TAMPERE UNIVERSITY OF TECHNOLOGY Department of Automation R E A L- T I M E C O N T R O L U S I N G WIRELESS LOCAL AREA NETWORK

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1 TAMPERE UNIVERSITY OF TECHNOLOGY Department of Automation PETR SIMACEK R E A L- T I M E C O N T R O L U S I N G WIRELESS LOCAL AREA NETWORK Master of Science Thesis The subject has been approved by the Department Council in its meeting of Examiner: Prof. Hannu Koivisto STREET ADDRESS POSTAL ADDRESS TELEPHONE TELEFAX Korkeakoulunkatu 3 P.O. Box 692 (+358) (+358) FIN TAMPERE FIN TAMPERE

2 Acknowledgement This research work was carried out during the years at the Tampere University of Technology, Tampere, Finland. The work was initiated by the Automation and Control Institute together with Metso Automation. I wish to express my gratitude to my thesis supervisor Prof. Hannu Koivisto for his guidance and encouragement during the project as well as for the opportunity to work in such a dynamic environment. I would also like to thank to M.Sc. Ari-Pekka Pura from Metso Automation for many constructive comments, questions and advices. I am very grateful to M.Sc. Jari Seppälä for the project managing, which he has been doing with an enthusiasm. I also thank to my colleague Vladimír Lucan, who has significantly reduced the time spent by collecting and evaluating data. I very much appreciate all support, which I have received from my friends: thank you Johano and Fantíku for enhancing the language of the thesis, thank you Máro for many useful discussions. Most of all I wish to convey my deepest gratitude to my parents, my fiancée, my sister and her son for their love, support and encouragement throughout all those years. Without their full support, it would have not been possible to accomplish this work. Tampere, May 2002 Petr Šimácek Mekaniikanpolku 6 A 11 FIN Tampere simacek@ac.tu.fi

3 1 Introduction Networking Network types Protocol Architecture The TCP/IP protocol architecture TCP/IP layers TCP and UDP Operation of TCP/IP The OSI protocol architecture WLANs Standardization b Standard Media Access Layer Physical Layer Frequency hopping spread spectrum (FHSS) Direct Sequence Spread Spectrum (DSSS) Comparison of FHSS and DSSS Services Security Difference between wireless and wired solution WEP Protocol Encryption Decryption WEP PRNG (RC4) Authentication Challenges of WLAN Transmission Power issue Collision avoidance issue (CSMA/CA) Methods for Measurement of Network Performance Delay: Latency and Round Trip Time... 40

4 4.2 Throughput Data Bandwidth Drop Rate Delay Variation or Jitter Experimental Part The goal of the experiment Experimental System Design Description of the system set-up The measurement Algorithms Measurement of a Round Trip Time and a Packet Error Rate Control Algorithm Measurement Data Rates Measurement Capacity Test Various Distance Testing Data flow analysis Description of Environment Results Discussion of Results Conclusion References.. 79 A Apendix A1 Description of Measurement area A2 Measurement conditions in scenario A A3 Measurement conditions in scenario B...86 A4 Measurement conditions in scenario C...88 A5 Degradation of throughput. 90

5 TAMPERE UNIVERSITY OF TECHNOLOGY Degree program in Automation Automation and Control Institute Petr Simacek: Real-time control using Wireless Local Area Network Process Automation Master of Science thesis, 94p. Examiner: prof. Hannu Koivisto Funding: Tekes, Metso Automation Department of Automation February 2002 This diploma thesis analyzes the capabilities of current Wireless Local Area Network (WLAN) according to standard IEEE b. The goal is to evaluate whether it is suitable as a communication media for time-critical applications within an industrial environment. The survey part of the thesis reviews basic networking technologies, emphasizing comparison of various network types. However, the major interest lies in Wireless Local Area Networking (WLAN) review, which comprehensively explains principles of transmission, collision avoidance as well as a growing issue of security. Moreover, it highlights the challenges of WLANs and presents methods for measurement of its performance. The main focus of this work is the performance measurement of a wireless network based on control systems embodying Programmable Logical Controllers (PLC) and measurement algorithms developed during the project. The experimental part describes the components and software used as well as the control established. The system is extensively tested and achieved results are analyzed in the measurement part.

6 List of abbreviations IEEE LAN WLAN MAN WAN WLAN IP TCP UDP HTTP SMPT FTP QoS PI DSSS FHSS IF RTS CTS ACK WEP PRNG AP WGB RTT Institute of Electrical and Electronic Engineering Local Area Network Wireless Local Area Network Metropolitan Area Network Wide Area Network Wireless Local Area Network Internet protocol Transmission Control Protocol User Datagram Protocol HyperText Transmission Protocol Simple Mail Transfer Protocol File Transfer Protocol Quality of Service Proportional and Integral Direct Spread Spectrum Frequency Hopping Spectrum Infrared Request to Send Clear to Send Acknowledgment Wired Equivalent Policy Pseudo Random Noise Generator Access Point Workgroup Bridge Round Trip Time

7 1 Introduction Real-time applications require very high reliability of a network performance. Therefore, initial research of the network is highly important. Many applications use standard communication media such as Profibus, CAN, Industrial Ethernet and even Internet for delivering the control, real-time, data. However, there are applications, which either do not allow wiring or are more convenient without wires; such applications include arrays of actuators or sensors, large industrial facilities, distant outdoor machines as well as biological implants. On contrary, cables cannot be simply replaced by a wireless local area network (WLAN), since many issues, such as environment, are significantly influencing the network performance. Consequently, security has to be ensured and interference avoided, otherwise the control system can fail or even collapse. Therefore, it is essential to understand the behaviour of WLAN under different conditions in order to avoid such circumstances. In a standard situation using conventional PC, commercial programs are available to analyse the performance of a WLAN. Yet, in industrial applications, when industrial PCs are used, there is not such an option. Thus, firstly, a simulation has been derived and later an experimental system has been designed and a program application was implemented within the system. This diploma thesis was written as a part of a TEKES project between Tampere University of Technology and Metso Automation. The purpose of it was to analyse the existing wireless communication and its capabilities of delivering time-critical data within industrial environment. A simulated model of a DC motor was developed and incorporated into the system together with a discrete PI controller. The simulated model provides a sufficient utility for future emphasis, such as implementing a real pilot process or enhancing the current b standard network by exploiting newer standards.

8 Introduction 8 Chapter two reviews networking basics, which are crucial for understanding the global issues of this thesis. The main focus of this chapter is on distinguishing the network types, their usage and their benefits. Further, important protocol types are discussed. Chapter three reviews modern WLAN technology, especially standard and defining its main characteristics. The main interest of this chapter is in explaining transmission technique including popular Direct-Sequence Spread Spectrum (DSSS) and Fast-Hopping Spread Spectrum (FHSS). The standard also defines Infrared transmission, which is not in the scope of this work and therefore remains neglected. This chapter also presents security issue, involving encryption and decryption mechanisms as well as authentication process. At the end of this section, two major difficulties including Transmission Power problem and Collision avoidance problem, are mentioned. Chapter four discusses the experimental part. A setup of the implemented system is presented as well as the algorithms used for controller, process simulator and performance measurements. At the end of this chapter the results obtained are demonstrated with explanation of all four measurement scenarios. The results are then summarized in the discussion part and in chapter six, where the conclusions were drawn.

9 2 Networking The modern electrical communication has started from the year 1835 with the invention of telegraph, which was the first revolutionary instrument allowing people to communicate. Since that time scientists have discovered many other ways of transmitting data, resulting in the phenomena of Internet. 2.1 Network types Network is typically defined as a collection of terminals, computers, servers and components, allowing an easy data flow and usage of common resources. There are several ways of assorting networks comprehending size, type of connection, media, protocol or use. From the Internet point of view the most characteristic difference comes with the size of the network. The size can be presented with three typical magnitudes: Local Area Network (LAN) Metropolitan Area Network (MAN) Wide Are Network (WAN) Due to the interest of this work, which is rather focused on LAN; WAN and MAN will be considered to be the same and will be always referred as to WAN. LAN is generally characterized by three basic features: A diameter of not more than few kilometres A total rate of at least several Mbps Complete ownership by a single organization

10 Networking 10 On the contrary, WAN typically covers whole countries, has slower data rates and the infrastructure is usually owned by multiple organizations. WANs normally consist of several LANs, which have been connected together using a third party networks. Internet could be defined as a superset of LANs. Vast amounts of WANs are connected together via so called Internet backbone networks. [12] Networks can be also separated by their communication media. The conventional way is a copper wire network such as networks using coaxial cable or a twisted pair. Optical fiber networks are a usual way to connect WAN and backbone networks. Recently they have also been dedicated for creating high speed LANs. WLANs, which are the main interest of this thesis, are the newest step in the networking technology; they are suitable mainly for small LANs such as office environments. The wireless networks are extensively explained in Chapter Protocol Architecture When computers, terminals or any other data processing devices exchange data, the procedures involved can be quite complex. For instance, transfer of a file between two computers requires not only data path, using direct connection or communication network, but more is needed. The following list demonstrates the most typical tasks to be performed: The source must either activate the direct data communication path or inform the communication network of the identity concerning the desired destination The source must ascertain that the destination system is prepared to receive data The file transfer application on the source system must ascertain that the file management program on the destination system is prepared to accept and store the file for this particular user If the file formats used on the two systems are incompatible, one or the other system must perform a format translation function

11 Networking 11 From the above list it is clear to see that there must be a high degree of cooperation between the two computer systems. Instead of implementing the logic for this as a single module the task is broken up into small subtasks, each of which is implemented separately. In a protocol architecture a modules are arranged in a vertical stack, where each layer performs a related subset of functions required to communicate with another system. Ideally, layers should be defined so that changes in one layer do not require changes in other layers. Communication then is achieved by having corresponding, or peer, layers in two systems communicate. The peer layers communicate by means of formatted blocks of data that obey a set of rules or conventions known as protocol. [21] 2.3 The TCP/IP protocol architecture TCP/IP is a result of protocol research and development conducted on the experimental packet-switched network, ARPANET, founded by the Defense Advanced Research Projects Agency (DARPA). This protocol suite consists of a large collection of protocols that have been issued as Internet standards by the Internet Architecture Board. [21] TCP/IP layers There is no official TCP/IP protocol stack, as there is in case of OSI. However, based on the protocol standards that have been developed, the communication tasks for TCP/IP can be organized into five relatively independent layers: Application layer Host-to-host, or transport layer Internet layer Network access layer Physical layer The physical layer covers the physical interface between a data transmission device such as computers and a transmission medium, the nature of the signals, the data rate, and related matters.

12 Networking 12 The network access layer is concerned with the exchange of data between an end system and the network to which is attached. The sending computer must provide the network with the address of the destination computer, so that the network may route the data to appropriate destination. The sending computer may request certain services, such as priority, that might be provided by the network. The special software used at this layer depends on the type of the network to be used; different standards have been developed for circuit switching, packet switching or local area networks. Thus, it makes sense to separate those functions by placing network access into separate layer. By doing this, the remainder of the communication software, above the network access layer, need not be concerned about the specifics of the network to be used. The network access layer is concerned with access to and routing data across a network for two end systems attached to the same network. In those cases where two devices are attached to different networks, procedures ate needed to allow data to traverse multiple interconnected networks. This is a function of the internet layer. The internet protocol (IP) is used at this layer to provide the routing function across multiple networks. This protocol is implemented in end systems as well as in routers. A router is a processor that connects two networks and whose primary function is to relay data from one to the other on its route from the source to the destination system. Regardless of the nature of the application that is exchanging the data, there is usually a requirement that the data is exchange reliably meaning that all data arrive at the destination application and that all data arrive at the same order, in which they were sent. This is responsibility of Host-t o- host layer, known also as transport layer. The transmission control protocol (TCP) is the most commonly used protocol to provide this functionality. Finally, the application layer contains the logic needed to support the various user applications. For each different type of application such as file transfer, a separated module is needed. [21]

13 Networking TCP and UDP TCP provides a point-to-point channel for applications that require reliable communication. The HyperText Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Telnet are all examples of applications that require a reliable communication channel. The order, in which the data are sent and received over the network, is critical to the success of these applications. When HTTP is used to read from a URL, the data must be received in the order in which it was sent. Otherwise, an error can occur in form of a corrupted file transfer, or some other invalid information. [13] In addition to TCP, there is one other transport layer protocol that is in common use as part of the TCP/IP protocol suite; UDP (User Datagram Protocol) is a protocol that sends independent packets of data, called datagrams, from one computer to another with no guarantee about arrival. UDP is not connection-based like TCP. The UDP protocol provides communication between two applications on the network, which is not guaranteed. UDP is not connection-based protocol like TCP. Rather, it sends independent packets of data, called datagrams, from one application to another. Sending datagrams is much like sending a letter through the postal service: The order of delivery is not important and is not guaranteed, and each message is independent of any other. For many applications, the guarantee of reliability is critical to the success of the transfer of information from one end of the connection to the other. However, other forms of communication do not require such strict standards. In fact, they may be slowed down by the extra overhead or the reliable connection may invalidate the service altogether. An example can be a clock server that sends the current time to its client when requested to do so. If the client misses a packet, there is a very little sense to resend it because when the client receives it on the second try the time will be incorrect. If the client makes two requests and receives the packets from the server out of order, it does not really matter because the client can recognize that the packets are out of order and make another request. The reliability of TCP is unnecessary in this instance because it causes performance degradation and may hinder the usefulness of the service [13]. Another

14 Networking 14 example of a service that does not need the guarantee of a reliable channel is the ping command. The purpose of the ping command is to test the communication between two programs over the network. In fact, ping needs to know about dropped or out-of-order packets to determine how good or bad the connection is. A reliable channel would invalidate this service altogether. [13] Figure 2.1 shows the header format for TCP, which is a minimum of 20 octets, or 160 bits. The Source Port and Destination Port fields identify the applications at the source and destination system that ate using this connection. The Sequence Number, Acknowledgement Number and Window field provide flow control and error control. The checksum is 16-bit frame check sequence used to detect errors in the TCP segment. In contrast, UDP header is presented in Figure 2.2. [21] bits Source port Destination port Sequence number Acknowledgement number Offset Resrvd U A P R S F Window Checksum Urgent pointer Option + Padding Figure 2.1. TCP Header [21] Source port Length bits Destination port Checksum Figure 2.2. UDP Header [21]

15 Networking Operation of TCP/IP Figure 2.3 indicates how the protocols are configured for communications. To make clear that the total communications facility may consist of multiple networks, the constituent Logical connection (TCP connection) Global network address Subnetwork attachment point address Network 1 Network 2 Figure 2.3. TCP/IP concept networks are usually referred to as subnetworks. Some protocols, such as Ethernet, is used to connect a computer to a subnetwork. This protocol enables the host to send data across the subnetwork to another host or in case of host on another subnetwork, to a router. It is important to note that IP is implemented in all end systems as well as in routers. On the other hand, TCP is implemented only in the end systems keeping track of block of data to assure that all are delivered reliably. [21] For success communication, every entity in the overall system must have a unique address. In fact, two levels of addressing are needed. Each host on the subnetwork must have a unique global address allowing the data to be delivered to the proper host. This

16 Networking 16 address in known as IP address. Each application within the host must have an address that is unique within the host. This allows the host-to-to-host protocol (TCP) to deliver the data to the process. These addresses are known as ports. [21] In order to control transmission operations at all levels, the sending process generates a block of data and passes it to the TCP. TCP may break this block into smaller pieces to make it more manageable. To the each of this piece, TCP appends control information in the TCP header, forming a TCP segment which includes Destination port, Sequence number and Checksum. Next, TCP hands each segment over to IP. These segments must be transmitted across one or more subnetworks and through one or more routers. This operation also requires the use of control information. Thus IP appends a header of control information to each segment to form a IP datagram. Finally, each IP datagram is presented to the network access layer for transmission to its destination. The network access layer appends its own header, too, creating a packet, or frame which is then transmitted. The whole process is demonstrated at Figure 2.4 Figure 2.4. Protocol data units in the TCP/IP architecture

17 Networking The OSI protocol architecture The OSI reference model was developed by the International Organization for Standardization (ISO) as a model for a computer protocol architecture and as a framework for developing protocol standards. It consists of seven layers: 1. Physical layer: Provides electrical, functional, and procedural characteristics to activate, maintain, and deactivate physical links that transparently send the bit stream; only recognises individual bits, not characters or multicharacter frames. 2. Data link layer: Provides functional and procedural means to transfer data between network entities and (possibly) correct transmission errors; provides for activation, maintenance, and deactivation of data link connections, grouping of bits into characters and message frames, character and frame synchronisation, error control, media access control, and flow 3. Network layer: Provides independence from data transfer technology and relaying and routing considerations; masks peculiarities of data transfer medium from higher layers and provides switching and routing functions to establish, maintain, and terminate network layer connections and transfer data between users. 4. Transport layer: Provides transparent transfer of data between systems, relieving upper layers from concern with providing reliable and cost effective data transfer; provides end-to-end control and information interchange with quality of service needed by the application program; first true end-to-end layer. 5. Session layer: Provides mechanisms for organising and structuring dialogues between application processes; mechanisms allow for two-way simultaneous or two-way alternate operation, establishment of major and minor synchronisation points, and techniques for structuring data exchanges. 6. Presentation layer: Provides independence to application processes from differences in data representation that is, in syntax; syntax selection and conversion provided by allowing the user to select a "presentation context" with conversion between alternative contexts.

18 Networking Application layer: Concerned with the requirements of application. All application processes use the service elements provided by the application layer. The elements include library routines which perform interprocess communication, provide common procedures for constructing application protocols and for accessing the services provided by servers which reside on the network [21] The designers of OSI assumed that this model and the protocols developed within this model would come to dominate computer communication, eventually replacing proprietary protocol implementation and rival multivendor models such as TCP/IP. This has not happened. Although many useful protocols have been developed in the context of OSI, the overall seven-layer model has not prospered. Instead, the TCP/IP architecture has come dominate. Perhaps, the most important reason is that the key TCP/IP protocols were mature and well tested at a time when similar OSI protocols were in development stage. Another reason is that the OSI model is unnecessarily complex compare to TCP/IP. Figure 2.5 shows a comparison of the TCP/IP and OSI protocol architecture. I S O O S I p r o t o c o l s t a c k TCP/IP protocol stack Application commands Application layer. Process/ Application Syntax management Presentation layer HTTP, SMPT, etc Synchronization and dialog Session layer Host-host control TCP & UDP End-to-end message transfer Transport layer Network routing and addressing Network layer Internet IP Framing and error control Data link layer Network Access Ethernet, Token-ring, etc. Electrical network interface Physical layer definitions Physical network Physical network Figure 2.5. comparison of the TCP/IP and OSI protocol architecture

19 3 WLANs The purpose of WLAN ( Wireless Local Area Network) is to provide all features and benefits of traditional LAN technologies such as Ethernet but wirelessly. WLAN works without limitations of cabling using either infrared light (IR) or radio frequencies (RF) as a medium. Currently, there is much excitement about the future possibilities, which can be realized by wireless networks: Wires, which earlier enabled the Internet revolution, have now, in many cases, simply become a barrier to proliferation. When nodes are disengaged, new options as well as new problems arise. For example, in recent decades, there has been much growth in embedded devices, such as those present in alarm clocks, automobiles, cellular phone and in fact all things around us. However, they have not been controlled in a harmony, since they have been disconnected from each other. In the future, toasters may coordinate with alarm clocks, travellers may acquire maps or video from local information offices, warehouses, automobiles may warn each other when they are braking, and wireless trading may become reality. Which applications will be successful and which of them will not, cannot be predicted, of course. When a myriad of embedded devices are wirelessly networked, people will find themselves confronted with legions of sensors and actuators. The challenge of synchronizing all these devices into a coherent control system will loom large. This chapter reviews modern WLAN technology by presenting a standard and defining its main characteristics. However the main interest of this chapter is in explaining transmission technique including popular Direct Sequence Spread Spectrum

20 WLANs 20 (DSSS) and Fast-Hopping Spread Spectrum (FHSS). This chapter also presents Security issue, involving encryption and decryption mechanisms as well as authentication process. At the end of this section, two major difficulties, including Transmission Power problem and Collision avoidance problem, are studied. 3.1 Standardization In 1997, the IEEE (Institute of Electrical and Electronic Engineering) approved a standard for wireless LANs called that provide data rates at 1 and 2 Mbps using frequency range from 2.4 GHz to 2,4835 GHz in North America and in Europe, while Japan regulatory bodies mandate that these networks operate between 2,471 and 2,497 GHz. [7] Two years later after the ratification, IEEE released a new and improved standard b, allowing data rates of up to 11 Mbps. Although b is achieving a certain level of success, the limited data rate is a handicap for some applications. In order to meet the needs of a truly high-speed LANthe IEEE has developed a in 1999, a new specification that represents the next generation of enterprise-class wireless LANs. The standard a features among others, following advantages: greater scalability, better interference immunity, and significantly higher speed, up to 54 Mbps and beyond, which simultaneously allows for higher bandwidth applications and more users. [15] Unlike b using 2.4 GHz specifications, IEEE a does not use a spreadspectrum scheme, defined later in this chapter, but rather uses orthogonal frequencydivision multiplexing (OFDM). OFDM, also called multicarrier modulation, uses multiple carrier signals (up to 52) at different frequencies, sending some of the bits on each channel. The possible data rates for IEEE a are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. Moreover, the 5.7 GHz band promises to allow for the next breakthrough data rate of 100 Mbps. [9]

21 WLANs b Standard b standard, as an enhanced version of standard, defines number of services as well as Media Access Layer (MAC) and Physical layer (PHY). It also defines one infrared transmission technique and two radio transmission techniques making use of spread spectrum method. Both layers as well as all three transmission techniques are depicted in Figure 3.1 Figure 3.1. IEEE standards mapped to the OSI reference model. Furthermore, the standard divides the spectrum into 14 different channels where the centre frequency of the first channel is GHz and subsequent channels are spaced 5 MHz apart. However, in North America, only 11 of these channels are used while in France Spain, the rest of Europe and Japan, there are own designated channel allocation as shown in Table 1.

22 WLANs 22 Table 1: Frequency Channel Assignment Channel Number USA (FCC) Center Frequency [MHz] Canada ( I C ) Center Frequency [MHz] Europe (ETSI) Center Frequency [MHz] S p a i n Center Frequency [MHz] France Center Frequency [MHz] Japan (MKK) Center Frequency [MHz] Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used Not used 2467 Not used Not used Not used 2472 Not used Not used Not used Not used Not used Not used Media Access Layer The MAC layer defines a way of accessing the physical layer and also controls the services related to the mobility management and the radio resources. It is similar to the wired Ethernet standard for data transmissions. The differences arise in the way how packets collisions are handled. In the wired standard, data packets are sent out to the network indiscriminately. The system uses additional measures only when some packets collide and therefore is not guaranteed that packets get to their destinations. In the standards, collision avoidance is implemented, known as CSMA/CA. This collision avoidance issue is carefully described in section

23 WLANs Physical Layer The physical layer specification includes two RF spread spectrum technologies and one Infrared technology (IF) with following properties: DSSS (Direct-sequence spread spectrum) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps, 2 Mbps, 5,5 Mbps and 11Mbps FHSS (F r e q u e n c y-hopping spread spectrum) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps. I F (Infrared) operating at wavelengths between 850 and 950 nanometers [nm] and achieving data rates of 1 Mbps and 2 Mbps. This technology will be neglected here due to the scope of this diploma thesis. The main focus of this section lies in spread spectrum method, which could be defined as: A radio technology that reduces the average power spectral density by spreading radio energy over a much wider bandwidth than is necessary for desired data rate. This decreases transmitted power to keep users from interfering with others in the same band. Spread Spectrum technology has been known for many decades. It was initially developed for military defense communication systems. Today, this technology is used in modern radio design for unlicensed commercial applications and has been positioned as an alternative to frequency bands requiring licenses. The knowledge gained over the years using this technology for military use, has led to spawning of a number of companies pioneering proprietary spread spectrum radio transceivers to the market. The demand for this technology did not expand commercially until the late 80, when the U.S. Federation Communication Commission (FCC) made changes to the spectrum allocation and opened three frequency bands of MHz, MHz and MHz. These frequencies were defined as the Industrial Scientific and Medical (ISM) band. In the recent years, the 900 MHz ISM band has been troubled with user overcrowding and interfering congestion. Much of the overcrowding was attributed to inadequate spectrum allocation and the lack of standards and interoperability. Unfortunately, this

24 WLANs 24 band suffers from spectral inefficiency because of the number of coexisting users. However, it is apparent that this band was the 1 st generation of wireless technology Problem of 900 MHz band yielded to the 2 nd generation, which is making use of 2.4 GHz band, which offers more spectrum bandwidth, permits higher data rates with substantial reduction of size and power. [17] Frequency hopping spread spectrum (FHSS) The FHSS physical layer uses a transmitter and receiver pre-defined pseudorandom hopping pattern which must hop across 75, 1 MHz frequency slots at least once every 400ms (see Figure 3.). Three sets of 22 hopping patterns are defined in the standard. Each frequency slot must be utilized at least once every 30 seconds. Frequency hopping systems typically use Gaussian Frequency Shift Keying modulation. The standard also specifies that the entire data frame must be sent before it hops to another frequency. This reduces the level of interference immunity. The FHSS systems commonly work with 1 Mbps data rate. One advantage to FHSS is that it allows for multiple networks to coexist in the same physical space [15]. An example of FHSS transmission is provided on Figure 3. Figure 3.2. FHSS Frequency modulation [18].

25 WLANs 25 Figure 3.2. An example of FHSS transmission [18] Direct Sequence Spread Spectrum (DSSS) DSSS works in a different manner then FHSS. The physical layer requires that each data bit is spread over a 22 MHz slice of the ISM band by mixing (XOR) each data bit with a high rate pseudorandom chipping code (formally known as PN code). After the XORing operation, the resulting data is PSK modulated onto the RF carrier. For interoperability, the only allowed chipping code is 11 chip Baker sequence. The Baker Code is unique because no section of it matches with any other part of the chip sequence unless the entire 11 chips are completely aligned. [15] This code, which is a random-like sequence of high and low signals, specifies the actual bit. If the code is inverted, then it represents the opposite bit in the data sequence. This frequency modulation, if the transmission is properly synchronized offers self-error correction, and thus, it has a higher tolerance for interference. Figure 3.4 shows, the chipping code for the DSSS transmission technique.

26 WLANs 26 Figure 3.3. The DSSS data coding [15]. As presented in Figure 3.4, the chipping code creates the spread spectrum profile being humped in the middle. Thus, the interference is greatest at the channel center and it rolls off at the edges. Figure 3.4. The DSSS Frequency modulation [18] Comparison of FHSS and DSSS The superiority of each technique has been researched and many aspects of each transmission technique are continually being shaped as this technology advances. However, it has been proved that although FHSS systems are more susceptible to noise during each one hop, in the long run, they can achieve almost error-free transmission as it

27 WLANs 27 hops around the band. Furthermore, FHSS systems are easier to implement as the modulation schemes are simpler and hence, require less processing power in the receiver and can be implemented with non-linear amplifiers. Another advantage of FHSS systems is its superiority in near-far performance and its ease of synchronization. However, the maximum achievable data rate complying with the standard is only 2 Mbps due to 1 MHz channel spacing requirement. Another drawback is that it must always have a positive signal to noise ratio (SNR) while DSSS systems can theoretically operate with a signal power lower than the noise power. FHSS also requires the use of tuneable synthesiser and tend to generate more interference on other systems. The primary advantage of DSSS systems is the ability to obtain much higher data rates by using more sophisticated modulation schemes. It can also operate below the noise level and can use non-tuneable synthesizers. Additionally, DSSS systems do not generate as much interference on other systems as FHSS systems do because they use smaller amount of overall spectrum. However, the complex modulation method used in DSSS requires linear amplifiers, which increase the cost of the product. Moreover, DSSS systems do not operate as well in near-far situations and it is more difficult to synchronize them [7]. The summary of advantages of each system can be found below, in Table 2. Table 2: Comparison of DSSS and FHSS characteristics DSSS Good range resolution Less interference on other systems Bandwidth tied to chipping rate Can operate below noise floor Susceptible to near-far degradation More efficient coherent modulation (uses more bits per symbol) Requires linear amplification; higher cost FHSS Poor range resolution More interference on other systems Bandwidth tied to transmitter tuning range Must have positive SNR Better near-far performance Less efficient non-coherent modulation Can use non linear amplifiers, easier to implement

28 WLANs Services This standard also defines a number of services that need to be supplied by the WLAN to provide functionality equivalent to that which is inherent to wired LANs. The services, listed below, are commonly considered as the most crucial. Association - Establishes an initial association between a station and an access point (AP). Before a station can transmit or receive frames on a wireless LAN, its identity and address must be known. For this purpose, a station must establish an association with an access point. The AP can then deliver this information to other access points to facilitate routing and delivery of addressed frames [9]. Function of AP is carefully presented in Experimental part of this diploma thesis. Reassociation - Enables an established association to be transferred from one access point to another, allowing a mobile station the freedom of movement. [9] Disassociation - A notification from either stations or an access point that an existing association is terminated. A station should give this notification before leaving an area or shutting down. However, the MAC management facility protects itself against stations that disappear without notification. [9] Authentication - Used to establish the identity of stations with each other. In a wired LAN, it is generally assumed that access to a physical connection conveys authority to connect to the LAN. This is not a valid assumption for a WLAN, in which connectivity is achieved simply by having an attached antenna that is properly tuned. The authentication service is used by stations to establish their identity for stations with which they want to communicate. The standard does not mandate any particular authentication scheme, which could range from a relatively insecure handshaking to a public-key encryption schemes. [9] Privacy - Used to prevent the contents of messages from being read by anyone else but the intended recipient. The standard also provides for the optional use encryption to

29 WLANs 29 assure privacy. Due to the considerably cogency, the security issue will be explained in the next chapter 3.3 Security Because WLAN uses a shared medium, everything that is transmitted or received over a wireless network can be intercepted. Encryption and authentication must be always considered when developing a wireless networking system. The goal of adding these security features is to make wireless traffic as secure as wired traffic. The IEEE b standard provides a mechanism to do this by encrypting the traffic and authenticating nodes via the Wired Equivalent Privacy (WEP) protocol. Whenever encryption and authentication are implemented in any system, three things must be considered: The customer need for privacy: How strong do the protocols need to be and how much they should cost. Ease of usability: If the security implementation is too difficult to use, then it will not be used. Government regulations: Encryption is viewed as munitions by many governments, including the US, so all encryption products are export controlled. The WEP protocol used in b balances all the above-mentioned considerations Difference between wireless and wired solution Many of the security issues facing wireless LANs are also issues facing wired LANs. Data transmitted on the wired LAN are incorrectly assumed to be protected because one needs to be physically in the building to access the network. This is largely untrue with corporate access to the Internet. Often, if users from inside can get out to the Internet, then hackers from outside can get into a network if proper precautions are not taken The main security issue with wireless networks, and especially radio networks, is that they intentionally radiate data over an area that may exceed its space limits, which the

30 WLANs 30 organization physically controls. For instance, b radio waves at 2.4 GHz easily penetrate building walls and are receivable from the parking facility lot and possibly a few blocks away. Someone can passively retrieve all sensitive information by using the same wireless Network Interface Card (NIC) from a distance without being noticed by network security personnel. This problem also exists with wired LAN networks, but in a smaller degree. Current flow through the wires emits electromagnetic waves that someone could receive by using sensitive listening equipment. However, a person would have to be much closer to the cable to receive the signal. These examples illustrate that both wireless and wired networks are subject to the same security risks and have the same issues. These include the following: Threats to physical security of a network (such as denial of service and sabotage). Unauthorized access Attacks from within the authorized users of networks community, (such as disgruntled, current, and ex-employees have been known to read, distribute, and alter valuable company data). In addition, measures taken to ensure the integrity and security of data in the wired LAN environment are also applicable to the WLANs as well. WLANs, such as b, include an additional set of security elements, namely WEP, which are not available in the wired world. Therefore, some people are of the opinion that a properly implemented protected wireless LAN is more protected than a wired LAN. On the other hand, it is important to emphasize that WEP was never intended to be a complete end-to-end security solution. It protects the wireless link between the client machines and access points. Whenever the value of the data justifies such concern, both wired and wireless LANs should be supplemented with additional higher-level security mechanisms such as an access control, an end-to-end encryption, a password protection, an authentication, virtual private networks, or firewalls. [10]

31 WLANs WEP Protocol The WEP ( Wireless Equivalent Policy) algorithm was selected to be applied in WLAN because it met several requirements including: self-synchronization, efficiency in computation, which guarantees that the WEP can be implemented either in hardware or in software, as well as the robustness of the algorithm. In this section basic principles of encryption and decryption technique used in WEP are examined Encryption Two processes are applied to the plaintext data. One encrypts the plaintext; the other protects against unauthorized data modification. Figure 3.5. Encryption algorithm. As can be seen from the Figure 3.5, the secret key (40-bits or 128-bits) is concatenated with an Initialization Vector ("IV", 24-bits) resulting in a 64-bit total key size in case of a 40-bits encryption. The resulting key is an input into the Pseudo-Random Number Generator (PRNG). The PRNG outputs a pseudorandom key sequence based on the input key. The resulting sequence is used to encrypt the data by doing a bitwise XOR. The result is encrypted bytes equal in length to the number of data bytes that are to be

32 WLANs 32 transmitted in the expanded data plus 4 bytes. This is because the key sequence is used to protect the Integrity Check Value (ICV, 32-bits) as well as the data. To protect against unauthorized data modification, an integrity algorithm (CRC-32) operates on the plaintext to produce the ICV. The cipher text, which is the resulting product, is accomplished by the following sequence of events: a) Compute the ICV using CRC-32 over the message plaintext b) Concatenate the ICV to the plaintext c) Choose a random initialization vector (IV) and concatenate this to the secret key d) Input the secret key + IV into the RC4 algorithm to produce a pseudorandom key sequence e) Encrypt the plaintext + ICV by doing a bitwise XOR with the pseudorandom key sequence under RC4 to produce the cipher text f) Communicate the IV to the peer by placing it in front of the ciphertext An example of encrypted data can be seen in Figure 3.6. The data frame incorporates the Initialization Vector (IV), plaintext and Integrity Check Value [11]. IV 4 Data (PDU) >=1 ICV 4 Size is in Octets 1 octet Init Vector Pad 6 bits Pad 2 bits Figure 3.6. WEP data frame.

33 WLANs Decryption In decryption, presented in Figure 3.8, the IV of the incoming message is used to generate the key sequence necessary to decrypt the incoming message (Figure 3.6). Combining the ciphertext with the proper key sequence yields the original plaintext and ICV. The decryption is verified by performing the integrity check algorithm on the recovered plaintext and comparing the output ICV' to the ICV transmitted with the message. If ICV' is not equal to ICV, the received message is an error, and an error indication is sent to the MAC management and back to the sending station. Mobile units with erroneous messages (due to inability to decrypt) are not authenticated [11]. Figure 3.7. Decryption algorithm WEP PRNG (RC4) RC4 was developed in 1987 by Ron Rivest. RC4 is a stream cipher that takes a fixedlength key and produces a series of pseudorandom bits that are XOR'ed with the plaintext to produce ciphertext and vice versa. It is the critical component of the WEP process, since it is the actual encryption engine. The IV extends the useful lifetime of the secret key and provides the self-synchronous property of the algorithm. The secret key remains constant while the IV changes periodically. Each new IV results in a new key sequence, thus there is a one-to-one correspondence between IV and the output. The IV may change as frequently as every

34 WLANs 34 message, and since it travels with the message, the receiver will always be able to decrypt any message. Therefore the data of higher layer protocols such as IP are usually highly predictable. An eavesdropper can readily determine portions of the key sequence generated by the Key-IV pair. If the same pair is used for successive messages, this effect may reduce the degree of privacy. Changing the IV after each message is a simple method of preserving the effectiveness of WEP [11]. RC4 is used in the popular SSL (Secure Socket Layer) Internet protocol and many other cryptography products. The benefits of using RC4 are as follows: a) The key stream is independent of the plaintext b) Encryption and decryption are fast, about 10 times faster than DES (D a t a Encryption Standard), which is an earlier used encryption algorithm. c) RC4 is simple enough that most programmers can quickly code it in software d) It is claimed that RC4 is immune to differential and linear cryptanalysis Authentication For appropriate communication with an Access Point, every device needs to be authenticated. This process is based on an authentication key, which is usually configured as follows: Open Key: Allows any device to authenticate and then attempt to communicate with the access point. If the access point is using WEP and the other device is not, the other device does not attempt to authenticate with the access point. If the other device is using WEP but its WEP keys do not match the keys on the access point, the other device authenticates with the access point but cannot pass data. Figure 3.8 shows the authentication sequence between a device trying to authenticate and an access point using open authentication.

35 WLANs 35 Figure 3.8. Sequence for Open Authentication -The device's WEP key does not match the access point's key, so it can authenticate but not pass data. Shared Key: The access point sends an unencrypted challenge text string to any device attempting to communicate with the access point. The device, requesting authentication encrypts the challenge text and sends it back to the access point. If the challenge text is encrypted correctly, the access point allows the requesting device to authenticate. Both the unencrypted challenge and the encrypted challenge can be monitored, however, which leaves the access point open to attack from an intruder who guesses the WEP key by comparing the unencrypted and encrypted text strings. Because of this weakness, Shared Key authentication can be less secure than Open authentication. Figure 3.9 shows the authentication sequence between a device trying to authenticate and an access point using open authentication.. Figure 3.9. Sequence for Shared Key Authentication - The device's WEP key matches the access point's key, so it can authenticate and communicate.

36 WLANs Challenges of WLAN Although Internet currently does not provide sufficient guaranteeing of QoS, future users may desire a network that has the same flexibility and low cost of the Internet but with end-to-end guarantees of the telephone networks. The following section explains some difficulties of WLAN as well as possible solutions. [2] The wireless medium is very unreliable, hence: transmissions are subject to obstacles, reflections, a multipath effect, and fading, all of them affect the quality of transmitted signal. Moreover, transmission can interfere with each other. All these contribute to make the packet reception significantly demanding. [2] Transmission Power issue Transmission power issue specifies, at what power level an incorporated node should transmit the signal. Clearly, the power level should be high enough that the intended receiver receives the signal of adequate power. On the other hand, too high transmission power should be avoided since it may cause interference. One should also keep in mind that a retransmission could be more efficient than broadcasting at very high power. An important challenge is in providing feedback signal to a transmitter, allowing it to regulate the transmission power level. It was shown that under some models, the system wide transport capacity of the WLAN is optimised when every hop covers a very short distance. Therefore, nodes should relay packets over very short distances to nearby nodes. This system is clearly allowing transmission at very low power. This strategy is preferred to broadcasting at a very high power level and reaching the remote destination in just one hop, which decreases the throughput capacity to all nodes. On contrary, a drawback of this solution is increased delay, caused by transferring through high number of nods. [2]

37 WLANs Collision avoidance issue (CSMA/CA) Collision avoidance issue is one problem, which arises from the nature of shared wireless medium. A collision can clearly occur when all nearby located nods transmit at the same time; therefore, in order to guarantee the correct transmission of a node, all other nodes should refrain from the broadcasting [2]. For better understanding of this difficulty, Figure 3.10 presents a scenario involving three transmitters ( T1, T2, T3) and one receiver ( R). Assuming T1, T2 and T3 are being all within a range of the reception by R. Consequently, if receiver R desires to successfully receive a packet from T1, then both T2 and T3 have to remain silent. Yet, this carrier sensing method does not guarantee that the conflict can be avoided. For instance, if T1 can hear T2 then it can sense the carrier of T2 and it refrains from transmitting at the same time. This avoids collision with T2; however, such a strategy does not work in avoiding collisions with all potential interferences. Suppose T3 can be heard by R, but cannot be heard by T1. Such a terminal is called hidden terminal. This often results in collisions at R, since the T1 cannot detect the transmission from the hidden terminal T3. [2] Range of R Range of T 1 T 1 R T 2 T 3 Figure Interference and hidden nodes.

38 WLANs 38 The existence of hidden nodes contributes to a development of a theory of nodes transmission scheduling, which guarantees that packets are correctly delivered to the intended receiver. This goal, leads to spatially reduced radio frequency spectrum, by restricting the range of a transmission, and thus, allowing a distant transmitter-receiver pair to carry on a conversation at the same frequency and at the same time. Therefore, transmission has to be scheduled in space as well as in time. The most widely used technique solving this problem can be seen in Figure The principle lies in employing reservation packets in order to allocate a channel locally (in space) for data packets. Supposing a node T has a data packet to send to a node R; it firstly sends out a RTS packet (request to send), which is heard by all nodes within the range of T. All neighbours of T then refrain silent for a certain time and let the node T transmit. If R is not currently under an order to remain silent, it sends a CTS (cl ear to Figure 3.11: Medium Access Control. send) packet to T. CTS will be also heard by all nodes within the range of R, which then should remain silent. When T receives the CTS packet than it sends the data to R successfully since all other nodes have been silenced. If R has received all data, Acknowledgement packet (ACK) is sent back to the T. At this time all neighbours are released from the silence. In case of a failure from any reason, node T restarts all the procedure. Such a procedure is called handshaking and it needs to be done for each and every data packet on each hop. [2]

39 WLANs 39 In order to evaluate this mechanism, many issues have to be analysed. These issues include questions such as: Is it possible to achieve higher throughput of a network with this mechanism? How well does this mechanism work in multihop system? How to increase the efficiency of this system in poor signal area, where RTS, CTS or ACK packets can be lost The problem lies in a viscous circle realising that the nodes have to coordinate their transmissions in order to communicate; however, this coordination is itself communication. Thus, communication needs coordination, which is also communication. The current research emphasises the need of communication with fewer coordination packets. This could be realised by giving information about transmission schedule of each node to every involved nodes this would give the possibility of self-organized transmission. [2]

40 4 Network Performance Metrics With a growing amount of real-time and multimedia applications, the network statistics evaluation has become essential part of networking. The importance of each statistic method is different depending on the current application. For instance, real -time application such as a real-time control or voice and video transfer, in which the delay is critical, can afford to lose some packets. On the other hand, for a data transfer delay may not be so important, but accurate and complete reception is the crucial problem The meaning of this section is to define some important network statistics, which will be later used in this diploma thesis. 4.1 Delay: Latency and Round Trip Time One of the key statistics used in evaluating the performance of a network connection is delay experienced by data, which travels from one host to another. The term Latency is used to describe this concept. However, one must take care in the use of this term because it is not always clear whether latency refers to the time to travel from one host to another, or the time required to transmit a packet and to receive an acknowledgment, or some other delays. A related term to latency is the round trip time (RTT) of a network connection. The RTT of a connection is defined as a time needed for a data packet to travel from one host to another and return back to the original host. Latency and round trip times are typically measured in milliseconds for IP-based networks. [19] 4.2 Throughput A Throughput is a measurement of the average rate that data (in bits) can be sent between one user and another and is typically reported in kilobits per second (kbps) or megabits

41 Methods for Measurement of Network Performance 41 per second (Mbps). The throughput of the same network connection can vary greatly depending on the protocol used for transmission such as UDP, TCP, the type of a data traffic being sent (such as HTTP, FTP, VoIP or others) as well as the quality and data bandwidth of a network connection. This is quite different from latency, which generally does not vary for different protocols or traffic types. Throughput is measured at the highest possible protocol level to reflect the performance that will be experienced by a user as accurately as possible. Throughput is, thus, computed using the amount of data in the payload area of the highest protocol layer (such as the UDP payload size) of the transmitted packet. Overhead due to protocol headers and checksum is not included in the calculation of the throughput as is defined in this thesis. [19] 4.3 Data Bandwidth The data bandwidth or channel data rate is the maximum available, raw rate at which data can be transmitted over a network connection. The data bandwidth of a connection is similar to the throughput of a connection except that the data bandwidth is the theoretical maximum rate at which data can be transmitted if all of the overhead and checksums of the protocols used is included and the multiple access protocol is completely efficient. Like throughput, data bandwidth is measured in units of bits per second. However, the data bandwidth of a network connection is always larger then the measured throughput of a connection, For example, later in the thesis, wireless LAN connections with 11 Mbps data bandwidths have been measured to have throughput of about 2 Mbps. [19] 4.4 Drop Rate The drop rate of a data connection is a common measure of the network performance. Packet can be lost by routers, during long trips across backbone networks due to collisions, but over short, LAN connections, typical wired or fiber optic transmission media have row bit error rates on the order of 10-6 to as low as With the addition of error checking in many packet transmission protocols, bit errors in wired and fiber-based data transmission are insignificant and, therefore, not often measured for LANs However,

42 Methods for Measurement of Network Performance 42 with the growing popularity of Voice over IP (VoIP) as well as of real time video streaming and use of wireless in data networks has meant that data packet are often dropped or excessively delayed with respect to real time requirements or shortcomings in wireless media. Thus, the drop rate of a network connection has grown in importance. This has resulted in increased interest in bit error rates and packet error rates. The B i t Error Rate (BER) is the percentage of bits that are received in error or not received of those that are sent. The Packet Error Rate (PER) is the percentage of packets that are dropped or received incorrectly of those that are sent. Attention must be paid in the use of term BER because it can be used to imply the percentage of errors in only payload data bits (such as excluding the header, footer, and checksum bits) or to the raw data bits (such as including all overhead and non payload data buts). [19] 4.5 Delay Variation or Jitter Delay Variation or Jitter is an important measure for qualifying data networks performance, especially for VoIP and video streaming applications in which the protocol relies on regular arrival rates of data packets. As a result, the delay of a packet sent from one host to another is extremely important, but so is the Delay Variation or Jitter. This is defined to be the average variation in the arrival time of a packet and is reported in milliseconds or other appropriate time scale. [19]

43 5 Experimental Part Metso Automation Company proposed testing of a self-designed PLC (programmable logical controller, formally known as PCR/ECR and defined in section 5.2.1) together with WLAN commercial products using DSSS transmission technique. The reason for testing was to find an equivalent communication medium to old-fashioned wires, which became obstacles in some cases. The performance of the wireless network is examined based on theoretical analysis as well as on the measurement. This chapter explains both techniques. 5.1 The goal of the experiment The main focus of this experimental part lies in testing the wireless network performance as part for control system, which is a time-critical application. For this purpose, a control system block scheme according to Figure 5.1 was shaped, taking into account disturbances caused by WLAN. This work tends to show whether the current WLAN can handle real time data transfer, rather than identify all possible sources of disturbance. Desired velocity e Controller u WLAN Servomotor Velocity y WLAN Feedback Figure 5.1: A control scheme

44 Experimental Part Experimental System Design The type of wireless network, which has been utilized in the experimental part of this diploma thesis has had many various names in the past, including terms such as packet radio networks or multihop radio networks. The current commonly used name for a network managed by an AP (Access Point) is an infrastructure network. The system designed according to the proposal functions in three consequential phases presented at Figure 5.2. In the first phase, the PLCs boot their operating system from an intended server. In the second phase a simulated DC motor is controlled by distant controller using the wireless network In the third phase, the Booting Servers monitors operations of both PLCs, giving the compete overview of the system Capacity test Industrial computers Booting Server WLAN hardware Figure 5.2. System functioning.

45 Experimental Part Description of the system set-up This section explains few terms regarding the current system represented in Figure 5.3. The set-up consists of several parts, including Programmable Logical Controllers (PLC) as well as devices used for wireless transmission such as Access Point and two Workgroup Bridges (WGB1 and WGB2). The Figure also shows two software tools used for programming and network monitoring. Internet Environment for development PC1 PC2 WGB1 PLC1 Discrete PI controller Firewall AP Server WGB2 PLC2 DC motor simulation Wireless connection Fixed connection Programming and monitoring Direction of network traffic Monitoring the network traffic by AiroPeak Figure 5.3. System setup..

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