IMPACT OF DIFFERENT BIT RATES ON PERFORMANCE

CHARACTERISTICS OF INDUSTRIAL WLAN SOLUTIONS

André Schimschar and Lutz Rauchhaupt

Institut für Automation und Kommunikation e.V. Magdeburg, Werner-Heisenberg-Str. 1, Magdeburg, Germany

Keywords: Communication Systems, Performance Analysis, Reliability Analysis, Transmission Characteristics.

Abstract: Wireless communication is already widely used in industrial automation applications. Many solutions are

available which consider industrial related requirements more or less. Thus, reliability and performance with

respect to the specific requirements are the main concerns of industrial automation system users. Some of

the wireless solutions base on the well-known Wireless LAN. The development of IEEE 802.11 was driven

by the demand on highest possible data rates e.g. for video streaming applications. This paper proves

whether the increased bit rates are appropriate to the requirements of industrial automation applications. A

test methodology is introduced which can be used to get the necessary characteristic parameters with respect

to the application field - industrial automation. The impact of bit rates on packet loss probability and

therefore on the performance is analysed. Together with the transmission time for a successful transmission

with one attempt a cost function has been developed. It shows for WLAN solutions, that the highest bit rate

is not adequate to meet the reliability and performance requirements of industrial automation applications.

1 INTRODUCTION

Wireless communication technologies are widely

spread in daily life. The price of wireless products is

thereby the main design aspect with respect to the

consumer market. Reliability is one of the minor

design goals. Therefore, almost everyone has had

negative experiences with such technologies and has

developed concerns regarding the usage of wireless

in industrial communication.

Also the most popular wireless communication

technology WLAN, which uses the probabilistic

media access control CSMA/CA, has disadvantages

concerning time behaviour and reliability of packet

transmissions. With setting up the bit rate the end-

user has to find a trade-off between transmission

time and loss probability of transmitted packets.

Following the common sense the user takes a high

bit rate for fast transmissions or a low bit rate for a

high reliability of communication.

In this paper a test methodology is described

which allows the investigation of wireless solutions

with respect to industrial automation application

requirements and conditions. This approach is used

to define appropriate test conditions and to set-up a

test system in order to investigate the influence of

different bit rates on performance and reliability.

The transmission time is measured and the

number of retransmission is investigated.

Furthermore the loss probability is derived from the

number of retransmission. Finally, a cost function

has been developed in order to provide a guideline to

find an appropriate trade-off between performance

and reliability.

The paper is structured as follows. In chapter 2

the test methodology is presented and the main

important influencing parameters are discussed and

its values are defined. In chapter 3 the characteristic

values are introduced which have been used for the

investigations and the test system and its

components are explained. The test results are

discussed in chapter 4. The cost function is derived

in chapter 5 and the conclusions out of it are

presented in chapter 6.

2 TEST METHODOLOGY

AND CONDITIONS

Because of the special requirements and conditions

of industrial automation applications a test

methodology has been developed by the authors (see

Rauchhaupt, L., Krätzig, M., 2008.). It considers the

387

Schimschar A. and Rauchhaupt L. (2010).

IMPACT OF DIFFERENT BIT RATES ON PERFORMANCE CHARACTERISTICS OF INDUSTRIAL WLAN SOLUTIONS.

In Proceedings of the 7th International Conference on Informatics in Control, Automation and Robotics, pages 387-394

DOI: 10.5220/0002942203870394

 SciTePress

application field, but is independent of a certain

wireless technology. All relevant influencing

parameters are taken into account (see Gnad, A.,

Krätzig, M., Rauchhaupt, L., Trikaliotis, S., 2008).

These parameters are configured well-directed or if

not possible documented. Here only the main

important influencing parameters are described

because of the limited space.

The focus lies on the impact of different bit rates

on the performance and reliability of WLAN

solutions. For WLAN systems according to

IEEE 802.11 this is related implicitly to the physical

layer coding and modulation schemes as discussed

later on.

Although the WLAN implementation (hardware

and software) has a noticeable influence on the time

behaviour (see e.g. Rauchhaupt, L., Krätzig, M.,

2008, Rauchhaupt, L., 2009) it is not considered

here. However, an industrial WLAN solution was

selected which provides the best performances in

terms of jitter of the transmission time (i.e. span) and

in terms of minimum outliers which means in

number and in value.

Also interferences of other wireless systems are

not considered. They are well investigated as

described e.g. in ZVEI, 2009.

Thus, a simple point-to-point topology is used

for the investigations consisting of a WLAN access

point and one client. The attenuation between the

two devices was 60 dB which is according to about

10 m line of sight. The WLAN channel 1 (2412

MHz) was chosen.

The test packets have been generated by the

client. They had a user data length of 64 octets

which is the minimum length for an Ethernet packet

and a typical length in Ethernet based automation

applications.

In previous investigations the dependency of the

performance from the value of the transmit time

interval became obvious. Therefore in these tests the

client generated the test packets with a random

transmit time interval between 15 ms and 25 ms.

The random generation is based upon a uniform

distribution function.

The sample size of each test case was 30,000

packet transmissions which resulted in test durations

of about 10 minutes.

NOTE: With the chosen implementation a bit

rate of 5.5 MBit/s was not adjustable. When this bit

rate has been configured the WLAN system showed

the same behaviour as for automatic bit rate control

mode. Therefore, the bit rate 5.5 MBit/s is not

considered here.

3 CHARACTERISTIC

PARAMETERS AND

TEST SYSTEM

The analysis of literature concerning the usage of

characteristic parameters to describe and assess

communication behaviour has shown that there are

remarkable differences. Moreover, the definitions

come mostly from the application field of Ethernet,

Internet or telecommunication which does not fit to

the application field of industrial automation (e.g. in

(EN 61491, 1999), (EN 61209, 200)).

Test Consumer

comm.ind

Test Producer

comm.req

Wireless System

under Test

Communication

Module

Communication

Module

transmission

delay

Communication

Interface

Communication

Interface

Figure 1: Definition of the transmission delay.

That is why it was necessary to find appropriate

definitions. The following characteristic parameters

are proposed to assess wireless communication

systems with respect to industrial automation

applications:

 Transmission time

 Update time

 Response time

 Data throughput

 Packet loss rate

 Residual error rate

 Activation time after energy saving mode

 Energy requirements

It has to be mentioned that it is not required nor

recommended to use all parameters at the same time

to characterise a communication solution for a

certain application. The definitions of the listed

characteristic parameters can be found in (VDI/VDE

2185, 2007).

In this paper we focus on the transmission time.

It is related to event driven applications. For

example when a work piece reaches a certain

position in order to be machined or when a fluid

reaches a defined level in a tank. In these cases it is

of interest as to how long it takes to transfer the

information from sensor to the control unit e.g.

programmable control logic (PLC). The appropriate

characteristic parameter of a communication system

to assess its behaviour is the transmission time.

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The definition of transmission time is based on a

producer consumer model as shown in Figure 1. It is

the time duration from the beginning of the handing

over of the first user data byte of a packet at the

communication interface in the test producer, up to

the handing over of the last user data byte of the

same packet at the communication interface at the

test consumer. It may be necessary to transmit

several telegrams between the communication

modules e.g. for acknowledgment. Furthermore,

network elements such as base stations may be

involved in the communication producing additional

delays. All these delays are covered by the

transmission time.

In order to assess the reliability the retry rate is

analysed. It is defined as the number of WLAN

packets that are necessary to transmit the content of

one generated test packet.

The system under test implements a widely used

radio technology - Wireless LAN and its application

interface is very common - Ethernet. Therefore,

standard measurement equipment can be used to

implement the above mentioned test methodology.

Ethernet

Tester

Access Point

(Consumer)

Client

(Producer)

Ethernet Ethernet

Power

Splitter

Dynamic

Attenuation

Antenna Cabel

Power

Splitter

56 dB

WLAN Packet

Analyser

3 dB 3 dB

20 dB

Figure 2: Test architecture.

The test architecture is shown in Figure 2. The

devices under test (WLAN client and WLAN access

point) are put into separate radio proof enclosures.

The test boxes achieve an attenuation of 90 dB

against the outer environment. The radio interfaces

of access point and client are connected by an

antenna cable. It is led via a power splitter and a

dynamically changeable attenuator. This approach

excludes interferences to the radio communication.

The dynamic attenuation has been configured to

a constant value of 57 dB. Together with the

attenuation of splitter and antenna cable the total

attenuation is 60 dB as specified for the test cases.

Besides the relevance of this value for automation

applications it has been shown that in this way the

transmit signal is low enough to avoid an

overmodulation and high enough to be unaltered.

The second output of the power splitter is connected

to another static attenuation of 56 dB in order to

connect a WLAN packet analyser. A second power

splitter is used in order to provide the signals to two

channels of the WLAN packet analyser. This

increases the reliability of the packet monitoring.

The total attenuation between client and WLAN

analyser is 62 dB. The total attenuation between

access point and analyser is more than 133 dB. Thus

the analyser captures only the packets transmitted by

the client.

The test packets are generated by an Ethernet

tester which transfers the data to the WLAN client.

As mentioned before the user data length is 64

octets. The generated packets are compliant to

PROFINET-IO telegrams. The advantage is the

specific frame type and content which simplifies the

identification of packets during analysis.

The packets transferred by the WLAN client are

monitored by WLAN analyser. If the packets are

successfully received by the WLAN access point

they are transferred to the Ethernet tester and are

monitored there also.

During analysis in a first step the user data

packets are filtered out of the packets monitored by

the WLAN analyser. Since every user data packet

can be identified by a unique payload it can be found

within the user data packets monitored by the

WLAN analyser. If a user data packet is listed more

than ones it means retries has been initiated by the

WLAN client because of missing

acknowledgements. This way the retry rate can be

calculated.

In addition a timestamp is included within the

user data. With help of this timestamp the Ethernet

tester is able to calculate the transmission time for

every successfully received packet.

4 TEST RESULTS

4.1 Transmission Delay

The measured transmission time consists of random

and constant components (see also Rauchhaupt, L.,

Krätzig, M., 2008.). Examples for constant

components are the frame spacing times or the

signal propagation delay on the wireless medium.

These components have the same values in every

measurement of a sample. The random nature of the

transmission time is being caused by latency of

application interface and implementation, by the

IMPACT OF DIFFERENT BIT RATES ON PERFORMANCE CHARACTERISTICS OF INDUSTRIAL WLAN

SOLUTIONS

389

technology variable, the number of packet retries

and the time allocation for additional connections.

Therefore, stochastic methods are required in

order to analyse the measurement results. Statistic

parameters such as mean value, median, standard

deviation, 95%-percentile (p95), maximum and

minimum value of a test case can be analysed.

0.5

1.5

2.5

3.5

4.5

0 102030405060

Bit ra te [MBit/s]

Transm ission delay [m s]

Max 1st attempt

Mean value

Max with retries

DSSS

OFDM

Figure 3: Transmission time for first attempt and including

all retries.

In Figure 3 then mean value of the transmission

time including all packets retries is depicted.

Furthermore the maximum transmission time values

out of all value which are received after the first

attempt is shown as well as the maximum

transmission time values of all successfully received

packets.

0.2

0.4

0.6

0.8

1.2

1.4

0 102030405060

Bit ra te [MBit/ s]

Transmission delay [ms]

DSSS

OFDM

Figure 4: Delay for transmissions without retries.

The first result is that the maximum values for a

certain bit rate are almost the same independent how

often a packet is retransmitted. Since there is no

contention on the medium it can be concluded that

the maximum values of the transmission time are

determined in the first line by the implementation

including interfaces from and to Ethernet.

The second result is that the value of the outliers

increases dramatically for 54 Mbit/s, while the mean

value is similar to all other bit rates.

0 100 200 300 400 500

-80

-70

-60

-50

-40

-30

Level [dBm]

Time [µs]

240 µs at 6 MBit/s

Figure 5: Medium utilisation time at 6 Mbit/s.

0 100 200 300 400 500

-80

-70

-60

-50

-40

-30

Level [dBm]

Time [µs]

190

s at 54 MBit/s

Figure 6: Medium utilisation time at 54 Mbit/s.

Since the influence of the number of

retransmissions is not very significant in Figure 4

only the mean values of the transmission time are

depicted for those packets which are received at the

first transmission attempt.

The diagram shows that at low bit rates the

differences between the neighbouring values are

high. The differences become smaller with higher bit

rates. However, from 12 Mbit/s on the increase of

the mean values is very low. The reason for that

behaviour is the time for a packet on the medium.

Figure 5and Figure 6 show the signal power level

of a WLAN data packet and an acknowledge frame

during transmission on the medium. It can be seen

that the increase of the bit rate by 9 (from 6 Mbit/s

to 54 Mbit/s) causes only a decrease of the medium

utilisation time by about 1.3 (from 240 µs to 190

µs). The reason is that because of the backward

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compatibility of the WLAN standard parts of a

packet are transferred with 1 Mbit/s or 2 Mbit/s.

With 64 octets user data only this aspect has

remarkable influence on the transmission time.



0.2

0.4

0.6

0.8

1.2

1.4

0 102030405060

Bit ra te [MBit/s]

Media utilization [ms]

DSS S

OFDM

Figure 7: Media utilization time for different bit rates.

Figure 7 illustrates the medium utilisation time

over the bit rate. As shown in Figure 5and Figure 6

the medium utilisation time is defined as time

duration from the beginning of a data packet

transmission up to the end of the related

acknowledgement. Retransmissions are not

considered. The curves in Figure 4 and Figure 7

show a similar characteristic. This underlines the

influence of the medium utilisation time on the

transmission time.

In addition the curves in Figure 7 indicate also

another effect. The WLAN standard IEEE 802.11

specifies different coding and modulation schemes

as listed in Table 1.

Table 1: Overview of the transmission method and

modulation for each data rate.

Data rate Method Modulation

1 MBit/s DSSS DBPSK

2 MBit/s DSSS DQPSK

5.5 MBit/s DSSS DQPSK

6 MBit/s OFDM BPSK

9 MBit/s OFDM BPSK

11 MBit/s DSSS DQPSK

12 MBit/s OFDM QPSK

18 MBit/s OFDM QPSK

24 MBit/s OFDM 16 QAM

36 MBit/s OFDM 16 QAM

48 MBit/s OFDM 64 QAM

54 MBit/s OFDM 64 QAM

Theses schemes are related to the configurable

bit rates. In particular the coding scheme Direct

Sequence Spread Spectrum (DSSS) and Orthogonal

Frequency Division Multiplex (OFDM) have

different impacts on the medium utilisation time.

Therefore, the curves are separated for theses coding

schemes in Figure 3, Figure 4 and Figure 7.

4.2 Number of Retransmissions

The bar chart in Figure 8 depicts the number of

packet transmissions needed for a correct reception

by the access point. A logarithmic scale is chosen

for the number of transmissions in order to see the

small number of retransmissions. The interesting

fact of this picture is that while using bit rates

related to DSSS coding the maximum number of

retransmission is one. Obviously this coding scheme

is more robust than OFDM.

0.1

100

1000

10000

100000

1 2 6 9 11 12 18 24 36 48 54

Bit rate [Mbit/s]

Number of packets

1. Transmission

2. Transmission

3. Transmission

4. Transmission

Figure 8: Number of packet retransmissions over bit rate.

For all bit rates related to DSSS the entire

number one retransmission is about ten. In test cases

with bit rates related to OFDM coding a number of

packet transmissions with at least two

retransmissions can be noticed. Especially for higher

bit rates also up to four retransmissions can be seen.

The diagram in Figure 9 illustrates the total

number of PROFINET-IO packet transmissions sent

by the WLAN client via the medium. This means the

transmission of all data packets including all

retransmissions. Figure 9 shows almost constant

values for DSSS bit rates. They are concentrated

next to the lower limit of 30000 packets. The OFDM

bit rates do not have such a constant behaviour.

However, for low OFDM bit rates only little

differences exist. At higher bit rates the number of

packet transmission increases. Especially for 54

Mbit/s the number of required transmission is

remarkable high. For this bit rate 31000 WLAN

packets are required to transmit 30000 Ethernet

packets. This results in a retry overhead of about 3.3

percent.

IMPACT OF DIFFERENT BIT RATES ON PERFORMANCE CHARACTERISTICS OF INDUSTRIAL WLAN

SOLUTIONS

391



29800

30000

30200

30400

30600

30800

31000

31200

0 102030405060

Bit rate [MBit/s]

Sum of transmitted packets

OFDM

DSSS

Figure 9: Total number of packet transmissions.

4.3 Loss Probability

Based on the discussion of retransmissions the loss

probability shall be determined. Out of the results

shown in Figure 8 a geometric distribution is

assumed. The geometric distribution deals with the

number of independent trials required for a single

success. A famous example of this distribution is

tossing a coin until it lands on heads.

The formula given in equation 1 describes the

expectation value E(X) of required transmissions for

each bit rate. Whereas N is the sample size, i is the

indicator of the transmission attempt and k

is the

frequency of an attempt within a test case.

()

⋅⋅=

∑

(1)

N in formula 1 has not been calculated as shown in

equation 2 since the sample size is well known for

the test cases.

∑

(2)

The estimation of the success probability p of a

packet transmission is equivalent to the reciprocal of

the expectation value E(X) of required transmissions

(Formula 3).

()

(3)

With the well-known equation 4 it is easy to get the

loss probability out of the success probability.

−=

(4)

Figure 10 illustrates the loss probability for

different bit rates, which has been calculated with

the last-mentioned equations. In this picture we

differentiate again between DSSS and OFDM

method. The diagram is similar to the one of Figure

9. The only difference is the normalised ordinate.

Additionally, the figure presents low loss

probabilities for DSSS bit rates. On the other hand

values for OFDM bit rates are essentially higher.

However, for a wide range the values are almost

constant. Furthermore, the abovementioned extreme

value at 54 MBit/s exists also in this figure. In

summary the diagram underlines the results

concerning the robustness of DSSS coding scheme.

OFDM bit rates are not recommended with respect

to a robust communication.

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 102030405060

Bit ra te [MBit/s]

Loss probability

OFDM

DSS S

Figure 10: Loss probability for different data rates.

5 COST FUNCTION

In order to consider all the above mentioned aspects

for the assessment of a wireless system a cost

function has been developed. It is a trade of between

(a high) transmission time and (a low) loss

probability. Therefore the values of the transmission

time measured in section 4.1 and the loss probability

values calculated in section 4.3 are used. For further

discussions the cost function shall depend on the bit

rate. In addition the values should not have a

dimension unit.

Therefore, the mean values of the transmission

time for packets without retransmissions are used.

Thus the influence of retransmissions on the time

behaviour is avoided. In order to get values free of

dimension units the transmission time values are put

in a ratio to the shortest transmission time mean

value. This value belongs to a bit rate of 54 MBit/s.

Thus the value of bit rate 54 MBit/s is assigned to

the ratio of one in the cost function. All other bit

rates have values greater than one. The ratio for 1

MBit/s is the highest and is almost three times

greater than for the reference bit rate.

The second part of the cost function is the loss

probability. It is multiplied by the transmission time

ratio. The complete formula is given in equation 5.

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392

()

() ()

(54 / )

tDR

CDR qDR

tMBits

=⋅

(5)

The cost for different bit rates is illustrated in the

diagram of Figure 11.



0.005773444

0.000617575

0.001798285

0.00162177

0.000747109

0.000899086

0.001564179

0.002632732

0.002842077

0.031164218

0.001415751

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 102030405060

Bit ra te [MBit/s]

Cost

Figure 11: Cost function of WLAN packet transmissions.

The diagram shows a general trend. The cost

values increase with higher bit rates. This means

better communication behaviour can be expected

with lower bit rates.

Taking a deeper look to the cost function the

minimum is visible at 11 MBit/s. As in the figures

before this diagram also offers the maximum at

54 MBit/s.

Bit rates lower than 24 MBit/s are almost in the

same range of values. However, there is no constant

increasing of cost values with higher bit rates. At bit

rates greater or equal than 24 MBit/s the trend of the

cost function rises noticeably.

6 CONCLUSIONS

The focus of this paper was the investigation of the

influence of bit rates on the reliability and

performance of WLAN systems with respect to

industrial automation applications. Thus the test

setup and the values of the influencing parameters

reflect the communication requirements of this

application field.

A first result of the measurements is the

confirmation of the generally way of thinking.

Higher bit rates cause shorter transmission time

values. On the contrary lower bit rates cause less

packet retransmissions. However, in the context of

industrial automation a more detailed look at the

results is necessary.

Thus there is an important finding, that there are

differences between the coding schemes DSSS and

OFDM. Obviously DSSS shows a more robust

behaviour than OFDM. DSSS is included in IEEE

802.11 b. In contrast IEEE 802.11 g uses OFDM in

favour. However, in order to be compatible with

IEEE 802.11 b devices IEEE 802.11 g also supports

bit rates using DSSS. With this background the user

can choose DSSS for more reliable connections or

OFDM for faster transmissions.

Another result is that the maximum values of the

transmission time are more influenced by the

implementation than by the number of

retransmissions. Thus optimisation potential of the

WLAN implementation can be detected using the

presented test approach. In fact there are remarkable

differences between industrial WLAN solutions as

investigated by the authors in other tests projects.

However, also the retransmissions play an

important role when a congested medium is taken

into account what was excluded in the given

investigations.

Therefore a cost function was introduced that

opens the possibility to consider transmission time

and required retransmissions for a successful

communication. It can help to find the appropriate

trade-off between a fast and a reliable wireless

connection. As shown in Figure 11 a transmission

with 11 MBit/s has the lowest cost value and is

therewith the best trade-off with respect to

considered application field. Also other bit rates of

the IEEE 802.11 b specification, which uses DSSS

and the corresponding modulations, showed an

appropriate behaviour. The worst behaviour offer

transmissions with 54 MBit/s. This fact disagrees

with the popular opinion that a high bit rate is the

best solution real-time automation applications. In

addition the investigations show that even the

medium utilisation with 54 Mbit/s is the highest

because of the retransmissions. This is an important

outcome with respect to the required efficient

spectrum use in order to support coexistence

between wireless systems.

Other interesting points of the explained

measurements are the lower costs of 12 Mbit/s and

18 MBit/s in comparison to 6 Mbit/s and 9 Mbit/s.

The rates of 24 Mbit/s and 36 MBit/s are also

acceptable.

As a conclusion the best bit rates for industrial

automation applications are the ones of the IEEE

802.11 b specification. With the focus on fast

transmissions bit rates between 12 Mbit/s and 36

MBit/s could be chosen. The bit rates 6 Mbit/s, 9

Mbit/s, 48 Mbit/s and 54 MBit/s should be avoided.

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