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Technical Report

Rev. 6.0

ABB Corporate Research

Issued by department ZCRD/SECRC/AN

Performance Evaluation of an ECMA368network in an electrical substationapplication based on IEC61850

Supervisor:

Jimmy Kjellsson

Author:Alberto Fiorenza

Acknowledgement

i

Abstract

An electrical substation is a subsidiary station of an electricity generation,transmission and distribution system where voltage is transformed from highto low or the reverse using transformers. All the devices that it contains arecontrolled, protected and monitored by an automation system (Substationautomation) that collects information from the process (power system) andperforms actions on it. Communication network represents a fundamentalelement of the whole automation system and network performances can havea critical impact on the control process. Today, in the Medium Voltage seg-ment, all the new products uses the IEC61850, an Ethernet-based standardthat implements multiple protocols and services for communication.UWB is an emerging technology, adopted by WiMedia Alliance to realizeWireless USB technology, that seems to be able to satisfy the IEC61850 re-quirements. The objective of the project is to simulate the performance of anetwork consisting of several UWB nodes communicating data according toa speci�c scheme (de�ned by an ABB switchgear application), in a simula-tion software environment. TrueTime, a Matlab/Simulink-based simulator,has been used and new functionalities have been added in the simulationtool to make it able to emulate an UWB network based on ECMA 368 stan-dard. In particular, we focused on network performances evaluating latencyintroduced by the communication combining transmissions of SMV (hardreal-time) and SCADA (best e¤ort) information in the same link. The twochannel access methods allowed by ECMA 368, DRP and PCA, permits tocreate a �exible system which can handle both periodical high priority datain combination with lower priority non-periodic data. Several con�gurationshave been analyzed in order to �nd the most suitable con�guration.

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Contents

Contents i

List of Figures iii

List of Tables v

List of Acronyms vi

1 Introduction 11.1 Outline Of The Thesis . . . . . . . . . . . . . . . . . . . . . . 3

2 Substation automation systems based on IEC61850 52.1 Electrical substation overview . . . . . . . . . . . . . . . . . . 52.2 Substation Automation systems (SAS) . . . . . . . . . . . . . 72.3 IEC61850 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3.1 Communication architectures . . . . . . . . . . . . . . 112.3.2 Data modeling approach . . . . . . . . . . . . . . . . . 122.3.3 Data exchange model . . . . . . . . . . . . . . . . . . . 132.3.4 SCADA Application . . . . . . . . . . . . . . . . . . . 162.3.5 Generic Object Oriented Substation Event (GOOSE) . 162.3.6 Sampled Measured Values (SMV) transmission . . . . . 17

3 Wireless communications 193.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.2 IEEE 802.11 (WI-FI) . . . . . . . . . . . . . . . . . . . . . . . 213.3 IEEE 802.15.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.4 IEEE 802.15.1 (BluetoothTM ) . . . . . . . . . . . . . . . . . . 29

i

CONTENTS

4 Ultra-Wideband Technology 324.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324.2 ECMA 368 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.2.1 PHY layer . . . . . . . . . . . . . . . . . . . . . . . . . 374.2.2 MAC layer . . . . . . . . . . . . . . . . . . . . . . . . . 43

5 IEC61850 on ECMA 368 49

5.1 ECMA368 for Sampled Measured Values transmission . . . . . 49

5.1.1 Time Synchronization . . . . . . . . . . . . . . . . . . 505.1.2 SMV on Distributed Reservation Protocol (DRP) . . . 515.1.3 SMV on Prioritized Contention Access (PCA) . . . . . 52

5.2 ECMA368 for SCADA transmission . . . . . . . . . . . . . . 53

6 Simulation enviroment and setups 546.1 Simulation Enviroment Overview: TrueTime . . . . . . . . . 54

6.1.1 Tool Introduction . . . . . . . . . . . . . . . . . . . . . 546.1.2 Tool Extension . . . . . . . . . . . . . . . . . . . . . . 55

6.2 Simulation Setups . . . . . . . . . . . . . . . . . . . . . . . . . 56

7 Simulation Results 627.1 SMV and SCADA data transmission in a superframe structure 627.2 SMV and SCADA data transmission in a contention-based

structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

8 Conclusions 71

A Simulation Results Plots 73

Bibliography 87

ii

List of Figures

2.1.1 Example of an outdoor electrical substation . . . . . . . . . . 62.1.2 Example of an indoor substation . . . . . . . . . . . . . . . . . 62.2.1 Example of an Intelligent Electronic Device (IED) . . . . . . . 72.2.2 Logical scheme of the three levels of a Substation Automation

system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.3 Vertical communication in the substation automation system . 92.2.4 Horizontal communication in the substation automation system 102.3.1 Examples of Ethernet architectures in Substation Automation 122.3.2 IEC61850 Class Model . . . . . . . . . . . . . . . . . . . . . . 132.3.3 The mapping of the IEC 61850 data model and services . . . . 142.3.4 Two-Party-Application-Association . . . . . . . . . . . . . . . 152.3.5 MultiCast-Application-Association . . . . . . . . . . . . . . . 152.3.6 Example of GOOSE transmission . . . . . . . . . . . . . . . . 172.3.7 Process connection with serial communication for SMV trans-

mission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.1.1 Wireless network scenarios . . . . . . . . . . . . . . . . . . . . 203.2.1 Spread-Spectrum system behavior . . . . . . . . . . . . . . . . 213.2.2 Multi-carrier system scheme . . . . . . . . . . . . . . . . . . . 233.2.3 Ideal OFDM transmitter . . . . . . . . . . . . . . . . . . . . . 243.2.4 Ideal OFDM receiver . . . . . . . . . . . . . . . . . . . . . . . 253.3.1 IEEE 802.15.4 and ZigBee protocol stack . . . . . . . . . . . . 263.3.2 802.15.4 channel allocation . . . . . . . . . . . . . . . . . . . . 283.3.3 802.15.4 Superframe . . . . . . . . . . . . . . . . . . . . . . . 293.4.1 Comparation between Bluetooth and ISO/OSI protocol stack

layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.1.1 UWB Emission Limits for outdoor communication systems . . 33

iii

LIST OF FIGURES

4.1.2 UWB Emission Limits for indoor communication systems . . . 334.1.3 Comparation between FCC(USA) and ETSI(Europe) masks

for UWB emission . . . . . . . . . . . . . . . . . . . . . . . . . 344.1.4 Comparation between FCC(USA), MIC(Japan) and ETRI(S.Korea)

masks for UWB emission . . . . . . . . . . . . . . . . . . . . . 344.1.5 Comparation between FCC(USA) and IDA(Singapore) masks

for UWB emission . . . . . . . . . . . . . . . . . . . . . . . . . 354.1.6 Comparation between Narrow-band and UWB PSDs . . . . . 364.2.1 ECMA368 Spectrum Allocation . . . . . . . . . . . . . . . . . 384.2.2 ECMA368 frequency band assignement. Detect and avoid

techniques mitigate interference potential by searching for broad-band wireless signals and then automatically switching theUWB device to another frequency to prevent any con�ict.) . . 39

4.2.3 Standard PPDU structure . . . . . . . . . . . . . . . . . . . . 414.2.4 ECMA368 Superframe structure . . . . . . . . . . . . . . . . . 434.2.5 Example of a Beacon Period (BP) . . . . . . . . . . . . . . . . 444.2.6 Typical scenario of joining devices . . . . . . . . . . . . . . . . 454.2.7 PCA protocol behaviour . . . . . . . . . . . . . . . . . . . . . 47

6.2.1 TrueTime blocks for ECMA368 simulations . . . . . . . . . . . 566.2.2 SMV on DRP with variable number of SMV samples per MAC

frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586.2.3 SMV on DRP with �xed number of SMV samples per MAC

frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596.2.4 TMT of MBOA DRP and PCA . . . . . . . . . . . . . . . . . 60

7.1.1 Latency trends for SMV data using DRP . . . . . . . . . . . . 637.1.2 Latency plots for a 9 nodes con�guration . . . . . . . . . . . . 647.1.3 Average latency trends for SCADA packets . . . . . . . . . . . 657.2.1 Latency trends for SMV packets using PCA . . . . . . . . . . 667.2.2 Maximum latency trends for SMV packets . . . . . . . . . . . 677.2.3 Average latency trends for SMV packets . . . . . . . . . . . . 687.2.4 Probability of latency > 3 ms . . . . . . . . . . . . . . . . . . 697.2.5 Average latency trends for SCADA packets . . . . . . . . . . . 70

iv

List of Tables

3.2.1 Main features of IEEE 802.11a/b/g physical layer . . . . . . . 253.3.1 Main parameters of 802.15.4 (ZigBee) physical layer . . . . . . 273.4.1 IEEE 802.15.1(Bluetooth) power classes . . . . . . . . . . . . 30

4.2.1 MB-OFDM PHY parameters . . . . . . . . . . . . . . . . . . . 404.2.2 Modulation modes and coding rates in MB-OFDM PHY . . . 404.2.3 PCA QoS Settings . . . . . . . . . . . . . . . . . . . . . . . . 46

6.2.1 Parameters for SMV and SCADA data . . . . . . . . . . . . . 60

v

List of Acronyms

AC Access Category

AIFS Arbitration Interframe Space

AC BE Access Category - Best E¤ort

AC BK Access Category - Background

AC VI Access Category - Video

AC VO Access Category - Voice

ACSI Abstract Communication Service Interface

BLER Block Error Rate

BP Beacon Period

BPSK Binary Phase Shift Keying

CCK Complementary Code Keying

CFP Contention-Free Period

CP Contention Period

CSMA/AMP Carrier Sense Multiple Access/Arbitration on Message Pri-ority

CSMA/CA Carrier Sense Multiple Access/Collision Avoidance

CSMA/CD Carrier Sense Multiple Access/Collision Detection

CW Contention Window

vi

LIST OF TABLES

DAA Detect and Avoid

DCF Distributed Coordination Function

DCM Dual-Carrier Modulation

DIFS DCF Interframe Space

DPSK Di¤erential Phase-Shift keying

DQPSK Di¤erential Quadrature Phase-Shift Keying

DRP Distributed Reservation Protocol

DS Direct Sequence

DSSS Direct Sequence Spread Spectrum

ECMA European Computers Manufacturers Association

EDCA Enhanced Distributed Channel Access

FCC Federal Communication Commission

FCS Frame Check Sequence

FEC Forward Error Correction

FFD Full Function Device

FFI Fixed Frame Interleaving

FHSS Frequency Hopping Spread Spectrum

FFT Fast Fourier Transform

GFSK Gaussian Frequency Shift Keying

GOOSE Generic Object Oriented Substation Event

GTS Guaranteed Time Slot

GW Gateway

HMI Human-Machine Interface

vii

LIST OF TABLES

IEC International Electrotechnical Commission

IED Intelligent Electronic Device

ISO/OSI International Organization for Standardization/Open System In-terconnection

MAC Medium Access Control

MAS Medium Access Slot

MBOA Multiband OFDM Alliance

MB-OFDM Multiband - Orthogonal Frequency-Division Multiplexing

MMS Manifacturing Message Speci�cation

MCAA Multicast Application Association

MSDU MAC Service Data Unit

MSVC Multicast Service

NCC Network Control Center

OSI Open System Interconnection

PCA Prioritized Channel Access

PHY Physical (layer)

PIFS PCF Inter-Frame Space

PLCP Physical Layer Convergence Protocol

PN Pseudonoise

PPDU PLCP Protocol Data Unit

PSD Power Spectral Density

QAM Quadrature Amplitude Modulation

QoS Quality of Service

viii

LIST OF TABLES

RFD Reduced Function Device

SAS Substation Automation System

SCADA Supervisory Control and Data Acquisition

SIFS Short Interframe Space

SIG Special Interest Group

SMV Sampled Measured Values

SNR Signal to Noise Ratio

SNTP Simple Network Time Protocol

STA Station

TBTT Target Beacon Transmission Time

TCP/IP Transmission Control Protocol/Internet Protocol

TDMA Time Division Multiple Access

TFC Time-Frequency Code

TFI Time-Frequency Interleaving

TPAA Two-Party Application Association

UDA Unused DRP Announcement

UDR Unused DRP Response

USVC Unicast Service

UWB Ultra Wideband

WIFI Wireless Fidelity

WLAN Wireless Local Area Network

WMAN Wireless Metropolitan Area Network

WPAN Wireless Personal Area Network

ix

LIST OF TABLES

WWAN Wireless Wide Area Network

WUSB Wireless USB

x

Chapter 1

Introduction

This thesis project was performed at the Automation Network departmentat ABB Corporate Research in Vasteras, Sweden. The purpose of the workwas to investigate the performance of a UWB communication network in anelectrical substation application.An electrical substation is a system involved in electricity generation,

transmission and distribution networks where voltage is transformed fromhigh to low or the reverse using transformers. This subsidiary station con-tains several electrical devices controlled, protected and monitored by anautomation system that collects information from the process (power sys-tem) and performs actions on it. A basic task is executed by sensors, thatacquire information by making measurements of voltages, currents, etc. onthree-phases electrical devices. This data is sent to the control layer by acommunication network that represents a fundamental element of the wholeautomation system. In fact, network performances have a critical impacton the control process, that could not work correctly if the communicationsystem doesn�t satisfy speci�c requirements. In the Medium Voltage seg-ment all new products uses the IEC61850, an Ethernet-based standard thatimplements multiple protocols and services for communication.Today, state of art in these applications is to use optical Ethernet. This

provides good performance and ensures galvanic isolation between di¤erentunits in an installation but it is both expensive and fragile. In order to makethe system cheaper and physically more robust, ABB has been working with anew concept where wireless communication is used within a well de�ned, con-�ned space, like rectangular tubes able to guide RF-waves. WLAN (802.11)communication is already used in substation automation to carry data, but

1

requirements for throughput, latency, jitter etc. are continuously increas-ing and new communication technologies need to be evaluated. Ultra WideBand [UWB] is especially interesting since it is an emerging technology whichis supported by major companies such as Intel (UWB will be used for e.g.wireless USB). In the speci�c context our work dealt with the performanceevaluation of a network consisting of several UWB nodes communicating dataaccording to a speci�c scheme. Simulations were made using TrueTime, aMatlab/Simulink-based software environment. For our purposes, we mod-i�ed the existing software by adding new functionalities in order to makeit able to emulate ECMA368, the standard that speci�es MB-OFDM UWBphysical layer and medium access layer for high-speed, short-range wirelessnetworks.In particular, we focused on network performances evaluating latency

introduced by the communication combining transmissions of Sampled Mea-sured Values [SMV] and Supervisory Control and Data Acquisition [SCADA]information in the same link. Latency is a critical parameter that can com-promise the correct behavior of the automation system. This requirementbecomes more pressing for SMV data. In fact, this information representscurrent and voltage measurements on substation devices and is necessary toacquire them in real time. We investigated the latency trend varying thenumber of nodes in the network and the medium access method used totransmit data. Indeed, the ECMA 368 standard allows two di¤erent mediumaccess methods. One, called Distributed Reservation Protocol (DRP), isbased on a TDMA scheme where the multiple access is performed reservingtime slots for devices in a superframe structure. The second method, calledPrioritized Contention Access (PCA), is a contention-based protocol basedon CSMA/CA with priority. The choice between DRP and PCA has a bigimpact on the latency trend when increasing the number of nodes. Our sim-ulations show that by using DRP the maximum latency increases linearlywith the number of nodes, instead PCA shows an exponential trend. Withregard to the average latency the network behavior is better using PCA.Future work of this project might be the development of a channel model

studying the propagation of UWB signals inside a waveguide. The ECMA368simulator can be extended taking into account this channel model. Further,it could be interesting to investigate network performance varying bit ratesas allowed by ECMA368 standard.

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1.1. OUTLINE OF THE THESIS

1.1 Outline Of The Thesis

The contents of the thesis are as follows:

� Chapter 2: Substation Automation systems based on IEC61850.This chapter will introduce electrical substations and its automationsystems that represents the scenario of our thesis work. First, a briefoverview of electrical substations will be given. Then, substation au-tomation systems and the IEC 61850, an international standard de-signed to support the communication of all functions being performedin the substation will be described.

� Chapter 3: Wireless communications. The aim of this chapteris to introduce wireless communication in order to have an overviewof principal technologies used today to implement WLAN and WPAN.In particular IEEE 802.11 (WiFi), IEEE 802.15.4 and IEEE 802.15.1(Bluetooth) will be brie�y described.

� Chapter 4: Ultra-Wideband technology. This chapter presentsa description of UWB. This emerging technology is having a rapiddevelopment and it is exciting interest of electronics technology majorssuch as Intel, Samsung, Philips, etc. Principal features and state ofart of UWB will be shown. In particular will be described ECMA 368standard, on which several future applications like Wireless USB orWireless FireWire will be based.

� Chapter 5: IEC61850 on ECMA368. Today, IEC61850 resortsto the utilization of Ethernet architectures for intercommunication. Inthis chapter it will be shown that UWB technology could be used in-stead of Ethernet in order to satisfy requirements dictated by IEC 61850applications.

� Chapter 6: Simulation environment and setups. This chapterbrie�y introduces the software simulation environment extended in or-der to comply with ECMA 368 standard. Further, it explains how thesimulator has been used in order to investigate network performanceand analyzes signi�cant parameters varied in simulations.

� Chapter 7: Simulation results. This chapter shows and analyzesresults obtained from simulations, evaluating the test-cases discussed in

3

1.1. OUTLINE OF THE THESIS

simulation setups. In particular, network performance are investigatedrespect to latency.

� Chapter 8: Conclusions. The last chapter presents the conclusionsof this thesis work. Some future work is also discussed.

4

Chapter 2

Substation automation systemsbased on IEC61850

2.1 Electrical substation overview

An electrical substation is a system involved in electricity generation, trans-mission and distribution networks where voltage is transformed from high tolow or the reverse. It uses step-up transformers for increasing voltage anddecreasing current and step-down transformers for decreasing voltage andincreasing current for domestic and commercial distribution. In addition totransformers, an electrical substation contains equipment for switching, pro-tection and control. There are circuit breakers for protection from damagescaused by overload or short circuits, isolators capable of dealing with smallcharging currents of busbars and connection, conductor systems, insulationand overhead line terminations[1]. Substations may be located undergroundor on the surface, properly fenced and grounded. High-rise buildings mayhave indoor substations. Indoor substations are usually found in urban ar-eas to reduce the noise from the transformers, or to protect switchgear fromextreme climate or pollution. Protection of equipments become necessary toavoid failures, lightning strikes, power disturbances and accidents. Thus, thesubstations should be properly controlled and monitored in order to take thenecessary precautions accurately and timely. Controllinf and monitoring op-erations are performed by a reliable and self healing system called SubstationAutomation (SA).

5

2.1. ELECTRICAL SUBSTATION OVERVIEW

Figure 2.1.1: Example of an outdoor electrical substation

Figure 2.1.2: Example of an indoor substation

6

2.2. SUBSTATION AUTOMATION SYSTEMS (SAS)

2.2 Substation Automation systems (SAS)

A Substation Automation system is able to guarantee the proper operationof electrical substations by rapidly responding to real time events with ap-propriate actions and ensuring to maintain uninterrupted power services tothe end users. It resorts to the utilization of multi-functional equipments,called Intelligent Electronic Devices (IEDs). These devices incorporate oneor more processors with capability of receiving or sending data/control fromor to an external source (for example electronic multifunction meters, digitalrelays, controllers.). They also issue control commands in case of anomaliesto maintain the desired status of power grid (e.g. tripping circuit breakers).

Figure 2.2.1: Example of an Intelligent Electronic Device (IED)

From a logical point of view, SA systems comprise three levels [2]:

� Station level: The station controls and monitors the system exe-cuting basic tasks as local operation of the switchgears, acquisitionof switchgear information and power system measurements and han-dling of events and alarms. It contains a substation host, a substationHuman-Machine Interface (HMI) and the Gateway (GW) to the remoteNetwork Control Center (NCC).

� Bay level: This level contains all the control and protection units.

� Process level: Consists of sensors and actuators that behave as processinterfaces to switchgears. In fact, sensors acquire information by mak-

7


ing measurements of voltages, currents, etc. on the electrical devicesand actuators perform actions on them, as, for example, closing cir-cuits.

Figure 2.2.2: Logical scheme of the three levels of a Substation Automationsystem

All the implemented levels in substation automation are interconnectedby (serial) communication links. It is possible to distinguish a vertical com-munication between the levels and an horizontal communication within thelevel [3].

� Vertical communication: Data is exchanged from the station leveldown to bay level (commands of any kind from the operators place) orreverse (binary indications like breakers or isolators position, measure-ments from instrument transformers and other sensors, events, alarms).This vertical communication is based on client-server concept and usesservices for reporting, commands and �le transfer. The reporting ismainly used for the communication from the bay level devices to the

8


station level devices. Data like status information events and measure-ments are sent using the reporting. Commands are used to control dif-ferent object within the system. The transfer service is used to transferdisturbance recorder �les from e.g. a protection device to the stationcomputer.

Figure 2.2.3: Vertical communication in the substation automation system

� Horizontal communication: In this communication, time criticalinformation exchange may be done using copper wiring and auxiliaryrelays or using the serial communication. This concerns exchange ofinformation between bays (e.g. station interlocking) and exchange ofinformation between functions inside the bay.

9

2.3. IEC61850

Figure 2.2.4: Horizontal communication in the substation automation system

Multiple protocols exist for substation automation, which include manyproprietary protocols with custom communication links. Interoperation ofdevices from di¤erent vendors would be an advantage to users of substationautomation devices. For this reason, the need of a standard through whichvendors and users may have bene�ts has emerged. This can guarantee afast and convenient communication as well as lower costs for installation,con�guration and maintenance.

2.3 IEC61850

IEC61850 [4] is the International Electrotechnical Commission�s internationalstandard for substation automation systems (SAS). It is designed to supportthe communication of all functions being performed in the substation. Themain goal of this standard is to furnish interoperability of IEDs, i.e. to al-low IEDs from di¤erent manufacturers to operate on the same network orcommunication path sharing information and commands. As a consequence,it supports the free allocation of functions to devices (IEDs) and, therefore,

10

2.3. IEC61850

supports any kind system philosophy covering di¤erent approaches in func-tion integration, function distribution, and SA architecture. In this way, thestandard wants to be future-proof, coping with fast development in communi-cation technology in the slow evolving application domain of power systems.In the next paragraphs the architectures used for communication inside SA,the data modeling approach and the data exchange model will be introduce,and typical applications for control or time-critical information exchange willbe described.

2.3.1 Communication architectures

IEC61850 bases its communication architectures on Ethernet [5]. This is theprevalent communication technology used in distribution automation dueto its cost savings and good functionality and performance. The standardo¤ers much �exibility in the choice of topologies and cabling. It supports dif-ferent communication architectures as, for example, cascading ,ring or star,commonly implemented with Ethernet switches with priority tagging. Thechoice of the architecture depends on the application performed on the sub-station and it is dictated by speci�c performance requirements. For example,IEC61850 requires that samples from the di¤erent sources should be takenin synchronism.Synchronisation is presently implemented using a dedicatedsynchronisation network distributing a one pulse per second signal or usingspeci�c protocols as IEEE 1588 [or SNTP]. For time synchronization, jitter inthe switch is also important. For long sequences of switches like in ring con-�gurations, the delay per switch will contribute to the response time. Latencyrepresents another fundamental prerequisite, that may be lower to guaranteethe good working of the system. This is further discussed in Chapter 5.

11

2.3. IEC61850

Figure 2.3.1: Examples of Ethernet architectures in Substation Automation

2.3.2 Data modeling approach

IEC61850 speci�es methods of data transfer and process data of the serversin the substation automation system. For that purpose, the standard uses anobject-oriented approach with Logical Nodes (LN) as core objects. [6] A logi-cal node is a functional grouping of data and represents the smallest function,which may be implemented independently in devices. Functions performedby SA can be split in parts residing in di¤erent IEDs but communicatingwhich each other (distributed function) and with parts of other functions.Logical Nodes are grouped in Logical Devices. By enabling or disabling aLogical Device, it is possible to enable and disable the contained group ofLogical Nodes together. Logical Devices are implemented in physical devices(IEDs).

12

2.3. IEC61850

Figure 2.3.2: IEC61850 Class Model

2.3.3 Data exchange model

Data model and services of the standard are mapped to a mainstream com-munication stack that follows two approaches: a Client-Server commu-nication approach and a Real Time Communication approach. It isprovided by the Abstract Communication Service Interface (ACSI) [7] thatfurnishes a common set of some 40 communication services for data access,reporting, logging, control applications and related support. In a client-server communication, the client controls the data exchange. Therefore,client-server communication is very �exible in terms of data to be trans-mitted. Compared to a master-slave system, the client-server concept allowsthe implementation of multiple clients in the same system. Client-server com-munication relies on the ISO/OSI model and consists of MMS, TCP/IP andEthernet with priority tagging. Manifacturing Message Speci�cation (MMS)is an application level protocol that provides peer-to-peer communicationover a network. This protocol de�nes communication between controllers aswell as between the engineering station and the controller. By using a con-�rmed transmission layer, the client-server communication is very reliablebut relatively time consuming and so it is not suited for time critical datatransmission.

13

2.3. IEC61850

In real time communications data is instead sent spontaneously withoutbeing requested from a Master or Client. This approach is followed for appli-cations that require a time critical exchange of binary information betweenfunctions located within the same bay or in di¤erent bays.

Figure 2.3.3: The mapping of the IEC 61850 data model and services

The communication between the various devices is achieved by an appli-cation model that describes two di¤erent methods for application association.They are the two-party application association (TPAA) and multicast appli-cation association (MCAA) [8].A two-party application association type shall provide a bi-directional

connection-oriented information exchange. The application association shallbe reliable and the information �ow shall be controlled end to end. Reliablemeans that the connection on which the application association relies pro-vides measures to notify reasons for non-deliverance of information in duetime. End-to-end �ow control means that sources of information do not sendmore information than the destination can bu¤er.

14

2.3. IEC61850

A multicast application association type shall provide a unidirectionalinformation exchange. Multicast information exchange shall be providedbetween one source (publisher) and one or many destinations (subscriber).Unidirectional information exchange shall provide su¢ cient information forthe receivers to uniquely interpret the context in which the exchange shallbe processed. The subscriber shall be capable to detect loss and duplicationof information received. The receiver shall not notify the loss of informationto its user and shall discard duplicated information.

Figure 2.3.4: Two-Party-Application-Association

Figure 2.3.5: MultiCast-Application-Association

15

2.3. IEC61850

Later, typical applications in substations will be described. In particularit will be explained how to use IEC 61850 for SCADA application, timecritical information exchange and connection of the primary process. [10]For the last two applications the standard provides exchange services calledGeneric Object Oriented Substation Event (GOOSE) and Sampled MeasuredValues (SMV).

2.3.4 SCADA Application

Supervisory control and data acquisition (SCADA) is one of the basic tasksof a substation automation system. It is related to human operation of thenetwork. Acquisition of information from high voltage equipment, handlingof events and alarms are performed by a local or remote operator. The datacommunication for this application is directed vertically from higher to lowerlevels or reverse. For this vertical relationship IEC 61850 uses the client-server concept. The server is the process or bay level IED, which providesall data to the client at station or any remote level. The client is tipically acomputer representing the operator�s work place. This approach is well suitedfor SCADA information, that is not time critical. A typical application ofSCADA is creation of alarm lists and event lists. For that purpose, IEC 61850de�nes a report service that is not accessing individual data, but group ofdata called datasets. Reports are sent to clients when an event on the systemoccurs. An event causing a transmission may be a change of a binary value,the crossing of a prede�ned alarm limit or the expiration of a cycle time.

2.3.5 Generic Object Oriented Substation Event (GOOSE)

IEC 61850 introduces a speci�c information exchange service for time criticalexchange of binary information between functions located within the samebay or in di¤erent bays, called Generic Object Oriented Substation Event(GOOSE) [8]. It is based on the publisher-subscriber concept and it is usedfor fast transmission of substation events, such as commands, alarms andindications. Messages are sent as multicast messages over the communicationnetwork and they are uncon�rmed. The publisher has no way of knowingif the subscriber has received the message. Because of this, the publishermust continuously transmit messages to the LAN. The triggering event forGOOSE messages may be a change of a value, a crossing of a boundary, etc.The content of messages is de�ned with a dataset (similar like for the report

16

2.3. IEC61850

model). Typical applications that resort to the utilization of GOOSE serviceare exchange of information between devices inside the bay or between devicesplaced in di¤erent bays. An example of the �rst case is the communicationbetween a logical device that control the distance protection and a logicaldevice recloser that will send an open command to the breaker. Breakerfailure protection is an example of communication between bays.

Figure 2.3.6: Example of GOOSE transmission

2.3.6 Sampled Measured Values (SMV) transmission

IEC 61850 de�nes a service for the exchange of information between the sub-station automation system and the high voltage equipment. Information issampled measured values like voltage and current waveforms, position andopen/close controls. The exchange may be done using copper wiring or usingserial communication. SMV supports two transmission methods: a Multi-cast service (MSVC) over Ethernet and a Unicast (point-to-point) service(USVC) over serial links [9]. Messages are shared among IEDs and they areuncon�rmed. As for the GOOSE message, the content of the message isde�ned with a dataset. Losses of same samples are handled without prob-lems by the receiving functions, e.g. by a protection algorithm. The triggerevent of sending these values is a clock event. An important aspect while us-ing sampled values in a power system is the phase relationship between thedi¤erent measured signals, in particular between current and voltage. IEC61850 is using the concept of synchronized sampling. All units performingsampling are globally synchronized with the required accuracy. The samplesare taken all at the same time. A typical application for the process connec-tion is the information transfer between instrument transformers, protectiondevices and circuit breakers. This information transfer is time critical. It hasa direct impact on the response time of the protection function.

17

2.3. IEC61850

Figure 2.3.7: Process connection with serial communication for SMV trans-mission

18

Chapter 3

Wireless communications

3.1 Overview

With the expression �wireless communications�, we mean the kind of commu-nication where the information transmission is assigned to electromagneticwaves without requiring a physical link between who sends the informationand who receives it.Thanks to this fundamental feature, during the last years a great interest

towards this kind of technology raised and hence there was a fast develop-ment.Actually, there are several wireless communication technologies that are

often very di¤erent, indeed it is possible to notice that wireless communica-tion system requirements can be very di¤erent depending on the application.Furthermore, in order to solve the same issues, parallel development of

various technologies followed and opposed approaches has been witnessed.Every wireless system has to combat transmission and propagation e¤ects

that are substantially more hostile than for a wired system. Critical technicalbottlenecks in a wireless link are the capacity of the radio channel, its unreli-ability due to adverse time-varying, multipath propagation and interferencefrom other transmissions.The developments in digital microelectronics and signal processing pro-

vided methods to overcome the anomalies of the mobile channel, therebyaccelerating the growth of wireless communication.For example, while traditional techniques such as maximum ratio com-

bining have been developed to optimally receive signal in the presence of

19

3.1. OVERVIEW

Gaussian noise only, current systems are totally adaptives and exploit sev-eral diversity techniques.Wireless networks can spread over di¤erent dimension areas and, conse-

quently, they can have di¤erent kinds of implementations.Depending on its extension, we can divide the wireless networks into four

categories:

� Wireless Wide Area Networks (WWANs): they can cover one or morecountries

� Wireless Metropolitan Area Networks (WMANs): they can cover a city

� Wireless Local Area Networks (WLANs): they can cover a building ora campus

� Wireless Personal Area Networks (WPANs): they can cover small dis-tances, less then 50 meters

Figure 3.1.1: Wireless network scenarios

In the next sections, the main technologies that are used today to imple-ment a wireless network will be shown. In particular, we will focuse on thetechnologies that are used to realize WLANs ( IEEE 802.11) and WPANs (IEEE 802.15.4, IEEE 802.15.1).

20

3.2. IEEE 802.11 (WI-FI)

3.2 IEEE 802.11 (WI-FI)

At the moment, most WLANs are implemented by using the standard IEEE802.11. In this technology, unlicensed bands are used to transmit data. Theperformance of this kind of networks are comparable with those of a wirednetwork; indeed today a WLAN reaches bit rates up to 54 Mb/s. The WLANstandard operates in the 2.4 GHz and 5 GHz Industrial, Science and Medical(ISM) frequency bands. It is speci�ed by the IEEE 802.11 standard and itcomes in many di¤erent variations like IEEE 802.11a/b/g/n. The applica-tion of WLANs have been most visible in the consumer market where mostportable computers support at least one of the variations.The physical layer of IEEE 802.11 uses spread spectrum techniques so

that the transmissions are reliable and secure. In fact if the receiver doesn�tknow the spread spectrum parameters, it detects a signal which appearsas noise thus Spread-spectrum signals are highly resistant to narrowbandinterference. The process of re-collecting a spread signal spreads out theinterfering signal, causing it to recede into the background. By taking ad-vantage of this feature, transmissions can share a frequency band with manytypes of conventional transmissions with minimal interference and with lowprobability of interception.

Figure 3.2.1: Spread-Spectrum system behavior

21

3.2. IEEE 802.11 (WI-FI)

There are two spread spectrum techniques :

� FHSS (Frequency Hopping Spread Spectrum)[11]: is a method of trans-mitting radio signals by rapidly switching a carrier among many fre-quency channels, using a pseudorandom sequence known to both trans-mitter and receiver.

� DSSS (Direct Sequence Spread Spectrum) [11]: It phase-modulatesa sine wave pseudorandomly with a continuous string of pseudonoise(PN) code symbols called "chips", each of which has a much shorterduration than an information bit. That is, each information bit ismodulated by a sequence of much faster chips. Therefore, the chip rateis much higher than the information signal bit rate. The receiver mustknow a priori the sequence of chips produced by the transmitter.

To reach 11Mb/s in IEEE 802.11b a series of codes called ComplementarySequence is used. The CCK (Complementary Code Keying) modulationtransmits data in symbols of eight chips, where each chip is a complex QPSKbit-pair at a chip rate of 11Mchip/s[12].The standards IEEE 802.11a and 802.11g reach a bit rate of 54 Mb/s by

using an OFDM (Orthogonal-Frequency-Division-Multiplexing) modulation.In this modulation, the whole channel band is divided in sub-bands so

that the PSD (Power Spectral Density) is almost constant in each sub-bandand several numbers of orthogonal sub-carriers are used to carry data. In thisway the data is divided into parallel data streams, one for each sub-carrier,and each of them is modulated with conventional modulation schemes. It ispossible to schematize an OFDM scheme in this way (Figure 3.2.2):

22

3.2. IEEE 802.11 (WI-FI)

CONVENTIONALMODULATION 1

CONVENTIONALMODULATION N1

CONVENTIONALMODULATION N

CONVENTIONALDEMODULATION 1

CONVENTIONALDEMODULATION N1

CONVENTIONALDEMODULATION N

CHANNEL

Figure 3.2.2: Multi-carrier system scheme

The i-th analog signal, if modulated with a QAM (Quadrature AmplitudeModulation), can be written as:

Xi (t) =1X

n=�1

NXi=1

s (t� nTs) Re�(an � jbn) ej2�fit

(3.2.1)

If it is assumed:

fi = fc +i

T(3.2.2)

where T is the symbol time in each sub-carrier,it can be written:

Xi (t) =1X

n=�1s (t� nTs) Re

n(ai;n + jbi;n) e

j2� iT ej2�fc

o(3.2.3)

Then, the channel input signal can be written as:

23

3.2. IEEE 802.11 (WI-FI)

X (t) =N�1Xi=0

Xi (t) =

N�1Xi=0

1Xn=�1

s (y � nTs) Ren(ai;n + jbi;n) e

j2� iT ej2�fc

o=

1Xn=�1

s (y � nTs) Re(ej2�fc

N�1Xi=0

(ai;n + jbi;n) ej2� i

T

)(3.2.4)

It is possible to notice that the expressionPN�1

i=0 (ai;n + jbi;n) ej2� i

T is aInverse Fourier Transform and then an OFDMmodulation allows for e¢ cientmodulator and demodulator implementation using the FFT algorithm on thereceiver side, and inverse FFT on the sender side. Although the principlesand some of the bene�ts have been known since the 1960s, OFDM is pop-ular for wideband communications today by way of low-cost digital signalprocessing components that can e¢ ciently calculate the FFT.An OFDM transmitter can be schematized as shown in Figure 3.2.3.

Figure 3.2.3: Ideal OFDM transmitter

Where s[n] is a serial stream of binary digits.

An OFDM receiver can be schematized as shown in Figure 3.2.4:

24

3.2. IEEE 802.11 (WI-FI)

Figure 3.2.4: Ideal OFDM receiver

The main features of the physical layer of IEEE 802.11a/b/g standardsare summarized in Table 3.2.1.

802.11b 802.11a 802.11gStandard approval July 1999 July 1999 June 2003Maximum bit rate 11Mb/s 54Mb/s 54Mb/sModulation CCK OFDM OFDM and CCKFrequencies 2.4-2.497 GHz 5.15-5.875 GHz 2.4-2.497 GHz

Table 3.2.1: Main features of IEEE 802.11a/b/g physical layer

For what concerns the MAC layer, the IEEE 802.11 [13] includes twoaccess mechanisms: the Distributed Coordination Function (DCF) and thePoint Coordination Function (PCF).The DCF uses a CSMA/CA access scheme: for a station (STA) to trans-

mit, it should sense the medium to determine if another STA is transmitting;if the medium is idle, then the transmission may proceed; if the medium isbusy, then the STA should defer until the end of the current transmission.The STA performs a back-o¤ procedure after deferral in order to minimizecollisions; the back-o¤ procedure starts after the medium has been sensedidle for a DCF Interframe Space (DIFS). When a unicast frame is correctlyreceived by an STA, the latter transmits an acknowledgment frame after aShort Interframe Space (SIFS), which is shorter than DIFS in order to pre-vent deferring stations from interrupting ongoing frame exchange sequences.

25

3.3. IEEE 802.15.4

If the Point Coordination Function is used, then the AP alternates aContention-Free Period (CFP) with a Contention Period (CP). During theCP, the above DCF transfer rules apply. During the CFP, the AP pollsthe stations that are in the CF poll list, using an implementation-dependentalgorithm. When polled, a station transmits an MSDU. The AP may alsouse the CFP to transmit frames addressed to associated stations. The �rstframe of the CFP is transmitted by the AP at regular intervals (TargetBeacon Transmission Time, TBTT) after the medium has been sensed idlefor a duration greater than a PCF Inter-Frame Space (PIFS), which is chosensuch that SIFS < PIFS < DIFS. Even though it has been shown that thePCF performs better than the DCF with real-time tra¢ c, it is neverthelessunable to guarantee that associated stations will have a parameterized QoS.

3.3 IEEE 802.15.4

The IEEE 802.15.4 [14] standard speci�es the physical and MAC layers forlow-rate wireless personal Area Networks (PAN). It is the basis for the ZigBeespeci�cation, that addresses the network and application layers for automa-tion and remote control applications.

Figure 3.3.1: IEEE 802.15.4 and ZigBee protocol stack

26

3.3. IEEE 802.15.4

The 802.15.4/ZigBee protocol stack is simple and �exible; it does notrequire any infrastructure and it is suitable for short-range communications(typically within the range of 10 meters). For these reasons, it features easeof installation, low cost, and a reasonable battery life of the devices. It alsofeatures activation and deactivation of the radio transceiver and transmittingas well as receiving packets across the physical medium. It operates in oneof the following three license-free bands:

� 868�868.6 MHz (e.g., Europe) with a data rate of 20 kbps;

� 902�928 MHz (e.g., North America) with a data rate of 40 kbps;

� 2400�2483.5 MHz (worldwide) with a data rate of 250 kbps.

The standard o¤ers two PHY options based on the frequency band. Bothare based on direct sequence spread spectrum (DSSS). The main parametersof the physical layer can be found in theTable 3.1.1.

Frequency Spreading parameters Data parametersband Chip Modulation Bit Symbols

rate rate(MHz) (kchips/s) (kb/s)868-868.5 300 BPSK 20 Binary902-928 600 BPSK 40 Binary

2400-2483.5 2000 O-QPSK 250 16-ary Orthogonal

Table 3.3.1: Main parameters of 802.15.4 (ZigBee) physical layer

There is a single channel between 868 and 868.6MHz, 10 channels between902.0 and 928.0MHz, and 16 channels between 2.4 and 2.4835GHz as shownin Figure 3.3.2. Several channels in di¤erent frequency bands enable theability to relocate within spectrum.

27

3.3. IEEE 802.15.4

Figure 3.3.2: 802.15.4 channel allocation

The MAC layer [15] de�nes two types of nodes: Reduced Function De-vices (RFDs) and Full Function Devices (FFDs). RFD are meant to im-plement end-devices with reduced processing, memory, and communicationcapabilities which implement a reduced set of functions of the MAC layer.In particular they can only join to an existing network and they depend onFFDs for communication. One RFD can be associated to only one FFD at atime. Example of RFD devices can be simple sensors or actuators like lightswitches, lamps and similar devices.The FFDs implement the full MAC layer and they can act as the Per-

sonal Area Network (PAN) coordinator or as a generic coordinator of a set ofRFDs. The PAN coordinator sets up and manages the network, in particularit selects the PAN identi�er and manages associations or disassociations ofdevices. The MAC protocol has two channel access methods : with or with-out a superframe structures. The channel access with superframe structureis used in star topologies (or in some limited cases in peer to peer topologies)and enables synchronization between nodes. On the other hand, the channelaccess without superframe structure is more general and can be used to sup-port communications in arbitrary peer to peer topologies. The CFP period isoptional and it is used for low-latency applications or applications requiringspeci�c data bandwidth. To this purpose the PAN coordinator may assignportions of the active superframe (called Guaranteed Time Slots or GTS) tospeci�c applications.

28

3.4. IEEE 802.15.1 (BLUETOOTHTM )

Figure 3.3.3: 802.15.4 Superframe

The PAN coordinator may optionally avoid the use of the superframestructure (referred to as a non beacon-enabled PAN). In this case The PANcoordinator never sends beacons and communication happens on the basis ofthe unslotted CSMA-CA protocol. The coordinator is always on and ready toreceive data from an end-device, while data transfer in the opposite directionis poll-based: the end device periodically wakes up and polls the coordinatorfor pending messages. Then the coordinator sends these messages or signalsthat no end-device is available.

3.4 IEEE 802.15.1 (BluetoothTM )

The Bluetooth Special Interest Group (SIG) [16] is a trade association com-prised of leaders in the telecommunications, computing, automotive, indus-trial automation and network industries that has been driving the develop-ment of Bluetooth wireless technology, a low cost short-range wireless speci�-cation for connecting mobile devices and bringing them to the market. Thistechnology is designed to be built into and connect electronic applianceswithout adding signi�cant costs.Bluetooth devices operate at 2.4 GHz in the licence-free ISM band (Indus-

trial, Scienti�c and Medical) where many applications operate; this meansthat Bluetooth must be robust regarding outside interference. The used

29


band, between 2.4000 - 2.4835 GHz, is divided into 1-MHz spaced chan-nels. Each Bluetooth device uses a transmission technique based on TDMA(Time Division Multiple Access): every time slot lasts 625 microseconds anda packet can be constituted by a single slot, 3 consecutive slots or 5 consec-utive slots. In addition, since Bluetooth uses the full ISM band, FrequencyHopping Spread Spectrum (FHSS) [1] technique is used to solve transmissionproblems. After each packet is sent, the radio frequency hops to another ra-dio channel, in both communicating devices, using a pseudorandom hoppingsequence. Bluetooth is a low power technology and this implies that thedistance between the devices must be short. From this point of view, thereare three di¤erent radio classes, which correspond to a di¤erent operationranges (see Table 3.4.1 ) :

Power Maximum Minimum MaximumClass Output Power Output Power distance1 100 mW (20dBm) 1 mW (0dBm) 100 meters2 2.5mW (4dBm) 0.25 mW (-6dBm) 20 meters3 1 mW (0dBm) - 10 meters

Table 3.4.1: IEEE 802.15.1(Bluetooth) power classes

There is also a minimum range of 10 cm to be respected between com-municating devices, because some receivers may be saturated.The initial standard of Bluetooth (versions 1.0 and 1.1) is called �Basic

Rate�, and the modulation used is GFSK (Gaussian Frequency Shift Key-ing). For the new standard called �Medium Rate�or in the latest version�Enhanced Data Rate�, two new modulation types have been selected: �/4-DQPSK and 8-DPSK, which are able to increase the maximum bit rate upto 2 Mbps and 3 Mbps, respectively. The standard model for every com-munication system, relative on the communication protocol stacks, is thefamiliar OSI (Open System Interconnection). Bluetooth follows a particularcommunication system, quite similar to the model, but they are not exactlymatched. In the Figure 3.4.1, the Bluetooth protocol stack layers and thedi¤erences among the two models are shown.

30


Figure 3.4.1: Comparation between Bluetooth and ISO/OSI protocol stacklayers

The Physical Layer is responsible for the electrical speci�cation of thecommunication device, including modulation and channel coding. In theBluetooth system, this is covered by the radio and part of the baseband. Theradio is the lowest layer. Its interface speci�cation de�nes the characteristicsof the radio front end, frequency bands, channel arrangements, permissibletransmit power levels, and receiver sensitivity level.The next layer is the baseband, which carries out Bluetooth�s physical

(PHY) and media access control (MAC) processing. This includes taskssuch as device discovery, link formation, and synchronous and asynchronouscommunication with peer.On 28 March 2006, the Bluetooth Special Interest Group announced its

selection of the WiMedia Alliance Multi-Band Orthogonal Frequency Divi-sion Multiplexing (MB-OFDM) version of UWB for integration with currentBluetooth wireless technology.

31

Chapter 4

Ultra-Wideband Technology

4.1 Overview

The Ultra-Wideband technology (UWB) has progressively attracted the re-searchers� attention. This technology exists already from 1980 and it hasbeen used mainly for radar applications. In 1998, the Federal Communica-tion Commission (FCC) proposed UWB transmissions in Part 15 of FCCReport and Order that concerns rules and regulations, mainly regarding un-licensed transmissions. A signal has been de�ned from FCC as UWB whenits fractional bandwidth (that is de�ned as the ratio of the signal bandwidthto the center frequency) is greater than 0.25 or it occupies 1.5 GHz or moreof spectrum [1]. In February 2002, the FCC allocated 7500MHz spectrum[17], from 3.1 to 10.6GHz, for unlicensed UWB communication applicationsthus allowing the use and the commercialization of new products that usethis technology.The rules were established after careful investigation of interference issues

such as multiple UWB transmitters and coexistence with global positioningsatellite (GPS) receivers. In Figure 4.1.1 and Figure 4.1.2 the FCC masksfor the average PSD of an UWB signal are shown [18].

32

4.1. OVERVIEW

Figure 4.1.1: UWB Emission Limits for outdoor communication systems

Figure 4.1.2: UWB Emission Limits for indoor communication systems

In the Figure 4.1.3, Figure 4.1.4 and Figure 4.1.5 it is possible instead tosee the masks adopted in Europe, Asia and Singapore, compared with theFCC rules limits [17].

33

4.1. OVERVIEW

Figure 4.1.3: Comparation between FCC(USA) and ETSI(Europe) masks forUWB emission

Figure 4.1.4: Comparation between FCC(USA), MIC(Japan) andETRI(S.Korea) masks for UWB emission

34

4.1. OVERVIEW

Figure 4.1.5: Comparation between FCC(USA) and IDA(Singapore) masksfor UWB emission

The UWB technology can be used in many applications, for exampleimaging systems, vehicular radar systems, communication and measurementsystems. [19]The combination of a big bandwidth and a relatively low centre frequency

provide two main characteristics of this technology. First, an UWB systemcan o¤er a very high temporal resolution; this feature is essential for rangingapplications and to reduce the multipath e¤ects. Secondly, among all systemsthat have the same bandwidth, UWB systems work at lower frequencies andthey have a greater probability of propagation through the materials.In Figure 4.1.6 the relative power spectral densities and bandwidths of

various wireless technologies is shown.

35

4.1. OVERVIEW

Figure 4.1.6: Comparation between Narrow-band and UWB PSDs

The considerable bandwidth of UWB systems not only provides hightemporal resolution but also provides many degrees of freedom for the use ofthis technology. Indeed, in addition to localization systems, there are severalapplications that use the UWB technology for high speed and short rangecommunication (WPANs) [24].Currently there are several companies that are developing the UWB tech-

nology for many application, both military and business, and there are severalapproaches. The main approaches being:

� MB-OFDM (Multi-Band Orthogonal Frequency DivisionMul-tiplexing ) UWB [20]: with this modulation the spectrum of thesignal is divided in sub-bands and several numbers of orthogonal sub-carriers are used to carry data. In this way the data are divided intoparallel data streams, one for each sub-carrier, and each of them is mod-ulated with conventional modulation schemes. This technology is sus-tained by the WiMedia Alliance which principal members are Philips,Intel, Time Domain, Samsung, StMicroelectronics among others.

� DS (Direct Sequence) UWB: In this case the big bandwidth is dueto the use, in the time domain, of brief duration and low energy pulses(that are base-band pulses). It is di¤erent from the spread spectrum(SS) systems, where the big bandwidth is due to the use of spreadingcodes. This technology was sustained by Freescale, Siemens, Motorola,Aether Wire, PulseLink, Tektronix etc.

36

4.2. ECMA 368

After the regulations conducted by FCC In February 2002, the interestfor UWB has exploded, the investments and the standards suggestion hasincreased. In this period IEEE released the IEEE 802.15.3 Standard.On January 19, 2006 IEEE 802.15.3a task group withdrew and the two

above mentioned technologies contended for the market. The main goal ofthese technologies is to implement a WPAN with high data rate, in order toreplace wired connections, like USB and IEEE 1394 (FireWire), with wirelessconnections (cable replacement).Freescale, in order to implement a technology called �Free Cable USB�,

followed an approach based on DS-UWB. WiMedia Alliance,instead, to real-ize a technology called �Wireless USB (WUSB)�, chose an approach basedon MB-OFDM UWB.It seems that the WiMedia Allinace has the technical, market, and polit-

ical power to carry their UWB solution to a dominant de facto standard inthe market with or without IEEE approval.The ECMA R International with the standard ECMA 368 [21] standard-

ize the MAC layer and the PHY layer adopted by the WUSB.

4.2 ECMA 368

The ECMA Standard speci�es the ultra wideband (UWB) physical layer(PHY) and medium access control (MAC) sublayer for a high-speed shortrange wireless network, utilizing all or part of the spectrum between 3 100 �10 600 MHz supporting data rates of up to 480 Mb/s.In the following sections, the main features of the physical (PHY) layer

and the medium access control (MAC) layer of the ECMA 368 standard willbe showed.

4.2.1 PHY layer

The 368 ECMA Standard speci�es the ultra wideband (UWB) physical layer(PHY) for a wireless personal area network (PAN), utilizing the unlicensed3 100 - 10 600 MHz frequency band, supporting data rates of 53,3 Mb/s,80 Mb/s, 106,7 Mb/s, 160 Mb/s, 200 Mb/s, 320 Mb/s, 400 Mb/s, and 480Mb/s.The spectrum is divided into 14 bands, each with a bandwidth of 528

MHz. The �rst 12 bands are grouped into 4 band groups consisting of 3

37

4.2. ECMA 368

bands. The last two bands are grouped into a �fth band group.The relationship between centre frequency and band ID number is given

by the following equation:

fc(nb) = 2904 + 258� nb (MHz) nb = 1; :::; 14 (4.2.1)

This de�nition provides a unique numbering system for all channels thathave a spacing of 528 MHz and lie within the band 3 100 - 10 600 MHz asshowed in Figure 4.2.1.

Figure 4.2.1: ECMA368 Spectrum Allocation

All of the spectrum between 3 100 - 10 600 MHz can be used only inUSA. In other countries there are di¤erent rules and only a limited numberof bands can be used. In Figure 4.2.2 the frequency band assignment forWUSB [22] in some important regions is shown.

38

4.2. ECMA 368

Figure 4.2.2: ECMA368 frequency band assignement. Detect and avoid tech-niques mitigate interference potential by searching for broadband wireless sig-nals and then automatically switching the UWB device to another frequencyto prevent any con�ict.)

To carrie the information a MultiBand Orthogonal Frequency DivisionModulation (MBOFDM) scheme combining with band hopping technique inthe UWB frequency band is used.The band hopping is carried out within each band group according to

the prede�ned Time-Frequency Codes (TFC) and with the frequency of eachhop per OFDM symbol. In total 30 logical channels are provided by usingthe multiband solution and the band hopping scheme. The MB-OFDM PHYparameters can be found in Table 4.2.1.

39

4.2. ECMA 368

Parameter ValueNumber of data subcarriers 100Number of piltot carriers 12Number of guard carriers 10Number of total subcarriers 122Subcarrier frequency spacing 4.125 MHzSymbol interval (TSYM) 312ns

Table 4.2.1: MB-OFDM PHY parameters

Data information is modulated over each data subcarrier either by Quadra-ture Phase Shift Keying (QPSK) or by Dual-Carrier Modulation (DCM).Frequency-domain spreading, time-domain spreading, and forward error cor-rection (FEC) coding are used to vary the data rates and to enhance therobustness of the data information against the wireless fading channel. TheFEC used is a convolutional code with coding rates of 1/3, 1/2, 5/8 and 3/4.The supported data rates, modulation modes and coding rates in MB-

OFDM PHY are listed in Table 4.2.2.

Data Modulation Coding(R) Codedbits Infobitsrate rate per6 per6

OFDM OFDMsymbols symbols

(Mb/s) (R) (NCBP6S) (NIPB6S)53.3 QPSK 1/3 300 10080 QPSK 1/2 300 150106.7 QPSK 1/3 600 200160 QPSK 1/2 600 300200 QPSK 5/8 600 375320 DCM 1/2 1200 600400 DCM 5/8 1200 750480 DCM 3/4 1200 900

Table 4.2.2: Modulation modes and coding rates in MB-OFDM PHY

The coded data is spread using a time-frequency code (TFC). The EcmaStandard speci�es two types of time-frequency codes (TFCs): one wherethe coded information is interleaved over three bands, referred to as Time-

40

4.2. ECMA 368

Frequency Interleaving (TFI); and, one where the coded information is trans-mitted on a single band, referred to as Fixed Frequency Interleaving (FFI).The standard MB-OFDMPhysical Layer Convergence Procedure (PLCP)

frame consists of a PLCP pre-amble, a PLCP header and a Frame Payload,as depicted in Figure 4.2.3. Both PHY and MAC layer headers are includedin the PLCP header, which has a �xed size of 25B and is always transmittedat the data rate of 39.4Mb/s. The PLCP frame can support the payload sizeup to 4095 bytes excluding the Frame Check Sequence (FCS) and Tail bits.In Figure 4.2.3 the standard PLCP Protocol Data Unit (PPDU) structure isshown.

Figure 4.2.3: Standard PPDU structure

It is possible to notice that the PPDU is composed of three major com-

41

4.2. ECMA 368

ponents: the PLCP preamble, the PLCP header, and the PHY Service DataUnit (PSDU).

� PLCP preamble: Aids the receiver in timing synchronization, carrier-o¤set recovery, and channel estimation. Two kinds of PLCP preamblesare de�ned in the MB-OFDM PHY, namely standard preamble andburst preamble, the latter for improving the burst transmission e¢ -ciency with high data rates. The burst preamble is shorter in lengththan the standard preamble. In a burst transmission, the �rst packetshall use the Standard PLCP preamble, while the remaining packetsmay use either the Standard PLCP or the burst PLCP preamble, aslong as the transmission data rate is higher than 200Mb/s. The stan-dard preamble is used by the �rst frame in a burst, all frames in anon-burst transmission and all frames transmitted with a data ratelower than or equals to 200Mb/s.

� PLCP header: the goal of this component is to convey necessary infor-mation about both the PHY and the MAC to aid in decoding of thePSDU at the receiver.

� PSDU: it is the last major component of the PPDU. This componentis formed by concatenating the frame payload with the frame check se-quence (FCS), tail bits, and �nally pad bits, which are inserted in orderto align the data stream on the boundary of the symbol interleaver.

It is possible to calculate the duration of a PPDU by the following formula:

Tpacket = (Nsync +Nhdr +Nframe)� TSYM (4.2.2)

where

Nsync = 30 is the number of symbols in a PLCP standard preamble

Nhdr = 12 is number of symbols in the PLCP header

Nframe = 6�l8�LENGTH+38

NIBP6S

mis the duration of the PSDU

TSYM = 312ns is the symbol interval

In theNframe expression d�e corresponds to the ceiling function, LENGTHis the payload length and NIBP6S is the number of info bits for 6 OFDM sym-bol depending on Data Rate.

42

4.2. ECMA 368

4.2.2 MAC layer

Di¤erent from the conventional centralized WPAN MACs, e.g. Bluetoothand IEEE 802.15.3 MACs, the MBOA MAC is based on a synchronized andtotally distributed solution. A distributed beaconing scheme is used for timesynchronization, network topology control and channel access coordination.No device acts as a central coordinator.In order to support Quality of Service (QoS) for both isochronous and

asynchronous tra¢ cs, MBOA MAC speci�es the reservation based Distrib-uted Reservation Protocol (DRP) and the contention based Prioritized Chan-nel Access (PCA) as medium access methods.The channel time resource is organized into �x-length superframes, which

comprise 256 Medium Access Slots (MASs). Each MAS lasts for 256�s. Atthe beginning of each superframe a Beacon Period (BP) consisting of n MASsis allocated for all devices to exchange beacons, as shown in Figure 4.2.4.

Figure 4.2.4: ECMA368 Superframe structure

After the BP there is the Data Period (DP) during which the data areexchanged among the devices. The DP can be divided into two intervals, onein which the nodes reserve MAS during the BP and one which is contention-based and where any node is allowed to initiate a transmission using thePCA protocol. This feature is very important and allows creating a systemcon�guration can handle both periodical high priority data in combinationwith lower priority non-periodic data.

43

4.2. ECMA 368

Beacon Period (BP)

Each superframe starts with a BP during which every device have to transmitits beacon and to listen to the remaining beacon slots sent by others devicesbelonging to network. An example of BP can be seen in Figure 4.2.5.

Figure 4.2.5: Example of a Beacon Period (BP)

The maximum length of the BP is de�ned as mMaxBPLength which isa multiple of MASs. Each MAS in the BP consists of three Beacon slots.During the BP devices sequentially broadcast Beacons at the base rate (i.e.53.3Mb/s). Each Beacon shall not exceed a length of mMaxBeaconLengthwhich is equal to

mBeaconSlotLength� SIFS �mGuardT ime (4.2.3)

mBeaconSlotLength is one third of a MAS, i.e. 85�s. The mGuardTime is12�s. Hence, a Beacon lasts at most 63�s. With every received beacon adevice learns about its direct neighborhood.Once a device is powered up it scans for an empty beacon slot during

three superframes. Then it announces its presence in a randomly chosenBeacon slot in between the highest-numbered Beacon slot and the end ofthe BP. If all Beacon slots are occupied, a device proceeds to send duringthe Signal Beacon Period and prolongs the BP by adding its Beacon to thesucceeding MAS of the BP of the next superframe.

44

4.2. ECMA 368

Figure 4.2.6: Typical scenario of joining devices

The total BP length is variable and depends on the system overall layout.It is not allowed to use more than 96 beacon slots( 32 MAS which correspondsto 8.129ms) in each superframe.

Data Period (BP)

The management of this part of the superframe, used to send data, is madeby using a contention based method (PCA) combining with a contention freemethod (DRP) based on TDMA.

Prioritized Contention Access (PCA) The PCA is very similar to theEnhanced Distributed Channel Access (EDCA) [23]. It is a contention-basedCarrier Sense Multiple Access/Collision Avoidance (CSMA/CA) scheme re-lying on a prioritized backo¤ procedure. Virtual stations of di¤erent priorityinside every device compete for the channel access.The standard ECMA 368 allows the followings Access Categories (AC),

each of them has a di¤erent priority:

� Background (AC_BK)

� Best E¤ort (AC_BE)

� Video (AC_VI)

45

4.2. ECMA 368

� Voice (AC_VO)

The main parameters with which the priorities are managed are summa-rized in Table 4.2.3.

Priority AC CW CW TXOP AIFSNmin max limit

1 AC_BK 15 1023 1 frame 72 AC_BK 15 1023 1 frame 70 AC_BE 15 1023 1 frame 43 AC_BE 15 1023 1 frame 44 AC_VI 7 511 1024�s 25 AC_VI 7 511 1024�s 26 AC_VO 3 255 256�s 17 AC_VO 3 255 256�s 1

Table 4.2.3: PCA QoS Settings

Prior to every transmission attempt a device has to sense the channelas idle for a static period called Arbitration Interframe Space (AIFS). TheAIFS is calculated by using the following expression.

AIFS[AC] = pSIFS +mAIFSN [AC]� pSlotT ime (4.2.4)

Afterwards, it has to keep on sensing the channel for multiples of a Slot-Time. For the current MBOA PHY the SlotTime is equal to 8 �s. Theamount of SlotTimes is a random number drawn from a uniformly distrib-uted interval of (0, CW). The initial value of CW is CWmin. The durationof AIFS and CWmin depend on the priority of the backo¤. Whenever thedevice senses the channel as idle it decreases its slot counter by one. If theslot counter reaches zero the device may transmit a data packet. If the devicesenses the channel as busy, it freezes its slot counter. After the channel issensed as idle for an AIFS period again, the backo¤procedure starts countingdown the remaining slots. With every failed transmission a device doubles itCW to reduce the probability of a collision with other devices [24].As an example, a PCA based channel access sequence is depicted in Figure

4.2.7.

46

4.2. ECMA 368

Figure 4.2.7: PCA protocol behaviour

Distributed Reservation Protocol (DRP) The Distributed Reserva-tion Protocol (DRP) provides a collision free channel access. It announcesfuture transmissions and thus allows devices to coordinate their channel ac-cess.Through beaconing, devices sharing the same BP can learn the MAS

occupation status and make their own reservation. The reservation is an-nounced by the owner device in its beacon and identi�ed with the start MASnumber and the duration in unit of MASs.There are several levels on which the reservation can be made according

to ECMA 368 standard, namely:

� Alien BP: prevents transmission during MAS occupied by an alien BP

� Hard: provides exclusive access to the medium for the reservation ownerand target. If there is any remaining time during a hard reservation,a frame exchange of Unused DRP Announcement (UDA) and UnusedDRP Response (UDR) allows other devices to reuse the residual reser-vation time by using PCA

� Soft: permits PCA, but the reservation owner has preferential access,indeed Only the owner of a reservation can access the medium withthe highest priority AIFS and without any perform backo¤. All otherdevices have to wait for an additional random time after AIFS accordingto PCA.

47

4.2. ECMA 368

� Private: provides exclusive access to the medium for the reservationowner and target. The access method and the data exchange techniqueare not de�ned by the standard. Unused time should be released forPCA

Using the DRP with an hard reservation, MBOA can support isochronousreal-time tra¢ c while the soft DRP can be useful when the owner of thereservation does not fully use the reserved MASs, other devices can still usethe unused MASs in the PCA mode [25].

Acknowledgment Policies

The ECMA 368 standard de�nes three acknowledgement policies: no ac-knowledgement (No-ACK), immediate acknowledgement (Imm-ACK) andblock acknowledgement (B-ACK).

� No-ACK: A frame with ACK policy set to No-ACK shall not be ac-knowledged by the recipient. The transmitting device MAC sublayerassumes the frame has been successfully transmitted and proceeds tothe next frame upon completion of current frame. All broadcast andmulticast frames shall have ACK Policy set to No-ACK.

� Imm-ACK: Each successfully receivedMac Protocol Data Unit (MPDU)is acknowledged after a Short Interframe Space (SIFS) period by thereceiver. The SIFS period is needed for transceiver (TRX) turnaroundand frame checking. It is used in between every frame exchange.

� Block-ACK: With No-ACK policy no ACK is generated at all. BurstACK policy increases the e¢ ciency, since a group of MPDUs is ac-knowledged with a single frame by the receiver.

48

Chapter 5

IEC61850 on ECMA 368

The IEC61850 standard for Substation Automation resort to the utilizationof the Ethernet technology for intercommunication, as mentioned in chapter2. Many applications have requirements regarding time synchronization, lowlevels of latency and jitter, which switched Ethernet is able to meet. Formedium voltage applications optical Ethernet is used to ensure galvanic iso-lation between di¤erent units in an installation. However, this usage is bothexpensive and fragile and good alternatives are now in course of study. Oneof them is the usage of a waveguide system for RF-waves in order to reducecosts and installation e¤ort. The basic idea is to use rectangular tubes totransmit RF signals. In this way, RF-waves are guided and basically freeof disturbances. The choice of a wireless technology that can ful�l the re-quirements of IEC61850 applications becomes fundamental. An emergingtechnology that seems to have good performance is the MB-OFDM UWBbased on standards ECMA368-369. It is having a rapid development andit is exciting interest of electronics technology majors as Intel, Samsung,Philips, etc. The high bandwidth (up to 480 Mb/s) permits the use of UWBin many applications, from multimedia to military communications.

5.1 ECMA368 for Sampled Measured Values

transmission

In chapter 2 the Sampled Measured Values service has been described. Itis a method for transmitting sampled measurements from transducers. For

49

5.1. ECMA368 FOR SAMPLED MEASURED VALUES TRANSMISSION

medium voltage applications each device typically produces a sample of 256bytes every 250 �s:The main requirements on this kind of communicationregard time-synchronization, latency, bandwidth and sampling interval. Inparticular they are:

� Time-synchronization of <25 �s

� Latency of <3 ms

� Bandwidth >10 Mbit/s

� Sampling interval >4kHz (allowed to bundle data as long as latencyrequirements are met)

Transmission of SMV data might be performed using the DistributedReservation Protocol (DRP), where nodes have access to the medium in re-served slots, or the Prioritized contention access (PCA), that provides di¤er-entiated, distributed contention access to the medium. In SMV applicationsthe higher layers don�t allow acknowledgement policies but ECMA 368 stan-dard o¤ers di¤erent ack methods at the MAC layer. Later on, main aspectsof the two protocols and some theoretical considerations about their use inorder to satisfy application requirements will be described.

5.1.1 Time Synchronization

ECMA368 standard o¤ers support for higher-layer time synchronization, thatis a fundamental requirement for SMV information in IEC61850 applications.In fact, the phase relationship between the di¤erent measured signals is fun-damental and samples must be acquired at the same time. The standardde�nes an optional MAC facility that enables layers above the MAC sublayer to accurately synchronize timers located in di¤erent devices. First, alow-level synchronization can be achieved in order to ful�ll the standard.The accuracy is de�ned as the maximum drift between synchronizations.

In the ECMA368 case it is function of the clock accuracy and the time elapsed(SynchronizationInterval) since a synchronization event. The maximum driftis calculated using the worst case value for clock accuracy and the longestSynchronizationInterval:

50


MaxDrift = mClockAccuracy (ppm) � 1E-6 � SynchronizationInterval(5.1.1)

where

SynchronizationInterval = (mMaxLostBeacons + 1) �mSuperframeLengthmClockAccuracy = 20 ppmmMaxLostBeacons = 3

mSuperframeLength = 65536 �s(5.1.2)

This guarantees a MaxDrift =10,5 �s:This accuracy could not be the same at the higher-levels. Jitter in the

system (up to 3 ms) must be considered. A solution that can achieve highaccuracy synchronization is the use of a proxy device that synchronizes itstime with a global clock on the outside network using a SNTP or IEEE 1588protocol. The application in the proxy periodically transmits packets to othernodes in the network containing time information. It keeps track of the o¤setbetween a reserved MAS used to transmit the clock-synchronization and theglobal clock. The applications in the nodes can use information received byproxy to adjust its local clock to concur with the global clock. This strategymake it possible to get an o¤set <11 �s between the clock in the proxy andthe clocks in the other nodes.

5.1.2 SMV on Distributed Reservation Protocol (DRP)

SMV data can be transmitted by nodes in the network during the dataperiod in the ECMA 368 superframe. After the reservation accorded inthe Beacon period, each node can have access to the medium in reservedslots. The standard describes several levels on which the reservation canbe made, as mentioned in chapter 4. Hard reservation is suitable for nodeswhich need to transmit every time, while Soft reservation is useful for nodesthat need to reserve periodical access to the medium to be used if an eventoccurs or a value changes. In particular, for SMV data, hard reservationhas been considered in this work. In order to ful�l latency requirements thenumber of nodes in the network has a big impact. A typical con�guration

51


in substation application consists in n nodes in the segment; n� 1 SMVandSCADA-producing nodes and one proxy device which communicates withthe outside network. The choice of an appropriate number of transmittingnodes has e¤ect on the Beacon Period length. Each BP has a duration ofa number of beacon slots (85�s) which is always a multiple of three. Sinceeach node needs a beacon slot for transmitting their information, the lengthof BP is depending of the number of nodes. In addition, two signaling slotshave to be considered. It is not allowed to use more than 96 beacon slots(32 MAS which corresponds to 8,129ms) in each superframe. Data periodcon�guration is also necessary in order to meet latency requirements. Asuitable con�guration is:

� Hard reservation for n�1 consecutive MAS (one for each SMV-producingnode)

� Leaves one MAS for PCA every n � 1 MAS, which can be used forSCADA tra¢ c

Since each MAS has a duration of 256 �s and considering one MAS foreach data-producing node, latency is function of the number of devices. Eachnode is enabled to transmit periodical data every NumberOfNodes�256�sexcept for once every superframe when the actual time between 2 consecutivetransmission opportunities will increase due to the beacon period.

5.1.3 SMV on Prioritized Contention Access (PCA)

Transmission of SMV data can be performed using the contention-based pro-tocol furnished by ECMA 368 standard. Unlike DRP, nodes don�t transmitdata in reserved MAS, but they have access to the medium depending ontheir priority and following the algorithm described in chapter 4. Once anode has obtained access, it has a transmission opportunity (TXOP) , aninterval of time to initiate transmission onto the medium. Sampled valuesrepresent information with a high priority sent periodically. It is possible tocreate a system con�guration which can handle both SMV data in combina-tion with lower priority non-periodic data. They are supervisory control anddata acquisition (SCADA) information, that will be presented in the nextparagraph.

52

5.2. ECMA368 FOR SCADA TRANSMISSION

5.2 ECMA368 for SCADA transmission

SCADA information is not time-critical data that comprise local and remoteoperation of high voltage equipment. A typical con�guration provides fortransmission of 2 packets of 183 bytes every second on average. Since con-trol data is fundamental for a substation automation system, the presenceof an acknowledgement policy is requested. The station controller receivesdata packets from equipment in the bay and process level and resend ackpackets of 53 bytes with an average delay of 15ms. This is performed at theapplication layer. By using the ECMA368 standard for SCADA transmis-sion, it is also possible to consider ack policies at the MAC layer, distinguingbetween an Immediate ACK and a burst ACK (see chapter 4). PCA proto-col seems to be well suggested for this kind of transmission. In particular,two system con�gurations can be studied. SCADA packets might be trans-mitted in a superframe structure, leaving a MAS for PCA after a certainnumber of reserved MAS for SMV transmission or by using only PCA. Inthis case SCADA information compete for the access to the medium withperiodical high priority data and they are supplied with a low priority. Bothcon�gurations will be examined in chapter 7, where performances have beeninvestigated.

53

Chapter 6

Simulation enviroment andsetups

6.1 Simulation Enviroment Overview: TrueTime

6.1.1 Tool Introduction

In this chapter the simulation software environment, TrueTime, used duringthe thesis work will be introduced. It has been modi�ed and extended inorder to make it capable of emulating the behavior of a UWB network.TrueTime [26] is a Matlab/Simulink-based simulator for real-time con-

trol systems, developed at Lund University, in Sweden, since 1999. It facil-ities co-simulation of controller task execution in real-time kernels, networktransmissions and continuous plant dynamics by providing a Simulink blocklibrary and a collection of MATLAB MEX �les. The application is writ-ten in Matlab code or in C++ and execution/transmission times must bespeci�ed by the developer. This approach makes TrueTime a very �exibleco-simulation tool. Hereafter the fundamentals Simulink blocks furnished bythe software will be brie�y described.

The TrueTime Kernel Block

The TrueTime Kernel block permits to emulate the behavior of a computernode with a generic real-time kernel, A/D and D/A converters, and network

54

6.1. SIMULATION ENVIROMENT OVERVIEW: TRUETIME

interfaces. It can be connected with ordinary Simulink blocks but it needsto be initialized before a simulation can be run. During a simulation, thekernel repeatedly calls tasks and interrupt handlers.

The TrueTime Wired Network block

The Wired Network block permits to simulate the physical-layer and themedium access layer of various local-area networks. It supports six simplemodes of networks: CSMA/CD (e.g. Ethernet), CSMA/AMP (e.g CAN),Round Robin (e.g. Token Bus), FDMA, TDMA and Switched Ethernet.Higher protocols such as TCP/IP are not simulated (but may be implementedas applications in the nodes).

The TrueTime Wireless Network block

The Wireless Network block works in the same way as the wired one andpermits to simulate communication between nodes taking also into accountthe path-loss of the radio signal. It is necessary to specify the true locationof the nodes with x and y inputs. Only two network protocols were sup-ported originally: IEEE 802.11b/g (WLAN) and IEEE 802.15.4 (WPAN).WirelessHART has been added in a recent work at ABB Corporate Researchwhile adding ECMA368 standard has been the purpose of our work.The radio model used includes support for:

� Ad-hoc wireless networks.

� Isotropic antenna.

� Inability to send and receive messages at the same time.

� Path loss of radio signals modeled as 1/d^a where d is the distancein meters and a is a parameter chosen to model the environment.

� Interference from other terminals.

6.1.2 Tool Extension

During this thesis work TrueTime has been modi�ed in order to emulateSMV and SCADA transmissions over a network based on ECMA368 stan-dard. Two di¤erent approaches have been followed to model the two medium

55

6.2. SIMULATION SETUPS

access methods de�ned by the standard (DRP and PCA). In both cases thenew functionalities of the simulator have been implemented utilizing the co-operation between C++ functions and speci�c TrueTime tasks (describedby m-functions). The main di¤erence is that in the DRP case each deviceperform its own MAC protocol, calling a C++ function, instead, in the PCAcase a centralized function manages the MAC protocol of all nodes. In addi-tion, the graphic interface has been modifed in order to simplify the settingup of parameters needed for simulations.

6.2 Simulation Setups

This chapter will explain how the TrueTime simulator has been used inorder to evaluate the behavior of a UWB network in an electrical substationapplication. Di¤erent test cases have been analyzed taking into accountspeci�c ABB requirements.The con�guration adopted for simulations is formed by n nodes of which

n�1 represent the network communication interfaces of IEDs and one is theproxy device that communicates with the outside network. An example of aTrueTime con�guration with 4 data-producing nodes and a proxy is shownin Figure 6.2.1.

Figure 6.2.1: TrueTime blocks for ECMA368 simulations

56


The purpose of simulations has been to �nd the optimal con�gurationby taking into account signi�cant parameters. These paramters have beenvaried in order to evaluate the impact of the network performance. Thefollowing main parameters have been considered:

� Number of nodes: Increasing the number of nodes implies thatthe tra¢ c over the network grows. It is interesting to evalutate howthis a¤ects the latency. Further, it is possible to notice that whena contention-based channel access protocol is used, by increasing thenumber of nodes the troughput and hence the e¢ cency decrease. Con-�gurations of 5,7,9,11,13,15,17 nodes have been used during the simu-lation.

� Number of SMV samples per MAC frame: It has been assumedthat SMV samples are bu¤erized and then aggregated in MAC frameswithin the device�s MAC layer. In particular, two di¤erent approacheshave been followed, depending on the channel access method used, DRPor PCA. In the DRP case, it has been assumed a cooperation betweenthe higher layers and the MAC layer, so that at the beginning of itsreserved MAS, a node transmits all the samples bu¤erized as from itslast transmission. In this way the number of SMV samples per MACframe must be variable. With this assumption the most of the latencyis introduced by the bu¤er and it is calculated as (assuming a samplerate of 4KHz):

V ariableBufferlatency = V ariableNbrofsamples� 250�s (6.2.1)

The latency introduced by the network is less then 250�s: Figure 6.2.2explains how it happens.

57


Figure 6.2.2: SMV on DRP with variable number of SMV samples per MACframe

� If the number of SMV samples per MAC frame is static, in the same waythe latency introduced by the bu¤er has a �xed value, as the following:

FixedBufferLatency = FixedNbrOfSamples� 250�s (6.2.2)

In this case the device wants to periodically send a MAC frame witha period of FixedBufferLatency: Since it is not possibile to havea synchronism between the superframe and this period, the latencyintroduced by the network reaches high values. This is explained byFigure 6.2.3.

58


Figure 6.2.3: SMV on DRP with �xed number of SMV samples per MACframe

In the PCA case, the number of SMV samples per MAC frame hasbeen assumed �xed. This parameter has a big impact on the networkperformance because increasing it the throughput grows. On the otherhand, increasing the number of bu¤erized samples, the MAC frameduration becomes longer and it will a¤ect latency.

Figure 6.2.4 [24] shows the relationship between the theoretical maxi-mum throughput and packet size for MBOA MAC with PHY mode of480Mb/s.

59


Figure 6.2.4: TMT of MBOA DRP and PCA

For con�gurations utilized in simulations each IED produces both SMVand SCADA data according to the following parameters:

Size(Byte) Rate(Hertz) Load(Mb/s)SMV 256B 4KHz SMVload � 8Mb=sSCADA 183B 2Hz SCADAload � 3Kb=s

Table 6.2.1: Parameters for SMV and SCADA data

The tra¢ c produced by each IED is:

IEDload = SMVload + SCADAload � 8Mb=s (6.2.3)

60


Thus, the throughput of the network (NT ) must ful�l the followingexpression:

NT > Nbr_of_IEDs� IEDload (6.2.4)

The proxy device transmits only ACK messages and its tra¢ c has notbeen taken into account. By resting on these considerations, the choiceof the number of SMV samples per MAC frame sets out a lower bound-ary for the number of nodes. Its impact on the latency will be shownwith simulation results.

� ACK policy: The choice of an ACK policy according to the stan-dard features in�uences in the same way the network performance. Insimulations, a No-ACK policy and a Imm-ACK policy have been con-sidered.

� Priority: In the ECMA 368 standard the PCA protocol allows a prior-ity management. In simulations the highest priority (7) and the lowestpriority (0) have been used for SMV data and for SCADA data respec-tively.

61

Chapter 7

Simulation Results

This chapter shows and analyzes the results obtained from simulations. Inparticular, the investigation of network performance has be done respect tolatency. Several test cases have been evaluated, according to con�gurationsdescribed in the last chapter.

7.1 SMV and SCADA data transmission in asuperframe structure

For the transmission of SMV data using DRP protocol, a good superframecon�guration is:

� hard reservation for n-1 consecutive MAS (one for each SMV-producingnode).

� 1 MAS for PCA transmission every n MAS used for SCADA transmis-sion.

where n is the number of nodes. With this con�guration it is possibleto theoretically calculate the maximum latency for SMV data. If the beconperiod occurs between two consecutive transmission opportunities for thesame device, the latency is calculated as following:

max_latency =��n+ 2

3

�+ n

�� 256�s (7.1.1)

62

7.1. SMV AND SCADA DATA TRANSMISSION IN A SUPERFRAMESTRUCTURE

where d�e is the ceiling function. Figure 7.1.1 shows how the theoreticalmaximum latency agrees with the simulation results. In addition the averagelatency trend andthe trend of probability that the latency is more than 3 ms are shown.

Figure 7.1.1: Latency trends for SMV data using DRP

The ACK policy choice doesn�t a¤ect network performance. In fact, us-ing the hard reservation, transmission and acknowledge happen in the sameMAS. In Figure 7.1.2 latency plots for 9 nodes are shown.

63

7.1. SMV AND SCADA DATA TRANSMISSION IN A SUPERFRAMESTRUCTURE

Figure 7.1.2: Latency plots for a 9 nodes con�guration

64

7.2. SMV AND SCADA DATA TRANSMISSION IN ACONTENTION-BASED STRUCTURE

Figure 7.1.3 shows the average latency trend for SCADA packets.

Figure 7.1.3: Average latency trends for SCADA packets

It is possible to notice that latency varies with the priority and the ACKpolicy. However, latency is not a critical parameter for SCADA data.

7.2 SMV and SCADA data transmission in acontention-based structure

Network performance has been investigated using PCA protocol for bothSMV and SCADA tra¢ c. Di¤erent requirements for the two types of data

65


have been managed with the use of dixoerent priorities. In particular, thehighest priority for SMV data and the lowest priority for SCADA data havebeen assumed. In the contention-based structure it was not possible to cal-culate an analytical value for maximum latency because of the stochasticbehaviour of the system. Unlike the superframe structure, the choice of anACK policy appreciably a¤ects performance. Figure 7.2.1 shows the latencytrends when varying ACK policies.

Figure 7.2.1: Latency trends for SMV packets using PCA

Using an imm-ACK policy for SMV data, the latency appreciably in-creases. Instead, the use of an imm-ACK policy for SCADA packets doesn�treduce network performance and it has therefore been assumed in next analy-sis. Figure 7.2.2 shows the maximum latency trends for SMV data whenvarying parameters described in the previous chapter.

66


Figure 7.2.2: Maximum latency trends for SMV packets

67


Figure 7.2.3 shows, instead, the average latency trends.

Figure 7.2.3: Average latency trends for SMV packets

In terms of maximum latency, best performance is obtained using DRP forSMV transmission. With regard to the average latency, the best performanceis obtained using PCA with no-ACK policy and aggregating 8 SMV samplesper MAC frame. In fact, a contention-based method achieves a more e¢ cientchannel utilization compared to a TDMA method. Further, the networkcapacity increases with increasing the MAC frame size [24], therefore, thechoice of a small number of SMV samples per MAC frame imposes an upperbound on the number of nodes that can belong to the network. In SMVtransmission case, the system reaches the saturation when the average latencyapproaches the value of the period with which devices want to send its data.

68


Next �gure (7.2.4) depicts the probability that a SMV sample has a latencymore than 3ms.

Figure 7.2.4: Probability of latency > 3 ms

69


The last �gure (7.2.5) shows the average latency trends for SCADA tra¢ cfor di¤erent con�gurations analyzed.

Figure 7.2.5: Average latency trends for SCADA packets

70

Chapter 8

Conclusions

The main aim of this work has been to evaluate performance of a networkconsisting of several UWB nodes communicating data according to a spe-ci�c scheme (de�ned by an ABB switchgear application), in a simulationsoftware environment. TrueTime, a Matlab/Simulink-based simulator hasbeen extended to comply with the ECMA368 standard. The latter has beenadopted byWiMedia Alliance for the development of Wireless USB technol-ogy and seems to be able to satisfy the IEC61850 requirements. In particu-lar, the simulator has been conceived to emulate transmission of SMV (hardreal-time) and SCADA (best-e¤ort) data over the network utilizing the twochannel access methods described by ECMA368, DRP and PCA. Two dif-ferent approaches have been followed in order to implement the protocols.For DRP, a C++ function that perform the MAC protocol for each nodehas been implemented, instead, for PCA, a centralized function performs theprotocol for all nodes. Speci�c TrueTime tasks have been used in coopera-tion with these functions to emulate transmission of SMV and SCADA datain the same link. In this way, it has been possible to analyze the latencyintroduced by the transmission and compare it with ABB requirements. Alltransmissions have been assumed in an ideal channel and using a data rateof 480Mb/s, allowed by ECMA368. Analyzing results obtained from simula-tions it has been noticed that best performance in terms of maximum latencyare guaranteed using DRP for SMV tra¢ c and leaving periodically a slot forSCADA transmission performed with PCA protocol. The utilization of PCAfor both SMV and SCADA data, assuming the highest priority for the �rstand the lowest priority for the second, gives the best performance in terms

71

of average latency. This is more evident when the number of SMV sam-ples aggregated in a MAC frame is increased. In fact, for contention-basedmethods, the network throughput increases when increasing the MAC framesize. Utilizing PCA for SMV transmission a performance degradation hasbeen noticed when a imm-ACK policy is used. Future work of this projectmight be the development of a channel model studying the propagation ofUWB signals inside a waveguide. The ECMA368 simulator can be extendedtaking into account this channel model. Further, it could be interesting toinvestigate network performance varying bit rates as allowed by ECMA368standard.

72

Appendix A

Simulation Results Plots

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Figure A.1: Latency spread for SMV tra¢ c using DRP

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Figure A.2: Accumulated latency for SMV tra¢ c using DRP

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Figure A.3: Latency spread and accumulated latency for SMVunacknowledged tra¢ c using PCA (2 samples per MAC frame)

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Figure A.4: Latency spread and accumulated latency for SMVacknowledged tra¢ c using PCA (4 samples per MAC frame)

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Figure A.5: Latency spread for SMV acknowledged tra¢ c using PCA (6samples per MAC frame)

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Figure A.10: Accumulated latency for SMV unacknowledged tra¢ c usingPCA (6 samples per MAC frame)

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Figure A.14: Accumulated latency for SMV unacknowledged tra¢ c usingPCA (8 samples per MAC frame)

86

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