Effect of Flat Fading in 802.11 MAC for Cross

Layer Evaluation Using Channel Emulator

Adriano Almeida Goes goes.adriano@gmail.com

Omar Carvalho Branquinho omar.branquinho@puc-campinas.edu.br

Norma Reggiani nreggiani@puc-campinas.edu.br

Douglas Zambianco

douglas.zambianco@gmail.com

Pontifícia Universidade Católica de Campinas

Centro de Ciências Exatas, Ambientais e de Tecnologias

Faculdade de Engenharia Elétrica

Rodovia Dom Pedro I km 136 – Campinas – Brasil

ABSTRACT WLAN 802.11 operating in 2,4 GHz are being intensely

implanted mainly in public environments. In these

environments mobility is always a present characteristic. The

objective of this work is to analyze the behavior of 802.11

MAC with flat fading. To reach this objective an emulation

system was developed to create the flat fading channel

phenomenon. With a WLAN and the emulator was possible to

show the cross layer effect, evaluating the WLAN

performance analyzes in mobility conditions. The

experiments evaluated: rate of transmission in each

modulation level, data rate in the transport layer, network

efficiency, jitter and throughput. The network information

was obtained through SNMP. The results show that the

802.11 MAC had problems to support services that need high

performance, as VoIP and Streaming. Therefore when the

receiver is in movement the degradation does not allow the

quality required by these types of services. Keywords PWLAN, WLAN, Rayleigh, Weibull, Flat fading,

wireless, MAC, IEEE802.11.

I. INTRODUCTION

Each time more 802.11 WLAN (Wireless Local Area

Network) [1], are being used in the market in wide scale. The

performance depends on the position of the user and its

mobility. WLAN used in public environment (PWLAN)

can be had accessed by diverse types of equipment, as for

example, cellular, palm tops, PDAs, laptops, among others

portable and mobile devices. The 802.11 MAC has an

anomaly effect that degraded the performance [2][3]. This

type of network supports well services that do not need high

performance such as, email, HTTP, ftp, among others.

However for services like video-on-demand, VoIP,

Streaming, among others, that require high availability and

functionality, there are serious doubts if the WLAN

performance will be good enough. In this work it is studied

the efficiency of the net and the effect of cross layer through

the emulation of the flat fading channel.

For mobile applications the attention must be in the

environment conditions, once there are diverse phenomena

that degrade the WLAN efficiency significantly, for example,

multi path. To analyze WLAN operating in these conditions

it is necessary a tool to reproduce these phenomena and take

into account the requirements of this type of application.

This article presents results of WLAN performance using a

work bench to emulate signal strength variation and evaluate

the efficiency with Weibull distribution [4] that takes in

consideration some factors normally not considered in other

models for indoors propagation.

The radio propagation tests to evaluate performance always

were a laborious task requiring sophisticated equipment. The

currently WLAN possess ways of measurement of the signal

intensity, signal-noise ratio, number of packets in each

modulation level, among others, through the protocol of

management of net SNMP (Simple Network Management

Protocol).

The objective of this work is to emulate the flat

fading phenomenon through a work bench and evaluate the

WLAN performance with different Weibull distribution

factor, to determine the cross layer effect.

The work is organized in the following form: in

section II presents the flat fading emulation process in 802.11

WLAN operating in 2,4 GHz [5,6,7]. In section III, the

software and the hardware used are described. Section IV

presents the results network efficiency, as well as an analysis

of the obtained results. Section V presents a preliminary test

in VoIP. Finally, section VI presents the conclusion.

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II. EMULATION OF FLAT FADING

To control the flat fading channel emulation was developed

a work bench with software (called SCLan5) and a radio

frequency hardware to emulate Weibull distribution, to

measure the network efficiency, to analyze the network

behavior, among others [6].

Before initiating any measurement it is necessary to

calibrate the bench work. This process consists basically of

sending definitive levels of attenuation to the control block

and observing the power of reception. However, this relation

is not enough to perform the calibration, because that the

attenuation level that must be characterized by the

distribution is the effective normalized tension, through the

Weibull distribution.

The expression below was used to equate these two values

of power:

m

ieffective

P

PV = (1)

Where, Pi is the received power and Pm is the logarithmic

average power of all received powers, both the two express in

Watts.

After concluding the calibration process, the algorithm that

emulates the flat fading channel on the transmitted signal

must be initiated, following the parameters described in Table

I.

TABLE I. PARAMETERS OF EXECUTION OF THE FLAT- FADING

Parameters Description Unit

Dref

VE

DP

Vrms

α

β

Referential Distance

Speed of Execution

Shunting line Standard Tension RMS

Path Loss

Environment Coefficient

meters

m/s

meters Volts

Dimensionless

Dimensionless

The signal that arrives at the receiver is a result of two

effects: one is the attenuation suffered by the transmitted

signal along its way to the receiver, and another is the fact

that the signal that arrives at the receiver is a result of

multipath propagation.

In order to describe the attenuation of the signal due to

the caracteristics of the environment, we used the shadowing

model given by [7]:

b

dBr

r Xd

d

dP

dP+

−=

00

log10)(

)(α (2)

where, Pr(d) is the surveyed power, Pr(d0) is the reference

power, α is the path loss factor, d is the distance measured,

d0 is the distance of reference and Xb is a log-normal

distribution. In this work we do not use the shadowing model

and concentrate the tests in the signal strength changing with

Weibull distribution. This is equivalent a device performing a

circle around the access point with fixed distance with

velocity that can be changed.

In order to describe the effect of summing the various

waves that come from the multipath propagation of the

transmitted wave, we used the Weibull distribution, given by:

α

βαααβ

−

−− ⋅=

x

exxf1)( (3)

The Rayleigh distribution is a particular case of the Weibull

distribution, if we consider in the last one α = 2, where

sigma is the variance of a gaussian variate [4]. So, the beta

parameter of the Weibull distribution is associated with the

variance of the variable considered. The Weibull distribution

is used to permit a grade of flexibility to emulate de

environment. This assumption is reasonable considering the

multi path and differences of obstacles in the indoor

environment. With the parameter β it is possible to change

from a very stable scenario to a severe one. This flexibility is

very interesting to evaluate the cross layer effect.

Through the described parameters in Table I, the software

generates a Weibull distribution and relates the obtained

values with the calibration table obtained in the beginning of

the process and stored it in a data base that contains the

effective tension. Based on this scenario, the software sends

the necessary parameters so that the control block attenuates

the RF signal following the characteristics of the Weibull

distribution, with a definite environment parameter β .

The Figure 1 shows a graphic obtained after sending the

values of Weibull distribution for different values of β . The

results for β =0,2, β =0,5 and β = 1, are showed by the grey,

blue and red lines respectively.

Figure 1. Weibull Distribution generated for software (SCLan5).

In the Figure 2 it can be verified the attenuation of the

transmitted wave described by the Equation (2) with different

distances an values of alpha (associated with different

environments), with the effect of flat fading described by the

Weibull distribution with β = 0,5:

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Figure 2. Distribution of Shadowing, with values of β (1,6;2;2,5,3 e 4).

Finally, it is possible to relate the effects caused by the

Weibull distribution with the efficiency of measured net with

protocol SNMP. In order to arrive in network efficiency it

was developed an expression that takes into account the

frames transmitted considering the rate used in the physical

layer by each frame. The result is a percentile evaluation to

monitor in real time the performance. The expression of the

effective rate is given by:

i

n

i T

i

efetiva rPNF

NFR ∑

=

=1

*[%] (4)

Where, NFi is the numbers of transmitted frames with rate i,

NFT is a total number of transmitted frames in a period of

time and Pri is the percent of the data rates compared with the

maximum rate.

III. TEST SYSTEM

In the process of emulation of the flat fading channel, the

components shown in the Figure 3 had been used.

AP

PC1

Shildet Box

PC2

WNIC

Coaxial cable

Control Terminal

Attenuator

LPT

Hardware

Control

Capture

Sniffer

(MAC)

Sniffer

(TCP)

Figure 3. Components used in the emulation of flat fading.

The AP controlled by the PC1 (microcomputer) and

confined in an armored box transmits a signal through a

coaxial cable. This signal is received by attenuator (AT) that

controls its power. The Div block is a RF splitter to permit

the sniffer to work directly in the RF signal.

To manage this attenuator, a control circuit (control block)

was set up, that consists of a digital-to-analogical converter

and a circuit that supplies an adjustment of the gain and Off-

set of the involved signal as showed in the Figure 4.

Figure 4. Control block.

This circuit is controlled by software located in the

“Terminal of Control” that supplies a tension to the

attenuator. Finally, the PC 2, through a plate WNIC (Wireless

Network Interface Card) PCMCIA receives this controlled

signal for the attenuator.

The software SCLan5 used to the control and management

of the system was developed not only to control the tension,

but also to treat the collected information obtained by the

SNMP protocol and to execute the algorithms for generation

of the considered effect, between them the Weibull

distribution.

The Figure 5 shows the bench work mounted for the

emulation of the flat fading channel in order to study the

effect cross layer caused by the physical layer in the superior

layers.

Figure 5. Organized system to emulate the flat fading channel.

In the Figure 5 is possible to identify in the right the

shielded box where the AP is conditioned. Besides it there are

the circuit of control and the changeable attenuator for

tension. The laptop works as a sniffer capturing packets

TCP/UDP directly in the RF signal.

To facilitate the visualization, the Figure 6 shows the

shielded box open with the AP connected to a pig tail and the

N connector with the coaxial cable that establishes

connection with the mechanism of attenuation and control of

the signal.

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Figure 6. Visualization of the emulation system.

In the Figure 6 it can be verified that the sniffer of the MAC

is directly connected with the box of control, that allows to

capture the packets of the transport layer (TCP/UDP) and to

analyze with software as Ethereal, Fluke (software for VoIP),

among others.

IV. RESULTS

After the emulation of the Weibull distribution for some

values of β as it was shown in Figure 1, graphical of jitter,

throughput, SNR, rate of transmission UDP and efficiency of

net had been captured.

Before, during and after the execution of the algorithm we

observed the power of the signal received in Netstumbler

software, presented in the Figure 7.

To facilitate the visualization of the presented phenomena,

each resultant graph will be divided in phases and numbered.

Figure 7. Visualization of power of the signal received for the Netstumbler.

At moment 1, the key of the controlling block is opened

without any effect of attenuation on the signal, characterizing

a communication without movement. At moment 2, the

execution of the Weibull distribution is started. The variation

of the signal is clearly observed in this moment. Finally, at

moment 3, the similar configuration of the moment 1 is

reestablished.

Figure 8. Graphs of the physical layer: Powers of signal, noise and SNR.

At the same time, through protocol SNMP, the SCLan5

software measures the SNR, signal and noise power, to relate

them later to the effect of cross layer. These three graphs are

shown in Figure 8.

In Figure 8, the power of the noise remains unchanged

during all the process, but the power of the signal and

consequently the SNR had been degraded significantly during

the emulation of the flat fading channel, what influences

directly the QoS of the application (VoIP or Streaming, for

example).

In Figure 9, it is observed that at moments 1 and 3 while the

Weibull distribution is not on, the net efficiency is 100%.

But when the signal is submitted to the flat fading channel,

a high degradation in the net efficiency is observed (equation

3), presented at moment 2. Moreover, it is noticed that there

were attempts carried through by the MAC protocol in

searching greater transmission rates, however without

keeping stability between the possible levels of transmission.

Figure 9. Graph of the efficiency for a net 802.11b.

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During the execution presented in Figure 9, it was possible

to identify a gradual reduction in the net efficiency. This

happens because the Access Point looks for lower rates in

order to stabilize the transmission. This induces the

transmission to become unstable and influences the superior

layers degrading and many times directly disabling services

as VoIP and Streaming.

Another important result obtained by the process is net

throughput. The Figure 10 shows a great variation of

throughput when the channel is flat fading. In this figure, β

was equal to 1,5.

Figure 10. Variation of throughput for the flat fading channel.

Already when the channel is not flat fading the throughput

practically remains unchanged, as shown in Figure 11.

Figure 11. Variation of trhoughtput for the channel without any movement.

With the same parameters of emulation UDP traffic was

created and these packets had been captured by the sniffer

using the Ethereal software. Through the analysis of these

packets, it was possible to determine the variation in the UDP

transmission rate for the normal channel and flat fading. The

Figure 12 shows the variation of the UDP transmission rate

captured in the channel without mobility.

Reception rate in channel without flat fading

622

755713

755768774725

847852

723736

841892

703

782

693750

956

797

883874

715

816

723762

721

947

864819

773

0

200

400

600

800

1000

1200

0,0 5,0 10,0 15,0 20,0 25,0 30,0 35,0

Time (min)

Rate (KB/s)

Figure 12. UDP transmission rate for 802.11b net with normal channel.

In the Figure 12 the rate variation in a station 802.11b

without movement is presented, that is, with a channel that is

not flat fading. In this situation, the logarithmic average

between rates is 786,04 KB/s and the standard deviation is

78,45 KB/s.

However, when the channel is flat fading there is a great

variation in the rate, as shown in the Figure 13.

Reception rate in flat fading channel

518

794

118

769

909944

101

862

148

238

115

173

487

729

621

918

991

89

910888

142107

51

910

71

285

150

55

395

914

118

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

0,0 5,0 10,0 15,0 20,0 25,0 30,0Time (min)

Rate (KB/s)

Figure 13. UDP transmission rate for 802.11b net with flat fading channel.

With the emulation of the flat fading channel a great

variability in the received rate can be verified. With this a

station 802.11 that it is in movement, can influence all the

others stations in the net because of its constant instability. In

this case, the logarithmic average of the received rate

decreased to 468,62 KB/s and the standard deviation

increased to 359,52KB/s, showing a great degradation in the

service.

Another important information is the jitter, that determines

the viability of transmissions UDP with a minimum of QoS.

In other words, it is the essential information to determine the

delay held in a VoiP connection in a flat fading channel. With

the same captured packets by the sniffer the graph of Figure

14 was generated, which shows the behavior of the jitter in a

flat fading channel.

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Flat Fading channel - Jitter

0

50

100

150

200

250

300

350

400

450

500

550

600

650

0 5000 10000 15000 20000 25000 30000 35000

Packages sequences

Jitter (ms)

Figure. 14. Jitter for net 802.11b with flat fading channel.

The logarithmic average of jitter joined in this situation was

49,11ms and its shunting line standard was 151ms.

With the jitter extracted from station 802.11b during the

emulation of flat fading channel, it can be verified that there

is a more variability and delay between the packets in this

case than the jitter extracted from a station without

movement. It is shown in the Figure 15.

Channel without flat fading - Jitter

0

5

10

15

20

25

30

35

40

45

50

55

60

65

500 5500 10500 15500 20500 25500 30500

Packages sequences

Jitter (ms)

Figure 15. Jitter for net 802.11b with channel not flat fading.

As shown in Figure 15, in a station without movement, the

jitter, besides being small, didn’t have a variability so

accented as the station submitted to the flat fading channel. In

this situation, the jitter was stable with a shunting line

standard of 4ms and a logarithmic average of 0,39ms.

V. RESULTS ON VOIP PLATFORM

As an extension of this work, services of VoIP had been

tested on the basis of a coder of available voice in the market

and some coefficients of environment.

When submitted to the flat fading channel, the results about

quality of the call, quality of net, jitter, loss of packets and

discarded packets, could have been compared with the

variation of the relation signal-noise and the variability of the

system when it is in movement.

The Figure 16 shows three graphs for the drawn up QoS of

VoIP with two different moments: before and during the

emulation of the flat fading channel. These tests had been

executed under a platform 802.11b and a environment

coefficient equal 1,5.

Figure 16. Jitter graphs, discarded packets and lost packets obtained from

software Fluke before and during the emulation of flat fading channel.

Through these graphs, it is possible to notice the

degradation in the parameters when the channel is flat fading,

what will influence the QoS of VoIP. Depending on the

environment this it can make the service unfeasible

independently of the power of received signal. In the Figure

17 it is possible to analyze the quality of the call presenting

the coefficient MOS LQ.

Figure. 17. Quality of the call graph MOS LQ obtain with software Fluke

before and during the emulation with a β equal 1,5.

An acceptable value for this coefficient MOS LQ is

between 3,5 and 4. With the emulation of flat fading channel,

the quality of call is bad because MOS LQ reaches values

below 3,5. This shows that a linking in these conditions

would not be possible.

Finally, the Figure 18 shows the factor R. This factor

defines the viability of to have or no a VoIP call.

Figure 18. Efficiency of net with controlled attenuation.

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This graph was generated from Fluke software and shows

that in the conditions presented and mainly with a value of β

equal 1.5, the degradation of the quality in the call is enough

to interrupt it and to make implantable the conversation.

Beyond the “cuts in the voice” was identified a great delay

and deformity in the transmitted sound.

VI. CONCLUSION

In this work we presented the results of performance

evaluation of 802.11 WLAN through a work bench

developed specially to test the behavior of the MAC in

different environments to analyze the cross layer effect.

To realize this test it was developed an emulator flat fading.

The developed system is all automatic allowing any type of

simulation and test with flat fading, emulating the movement

of users of a WLAN, predominant characteristic found in

PWLAN.

Through the results it was verified that there is a reaction of

802.11 MAC to the variation of the signal and does not

present stability when submitted the wireless device to move

in high speed, that is, when a mobile receiver is in movement

the network efficiency presented was inadequate for

applications with high rates and real time requirements.

Another interesting point is that the network efficiency is

better when the receiver is stopped than when it is in

movement, even if the received power in the stopped receiver

was smaller than the average power received in movement.

A type of timeout was identified on the stabilization

process of the signal in the sub layer MAC when it is

submitted to the high variance, or either, high mobility.

During the execution of the tests it was identified a low

effectiveness in working with WLAN in environments with

other systems and with mobility. This work will have its

evolution in the direction to evaluate the parameters

necessary for identification of the points of deficiency in

many services with high performance and to consider

solutions to WLANs that work with this type of service. It

will also be used to assist implantation of PWLAN standard

802.11 in public environments with high mobility.

VII. ACKNOWLEDGEMENT

The authors are thankful the support of laboratory WCN

(Wireless Competence Network) of the INTEL in the Institute

of Computation of UNICAMP.

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