Institutionen för systemteknik Department of Electrical Engineering Actuator Networks

Institutionen för systemteknik Department of Electrical Engineering Actuator Networks

Institutionen för systemteknik
Department of Electrical Engineering
Examensarbete
Media Access Control for Wireless Sensor and
Actuator Networks
Examensarbete utfört i Kommunikationssystem
vid Tekniska högskolan i Linköping
av
Muaz Un Nabi
LiTH-ISY-EX--12/4603--SE
Linköping 2012
Department of Electrical Engineering
Linköpings universitet
SE-581 83 Linköping, Sweden
Linköpings tekniska högskola
Linköpings universitet
581 83 Linköping
Media Access Control for Wireless Sensor and
Actuator Networks
Examensarbete utfört i Kommunikationssystem
vid Tekniska högskolan i Linköping
av
Muaz Un Nabi
LiTH-ISY-EX--12/4603--SE
Handledare:
Hien Quoc Ngo
isy, Linköpings universitet
Mikael Gidlund
ABB AB
Johan Åkerberg
ABB AB
Examinator:
Eleftherios Karipidis
isy, Linköpings universitet
Linköping, 11 July, 2012
Avdelning, Institution
Division, Department
Datum
Date
Division of Communication Systems
Department of Electrical Engineering
Linköpings universitet
SE-581 83 Linköping, Sweden
Språk
Language
Rapporttyp
Report category
ISBN
Svenska/Swedish
Licentiatavhandling
ISRN
Engelska/English
Examensarbete
C-uppsats
D-uppsats
Övrig rapport
2012-07-11
—
LiTH-ISY-EX--12/4603--SE
Serietitel och serienummer ISSN
Title of series, numbering
—
URL för elektronisk version
http://www.commsys.isy.liu.se
Titel
Title
Media Access Control for Wireless Sensor and Actuator Networks
Författare Muaz Un Nabi
Author
Sammanfattning
Abstract
In a wireless network, the medium is a shared resource. The nodes in the network
negotiate access of the shared resource using the Medium Access Control (MAC)
protocol. The design of a MAC protocol for a sensor node is not the same as
that for a wireless transceiver. Due to the transceiver characteristics, the MAC
protocol design is limited in terms of medium access methods. However, in most
cases, the protocols rely on simple access methods i.e. Time Division Multiple
Access (TDMA) or Carrier Sense Multiple Access / Collision Avoidance (CSMA /
CA).
Control and monitoring applications, running over a wireless network, are
typical examples of Wireless Sensor Actuator Network (WSAN) application in
industries. In an industrial network, the message deliveries must be time-bounded
otherwise, they are of no use.
This report aims to present the thesis work carried out at ABB AB, Västerås.
The purpose of this thesis was to compare the performance of WLAN and
WirelessHART when it comes to control applications. For the purpose of WLAN,
the media access schemes are analyzed in terms of deadline misses. There are
other metrices for the performance evaluation but our focus was on the latency,
since it is very important in the field of industrial automation. NS-2 was used for
the purpose of MAC layer analysis and it is also shown that PCF gives better
performance as compared to DCF, in terms of deadline misses. Finally, WLAN is
proven to accommodate more control loops as compared to WirelessHART for a
given scenario.
Nyckelord
Keywords
NS-2, WSN, IWLAN, DCF, PCF
Abstract
In a wireless network, the medium is a shared resource. The nodes in the network
negotiate access of the shared resource using the Medium Access Control (MAC)
protocol. The design of a MAC protocol for a sensor node is not the same as
that for a wireless transceiver. Due to the transceiver characteristics, the MAC
protocol design is limited in terms of medium access methods. However, in most
cases, the protocols rely on simple access methods i.e. Time Division Multiple Access (TDMA) or Carrier Sense Multiple Access / Collision Avoidance (CSMA / CA).
Control and monitoring applications, running over a wireless network, are typical
examples of Wireless Sensor Actuator Network (WSAN) application in industries.
In an industrial network, the message deliveries must be time-bounded otherwise,
they are of no use.
This report aims to present the thesis work carried out at ABB AB, Västerås. The
purpose of this thesis was to compare the performance of WLAN and WirelessHART
when it comes to control applications. For the purpose of WLAN, the media access
schemes are analyzed in terms of deadline misses. There are other metrices for the
performance evaluation but our focus was on the latency, since it is very important
in the field of industrial automation. NS-2 was used for the purpose of MAC layer
analysis and it is also shown that PCF gives better performance as compared to
DCF, in terms of deadline misses. Finally, WLAN is proven to accommodate more
control loops as compared to WirelessHART for a given scenario.
v
Acknowledgments
First of all, I would like to thank ABB AB for giving me this opportunity to work
with them. I am grateful to Mikael and Johan for sparing their valuable time, to
supervise me for this master thesis, during my stay at ABB AB.
Also, I would like to thank Hien and Eleftherios for their support at the university.
Muaz Un Nabi. July, 2012
vii
Contents
1 Introduction
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Purpose and Goals . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Disposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Introduction to Wireless Local Area Networks
2.1 Basic Architecture of IEEE 802.11 WLAN . . . .
2.2 802.11 PHY Layer . . . . . . . . . . . . . . . . .
2.3 PHY Extensions . . . . . . . . . . . . . . . . . .
2.4 IEEE 802.11n . . . . . . . . . . . . . . . . . . . .
2.4.1 Guard Interval . . . . . . . . . . . . . . .
2.4.2 High Rate FEC Codes . . . . . . . . . . .
2.4.3 MIMO Technology . . . . . . . . . . . . .
2.4.4 Modes of Operation . . . . . . . . . . . .
3 IEEE 802.11 MAC Layer
3.1 Distributed Coordination Function
3.1.1 Working Mechanism . . . .
3.1.2 Problems with the DCF . .
3.2 Point Coordination Function . . .
3.2.1 Working Mechanism . . . .
3.2.2 Problems with the PCF . .
3.2.3 Polling Mechanism . . . . .
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4 Wireless in Industrial Automation
4.1 Industrial Automation Requirements .
4.2 Standards . . . . . . . . . . . . . . . .
4.2.1 Controller Area Network . . . .
4.2.2 PROFINET I/O . . . . . . . .
4.2.3 Bluetooth . . . . . . . . . . . .
4.2.4 Wireless HART . . . . . . . . .
4.2.5 IWLAN . . . . . . . . . . . . .
4.3 Advantages of Wireless Technologies in
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Industrial
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Automation .
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x
Contents
5 Performance Analysis of IEEE 802.11 PHY
5.1 IEEE 802.11n Simulink Model . . . . . . . . . . . . .
5.1.1 Model Description . . . . . . . . . . . . . . .
5.1.2 Channels . . . . . . . . . . . . . . . . . . . .
5.1.3 Parameters . . . . . . . . . . . . . . . . . . .
5.1.4 Results . . . . . . . . . . . . . . . . . . . . .
5.2 Throughput and Delay Limits . . . . . . . . . . . . .
5.2.1 Assumptions . . . . . . . . . . . . . . . . . .
5.2.2 Derivation of MT and MD for IEEE 802.11a
5.2.3 Derivation of MT and MD for IEEE 802.11b
5.2.4 Parameters . . . . . . . . . . . . . . . . . . .
5.2.5 Results . . . . . . . . . . . . . . . . . . . . .
5.3 Packet Aggregation . . . . . . . . . . . . . . . . . . .
5.3.1 DSDSRFA . . . . . . . . . . . . . . . . . . .
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27
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6 Performance Analysis of IEEE 802.11 MAC
6.1 NS-2 . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.1 Overview . . . . . . . . . . . . . . . . . . . .
6.1.2 Network Components . . . . . . . . . . . . .
6.1.3 Tracing . . . . . . . . . . . . . . . . . . . . .
6.2 Comparison between PCF and DCF . . . . . . . . .
6.2.1 Parameters . . . . . . . . . . . . . . . . . . .
6.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3.1 Deadline misses of PCF and DCF for 1 Mbps
6.3.2 Deadline misses of PCF and DCF for 2 Mbps
6.4 Observations . . . . . . . . . . . . . . . . . . . . . .
6.4.1 Deadline Misses . . . . . . . . . . . . . . . . .
6.4.2 Bandwidth Efficiency . . . . . . . . . . . . .
6.5 Comparison between WLAN and WirelessHART . .
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7 Conclusions and Future Work
7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
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68
Bibliography
69
Contents
xi
List of Figures
1.1
1.2
Wireless Sensor Networks (Courtesy of ABB) . . . . . . . . . . . .
The Basic Block Diagram of the Wireless System (Courtesy of ABB)
2.1
2.2
2.3
2.4
2.5
Basic WLAN Architecture . . . . . . . .
IEEE 802.11 Protocol Reference Model .
General IEEE 802.11a Frame Format . .
General IEEE 802.11n Frame Format . .
The HT Control Field . . . . . . . . . .
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3.1
3.2
Graphical Representation of CSMA / CA . . . . . . . . . . . . . .
Graphical Representation of PCF . . . . . . . . . . . . . . . . . . .
14
17
4.1
Wireless HART Mesh Network (Courtesy of ABB) . . . . . . . . .
23
5.1
5.2
5.3
5.4
5.5
5.6
30
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36
5.15
5.16
5.17
PER vs SNR for MCS = 13, 14 and 15 (AWGN, 2x2 Direct-Map)
PER vs SNR for MCS = 13, 14 and 15 (Ch D nLoS, 2x2 Direct-Map)
PER vs SNR for MCS = 13, 14 and 15 (Ch D nLoS, 2x2 Beamforming)
PER vs SNR for MCS = 13, 14 and 15 (Ch D nLoS, 4x2 STBC) .
Ideal TUL for frame size = 1000 Bytes . . . . . . . . . . . . . . . .
Maximum Throughput vs Payload size for different Data Rates
(IEEE 802.11a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Minimum Delay vs Payload size for different Data Rates (IEEE
802.11a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bandwidth Efficiency for different Data Rates (IEEE 802.11a) . . .
Effect on Throughput by doubling the Rate (2x6=12Mbps)(IEEE
802.11a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Effect on Throughput by doubling the Rate (2x54=108Mbps)(IEEE
802.11a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Percentage increase in Throughput by doubling the Rate (IEEE
802.11a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Throughput when the Channel is adding Noise (IEEE 802.11a) . .
Maximum Throughput vs Payload size for different Data Rates
(IEEE 802.11b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Minimum Delay vs Payload size for different Data Rates (IEEE
802.11b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Concept of DSDSRFA . . . . . . . . . . . . . . . . . . . . . . . . .
Comparison of TUL . . . . . . . . . . . . . . . . . . . . . . . . . .
Comparison of MT . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1
6.2
6.3
6.4
6.5
6.6
Simplified User0 s View of NS
Class Hierarchy (Partial) . . .
Node (Unicast and Multicast)
Link . . . . . . . . . . . . . .
Insertion of Trace objects . .
Trace Format example . . . .
5.7
5.8
5.9
5.10
5.11
5.12
5.13
5.14
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xii
Contents
6.7
6.8
6.9
6.10
6.11
6.12
6.13
6.14
6.15
6.16
6.17
6.18
6.19
6.20
6.21
6.22
6.23
6.24
6.25
6.26
Five Node Network Topology . . . . . . . . . . . . . . . . . . . . .
Encountered delay vs Packets received for 5 nodes and 1000 bytes
Encountered delay vs Packets received for 10 nodes and 1000 bytes
Encountered delay vs Packets received for 10 nodes and 1500 bytes
Encountered delay vs Packets received for 15 nodes and 512 bytes
Encountered delay vs Packets received for 20 nodes and 5 bytes . .
Encountered delay vs Packets received for 5 nodes and 100 bytes .
Average end-to-end delay vs cbr interval (1 Mbps) . . . . . . . . .
Average end-to-end delay vs cbr interval (1 Mbps) . . . . . . . . .
Average end-to-end delay vs cbr interval (1 Mbps) . . . . . . . . .
Encountered delay vs Packets received for 5 nodes and 1000 bytes
Encountered delay vs Packets received for 10 nodes and 1000 bytes
Encountered delay vs Packets received for 10 nodes and 1500 bytes
Encountered delay vs Packets received for 15 nodes and 512 bytes
Encountered delay vs Packets received for 20 nodes and 5 bytes . .
Encountered delay vs Packets received for 5 nodes and 100 bytes .
Average end-to-end delay vs cbr interval (2 Mbps) . . . . . . . . .
Average end-to-end delay vs cbr interval (2 Mbps) . . . . . . . . .
Average end-to-end delay vs cbr interval (2 Mbps) . . . . . . . . .
The Basic Block Diagram of the Wireless System (Courtesy of ABB)
50
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59
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60
61
61
62
65
Contents
xiii
List of Tables
2.1
2.2
2.3
IEEE 802.11 Standards . . . . . . . . . . . . . . . . . . . . . . . .
Rate Dependent Parameters for IEEE 802.11a . . . . . . . . . . . .
Rate Dependent Parameters for IEEE 802.11n . . . . . . . . . . .
6
9
11
4.1
Typical Requirements for Industrial Wireless Sensor and Actuator
Networks in the Process Automation Domain . . . . . . . . . . . .
20
5.1
5.2
5.3
Parameters used for Simulating IEEE 802.11n PHY Simulink Model 28
MCS Parameters for 20 Mhz, NSS = 2, NES = 1, EQM . . . . . . 29
MAC and PHY Parameters for 802.11 a/b . . . . . . . . . . . . . . 35
6.1
6.2
6.3
6.4
Parameters for PCF Vs. DCF Comparison
Deadline Comparison of PCF and DCF .
Bandwidth Efficiency (PCF Vs. DCF) . .
Bandwidth Efficiency (PCF Vs. DCF) with
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Varying
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Number of Nodes
49
63
64
65
xv
xvi
Contents
Abbreviations
Abbreviations
AWGN
BCC
CAN
CBR
CFP
CP
CSMA/CA
CW
DCF
DIFS
DSSS
ELF
EQM
FCS
FHSS
FIFO
FTP
GI
HART
ISI
ISM
IWLAN
MAC
MCS
MIMO
OFDM
PCF
PDR
PER
PHY
PIFS
PSDU
RR
SIFS
SM
STBC
UDP
WLAN
WSN
Description
Additive White Gaussian Noise
Binary Convolutional Codes
Controller Area Network
Constant Bit Rate
Contention Free Period
Contention Period
Carrier Sense Multiple Access/Collision Avoidance
Contention Window
Distributed Coordination Function
DCF Inter-Frame Sequence
Direct Sequence Spread Spectrum
Effort Limited Fair
Equal Modulation
Frame Check Sequence
Frequency Hopping Spread Spectrum
First-In-First-Out
File Transfer Protocol
Guard Interval
Highway Addressable Remote Transducer
InterSymbol Interference
Industrial, Scientific and Medical
Industrial Wireless Local Area Network
Media / Medium Access Control
Modulation and Coding Scheme
Multiple Input Multiple Output
Orthogonal Frequency Division Multiplexing
Point Coordination Function
Packet Delivery Ratio
Packet Error Rate
Physical
PCF Inter-Frame Sequence
Physical Service Data Unit
Round Robin
Short Inter-Frame Sequence
Spatial Multiplexing
Space Time Block Coding
User Datagram Protocol
Wireless Local Area Network
Wireless Sensor Networks
Chapter 1
Introduction
This Master of Science thesis is a partial fulfillment for the Master program in
Communication Systems at Linköping University. The thesis work was carried out
at the Corporate Research Center (CRC) of ABB AB in Västerås and this report
aims to present the work carried out during the period and the corresponding results.
In the modern world, industries are in the process of competing with each other
in order to have an enhanced performance. In the field of industrial automation,
low-cost and efficient industrial automation systems are required for the purpose.
The application of wireless technologies, in industrial automation applications, are
at a high pace. Due to the robust requirements of an industry, sometimes it is
difficult to install a cable-based system and in most of the cases, it is impossible
to do so. To have a cost-efficient system and increase the flexibility, wireless
technologies are increasingly being deployed to overcome the shortcomings of a
cable-based system. The most popularly used wireless-based standards, for industrial automation applications, are the Bluetooth [1], ZigBee, WirelessHART [2]
and now the Wireless Local Area Network, also termed as IWLAN.
1.1
Background
In a wireless network, the medium is a shared resource. The nodes in the network
negotiate access of the shared resource using the Medium Access Control (MAC)
protocol. The design of a MAC protocol for a sensor node is not the same as that
for a wireless transceiver. Due to the transceiver characteristics, the MAC protocol
design is limited in terms of medium access methods. However, in most cases,
the protocols rely on simple access methods i.e. Time Division Multiple Access
(TDMA) or Carrier Sense Multiple Access / Collision Avoidance (CSMA / CA).
Control and monitoring applications, running over a wireless network, are typical
examples of Wireless Sensor Actuator Network (WSAN) application in industries.
A general architecture of a Wireless Sensor Network is shown in Figure 1.1.
1
2
Introduction
Figure 1.1: Wireless Sensor Networks (Courtesy of ABB)
To control an unstoppable process, message deliveries over the network must
be time-bounded; the network must offer reliabilities and delays comparable to
a point-to-point network. A typical example, showing the importance of timebounded message deliveries, is of a stone crushing machine in an industry. This
kind of machine requires a start and a stop signal, which should be transmitted
and received in a specified amount of time. For instance, due to some calculation
error, a stone entering the crusher exceeds the size accommodated by the machine.
To prevent any hazardous event, the stop signal should be sent and received in
a pre-defined amount of time. If these message deliveries are not time-bounded,
the blockade can cause a loss up to $ 50,000 to $ 100,000 per hour. More details,
about the time-bounded critical application areas, are in Section 4.1.
This is a major challenge and the thesis aims to investigate the problem further.
1.2
Purpose and Goals
In the modern world of today, WLANs are considered to be an important part
of one0 s life. The aim of this master thesis is to investigate the medium sharing
techniques for industrial automation applications. The specific question this thesis
wants to answer is: Can WLAN perform better than WirelessHART for industrial
automation applications?
Figure 1.2 shows a top-level block diagram of a Wireless Sensor and Actuator
1.3 Disposition
3
network. The Sensor Node senses the application controlled variable from the
Process Plant and sends it ahead to the Gateway. The Controller is responsible
for the computation of the control signals. It receives the data from the Gateway,
performs the required computations and sends it back to the Gateway. It then
further sends the data to the Actuator, which performs the required operation
on the Process Plant. This whole process is termed as one Control loop. We are
interested to answer the question: Is WLAN better than WirelessHART when it
comes to control applications?
Figure 1.2: The Basic Block Diagram of the Wireless System (Courtesy of ABB)
When it comes to control applications, which require a fast update frequency
rate, can WLAN perform better than WirelessHART? Furthermore, The MAC
schemes for WLAN, are investigated. The results are analyzed under different
scenarios to conclude the above stated questions.
1.3
Disposition
The remaining of the report is as follows:
• Chapter 2 explains the theoretical background of IEEE 802.11 WLAN and
the physical layer characteristics.
• Chapter 3 describes the IEEE 802.11 MAC layer and the MAC access techniques.
• Chapter 4 explains the application of wireless technology in the field of
industrial automation, some standards and protocols and the concept of
IWLAN.
• Chapter 5 shows the IEEE 802.11 physical layer analysis and theoretical
limitations on throughput and delay.
• Chapter 6 explains the MAC layer analysis.
4
Introduction
• Chapter 7 highlights the conclusions and a guide for future work.
Chapter 2
Introduction to Wireless
Local Area Networks
Today, wireless technologies are increasingly being used in all kinds of applications.
Whether it be in home or offices to large-scale industrial applications, wireless
technologies have evolved in every walk of life.
A simple scenario of a Wireless Local Area Network (WLAN) is a few computers
connected, wirelessly, via an access point (AP). An AP acts as a gateway between
the users or stations and the wider internet. Once the station is connected to
the internet via an AP, it can roam freely within the range wirelessly. The range
depends on the device acting as an AP. The IEEE 802.11 defines several protocols
used within the WLANs. Table 2.1 [3], [4] and [5] briefly explains different IEEE
802.11 protocols:
Standard
802.11
Release
1997
802.11a
1999
802.11b
1999
802.11e
2005
802.11g
2003
802.11h
2004
Description
Basic standard, uses ISM band of 2.4 GHz and
maximum supported data rate is of 2 Mbps.
Extension of 802.11 supporting a maximum
data rate of 54 Mbps and uses the ISM band
of 5 GHz.
Extension of 802.11 supporting a maximum
data rate of 11 Mbps and uses the ISM band
of only 2.4 GHz.
Enhanced QoS, resource allocation of bandwidth according to the content and the use of
Wireless Multi Media (WMM).
Same as 802.11a, however the ISM band is of
2.4 GHz for 802.11b backward compatibility.
Standard for the dynamic frequency selection
and power limitations in order to meet the
requirements for the 5 GHz band in Europe.
5
6
Introduction to Wireless Local Area Networks
802.11i
2004
802.11n
2008
802.11ac
2012
Standard for the security access control and
data encryption for 802.11, Wifi Protected Access 2 (WPA2) is a security certification program of this standard.
Uses the ISM band of 2.4 / 5 GHz, supports
a maximum data rate of 72.2 Mbps with 20
MHz bandwidth, supports a maximum data
rate of 150 Mbps with 40 MHz bandwidth and
backward compatible with 802.11g.
Uses the ISM band of 5 GHz, supports a maximum data rate of 867 Mbps with 80 MHz
bandwidth and a maximum data rate of 6.93
Gbps with 160 MHz bandwidth.
Table 2.1: IEEE 802.11 Standards
2.1
Basic Architecture of IEEE 802.11 WLAN
The IEEE 802.11 standard consists of two basic components: The PHY layer
specifications and the MAC layer specifications. The detailed description is given
in [3]. Figure 2.1 [6] illustrates how a simple WLAN looks like:
Figure 2.1: Basic WLAN Architecture
A distributed system (DS) connects one AP to another to form a larger network. Its
main purpose is to pass data from the source AP to the destination AP. An access
point (AP) gives wireless connectivity to other devices in the network. It can also
serve as the center of a wireless network. The transmission of data from one station
to the other is performed by a wireless medium provided. For this purpose, IEEE
has defined standards [3]. These PHY access techniques are discussed in Section 2.2.
A Basic Service Set (BSS) is considered to be an essential part of a WLAN
2.2 802.11 PHY Layer
7
network. A BSS is generally a collection of a number of stations that are controlled
by a coordination function. These coordination functions are explained in detail in
Chapter 3. Generally, two kinds of network implementations exist i.e. Independent BSS (IBSS) or the ad hoc network and Extended Service Set (ESS) or an
infrastructure network.
• Ad hoc Network: An ad hoc network consists of a number of networks within
a single BSS. Since it has no central server, all the stations, within the BSS,
can communicate freely with one another.
• Infrastructure Network: On the other hand, an infrastructure network consists
of an AP which serves as a server in the network. One can say that in such a
network, multiple BSSs are connected together via APs.
Infrastructure networks combine to form an ESS. In an ESS, multiple BSSs are
connected via a common DS to enhance the coverage area.
2.2
802.11 PHY Layer
In 1997, the IEEE released the 802.11 standard [3] which defines the physical
(PHY) and the media access control (MAC) layer for WLANs. The IEEE 802.11
WLAN standard describes three wireless data exchange techniques for the PHY
layer i.e. Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread
Spectrum (DSSS) and IR. The physical layer of 802.11 constitutes of two parts
i.e. the Physical Layer Convergence Protocol (PLCP) and the Physical Medium
Dependent (PMD) sub layers [3]. The PLCP uses different 802.11 media access
methods to analyze the data being sent or received. Under the supervision of PLCP,
PMD performs data transmission or reception and modulation or demodulation
via accessing the air directly. Figure 2.2 [3] illustrates the 802.11 protocol reference
model.
• FHSS: The main idea behind FHSS is to transmit the data on a particular
frequency for a very short duration of time and then hop to another frequency
value. This frequency is a pre-set value and is known to both, the sender and
the receiver. The FHSS uses the 2.4 GHz of ISM band. The center frequency
of the first channel is 2.402 GHz and the rest of the channels are at a distance
of 1MHz from each other. The FHSS creates three different sets of hopping
sequences and each set consists of 26 hopping sequences [7]. The purpose of
these sequences is to allow multiple BSSs to exist in the same location. The
1 Mbps access rate uses two-level Gaussian Frequency Shift keying (GFSK).
In GFSK, 1 is coded as fc+f and 0 is coded as fc-f where fc is the center
frequency. For the access rate of 2 Mbps, a four-level GFSK is used.
• DSSS: In DSSS, the transmitted data is multiplied with a pseudo random
binary sequence, which has a higher bit rate. It uses the same 2.4 GHz ISM
8
Introduction to Wireless Local Area Networks
Figure 2.2: IEEE 802.11 Protocol Reference Model
band but unlike FHSS, DSSS uses Differential Binary Phase Shift Keying
(DBPSK) for the 1 Mbps access rate and Differential Quadrature Phase Shift
Keying (DQPSK) for the 2 Mbps access rate.The PHY access schemes are
explained in [8] in detail.
2.3
PHY Extensions
In 1999, there were two extensions namely 802.11b and 802.11a. The IEEE 802.11b
is based on the preceding definition of the PHY with DSSS. However, the data rate
is extended to 11 Mbps. The modulation scheme used is called Complementary
Code Keying (CCK) [9]. The other extension i.e. 802.11a used the 5.2 GHz band to
stipulate a new radio-based PHY. It uses a modulation scheme called Orthogonal
Frequency Division Multiplexing (OFDM) [10] and hence providing a data rate of
54 Mbps. In practice, IEEE 802.11b only provided up to 5 Mbps and IEEE802.11a
provided almost half of the PHY data rate [11].
The major problem with these amendments was their use of different ISM bands.
For the co-existence of both 11a and 11b required production cost to make the two
standards compatible with each other. The IEEE later proposed 802.11g standard
to resolve these issues. IEEE 802.11g has the same characteristics as IEEE 802.11a,
however, based on the 2.4 MHz band. This extension resolved the issue of backward
compatibility but came up with some new things to consider for instance, the
simultaneous high quality video streaming for multiple customers. Table 2.2 [4]
shows the rate dependent parameters for IEEE 802.11a. Figure 2.3 [4] gives a
general overview of the IEEE 802.11a frame format.
2.4 IEEE 802.11n
Mode
1
2
3
4
5
6
7
8
Modulation
BPSK
BPSK
QPSK
QPSK
16-QAM
16-QAM
16-QAM
64-QAM
9
Code Rate
1
/2
3
/4
1
/2
3
/4
1
/2
3
/4
2
/3
3
/4
Data Rate
6 Mbps
9 Mbps
12 Mbps
18 Mbps
24 Mbps
36 Mbps
48 Mbps
54 Mbps
NDBP S
24
36
48
72
96
144
192
216
Table 2.2: Rate Dependent Parameters for IEEE 802.11a
Figure 2.3: General IEEE 802.11a Frame Format
2.4
IEEE 802.11n
Due to the shortcomings of the previous IEEE standards and the growing demands
of high throughputs, IEEE came up with a new standard i.e. 802.11n in 2003.
The initial target of this standard was to provide a data rate of a minimum of
100 Mbps, which is almost twice as the rates supported by previous standards. A
special Task Group (TGn) was formed and given the job to do so. The upper limit
for the PHY layer data rate was assigned to be 600 Mbps. The current version of
the IEEE 802.11n standard is defined in [5].
In order to provide this huge amount of data rate, improvements have to be
made in the existing MAC and PHY layers. Following are the PHY layer characteristics for the IEEE 802.11n standard.
10
Introduction to Wireless Local Area Networks
Figure 2.4 [5] depicts the general frame format for IEEE 802.11n. Compar-
Figure 2.4: General IEEE 802.11n Frame Format
ing it with the original 802.11 frame format [3], one can observe two additions
namely, the QoS Control and High-Throughput (HT) control.
The HT control field is shown in Figure 2.5 [5]:
Figure 2.5: The HT Control Field
Note: Following are simply the highlights for the IEEE802.11n amendments.
For detailed description, refer to [11], [5] and [12].
2.4.1
Guard Interval
IEEE 802.11n uses OFDM [10] as a modulation technique. Each OFDM symbol
constitutes of a guard interval. The purpose of guard interval is to reduce the
Inter Symbol Interference (ISI). The duration of the guard interval depends on the
environment conditions, which is normally a multipath environment. For IEEE
802.11 a/g, the length of the guard interval is 0.8 µs. for IEEE 802.11n it is
the same, however, it can be lowered as low as 0.4 µs depending on the channel
conditions. This reduction in guard interval increases the data rate by a value of
10 % [12].
2.4.2
High Rate FEC Codes
Mode
BPSK
QPSK
QPSK
16-QAM
16-QAM
Modulation
1
/2
1
/2
3
/4
1
/2
3
/4
Code Rate
1
2
3
4
5
Data Rate
6.5 Mbps
13 Mbps
19.5 Mbps
26 Mbps
39 Mbps
2.4 IEEE 802.11n
64-QAM
64-QAM
64-QAM
11
2
/3
/4
5
/6
3
6
7
8
52 Mbps
58.5 Mbps
65 Mbps
Table 2.3: Rate Dependent Parameters for IEEE 802.11n
As in IEEE 802.11a, the BPSK modulation, having code rate of 3 /4 , is removed
from the IEEE 802.11n standard. It is not at all suitable for the performance
improvement at any SNR value [13]. Table 2.3 [14] shows an addition in the form
of 5 /6 puncturing for a 64 QAM. This scheme has been tested for a 10 % increase
in the data rate [12] . This Modulation and Coding Scheme (MCS) [5] works well
if the channel is in good condition.
2.4.3
MIMO Technology
The major advancement in the IEEE 802.11n was the inclusion of the Multiple
Input Multiple Output (MIMO) transmission technology. In simple terms, MIMO
implies the usage of more than one antenna at both, the transmitter and the
receiver side. MIMO technology has been applied on large scales, since it is capable
of providing enhanced data rates at a minimal cost of extra transmitted power. In
IEEE 802.11n, the MIMO techniques are usable along with three other techniques
namely, Transmit Beamforming, Space Time Block Coding (STBC) and Spatial
Multiplexing (SM) [14].
• Transmit Beamforming: Beamforming is a technique, applied at the transmitting antennas (MIMO) to improve SNR. In MIMO scheme, multiple antennas
for the transmission and reception of radio signals. These radio signals, due
to multipath environment, follow different paths before reaching the receiver
side. They arrive at different times thus affecting the phase of the transmitted
signals. Beamforming wisely sets the phase at the transmitter in order to
increase the SNR at the receiver end. However, if the channel conditions
are known to the transmitter and the receiver, this process can further be
enhanced but it involves an extra overhead. Therefore this technique is
recommended to use with one receiver.
• Space Time Block Coding: STBC corresponds to transmitting the same signal
on multiple transmitter antennas for the purpose of redundancy. Since they
follow the multipath scheme, the same signal travels via different conditions
before reaching the receiver. For the signal to reach the receiver safe and
sound, it is advised to send more copies of the signal. [15] can be referred for
more details about beamforming Vs. STBC performance.
• Spatial Multiplexing: SM is a transmission technique which parses transmitted data streams into sub-streams and sends them with the help of multiple
antennas with one sub-stream per antenna. Each individual sub-stream
12
Introduction to Wireless Local Area Networks
practices the effects of the multipath environment. At the transmitter side,
special mechanisms are applied to re-group the sub-streams. SM is more
effective in terms of providing high data rates at high SNR values. SM can
be employed along with the STBC but in this case, only one sub-stream will
be transmitted via STBC and the rest will be sent via SM [5].
2.4.4
Modes of Operation
In IEEE 802.11n, the PLCP preamble [3] is also modified, depending on the mode
of operation. There are three modes of operation: Green field mode, Non-HT mode
and Mixed mode.
In Green field mode, only IEEE 802.11n stations are involved in the process
of communication. In Non-HT mode, legacy 802.11 station and Mixed mode
corresponds to a combination of both. The first one is optional but the rest are
compulsory.
Chapter 3
IEEE 802.11 MAC Layer
The MAC layer of IEEE 802.11 specifies the schemes to access the channel in the
process of communication. The two techniques are namely Distributed Coordination
Function (DCF) and Point Coordination Function (PCF).
3.1
Distributed Coordination Function
The DCF mechanism is the conventional scheme for the channel access. As the
name says, DCF does not require a master station to control the sharing mechanism.
It uses the concept of Distributed Coordination. DCF uses Carrier Sense Multiple
Access with Collision Avoidance (CSMA/CA) and a random exponential backoff [16]
to decide the control of channel. The backoff interval is randomly defined between
0 and the length of the Contention Window (CW). In other words, one can say
that DCF is used for the contention dependent services. DCF is particularly for
asynchronous data transmission e.g. e-mail or web browsing. These data exchanges
are not time dependent. The technique CSMA/CA is the reason why its time
independent, since when it senses the medium to be free, it allows a particular
station to transmit without a time bound [16].
3.1.1
Working Mechanism
The working mechanism of CSMA/CA, for the DCF scheme is shown in Figure
3.1. In CSMA/CA, the station senses the channel to be idle, following Wait and
Listen scheme. It keeps on observing the required channel. As long as there are
transmissions going on the channel, it waits. When it senses the channel to be
idle, the station starts sending data packets [17]. In DCF, when the channel is
sensed idle, the station will wait for another DCF Interframe Space (DIFS) before
starting the transmission. This DIFS interval varies depending upon the type of
the physical layer [18]. After the transmission, the transmitter waits for an ACK.
If the ACK is not received, it assumes that the packet maybe lost. As a result,
it tries for a re-transmission. When the channel is sensed idle, the backoff timer,
set to some value, is decreased. The backoff timer is selected, randomly, between
13
14
IEEE 802.11 MAC Layer
zero and the maximum size of contention window. In Figure 3.1, the minimum size
of contention window is 31 and the maximum size of contention window is 1023
(these values are for IEEE 802.11b). The station is allowed to transmit only when
the backoff timer reaches 0 [19]. Every station has to come up with a CW. This
CW determines the waiting time for a station (in terms of time slots) before it can
initiate a transmission. The size of the contention window is 2K - 1. Each station
has a Short Interframe Space (SIFS) to transmit the ACK. Since SIFS shorter then
DIFS, no medium sensing is required by the station before sending the ACK. The
SIFS also depend on the type of PHY and are listed in [3].
Figure 3.1: Graphical Representation of CSMA / CA
3.2 Point Coordination Function
3.1.2
15
Problems with the DCF
The Hidden node problem is a major hurdle in the implementation of DCF. This
problem is resolved by using Request to Send (RTS) and Clear to Send (CTS)
flags. However, RTS/CTS should be efficiently implemented, since they deliberately
increase the overhead and as a result, affect the total throughput of the system. A
wise method, for instance, is to implement RTS/CTS algorithm at the user wireless
NICs [20]. The drawbacks of using DCF for real-time communication applications
are listed below: [20] and [21]
• Cannot support QoS.
• The guarantee of the packet transmission, during a specified time, is not
confirmed.
• DCF cannot differentiate between different types of data and hence has no
concept of prioritization.
• It is not suitable for highly loaded traffic since it uses wait and listen concept.
It will eventually affect the total throughput of the system.
• Theory of contention.
• Even if RTS/CTS are used, in the presence of RF interference, it will sense the
channel to be busy and will not allow initiating the process of transmission.
• Denials of Service (DoS) attacks also affect the overall performance of the
system using DCF as a channel access scheme.
The above mentioned drawbacks of DCF are tackled by PCF, which is a polling
based medium access mechanism devised by IEEE 802.11.
3.2
Point Coordination Function
PCF is a polling based algorithm for MAC layer. The main aspect of this technique
is that there is single Point Coordinator (PC), usually an Access Point (AP),
which keeps on polling the stations within the Basic Service Set (BSS) to initialize
the data transmission process. PCF is basically designed for synchronous data
transmission i.e. real time data such as video etc. synchronous means that the
data transmission is time-bounded i.e. a process has to occur in a specific interval
of time. The PCF process initiates with a Contention-free Period (CFP). However
it also has a room for Contention Period (CP) [16]. The time access for the channel
is divided into these CFP and CP. A combination of alternating CFP and CP is
called a SuperFrame [19].
3.2.1
Working Mechanism
Before PCF initializes and when the AP is in CP, the stations requiring transmission
of some data request the PC in AP to add them in the polling list. Reason being
16
IEEE 802.11 MAC Layer
once the technique begins, the PC will only poll those stations for the channel
access which are in its polling list. Once the AP is activated with PCF mode, the
PC polls the stations, in its list, to access the channel by sending a BEACON signal.
A BEACON frame consists of three things: the maximum duration of the CFP,
the BEACON interval and the BSS identifier. Before sending the BEACON signal,
the PC will wait for a PCF Interframe Space (PIFS). Once there is no activity, the
PC starts with the polling process by following the polling list. Another important
thing is that PIFS is less than DIFS. As a result, PCF always gets priority over
DCF and no interruption is encountered [16]. PIFS for different PHY types are
listed in [3].
Each station maintains its own Network Allocation Vector (NAV) [16]. This
NAV stores the maximum duration of the CFP so that the stations cannot occupy
the channel (since the stations to get access to the channel are polled by the PC
and they already exist in the polling list). After the exchange of some signals,
the station is ready to transmit. If the station has nothing to transfer, it has
to send a NULL frame. In the same way, the PC continues the polling process
following the list maintained in the CP. Since PCF also has the provision of CP,
when the AP reaches that state, the channel access scheme is switched to DCF.
Figure 3.2 [16], [19] and [21] shows the general working of the PCF scheme.
3.2.2
Problems with the PCF
Although PCF is a lot better in terms of QoS as compared to DCF, it still has
some shortcomings, some of which are discussed as follows [19] and [22].
• Polling algorithms are important aspect for effective implementation of PCF.
In most of the cases, RR scheme is used. Its drawback is that it cannot
differentiate between the types of data traffic and which type should be given
preference.
• The time span of the SuperFrame (SF) is important, since it directly affects
the total delay encountered by system.
• As described above, a SF alternates between CP and CFP. This increases
the overhead and affects the throughput of the system.
• In PCF, the PC lacks the tendency to determine the queuing order.
• PC does not have the inner conditions of the channel to be accessed. Maybe,
it will keep on polling the station when the channel is in bad condition and
it will result in a drop of throughput.
3.2.3
Polling Mechanism
In PCF, the polling schemes play an important role in the efficient application of
the technique. Some of the polling schemes, for the efficient utilization of PCF in
terms of multimedia services, are proposed in [21]. The schemes are Round-Robin
3.2 Point Coordination Function
17
Figure 3.2: Graphical Representation of PCF
(RR), First-In-First-Out (FIFO), Priority and Priority-ELF (Effort-Limited Fair).
RR, FIFO and priority schemes have one thing in common. When the PC is
transmitting frames to a particular frame, it also combines the polling frames with
it. If a station requires an ACK from the PC, instead of sending it separately, PC
combines the ACK with the transmitted data packet. This whole process is for the
efficient utilization of the wireless medium.
• Round Robin: In this polling method, the CFP is set to a specific value,
say Max_CFP. Once the PC has access over the channel, it searches for the
station with the minimum address in the BSS. Once the PC finds that it
has to perform data transmission with the station, it starts with the PCF
procedure by sending the respected frame. There is a particular list of stations
that are being served by a BSS. In RR, the ACK from the station is not
that important. Even if the PC does not receive the ACK, it will switch to
the next station according to the maintained polling list. Once the PC has
entertained all of the stations, it re-starts the process with the lowest address
station. This continues until we have reached Max_CFP.
• First-In-First-Out: This scheme is quite clear from its name. The station
to be polled depends on the queue maintained in the PC0 s polling list. The
difference between FIFO and RR is that when the station has not received
18
IEEE 802.11 MAC Layer
the ACK from the polled station, it assumes that there has been some
transmission error and it prepares for the re-transmission. It also maintains
a counter for the number of retries and continues to transmit until it has
reaches the maximum value of the number of retries. When the PCs queue is
empty, it acts as RR but as soon as any frame enters the queue, it switches
to the FIFO scheme.
• Priority: In this scheme, priority is given to the station having the highest
Type of Service (TOS). Once the PC recognizes the station with the highest
TOS, the process of frame transmission begins. In the same way, the PC
switches to the station with the second highest TOS after the data transmission of the first one. The PC will serve all the stations with the TOS greater
than 1 and when it comes to Best Effort Traffic (e.g. data traffic), the PC
adapts the RR scheme for polling. In this scheme, every station has a flag
segment in the MAC header. The purpose of this flag is to tell the PC that a
particular station has data to send or receive. This flag comes into play once
the station is polled by the PC. In the response, if the flag is set to TRUE,
the PC will have to send a poll frame to that station.
• Priority ELF: In ELF, there is a threshold i.e. the per-flow threshold. The
ELF scheduling confirms that the data flows, experiencing an error rate below
this per-flow threshold, will get their required services. In this scheme, the
priority scheme utilizes the concept of ELF scheduling to perform the polling
mechanism. The mechanism of TOS is the same as in the Priority scheme.
However, in priority ELF, the PC also keeps track of the counter exhibiting
the amount of data transmitted by a station in a particular period of time.
It should be greater than 1. If the counter value is less than 1, the PC will
not poll the corresponding station for the data transmission.
The management of the polling list is very important. An efficient way to do this
is shown in [22]. There are three main aspects when considering the polling mechanism namely, throughput, performance and the fairness of the channel accession.
The polling list is divided into 4 categories: good channel with data (GD), good
channel with no data (GN), bad channel (BC) and no response (NR) [22].
The PC should give the GD with the highest priority, followed by GN, BC and
finally NR. In case of GD, the sorting of the stations is necessary. This sorting
depends if the station has uplink, downlink or both kinds of data to transmit.
There is a new variable i which assigns the priority of stations within the GD. The
formula for channel efficiency calculation is given in [22]. Once getting these values,
sorted in descending order and then a polling scheme is applied to the new polling
list (the scheme is Non-Preemptive: the PC continues to poll the station as long as
it has data to transmit or receive). This will have a positive impact on the total
throughput of the system. One of the dis-advantages of using NP polling scheme is
that it does not caters for the fairness. For fairness, RR polling scheme is more
suitable. An efficient polling algorithm, which shows better results as compared to
the RR or NP polling schemes, is also proposed in [22].
Chapter 4
Wireless in Industrial
Automation
The deployment of WLANs in industries is getting quite common in the world of
today such as the control and automation industry. An industrial environment
may include hostile surroundings such as extreme heat and movement of heavy
machinery. In these conditions, sometimes, it becomes almost impossible to deploy
wired communication systems. These conditions and scenarios gave rise to the
extensive use of wireless LANs in the industrial industries. One of the major
vendors for the IWLAN devices, deployed in various industries, is Siemens [23].
Wireless Sensor Networks (WSNs) are also getting popular among the industrial applications. WirelessHART [2] is the industrial standard which provides a
seamless wireless communication to be used in industries. WirelessHART provides
a solution for the replacement of wired fieldbuses in the automation industry in
order to reduce the cost and the mechanical wear and tear. Fieldbuses are basically
computer network protocols and they support real-time distributed controls among
communications. These fieldbuses lie between the Programmable Logic Controllers
(PLC) and the devices which are actually performing the work such as sensors and
actuators. With the help of IWLANs, the distributed I/O devices can communicate
wirelessly. Some of the common applications are the driverless transport machines,
conveyor belts, and remote control applications [24].
4.1
Industrial Automation Requirements
The use of WLAN for Industrial Automation is increasing day by day. However,
the conventional IWLAN scheme is not suitable for all the Industrial applications,
deterministic communication, being the major one. For effective utilization of
WLAN in industrial automation, there are some requirements which have to be
met. From [25] and [26], the major industrial automation requirements are listed
below:
19
20
Wireless in Industrial Automation
• Safety and Security requirements: Safety, in terms of equipments and people
working in the industry, should be the first priority before deploying any kind
of system. Network security is also an important factor. It should be taken
into account that the transmitted data is safe from unauthorized users and
avoiding access to hackers.
• Delay requirements: In industrial automation, data must be transmitted with
the minimum possible delay. In order to have a precise control communication,
latency and jitter should be minimized.
• Deterministic requirements: The data transmission should be time-bounded.
Deterministic communication is very important in industrial automation.
Incase of mobility, this requirement is necessary hence giving rise to real-time
handover.
• Network Redundancy: In a given network, there should not be a single point
of faliure. For instance if something goes down at one point, it should not
affect the whole system and it should continue with the production.
• Support for large number of devices: The network topology should be flexible
i.e. any addition to the existing network should be possible. In a given
industrial environment, the network might have nodes of the order of hundreds.
For conventional IEEE 802.11, the efficiency is reduced with the increase in
the network size.
In an industrial application, latency is very important. Usually, the transmitted
signals are of very short length and if the transmission is not within the specified
deadline, it is of no use. Table 4.1 [26] highlights some of the requirements for the
successful deployment of a large scale Wireless Sensor Network (WSN).
Sensor Network Application
Monitoring and supervision
Vibration sensor
Pressure sensor
Temprature sensor
Gas detection sensor
Closed loop control
Control valve
Pressure sensor
Temprature sensor
Flow sensor
Torque sensor
Variable speed drive
Interlocking and Control
Proximity sensor
Motor
Valve
Protection relays
Delay
Range
Battery Life
Update Freq.
Security
s
ms
s
ms
100
100
100
100
m
m
m
m
3
3
3
3
years
years
years
years
sec - days
1 sec
5 sec
1 sec
low
low
low
low
ms
ms
ms
ms
ms
ms
100
100
100
100
100
100
m
m
m
m
m
m
>
>
>
>
>
>
5
5
5
5
5
5
years
years
years
years
years
years
10 - 500
10 - 500
500 ms
10 - 500
10 - 500
10 - 500
ms
ms
ms
ms
ms
medium
medium
medium
medium
medium
medium
ms
ms
ms
ms
100
100
100
100
m
m
m
m
>
>
>
>
5
5
5
5
years
years
years
years
10
10
10
10
ms
ms
ms
ms
medium
medium
medium
medium
-
250
250
250
250
Table 4.1: Typical Requirements for Industrial Wireless Sensor and Actuator
Networks in the Process Automation Domain
4.2 Standards
21
• Monitoring and Supervision: Sensors providing diagnostics and monitoring
functions lie in this category. The update time for these applications typically
varies from 1 sec to several days. These type of applications are insensitive
to packet losses and / or jitter, since they are reserved for monitoring and
supervision purposes.
• Closed Loop Control: The main purpose of closed loop is to stabilize the
process by controlling the actuators using preset readings. Typically, these
kind of applications are affected by delays and jitter. These applications can
bear the loss of packets and the wait time for retransmissions to some extent.
• Interlocking and Control: Interlocking and control are very important in terms
of a large control application. They correspond to discrete signaling. For
example, to enable a start command for a particular control application, there
might be several conditions to be fulfilled. These start, stop etc commands
are termed as interlocks and are highly sensitive to delays. Generally, these
applications are required to start or stop a particular machine and hence can
cause a lot of damage if we have deadline misses.
4.2
Standards
Depending on the requirements, there are different standards and protocols for
data transfer in industrial applications. In some cases, data is tranmitted via cable
networks e.g. CAN, EtherCAT and PROFINET I/O. Bluetooth, Wireless HART
and IWLAN are the wireless networks used for the purpose of data transfer.
4.2.1
Controller Area Network
CAN bus is the most extensively used industrial cable network. CAN bus was
originally developed to be used with the automobile industry. However, due to
some of its properties, such as speed, cost-effectiveness and utilization in real time
applications, CAN bus has made its way into other industrial applications.
For a given scenario, where a large number of short messages are to be transmitted for multiple users, CAN bus is ideal to use. CANopen and DeviceNet are
some of the protocols used in industrial applications. However, users can also
design their own protocols according to the application requirements.
4.2.2
PROFINET I/O
PROFINET IO is an Ethernet based fieldbus protocol used in the automation
industry. It is designed for the transmission of both real-time and non-real time
data. For that, it uses a switched 100 Mbps network [27]. It is one of the IEC 61784
standards. Since real time data transmissions are of great importance industries,
22
Wireless in Industrial Automation
PROFINET IO has a dedicated layer for that which lies above the Ethernet [27].
PROFINET IO is termed as a unique Ethernet based protocol and it has some
benefits over the traditional physical layers [28]:
• Provides high speed i.e. it skips the TCP/IP stack and the time required for
the message delivery in case of real-time data traffic.
• Unified integration with PROFIBUS [29].
• Supports time-dependent applications.
• Easy deployment.
PROFINET IO is divided into three types of devices: IO-Controllers, IODevices IO-Supervisors. IO-Controllers initiate the automation process whereas
the IO-Supervisors are the end point devices e.g. monitoring or the diagnostic
tools. IO-Devices serve as moderators between these end point devices e.g. sensors,
actuators etc [28]. 1412 bytes is the maximum payload size for PROFINET IO [27].
This is to avoid fragmentation in case of real-time transmissions.
4.2.3
Bluetooth
The wireless technology, Bluetooth, serves the purpose of data exchange over short
ranges. Generally, bluetooth has a range of approximately 10 m but this range
can be extended upto several hundred meters by using a long-range module. It
has a maximum data throughput of 780 Kbps. However, Bluetooth v2.1+EDR
(Enhanced Data Rate) gives an approximate data throughput of 2.1 Mbps [30]. In
terms of delay, the bluetooth technology provides a latency of 5-10 ms. Considering the above mentioned properties, the Bluetooth technology (IEEE 802.15.1),
in terms of industrial applications, serves the purpose of wirelessly integrating
various automation devices. Devices which are cost-effective and have low power
consumption are practically used within the Bluetooth technology.
Using the Bluetooth PHY and a proprietary MAC, ABB has developed the Wireless
Interface to Sensors and Actuators (WISA) system [1]. The communication between
the sensors, equipped with the WISA technology, takes place in accordance with
the IEEE standard radios which are same as that of bluetooth. The protocol for
communication is designed by ABB, which works on the principle of TDMA and
frequency hopping to provide enhanced and collision-free transmission. The design,
implementation and working of the WISA system is explained in [31].
4.2.4
Wireless HART
HART stands for Highway Addressable Remote Transducer. WirelessHART is
termed as a wireless sensor networking technology. It uses mesh-network topology.
It is considered to be a robust wireless protocol for various industrial applications e.g. process management, control and asset management applications. It
4.2 Standards
23
utilizes the 2.4 GHz ISM band with IEEE 802.15.4 compatible Direct Sequence
Spread Spectrum, channel hopping and Time Division Multiple Access [27]. The
key characteristics of WirelessHART are Simple (ease of installation), Reliable
(robust, compatible with other wireless technologies) and Security (uses Advanced
Encryption Standard (AES) for security) [2].
Figure 4.1: Wireless HART Mesh Network (Courtesy of ABB)
Figure 4.1 depicts the general network of wirelessHART. A WirelessHART networks constitutes of 5 parts:Gateway: provides connection between the wireless
and the control network, Access Point: resides inside the gateway, provides the
radio interface, Network Manager: responsible for the wireless network management, configures and sustains the meshed network, Security Manager: sharing and
management of security keys, also holds the authorization list, Field Devices: end
point devices, performing the real operation. [2] and [27]
In a WirelessHART wireless network, each field device in the mesh network acts as
a router. Even if the data packet is not assigned to the corresponding device, it
will redirect the package to the respective device using its routing capabilities. It
is the responsibility of the Network Manager to determine and establish the most
efficient routes for the data transfer. It also adds the concept of redundancy. E.g.
if a particular path is congested, the network manager will assign another path for
the data to travel, which is going to be reliable and efficient also.
WirelessHART works on the principle of Time Division Multiplexing (TDMA) and
uses the concept of Frequency Hopping Spread Spectrum (FHSS). With TDMA,
the participants are allocated a slot of 10 ms each, for transmission of data, whether
it is from the sensor node to the gateway or from the gateway to the actuator.
24
Wireless in Industrial Automation
This synchronization allows to have a reliable and collision-free transmission. Due
to these properties, WirelessHART is ideal for industrial automation applications.
As it is said, every cloud has a silver lining. WirelessHART also comes with
some drawbacks. When it comes to applications requiring a fast update frequency
rate, WirelessHART fails due to the predefined 10 ms slot. Considering the control
loop defined in Figure 1.2, an application with an update frequency rate of 10 ms
will not be supported by WirelessHART since it will take 10 ms time from sensor
to the gateway and another 10 ms delay from the gateway to the actuator. This
means that the total delay time will be 20 ms plus the time taken by the controller
and the process plant. Another drawback is the limitation on the data payload
size (0-127 bytes).
4.2.5
IWLAN
The use of WLAN for industrial applications is termed as IWLAN. In an industrial
environment, sometimes it is not feasible to use wired connectivity for data transfer.
Also, on many occasions, it is impossible to deploy wired connections between the
devices. IWLAN application arises in these situations. Some of the applications of
IWLAN are in Automation Control, Manufacturing Industry, Process Control and
Motion Control.
The application of wireless communication technology in the control industry
implies to the extension of a particular network to physical locations, some of which
are not easily accessible using a wired connection. In an industrial environment,
due to some physical parameters, robustness is required and there are some areas,
where it is not possible for cable network to arrive. As a solution, the concept of
IWLAN is introduced.
As discussed in Chapter 3, there are two multiple access schemes for IEEE 802.11
namely, DCF and PCF. DCF being the non-deterministic scheme and PCF being
the deterministic scheme. Generally, for the IWLAN implementation, CSMA/CA
is used for the purpose of medium access which corresponds to the DCF scheme.
This method is also termed as the Contention Scheme where each node, in the
network, has to content with each other in order to win the medium access. The
details are explained in Section 3.1. It can be seen that CSMA/CA is based on
the principle of deterministic scheduling. Since the medium is shared by multiple
nodes, it also increases the transmission delay, encountered by the transmitted
data packets.
In industrial applications i.e. control applications etc, latency plays an important
role and there are strict thresholds for this purpose. In case of CSMA/CA, the
system will encounter longer delay, which will eventually affect the overall system
performance. PCF is considered to be a deterministic approach and is thought to
give better results in terms of throughput and more importantly, delay. Section 3.2
explains the access scheme in detail.
4.3 Advantages of Wireless Technologies in Industrial Automation 25
4.3
Advantages of Wireless Technologies in Industrial Automation
The wireless sensor network technology is increasingly being implemented in modern
world Industrial Automation applications. Following are some of the highlighted
benefits of using WSN in industrial Automation. The wireless technologies are
increasingly being implemented in modern world Industrial Automation applications.
Following are some of the highlighted benefits of using wireless networks in industrial
Automation.
• Cost: Cost effective techniques play an important role in Industrial automation. Using wireless networks in industrial applications highly reduces the
cost and are easy to deploy. As an example, green field installation could
cost upto $ 200 per meter to deploy a wired network in an industrial field
and around $ 1000 in an offshore installation. In addition, maintainance,
troubleshooting etc are required on regular basis which increases the cost and
requires more time.
• Flexibility: With the use of wireless technologies, it is possible to extract useful
data from equipments, which used to be non-instrumented previously. The
flexibility factor come with the benefit that the deployed wireless networks in
automation industries can be utilized to temporarily measure certain process
values without even having a wired network installed.
• Emerging Applications: Wireless applications such as wireless control applications, empoweing mobile workers, integration with the non-traditional signals
etc arise with the use of wireless technologies in Industrial automation. These
emerging wireless applications are useful in terms of Industrial applications.
• Availability: In order to prevent any catastrophic event, communication in
an Industrial application should be deterministic. For instance, any precision
error in a stone crushing industry can cause a loss upto $ 50,000 to $ 100,000
per hour. Wireless technologies, with the properties of being deterministic,
are useful for industrial automation applications.
Chapter 5
Performance Analysis of
IEEE 802.11 PHY
The following Sections contain the simulational results, along with the arguments,
for the IEEE 802.11 PHY. For the purpose, MATLAB is used.
5.1
IEEE 802.11n Simulink Model
The purpose of analyzing the IEEE 802.11n PHY simulink model is to study the
IEEE 802.11 physical layer (Section 2.2) and observe the behavior under different
circumstances. The simulation reflects an indoor environment. The effects of MIMO
(Section 2.4.3), along with STBC and transmit beamforming are also analyzed.
5.1.1
Model Description
The generic Simulink model for IEEE 802.11n is available at [32]. Following are
the characteristics of the model:
• 802.11n PHY model.
• All the optional and compulsory data rates are supported i.e. 6, 9, 12, 18,
24, 36, 48 and 54 Mbps.
• BPSK, QPSK, 16- and 64-QAM modulation schemes supported.
• Convolutional coding with supporting rate: 1 /2 , 2 /3 , 3 /4 and 5 /6 .
• OFDM with 20 MHz supported: 52 data carriers.
• MIMO Detection.
• TGn channel supported.
27
28
5.1.2
Performance Analysis of IEEE 802.11 PHY
Channels
This Section is about the MIMO channel modeling for indoor wireless communications. However, having a single transmit antenna and a single receive antenna is
considered to be a part of MIMO. Large-scale propagation and Small-scale propagation, are the two types of propagation models in a wireless channel model. The
former deals with the signal losses caused over distance and the latter corresponds
to the signal losses caused by small changes in the distance. The details about the
propagation models are in [11].
For indoor wireless communications, a number of MIMO channel models, based on
the TGn specifications, are designed and are detailed in [33]. The High Throughput
Task Group (TGn) has designed six channel models, namely A - F. These channel
models are designated for different indoor configurations. Channels A - C are
specifiied for smaller environments i.e. residential areas and channels D - F are
specified for larger indoor areas such as office-type or industrial environments. In an
industry, multi path effect is an important factor to consider, since there is no LoS
communication in industrial applications. Also, the rms delay factor, of the channel
selected, reflects an indoor industrial environment. Here, channel D, with nLoS,
is used for the simulation purposes, which is a Rayleigh fading MIMO channel,
corresponds to larger areas and has an rms delay spread of 50 ns. Normally, this
category of channel models (D - F) show an rms delay spread of 50 to 150 ns [11].
On the other hand, channels A - C encounter an rms delay spread varying from 0
to 30 ns [11]. In wireless communications, rms delay spread corresponds to the
multipath properties of a communication channel. In simple words, it is the time
duration between the arrival of earliest and the latest multipath component.
5.1.3
Parameters
Table 5.1 [5] shows the parameters used for the simulation purposes.
Attributes
Channel
MCS
Payload Size
MIMO Configurations
Value
AWGN and TGn
13, 14 and 15
1000 Bytes
2x2
2 x 2 with beamforming
4 x 2 with STBC
Table 5.1: Parameters used for Simulating IEEE 802.11n PHY Simulink Model
As stated above, the Simulink model for IEEE 802.11n supports OFDM with
20 MHz. Corresponding to that, three values for Modulation and Coding Scheme
(MCS) i.e. 13, 14 and 15 are used in the simulation. Table 5.2 shows the rate
dependent parameters when the number of spatial streams (NSS ) is equal to 2.
5.1 IEEE 802.11n Simulink Model
MCS
Modulation R
13
14
15
64-QAM
64-QAM
64-QAM
2
NBP SCS (iss ) NSD
/3 6
/4 6
5
/6 6
3
29
52
52
52
NSP
NCBP S
NDBP S
4
4
4
624
624
624
416
468
520
Data
Rate(Mbps)
104.0
117.0
130.0
Table 5.2: MCS Parameters for 20 Mhz, NSS = 2, NES = 1, EQM
NOTE: The data rates in the last column correspond to a Guard Interval (GI)
of 800 ns.
The parameters in Table 5.2 are defined as follows:
NSS = Number of spatial streams
MCS = Modulation and Coding scheme index
R = Code rate
NBP SCS (iss ) = Number of coded bits per single carrier for each spatial stream,
iss = 1,...,NSS
NSD = Number of complex data numbers per spatial stream per OFDM symbol
NSP = Number of pilot values per OFDM symbol
NCBP S = Number of coded bits per OFDM symbol
NDBP S = Number of data bits per OFDM symbol
NES = Number of BCC encoders for the DATA field
The transmit beamforming is a method in which the spatial streams, for the
signal to be transmitted, are determined using the channel estimates. The purpose
of beamforming is to improve the signal strength at the receiving antenna. Beamforming can either be explicit or implicit. In the former technique, the required
matrices, for the purpose of beamforming, are send by the receiver side. In the
latter method, the transmitter side uses the transpose of its own channel estimation
and performs beamforming. Explicit beamforming comes with the assumption that
the channel response, from the transmitter to the receiver antenna, and vice versa
is same.
5.1.4
Results
Following are the Packet Error Rate (PER) vs Signal to Noise Ratio (SNR) curves,
obtained after running 15 number of simulations.
Figure 5.1 depicts PER Vs. SNR curve for MCS values of 13, 14 and 15 in
the presence of an AWGN channel. MIMO technique, with 2 transmitter and 2
receiver antennas is used.
The performance of IEEE 802.11n, in the presence of channel D nLoS, is shown in
Figure 5.2. The channel is a typical delay spread channel [11]. It can be observed
that at lower values of SNR, PER is on the higher side.
30
Performance Analysis of IEEE 802.11 PHY
Figure 5.1: PER vs SNR for MCS = 13, 14 and 15 (AWGN, 2x2 Direct-Map)
Figure 5.2: PER vs SNR for MCS = 13, 14 and 15 (Ch D nLoS, 2x2 Direct-Map)
5.1 IEEE 802.11n Simulink Model
31
Figure 5.3: PER vs SNR for MCS = 13, 14 and 15 (Ch D nLoS, 2x2 Beamforming)
Figure 5.4: PER vs SNR for MCS = 13, 14 and 15 (Ch D nLoS, 4x2 STBC)
32
Performance Analysis of IEEE 802.11 PHY
Therefore, in order to attain the same performance as AWGN channel, a higher
averaged-SNR is required. The MIMO configuration is same as before i.e. 2
transmitter and 2 receiver antennas.
Figures 5.3 show the effect of transmit beamforming (Section 2.4.3). The purpose
of applying beamforming is already explained in Section 5.1.3. One can see a
visible improvement in SNR (although minute, 2-3 dB). The reason for this small
improvement in SNR is due to the use of equal number of spatial streams and
transmitter antennas i.e. 2. This reduces the diversity order.
Finally, the effect of STBC (Section 2.4.3) is shown in Figure 5.4. The performance
is close to AWGN and an almost 5-8 dB improvement in SNR as compared to 2 x 2
channel D nLoS and beamforming. For this scenario, 4 transmitter and 2 receiver
antennas are used. This performance improvement is due to the increase in the
transmit diversity, which is due to the use of 4 x 2 transmission.
Data rate increases with the increase in MCS value (Table 5.2). High data rate
corresponds to high signal transmission power which directly affects the SNR at a
given value of PER. This can be seen in Figures 5.1, 5.2, 5.3 and 5.4.
5.2
Throughput and Delay Limits
This Section describes the theoretical limitations, in terms of throughput and delay,
for IEEE 802.11a and IEEE 802.11b. It also shows the impact of using aggregation
on the throughput for different data rates. The equations for the throughput and
the delay analysis are adapted from [34] and [35].
5.2.1
Assumptions
• The channel is functioning under ideal conditions i.e. there is no InterSymbol
Interference (ISI) and there is no noise added by the channel. In non-ideal
case, the achieved throughput is always less than Maximum Throughput
(MT) and the experienced delay is always greater than Minimum Delay (MD).
• For a particular transmission cycle, only one station is able to send data
packets and the rest of the nodes in the network can only receive and
acknowledge packet reception.
5.2.2
Derivation of MT and MD for IEEE 802.11a
The following derivation is based on [34] and [35].
A DCF transmission cycle constitutes of a DIFS interval, Backoff time, an SIFS
interval, Data and Acknowledgement transmission time.
TDCF = TDIF S + TB + TDAT A + TSIF S + TACK
(5.1)
5.2 Throughput and Delay Limits
33
TB is the average backoff time and is given by:
TB =
CWmin σ
2
(5.2)
CWmin = Minimum contention window size
σ = Slot time
The data transmission time (TDAT A ) is given by:
TDAT A = TP + TP HY hdr + TM AChdr + TP AY LOAD
(5.3)
TP = Physical preamble transmission time
TP HY hdr = PHY header transmission time
TM AChdr = MAC header transmission time
TP AY LOAD = Data payload transmission time
Similarly, the acknowledgement transmission time (TACK ) is given by:
TACK = TP + TP HY hdr + TACK_f rame
(5.4)
TACK_f rame = Acknowledgement frame transmission time
From Figure 2.3, the DATA field corresponds to the number of coded bits in
an OFDM symbol [4]. For the purpose, the message length in increased to be the
multiple of data bits per OFDM symbol (NDBP S ). Therefore, to cater the pad
bits, a ceiling function is used. We have:
16 + 6 + 8LM AChdr + 8LP AY LOAD
TDAT A = TP + TP HY hdr + τ
(5.5)
NDBP S
LM AChdr = MAC header length in bytes
LP AY LOAD = Data payload length in bytes
τ = OFDM symbol delay
NDBP S = Data bits per OFDM symbol
In (5.5), let us consider that the fraction, within the ceil function, be denoted by z.
For IEEE 802.11a, the value of LM AChdr is 28 bytes, corresponding to the data
rates i.e. 6, 9, 12, 18, 24, 35, 48 and 54 Mbps, NDBP S is 24, 36, 48, 72, 96, 144,
192 and 216 respectively. In Figure 2.3, the PSDU consists of 28 bytes of MAC
header, 0 - 2312 bytes of data payload and 4 bytes of FCS. So, for a given data
rate, say 6 Mbps, dze ≥ 3.
The same follows for the acknowledgement transmission time:
16 + 6 + 8LACK_f rame
TACK = TP + TP HY hdr + τ
NDBP S
LACK_f rame = Acknowledgement frame length in bytes (14 bytes)
(5.6)
34
Performance Analysis of IEEE 802.11 PHY
From (5.1),
CWmin σ
TDCF = TDIF S +
+ 2TP + 2TP HY hdr + TSIF S
2
16 + 6 + 8LM AChdr + 8LP AY LOAD
16 + 6 + 8LACK_f rame
+τ
+τ
NDBP S
NDBP S
(5.7)
The maximum throughput (MT) is given by:
MT =
8LP AY LOAD
TDCF + 2δ
(5.8)
Table 5.3 shows the parameters for IEEE 802.11a/b. Note that NDBP S is rate
dependent as shown in Table 2.2. Applying the limit theorem (NDBP S →∞), gives
us the Throughput Upper Limit (TUL).
TUL =
8LP AY LOAD
2TP + 2TP HY hdr + 2δ + TDIF S + TB
(5.9)
The time between the starting of a transmission cycle and the packet0 s successful
reception is termed as Packet Delay. The minimum encountered delay consists of
a DIFS interval, backoff time and data transmission time.
M D = TDIF S + TB + TDAT A
(5.10)
The values of TDIF S , TB and TDAT A have already been derived in the first part
this Section. Substitution gives us:
CWmin σ
16 + 6 + 8LM AChdr + 8LP AY LOAD
M D = TDIF S +
+TP +TP HY hdr +δ+τ
2
NDBP S
(5.11)
Note that NDBP S is rate dependent as shown in Table 2.2. Applying the limit
theorem (NDBP S →∞), gives us the Delay Lower Limit (DLL).
DLL = TP + TP HY hdr + δ + TDIF S + TB
(5.12)
δ in (5.9) and (5.12) is the propagation delay.
5.2.3
Derivation of MT and MD for IEEE 802.11b
The equations for MT and MD, for IEEE 802.11b are the same as those for IEEE
802.11a i.e. (5.8) and (5.11), respectively. The only difference comes in the data
transmission time (TDAT A ) and the acknowledgement transmission time (TACK ).
In IEEE 802.11b, ratio of the packet size to the rate, at which the transmission
occurs, equals the transmission time. Therefore, for IEEE 802.11b, (5.3) takes the
form as follows:
TDAT A = TP + TP HY hdr +
8LM AChdr + 8LP AY LOAD
1000000RDAT A
(5.13)
5.2 Throughput and Delay Limits
35
RDAT A = Transmission rate
Similarly, for TACK , (5.4) takes the form as follows:
TACK = TP + TP HY hdr +
8LACK_f rame
1000000RACK
(5.14)
RACK = Control rate
The equations for the Throughput Upper Limit (TUL) and the Delay Lower
Limit (DLL) are the same as (5.9) and (5.12), respectively. The supported rates
for IEEE 802.11b are 1, 2, 5.5 and 11 Mbps. Therefore, the (data rate, control
rate) pairs are (1, 1), (2, 2), (5.5, 2) and (11, 2).
5.2.4
Parameters
Table 5.3 [5] defines the attributes and their corresponding values used for the
simulations to follow in this Section.
Attributes
TSIF S (µs)
Slot time - σ (µs)
TDIF S (µs)
TP HY hdr (µs)
TP (µs)
CWmin (slots)
MAChdr (bytes)
CRC (bytes)
Propagation delay - δ (µs)
OFDM symbol delay - τ (µs)
PHY layer peak rate (Mbps)
802.11a
16
9
34
4
16
15
28
4
1
4
54
802.11b
10
20
50
48
144
31
28
4
1
11
Table 5.3: MAC and PHY Parameters for 802.11 a/b
5.2.5
Results
In real-time applications, we come across some limitations for the available throughput and encountered delay. Figure 5.5 shows the throughput upper limit (TUL)
and the ideal throughput attained when the frame size is fixed to 1000 bytes [35].
It is observed that one cannot exceed 50 Mbps throughout with a payload size of
1000 bytes. Figure 5.6 shows the maximum achievable throughput for different
data rates with a varying payload size. For a payload size of 1000 bytes at data
rate of 54 Mbps, the throughput is almost 25 Mbps. However, for higher rates the
performance is not good.
36
Performance Analysis of IEEE 802.11 PHY
Figure 5.5: Ideal TUL for frame size = 1000 Bytes
Figure 5.6: Maximum Throughput vs Payload size for different Data Rates (IEEE
802.11a)
5.2 Throughput and Delay Limits
37
This shows that in practice, the achievable maximum throughput is always less
than the actual rate.
Figure 5.7 illustrates the Delay lower limit (DLL) and the delay experienced
for different data rates. The DLL is the same for all the rates i.e. 121.5 µs.
Figure 5.7: Minimum Delay vs Payload size for different Data Rates (IEEE 802.11a)
For a payload size of 1000 bytes and rate of 54 Mbps, the maximum delay is
almost 280 µs.
The Bandwidth efficiency is defined as the ratio of throughput and the data
rate. Figure 5.8 depicts the bandwidth efficiency curves for different rates. For a
payload size of 1000 bytes, the bandwidth efficiency for 54 Mbps is almost 46 %.
The bandwidth efficiency for other rates can be seen from Figure 5.8.
However, by doubling the rate, one can have an enhanced throughput. Figures 5.9
and 5.10 show the effect of doubling the rate for 6 Mbps and 54 Mbps, respectively.
For 2 x 6 = 12 Mbps, we have an increase of around 6 dB. For 2 x 54 = 108
Mbps, we experience an increase of almost 10 dB. Figure 5.11 shows the percentage
increase in throughput by doubling the rate for different rates.
38
Performance Analysis of IEEE 802.11 PHY
Figure 5.8: Bandwidth Efficiency for different Data Rates (IEEE 802.11a)
Figure 5.9: Effect on Throughput by doubling the Rate (2x6=12Mbps)(IEEE
802.11a)
5.2 Throughput and Delay Limits
39
Figure 5.10: Effect on Throughput by doubling the Rate (2x54=108Mbps)(IEEE
802.11a)
Figure 5.11: Percentage increase in Throughput by doubling the Rate (IEEE
802.11a)
40
Performance Analysis of IEEE 802.11 PHY
Figure 5.12: Throughput when the Channel is adding Noise (IEEE 802.11a)
In Figures 5.5 to 5.11, an ideal channel was considered (Section 5.2.1). Figure 5.12
depicts the actual effective throughput when the frame size is varying and the PHY
layer data rate is 216 Mbps. The sharp curve for BER = 10−3 is showing the concept
of CSMA. The lower the bit error rate is, the more the effective throughput we have.
5.2 Throughput and Delay Limits
41
Figure 5.13: Maximum Throughput vs Payload size for different Data Rates (IEEE
802.11b)
Figure 5.14: Minimum Delay vs Payload size for different Data Rates (IEEE
802.11b)
42
Performance Analysis of IEEE 802.11 PHY
Figure 5.13 shows the TUL and the MT for IEEE 802.11b for different data
rates. It can be observed that, at a payload size of 1000 bytes, the TUL is almost
11.5 Mbps. This shows that, even if the rate maximized, there exists a limitation
to maximum achievable throughput.
Figure 5.14 shows the DLL and the MD for IEEE 802.11b for different data
rates. The lowest achievable delay, in the case of IEEE 802.11b, is almost 523 µs.
5.3
Packet Aggregation
The IEEE 802.11 MAC has overhead which affects the effective throughput. [36]
suggests some IEEE 802.11 enhancements for the MAC overhead improvement.
One method, along with the simulation results is explained in this Section. The
reader can refer to [36] for the details for other techniques.
5.3.1
DSDSRFA
Figure 5.15 [36] depicts the concept of Distributed, Single-Destination and SingleRate Frame Aggregation (DSDSRFA). It shows k PHY frames being sent without
any space involved.
Figure 5.15: Concept of DSDSRFA
The start of the frame aggregation is initialized by PHY frame 1 and PHY frame k
indicates that the frame aggregation has ended. There are k PHY frames in frame
aggregation.
5.3 Packet Aggregation
Figure 5.16: Comparison of TUL
Figure 5.17: Comparison of MT
43
44
Performance Analysis of IEEE 802.11 PHY
Figures 5.16 and 5.17 show the comparison of the original and the aggregated
versions in terms of TUL and MT, respectively. For both simulations the value
of k is taken as 2. The data rate used is 54 Mbps. It is clearly seen that there is
an improvement in both, TUL and MT, when frame aggregation is applied. For
a data payload size of 1500 bytes, the improvement in TUL is approximately 58
Mbps. For a same payload size, the improvement is MT is about 6 Mbps.
Chapter 6
Performance Analysis of
IEEE 802.11 MAC
The following Sections contain the simulational results, along with the arguments,
for the IEEE 802.11 MAC layer. For the purpose, NS-2 [37] is used.
6.1
6.1.1
NS-2
Overview
NS-2 is an object-oriented, event driven network simulator which is developed at
UC Berkley. Essentially, NS-2 is written in C++ and OTcl. OTcl is an objectoriented subdivision of Tcl [38]. At present, NS-2 is being widely used for the
network simulation of local and wide area networks. NS-2 is open source and [39]
states the contributions made to the NS-2. NS-2 is implemented using C++ and
OTcl. There are always two corresponding hierarchies for every protocol or network
objects implemented in NS-2, the compiled C++ hierarchy and the interpreted
OTcl hierarchy [40]. Figure 6.1 [40] shows a simplified user0 s view to NS.
6.1.2
Network Components
An NS network comprises of Compound network components and Basic network
components. A partial class hierarchy is shown in Figure 6.2 [40]. TclObject acts
as the root i.e. it is the superclass for all the OTcl library objects. As we move
down the hierarchy, there is NsObject, which is the superclass for all the basic
networking components. These basic network components are responsible for tasks
such as packet handling. The network may also contain some compound network
objects e.g. nodes and links. Possible number of output data paths decide whether
the basic network components branch into Connector or Classifier. A single output
data path corresponds to the Connector class and multiple output data paths
correspond to the Classifier class.
45
46
Performance Analysis of IEEE 802.11 MAC
Figure 6.1: Simplified User0 s View of NS
Figure 6.2: Class Hierarchy (Partial)
One of the essential components of NS-2 is a Node. A node entry object and
a classifier make up a node. Figure 6.3 [40] shows the structure of nodes in NS-2.
By default, NS has Unicast nodes. A unicast node consists of an address classifier
and a port classifier. The former is responsible for unicast routing. In case of
multicast operations, it is possible to creat Multicast nodes. Multicast nodes have
the same structure of unicast nodes. However, in addition to that, they have a
classifier to distinguish between unicast and multicast packets and a multicast
classifier to undertake multicast routing.
6.1 NS-2
47
Figure 6.3: Node (Unicast and Multicast)
Another important component of NS is a Link. Generally, the link object corresponds to a uni-directional simplex link. A bi-directional duplex link can be created
by using the duplex-link member function of the Simulator object. It generates
two simplex links in both the directions as shown in Figure 6.4 [40].
Figure 6.4: Link
During network generations, an Agent can either be in an active or a reactive state.
On the other hand, nodes and links act as reactive elements. Routing agents is
such an example. Packets are actively generated by the routing agents and are
sent to other agents, in the same line, in order to define possible routes within the
network.
Traffic generators are responsible to generate data within the network. They
48
Performance Analysis of IEEE 802.11 MAC
can either be a data source (e.g. CBR) or an application simulating source (e.g.
FTP).
6.1.3
Tracing
NS-2 has a unique way of analyzing the results obtained after running the simulations. For the purpose, trace objects i.e. EnqT, DeqT, DrpT and RecvT are
inserted within the corresponding link objects. Figure 6.5 [40] depicts the trace
object insertion structure. As a result, useful information, about the packets
passing through the trace objects, is stored in a predefined trace file. Figure 6.6 [40]
shows a general format of a trace file.
Figure 6.5: Insertion of Trace objects
Figure 6.6: Trace Format example
The segments of the trace file in Figure 6.6 are defined as follows:
• event: Each trace file starts of with an event descriptor. +,−, d and s are
some event descriptors which specify enqueue, dequeue, packet drop and
packet receive respectively.
• time: For a particular event, this part gives the simulation time in sec.
• from node and to node: Source and destination nodes.
• pkt type and pkt size: Type of the packet e.g. cbr, tcp, ack etc and the
packet size in Bytes.
• fid: IPv60 s flow id. The user can define this fid in the input OTcl script.
• src addr and dst addr: Addresses for the source and destination in the form
of node.port.
6.3 Results
49
• seq num: This part specifies the network layer protocol packet sequence
number. For the case of UDP, this field is not used but NS keeps track for
the sequence number for future analysis.
• pkt id: Since this packet id is unique, it serves as an important part of the
trace file.
6.2
Comparison between PCF and DCF
In Industrial Automation, latency is an important factor. Usually, the transmitted
signals are of very short length and if they are not received in time, they are of no
use. The purpose of this Section is to compare PCF and DCF, in terms of deadline
misses, under different scenarios. NS2 [37] is used for the simulation purposes.
6.2.1
Parameters
Table 6.1 shows the attributes, and their values, used in the simulations.
Attributes
Slot time (µs)
SIFS(µs)
DIFS (µs)
PIFS (µs)
Transmit power (W )
Transmission range (m)
Simulation time (sec)
CWmin (slots)
CWmax (slots)
PLCP preamble (bits)
PLCP header (bits)
Beacon period (TU )
CFPMaxDuration (TU )
Values
20
10
16
12
0.2818
250
600
31
1023
144
48
200
180
Table 6.1: Parameters for PCF Vs. DCF Comparison
Note: TU is Time Unit. 1 TU = 1024 µs.
6.3
Results
This Section explains the generated scenario, along with the results and observations.
The PCF patch for ns2 is available at [41]. The network size and data payload is
varied from 5 to 20 nodes and 5 to 1500 bytes respectively. For simulating PCF,
the number of priority nodes is also varied. All these scenarios are simulated with
1 and 2 Mbps of data rates. Figure 6.7 shows a simple five node star topology. PC
is the server which is the point coordinator for PCF and base station incase of
50
Performance Analysis of IEEE 802.11 MAC
DCF. The PC is a wired station and is connected to the wired backbone. The PC
has a wireless range and the nodes (N) are randomly distributed within the range.
Exchange of data packets is done between the PC and the nodes.
Wired Backbone
PC
N
N
N
N
N
Figure 6.7: Five Node Network Topology
In the following, different environments are listed, under which PCF and DCF are
compared.
• 5 nodes, 1000 bytes.
• 10 nodes, 1000 bytes.
• 10 nodes, 1500 bytes.
• 15 nodes, 512 bytes.
• 20 nodes, 5 bytes.
• 20 nodes, 100 bytes.
6.3 Results
6.3.1
51
Deadline misses of PCF and DCF for 1 Mbps
Figures 6.8 to 6.13 depict the encountered delay vs number of received packets, in
seconds, for the above mentioned scenarios for 1 Mbps.
The deadline point is highlighted by a blue horizontal line in the Figures. This
is also termed as the cbr interval i.e. the time between packet transmissions. As
mentioned earlier, time is very crutial for Industrial applications. So, it is important
for the packet to be received within the specified deadline. The x-axis shows the
number of packets received and the experienced end-to-end delay is shown along the
y-axis. By closely observing the Figures, one can see that the DCF access scheme
is encountering deadline at an higher cbr interval than the PCF access scheme.
The DCF and PCF schemes are explained in Sections 3.1 and 3.2 respectively.
In case of DCF, the concept of CSMA / CA is used for the channel access.
According to CSMA / CA, the station senses the channel to be idle. Once it has
sensed the channel to be idle, the station waits another DIFS before starting the
data transmission process. It also has a backoff timer which is reduced whenever
the channel is sensed idle. This backoff timer is set randomly varying from 0 to
the maximum size of contention window (CW).
For the PCF access scheme, the concept of polling is used. The PC already
maintains a polling list before the data transmission starts. There is a superframe
or the beacon period which has the provision of both, the CFP and the CP. Once
the process starts, the PC waits a PIFS before it sends the beacon signal to the
stations, that are already in the PC’s polling list. Since, PCF has the provision of
both, the CFP and the CP, and PIFS is less than DIFS, PCF scheme always gets
priority over the DCF channel access scheme.
Let us discuss Figure 6.8 in detail. The rate is 1Mbps, we have a setup with
5 active nodes and a payload size of 1000 bytes. One can see that when the cbr
interval is set to 0.0499 sec, the DCF is encountering deadline misses at an higher
cbr interval than the PCF scheme.
The reason for the DCF scheme to encounter deadline misses at an higher cbr
interval than the PCF scheme is due to the use of CSMA / CA. In DCF, since
the nodes have to content for the channel access and it depends on the CW size,
there is an increase in the delay factor, causing the deadline misses. On the other
hand, for the PCF scheme, the concept of polling is used for the maximum size
of the predefined contention free period, which overcomes the channel access fight
between the stations in the polling list of the PC.
However, as already stated above, PCF has both CFP and CP. When the CFP
end is received, the channel access scheme automatically switches back to the
conventional DCF channel access scheme.
52
Performance Analysis of IEEE 802.11 MAC
Figure 6.8: Encountered delay vs Packets received for 5 nodes and 1000 bytes
Figure 6.9: Encountered delay vs Packets received for 10 nodes and 1000 bytes
6.3 Results
53
Sudden spikes in case of PCF are due to that reason.
The rest of the Figures (Figures 6.9 to 6.13) show the same behavior under
different scenarios.
Figure 6.10: Encountered delay vs Packets received for 10 nodes and 1500 bytes
Figures 6.14 to 6.16 show the average end-to-end delay vs cbr interval for a
data rate of 1 Mbps under the scenarios mentioned in Section 6.3.
The deadline is marked with "deadline" in the Figures. In Figure 6.14, PCF
and DCF are compared for two cases i.e. 20 nodes, 5 bytes and 5 nodes, 1000 bytes.
The increase in the difference between the average end-to-end delay, at the point
of deadline (0.042 sec for 20 nodes, 5 bytes and 0.0499 sec for 5 nodes, 1000 bytes),
shows that DCF encounters deadline misses.
Same explaination is valid for the rest of the cases (Figures 6.15 and 6.16).
54
Performance Analysis of IEEE 802.11 MAC
Figure 6.11: Encountered delay vs Packets received for 15 nodes and 512 bytes
Figure 6.12: Encountered delay vs Packets received for 20 nodes and 5 bytes
6.3 Results
55
Figure 6.13: Encountered delay vs Packets received for 5 nodes and 100 bytes
Figure 6.14: Average end-to-end delay vs cbr interval (1 Mbps)
56
Performance Analysis of IEEE 802.11 MAC
Figure 6.15: Average end-to-end delay vs cbr interval (1 Mbps)
Figure 6.16: Average end-to-end delay vs cbr interval (1 Mbps)
6.3 Results
6.3.2
57
Deadline misses of PCF and DCF for 2 Mbps
Figures 6.17 to 6.22 depict the encountered delay vs number of received packets, in
seconds, for the above mentioned scenarios (Section 6.3) for 2 Mbps.
Figure 6.17: Encountered delay vs Packets received for 5 nodes and 1000 bytes
The blue horizontal line marks the deadline. In Figure 6.17, the cbr interval
or the deadline is 0.0285 sec. From the Figure, it is quite evident that DCF encounters deadline misses at an higher cbr interval than PCF. The same explanation
goes for the rest of the Figures.
The reason for DCF, to encounter deadline misses at an higher cbr interval than
PCF, is the same as described in Section 6.3.1. The only difference is that we can
observe an enhanced performance, in all cases, since the data rate is 2 Mbps. High
data rate means that the transmission power is high, which directly affects the
encountered delay time.
58
Performance Analysis of IEEE 802.11 MAC
Figure 6.18: Encountered delay vs Packets received for 10 nodes and 1000 bytes
Figure 6.19: Encountered delay vs Packets received for 10 nodes and 1500 bytes
Figures 6.23 to 6.25 show the average end-to-end delay vs cbr interval for a
data rate of 2 Mbps under the scenarios mentioned in Section 6.3. The deadline is
6.3 Results
59
Figure 6.20: Encountered delay vs Packets received for 15 nodes and 512 bytes
Figure 6.21: Encountered delay vs Packets received for 20 nodes and 5 bytes
60
Performance Analysis of IEEE 802.11 MAC
Figure 6.22: Encountered delay vs Packets received for 5 nodes and 100 bytes
marked with "deadline" in the Figures. In Figure 6.23, PCF and DCF are compared
for two cases i.e. 20 nodes, 5 bytes and 5 nodes, 1000 bytes. The increase in the
difference between the average end-to-end delay, at the point of deadline (0.035
sec for 20 nodes, 5 bytes and 0.0285 sec for 5 nodes, 1000 bytes), shows that DCF
encounters deadline misses.
Same explaination is valid for rest of the cases.
6.3 Results
Figure 6.23: Average end-to-end delay vs cbr interval (2 Mbps)
Figure 6.24: Average end-to-end delay vs cbr interval (2 Mbps)
61
62
Performance Analysis of IEEE 802.11 MAC
Figure 6.25: Average end-to-end delay vs cbr interval (2 Mbps)
6.4
6.4.1
Observations
Deadline Misses
Table 6.2 shows the points, at which PCF and DCF are encountering deadline
misses, when different parameters are varied. An important observation is that
when the number of stations is increased, there is a rapid improvement in the PCF
channel access scheme as compared to the DCF scheme in terms of deadline misses.
When the network size increases, in DCF, there are more nodes in the process of
contention and the probability of collision increases. When collision occurs, the
CW increases about the power of 2 and eventually it encounters deadline misses
earlier as compared to PCF. For the PCF, when we have more nodes and increase
in the priority nodes, maximum data transfer will take place in the CFP. Hence,
difference between the deadline misses (PCF Vs. DCF) is more obvious in a bigger
network. Another important observation is that when the packet size increases,
the improvement level of PCF falls down. Here, we can say that there is a trade-off
between the packet size and the number of nodes.
6.4 Observations
Scenario
1 Mbps
Payload size = 100 bytes
5 nodes
10 nodes
15 nodes
20 nodes
Payload size = 512 bytes
5 nodes
10 nodes
15 nodes
20 nodes
Payload size = 1000 bytes
5 nodes
10 nodes
15 nodes
20 nodes
2 Mbps
Payload size = 100 bytes
5 nodes
10 nodes
15 nodes
20 nodes
Payload size = 512 bytes
5 nodes
10 nodes
15 nodes
20 nodes
Payload size = 1000 bytes
5 nodes
10 nodes
15 nodes
20 nodes
63
PCF
DCF
12
22
35
51
14
28
43
59
ms
ms
ms
ms
ms
ms
ms
ms
41 ms
65 ms
86 ms
119 ms
42 ms
69 ms
90 ms
126 ms
47 ms
97 ms
148 ms
197 ms
50 ms
99 ms
151 ms
201 ms
11
17
28
38
ms
ms
ms
ms
12
22
33
50
ms
ms
ms
ms
22
32
50
68
ms
ms
ms
ms
23
38
56
76
ms
ms
ms
ms
26 ms
53 ms
86 ms
109 ms
28 ms
57 ms
89 ms
114 ms
Table 6.2: Deadline Comparison of PCF and DCF
64
6.4.2
Performance Analysis of IEEE 802.11 MAC
Bandwidth Efficiency
Bandwidth efficiency or the bandwidth utilization is another important factor,
which helps us in the process of investigating the behaviour of PCF and DCF,
under different scenarios. Table 6.3 shows the utilization of bandwidth for PCF
and DCF, for a given scenario.
Payload Size (bytes)
100
200
300
400
500
Bandwidth
Efficiency (%)
PCF
DCF
47
32
69
48
77
60
82
65
85
75
Table 6.3: Bandwidth Efficiency (PCF Vs. DCF)
In terms of bandwidth efficiency, PCF has a better performance than DCF. The
scenario shown in Table 6.3 is for a simple 2-node network. The data transmission
rate is of 1 Mbps, 10 ms is the cbr interval or the update frequency and the data
payload size is varied from 100 bytes to 500 bytes. When we increase the number
of stations in the network, the bandwidth utilization for PCF remains the same.
The reason is simply that PCF is deterministic. Since the delay time is same
for all of the transmissions, increasing the number of nodes will also increase the
overall system delay in a deterministic way. For example, a network has 5 nodes.
If each nodes has a delay time of 1 ms, then the overall system delay will be 5 ms.
However, for the case of DCF, the encountered delay consists of a random backoff.
With the increase in number of nodes, the probability of collision also increases
which consequently, increases the random backoff. In CSMA/CA, the nodes sense
the channel to be idle before starting to send data. If a node never senses the
channel to be idle, there will be an increase in the random backoff, which increases
the overall system delay. The working mechanism of CSMA / CA is explained in
Section 3.1.1. This behaviour is shown in Table 6.4.
6.5 Comparison between WLAN and WirelessHART
Payload Size (bytes)
100
5 nodes
10 nodes
200
5 nodes
10 nodes
300
5 nodes
10 nodes
400
5 nodes
10 nodes
500
5 nodes
10 nodes
65
Bandwidth
Efficiency (%)
PCF
DCF
47
47
28
19
69
69
35
27
77
77
51
40
82
82
59
40
85
85
65
58
Table 6.4: Bandwidth Efficiency (PCF Vs. DCF) with Varying Number of Nodes
6.5
Comparison between WLAN and WirelessHART
This Section explains, for a given scenario, how WLAN is better then WirelessHART
in terms of control loops. We have an update frequency of 20 ms and a data payload
size of 100 bytes.
Figure 6.26: The Basic Block Diagram of the Wireless System (Courtesy of ABB)
For WirelessHART, we have a 10 ms fixed time slot, since it follows the concept of
TDMA for channel access. This means that there is a fixed window for the data
66
Performance Analysis of IEEE 802.11 MAC
transmission and the reception of acknowledgement. Ideally, with a data rate of
250 Kbps, maximum supported by WirelessHART, a packet takes around 4 ms to
travel from a source to a destination. Figure 6.26 shows a top-level block diagram
for a wireless sensor and actuator network. Considering a packet travelling from
the Sensor Node to the Actuator, via the Gateway, for the case of wirelessHART,
it will require two time slots, 10 ms each, to complete the one way process, since
the slot is reserved for this purpose. The time required by the Controller and the
Process Plant will be the same for both, WirelessHART and the WLAN. Therefore,
the difference in the number of control loops is affected by the time from the Sensor
Node to the Actuator, via the Gateway.
On the other hand, WLAN encounters a delay of only 1.72 ms. This means
that with an update frequency of 20 ms, WirelessHART can only entertain 1
control loop (ideally), whereas WLAN can squeeze in 5 control loops.
Chapter 7
Conclusions and Future
Work
7.1
Conclusions
The use of Wireless Local Area Networks, in various applications, is at a high
pace. Recently, their use in industrial applications has been started. In the field of
industrial automation, there is a strict requirement for a signal to be received with
in the specified deadline otherwise it is of no use. Encountering a deadline miss
can cause a lot of damage in terms of money and personal health.
The target of this thesis was to investigate the IEEE 802.11 PHY and MAC
layer for its use in industrial automation. Section 5.1 shows the PHY layer analysis
using the IEEE 802.11n simulink model. This simulation was carried out under
the influence of a larger indoor area using different conditions. The results show
that when the channel is AWGN, the effects on the PER Vs. SNR curves are not
that severe. This is because in an AWGN channel, the only impairment is the
white noise with a constant spectral density and it does not include the effects of
fading. The affect of fading is observed when a n-LoS channel D is used. In this
case, PER is higher at lower values of SNR. This behavior was catered by using
the concept of beamforming which shows some improvements. However, the use of
STBC shows good improvements since, there is an increase in the transmit diversity.
Section 5.2 shows the theoretical upper and lower bounds on the throughput
and the delay, respectively. These limitations show that by simply increasing
the available data rate, without reducing the overhead, these values are bounded.
Therefore, overhead reduction is necessary in order to have an enhanced performance in terms of throughput and delay. This problem ca be solved, to some
extent, by using packet aggregation (Section 5.3).
Section 6.2 investigates the main target of this thesis, i.e. the comparison of
67
68
Conclusions and Future Work
MAC access schemes (PCF Vs. DCF) in terms of deadline misses. PCF and DCF
were investigated under various scenarios. Observing the MAC schemes in different
environments leads us to the conclusion that PCF has a far better performance
than DCF in terms of deadline misses. From Table 6.2, it can be concluded that
there exists a trade-off between the packet size and the number of stations involved
in the network. However, increasing the available data rate and reducing the
overhead, can enhance the performance of IEEE 802.11 (WLAN) in industrial
applications. PCF is also efficient, in terms of bandwidth utilization, as compared
to DCF, with varying number of nodes.
Applications requiring a fast update of frequency cannot be handled easily by the
wirelessHART. For example, with an update frequency of 10 ms, WirelessHART
fails. Here, WLAN comes into play. However, it is not wise to use WLAN for
control applications with slow update frequency rate, since WLAN requires more
transmission power as compared to WirelessHART. WLAN is also suitable for
monitoring applications where a large amount of data payload size is required.
Finally, for a given scenario, more control loops can be squeezed in, in case of
WLAN than WirelessHART.
7.2
Future Work
In any field of study, there is always room for further research. As explained
in Chapter 3, the PCF channel access scheme works on the principle of polling.
However, the current PCF implementation has some deficiencies. The PC maintains a polling list, which contains the stations who need to use the channel for
the purpose of data transmission. Currently, the polling mechanism is not adaptive i.e. there is no proper method to detect the level of emergency among the
stations. This is very important in terms of industrial automation applications.
This issue can be solved by putting the high-risk sensors in the priority list but
is open for future research. A solution might be by keeping the count of recent
polls and put the stations, with maximum number of recent polls, in the priority list.
In the current implementation of PCF access scheme, if a polled station has
no data to send, the PC waits until the poll times out. However, if the polled
stations have a NODATA frame and the stations have no data to transmit, they
can send this NODATA frame to the PC, notifying that the PC should move on to
the next station in its polling list. This can save the waiting time of the PC and
consequently, reduce the encountered delay time.
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