

terval set at 200ms allowed ConTPC to continue to

outperform tansmissions at maximum power for our

eleven node test bed.

6.6 ConTPC Parameters Evaluation

ConTPC uses two conﬁgurable parameters P

safety

and

thr to control its aggressiveness. The P

safety

is a

safety margin, which deﬁnes the minimum range of

the ﬂat region around the maximum power. The thr

is the threshold value which deﬁnes if the region is

still ﬂat around the the maximum power. Thus, if

one would set P

safety

to be 0dBm, and the thr to

be equal to 0.9, then all network links would use a

transmit power which results in the delivery rate of

0.9 ∗ dlr(18dBm). These settings would effectively

reduce the transmit power on all the network links.

In Figure 15 we have provide evidence that the

opportunistic approach where (P

safety

= 0dBm) re-

sults in the worst performance. The conservative ap-

proach ( where P

safety

>= 1dBm) is much better. It

can be observed that the highest average throughput

is achieved if we select a low threshold thr = 0.8 and

use a wide power safety margin as P

safety

= 4dBm.

Setting a too high a threshold of around thr = 0.95

for a link with a high variance in the delivery rate vs

power curve could cause ConTPC to decide that the

WINSYS 2008 - International Conference on Wireless Information Networks and Systems

100

120

140

160

0 1 2 3 4 5

Avg. Throughput [kbps]

TX Power safety margin [dBm]

thr 0.8

thr 0.85

thr 0.90

thr 0.95

Figure 15: Evaluation of importance of P

safety

and thr pa-

rameters.

ﬂat area is quite narrow. This could effectivelydisable

ConTPC on links with high variance of the delivery

rate vs power characteristics. However, threshold val-

ues of around 0.8 ensure that ConTPC correctly ﬁnds

the boundary of the ﬂat area despite some variability.

While a large P

safety

(above 3dBm) ensures that only

good quality links reduce their power. Consequently,

by being conservative this allows for the efﬁcient op-

eration of ConTPC.

7 CONCLUSIONS

In this paper we have presented the ConTPC mecha-

nism which is a conservative transmit power control

scheme. It does not seek to reduce the transmit power

for all the networks links because such an approach

usually penalises low quality links. Instead, it only

reduces the transmit power for links on which such a

reduction does not affect the delivery rate.

We have demonstrated that the performance of

ConTPC is topology dependent. The highest through-

put gain can be observed in nonuniform topologies

such as the disjoint node pairs case, when multiple

transmissions may occur simultaneously. However, in

a typical indoor deployment, as for example in case

of our test bed, such a scheme continues to outper-

form the ﬁxed power levels case, resulting in a 15%

throughput increase when compared with ﬁxed power

operating at its maximum value.

We have also shown that ConTPC not only in-

creases the overall throughput but also increases the

fairness by reducing contention. Moreover, it is pos-

sible to control the overhead of ConTPC while main-

taining its high throughput gain.

ACKNOWLEDGEMENTS

The authors acknowledge the support of Enterprise

Ireland (under the Commercialisation Fund 2007)

and Science Foundation Ireland (under the grant

03/IN3/I396).

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