Lecture notes on Mobile communications

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SCHILLER Schiller_ppr 9/19/07 3:38 PM Page 1 Second Edition JOCHEN SCHILLER The mobile communications market remains the fastest growing segment of the global computing and communications business.The rapid progress and convergence of the field has created a need for new techniques and solutions, knowledgeable professionals to create and implement them, and courses to teach the background theory and technologies while pointing the way towards future trends. In this book Jochen Schiller draws on his extensive experience to provide a thorough grounding in mobile communications, describing the state of the art in industry and research while giving a detailed technical background to the area.The book covers all the important aspects of mobile and wireless communications Second Edition from the Internet to signals, access protocols and cellular systems, emphasizing the key area of digital data transfer. It uses a wide range of examples and other teaching aids, making it suitable for self-study and university classes. The book begins with an overview of mobile and wireless applications, covering the history and market, and JOCHEN SCHILLER providing the foundations of wireless transmission and Medium Access Control. Four different groups of wireless network technologies are then covered: telecommunications systems, satellite systems, broadcast systems and wireless LAN.The following chapters about the network and transport layers address the impairments and solutions using well-known Internet protocols such as TCP/IP in a mobile and wireless environment.The book concludes with a chapter on technologies supporting applications in mobile networks, focusing on the Web and the Wireless Application Protocol (WAP). Each chapter concludes with a set of exercises for self-study (with solutions available to instructors) and references to standards, organizations and research work related to the topic. Second New to this edition ➤ Integration of higher data rates for GSM (HSCSD, GPRS) Edition ➤ New material on 3rd generation (3G) systems with in-depth discussion of UMTS/W-CDMA ➤ Addition of the new WLAN standards for higher data rates: 802.11a, b, g and HiperLAN2 ➤ Extension of Bluetooth coverage to include IEEE 802.15, profiles and applications ➤ Increased coverage of ad-hoc networking and wireless profiled TCP ➤ Migration of WAP 1.x and i-mode towards WAP 2.0 Jochen Schiller is head of the Computer Systems and Telematics Working Group in the Institute of Computer Science, Freie Universität Berlin, and a consultant to several companies in the networking and communications business. His research includes mobile and wireless communications, communication architectures and operating systems for embedded devices, and QoS aspects of communication systems. Cover image © Photonica ADDISON ADDISON-WESLEY www.pearson-books.com WESLEY A Pearson Education book02Chap01 8804 (1-24) 30/5/03 11:03 am Page 1 Introduction 1 Downloaded from www.books4career.blogspot.com hat will computers look like in ten years? No one can make a wholly accurate prediction, but as a general feature, most computers will cer- Wtainly be portable. How will users access networks with the help of computers or other communication devices? An ever-increasing number with- out any wires, i.e., wireless. How will people spend much of their time at work, during vacation? Many people will be mobile – already one of the key charac- teristics of today’s society. Think, for example, of an aircraft with 800 seats. Modern aircraft already offer limited network access to passengers, and aircraft of the next generation will offer easy Internet access. In this scenario, a mobile network moving at high speed above ground with a wireless link will be the only means of transporting data to and from passengers. Think of cars with Internet access and billions of embedded processors that have to communicate with, for instance, cameras, mobile phones, CD-players, headsets, keyboards, intelligent traffic signs and sensors. This plethora of devices and applications show the great importance of mobile communications today. Before presenting more applications, the terms ‘mobile’ and ‘wireless’ as used throughout this book should be defined. There are two different kinds of mobil- ity: user mobility and device portability. User mobility refers to a user who has access to the same or similar telecommunication services at different places, i.e., the user can be mobile, and the services will follow him or her. Examples for mechanisms supporting user mobility are simple call-forwarding solutions known from the telephone or computer desktops supporting roaming (i.e., the desktop looks the same no matter which computer a user uses to log into the network). 1 With device portability, the communication device moves (with or without a user). Many mechanisms in the network and inside the device have to make sure that communication is still possible while the device is moving. A typical example for systems supporting device portability is the mobile phone system, where the system itself hands the device from one radio transmitter (also called a base sta- tion) to the next if the signal becomes too weak. Most of the scenarios described in this book contain both user mobility and device portability at the same time. 1 Apart from the term ‘portable’, several other terms are used when speaking about devices (e.g., ‘mobile’ in the case of ‘mobile phone’). This book mainly distinguishes between wireless access to a network and mobility of a user with a device as key characteristics. 102Chap01 8804 (1-24) 30/5/03 11:03 am Page 2 2 Mobile communications With regard to devices, the term wireless is used. This only describes the way of accessing a network or other communication partners, i.e., without a wire. The wire is replaced by the transmission of electromagnetic waves through ‘the air’ (although wireless transmission does not need any medium). A communication device can thus exhibit one of the following characteristics: ● Fixed and wired: This configuration describes the typical desktop computer in an office. Neither weight nor power consumption of the devices allow for mobile usage. The devices use fixed networks for performance reasons. ● Mobile and wired: Many of today’s laptops fall into this category; users carry the laptop from one hotel to the next, reconnecting to the company’s network via the telephone network and a modem. ● Fixed and wireless: This mode is used for installing networks, e.g., in his- torical buildings to avoid damage by installing wires, or at trade shows to ensure fast network setup. Another example is bridging the last mile to a customer by a new operator that has no wired infrastructure and does not want to lease lines from a competitor. ● Mobile and wireless: This is the most interesting case. No cable restricts the user, who can roam between different wireless networks. Most technol- ogies discussed in this book deal with this type of device and the networks supporting them. Today’s most successful example for this category is GSM with more than 800 million users. The following section highlights some application scenarios predestined for the use of mobile and wireless devices. An overview of some typical devices is also given. The reader should keep in mind, however, that the scenarios and devices discussed only represent a selected spectrum, which will change in the future. As the market for mobile and wireless devices is growing rapidly, more devices will show up, and new application scenarios will be created. A short history of wireless communication will provide the background, briefly summing up the development over the last 200 years. Section 1.3 shows wireless and mobile communication from a marketing perspective. While there are already over a billion users of wireless devices today and the wireless business has experienced some problems in the last few years, the market potential is still tremendous. Section 1.4 shows some open research topics resulting from the fundamen- tal differences between wired and wireless communication. Section 1.5 presents the basic reference model for communication systems used throughout this book. This chapter concludes with an overview of the book, explaining the ‘tall and thin’ approach chosen. Tall and thin means that this book covers a variety of different aspects of mobile and wireless communication to provide a com- plete picture. Due to this broad perspective, however, it does not go into all the details of each technology and systems presented.02Chap01 8804 (1-24) 30/5/03 11:03 am Page 3 Introduction 3 1.1 Applications Although many applications can benefit from wireless networks and mobile communications, particular application environments seem to be predestined for their use. The following sections will enumerate some of them – it is left to you to imagine more. 1.1.1 Vehicles Today’s cars already comprise some, but tomorrow’s cars will comprise many wireless communication systems and mobility aware applications. Music, news, road conditions, weather reports, and other broadcast information are received via digital audio broadcasting (DAB) with 1.5 Mbit/s. For personal communica- tion, a universal mobile telecommunications system (UMTS) phone might be available offering voice and data connectivity with 384 kbit/s. For remote areas, satellite communication can be used, while the current position of the car is determined via the global positioning system (GPS). Cars driving in the same area build a local ad-hoc network for the fast exchange of information in emer- gency situations or to help each other keep a safe distance. In case of an accident, not only will the airbag be triggered, but the police and ambulance service will be informed via an emergency call to a service provider. Cars with this technol- ogy are already available. In the future, cars will also inform other cars about accidents via the ad-hoc network to help them slow down in time, even before a driver can recognize an accident. Buses, trucks, and trains are already transmit- ting maintenance and logistic information to their home base, which helps to improve organization (fleet management), and saves time and money. Figure 1.1 shows a typical scenario for mobile communications with many wireless devices. Networks with a fixed infrastructure like cellular phones (GSM, UMTS) will be interconnected with trunked radio systems (TETRA) and wireless LANs (WLAN). Satellite communication links can also be used. The networks between cars and inside each car will more likely work in an ad-hoc fashion. Wireless pico networks inside a car can comprise personal digital assistants (PDA), laptops, or mobile phones, e.g., connected with each other using the Bluetooth technology. This first scenario shows, in addition to the technical content, something typical in the communication business – many acronyms. This book contains and defines many of these. If you get lost with an acronym, please check the appendix, which contains the complete list, or check the terms and definitions database interactive (TEDDI) of ETSI (2002). Think of similar scenarios for air traffic or railroad traffic. Different prob- lems can occur here due to speed. While aircraft typically travel at up to 900 km/h and current trains up to 350 km/h, many technologies cannot oper- ate if the relative speed of a mobile device exceeds, e.g., 250 km/h for GSM or 100 km/h for AMPS. Only some technologies, like DAB work up to 900 km/h (unidirectional only).02Chap01 8804 (1-24) 30/5/03 11:03 am Page 4 4 Mobile communications Figure 1.1 A typical application of mobile communications: road traffic UMTS, WLAN, DAB, GSM, cdma2000, TETRA, ... Personal Travel Assistant, DAB, PDA, laptop, GSM, UMTS, WLAN, Bluetooth, ... 1.1.2 Emergencies Just imagine the possibilities of an ambulance with a high-quality wireless con- nection to a hospital. Vital information about injured persons can be sent to the hospital from the scene of the accident. All the necessary steps for this particu- lar type of accident can be prepared and specialists can be consulted for an early diagnosis. Wireless networks are the only means of communication in the case of natural disasters such as hurricanes or earthquakes. In the worst cases, only decentralized, wireless ad-hoc networks survive. The breakdown of all cabling not only implies the failure of the standard wired telephone system, but also the crash of all mobile phone systems requiring base stations 1.1.3 Business A travelling salesman today needs instant access to the company’s database: to ensure that files on his or her laptop reflect the current situation, to enable the company to keep track of all activities of their travelling employees, to keep data- bases consistent etc. With wireless access, the laptop can be turned into a true mobile office, but efficient and powerful synchronization mechanisms are needed to ensure data consistency. Figure 1.2 illustrates what may happen when employ- ees try to communicate off base. At home, the laptop connects via a WLAN or LAN and DSL to the Internet. Leaving home requires a handover to another tech- nology, e.g., to an enhanced version of GSM, as soon as the WLAN coverage ends. Due to interference and other factors discussed in chapter 2, data rates drop while cruising at higher speed. Gas stations may offer WLAN hot spots as well as gas. Trains already offer support for wireless connectivity. Several more handovers to different technologies might be necessary before reaching the office. No matter ad-hoc02Chap01 8804 (1-24) 30/5/03 11:03 am Page 5 Introduction 5 Figure 1.2 LAN, WLAN GSM 53 kbit/s UMTS, GSM LAN Mobile and wireless 780 kbit/s Bluetooth 500 kbit/s 115 kbit/s 100 Mbit/s, WLAN services – always best 54 Mbit/s connected UMTS, DECT 2 Mbit/s GSM/EDGE 384 kbit/s, GSM 115 kbit/s, UMTS, GSM WLAN 780 kbit/s WLAN 11 Mbit/s 384 kbit/s when and where, mobile communications should always offer as good connectiv- ity as possible to the internet, the company’s intranet, or the telephone network. 1.1.4 Replacement of wired networks In some cases, wireless networks can also be used to replace wired networks, e.g., remote sensors, for tradeshows, or in historic buildings. Due to economic reasons, it is often impossible to wire remote sensors for weather forecasts, earthquake detection, or to provide environmental information. Wireless con- nections, e.g., via satellite, can help in this situation. Tradeshows need a highly dynamic infrastructure, but cabling takes a long time and frequently proves to be too inflexible. Many computer fairs use WLANs as a replacement for cabling. Other cases for wireless networks are computers, sensors, or information dis- plays in historical buildings, where excess cabling may destroy valuable walls or floors. Wireless access points in a corner of the room can represent a solution. 1.1.5 Infotainment and more Internet everywhere? Not without wireless networks Imagine a travel guide for a city. Static information might be loaded via CD-ROM, DVD, or even at home via the Internet. But wireless networks can provide up-to-date information at any appropriate location. The travel guide might tell you something about the history of a building (knowing via GPS, contact to a local base station, or trian- gulation where you are) downloading information about a concert in the building at the same evening via a local wireless network. You may choose a seat, pay via electronic cash, and send this information to a service provider (Cheverst, 2000). Another growing field of wireless network applications lies in entertainment and games to enable, e.g., ad-hoc gaming networks as soon as people meet to play together. 1.1.6 Location dependent services02Chap01 8804 (1-24) 30/5/03 11:03 am Page 6 6 Mobile communications Many research efforts in mobile computing and wireless networks try to hide the fact that the network access has been changed (e.g., from mobile phone to WLAN or between different access points) or that a wireless link is more error prone than a wired one. Many chapters in this book give examples: Mobile IP tries to hide the fact of changing access points by redirecting packets but keep- ing the same IP address (see section 8.1), and many protocols try to improve link quality using encoding mechanisms or retransmission so that applications made for fixed networks still work. In many cases, however, it is important for an application to ‘know’ some- thing about the location or the user might need location information for further activities. Several services that might depend on the actual location can be distinguished: ● Follow-on services: The function of forwarding calls to the current user location is well known from the good old telephone system. Wherever you are, just transmit your temporary phone number to your phone and it redi- 2 rects incoming calls. Using mobile computers, a follow-on service could offer, for instance, the same desktop environment wherever you are in the world. All e-mail would automatically be forwarded and all changes to your desktop and documents would be stored at a central location at your com- pany. If someone wanted to reach you using a multimedia conferencing system, this call would be forwarded to your current location. ● Location aware services: Imagine you wanted to print a document sitting in the lobby of a hotel using your laptop. If you drop the document over the printer icon, where would you expect the document to be printed? Certainly not by the printer in your office However, without additional information about the capabilities of your environment, this might be the only thing you can do. For instance, there could be a service in the hotel announcing that a standard laser printer is available in the lobby or a color printer in a hotel meeting room etc. Your computer might then trans- mit your personal profile to your hotel which then charges you with the printing costs. ● Privacy: The two service classes listed above immediately raise the question of privacy. You might not want video calls following you to dinner, but maybe you would want important e-mails to be forwarded. There might be locations and/or times when you want to exclude certain services from reaching you and you do not want to be disturbed. You want to utilize loca- tion dependent services, but you might not want the environment to know exactly who you are. Imagine a hotel monitoring all guests and selling these profiles to companies for advertisements. ● Information services: While walking around in a city you could always use 2 Actually, this is already done with the phone network – your phone just handles some signalling.02Chap01 8804 (1-24) 30/5/03 11:03 am Page 7 Introduction 7 your wireless travel guide to ‘pull’ information from a service, e.g., ‘Where is the nearest Mexican restaurant?’ However, a service could also actively ‘push’ information on your travel guide, e.g., the Mexican restaurant just around the corner has a special taco offer. ● Support services: Many small additional mechanisms can be integrated to support a mobile device. Intermediate results of calculations, state informa- tion, or cache contents could ‘follow’ the mobile node through the fixed network. As soon as the mobile node reconnects, all information is avail- able again. This helps to reduce access delay and traffic within the fixed network. Caching of data on the mobile device (standard for all desktop systems) is often not possible due to limited memory capacity. The alterna- tive would be a central location for user information and a user accessing this information through the (possibly large and congested) network all the time as it is often done today. 1.1.7 Mobile and wireless devices Even though many mobile and wireless devices are available, there will be many more in the future. There is no precise classification of such devices, by size, shape, weight, or computing power. Currently, laptops are considered the upper 3 end of the mobile device range. The following list gives some examples of mobile and wireless devices graded by increasing performance (CPU, memory, display, input devices etc.). However, there is no sharp line between the cate- gories and companies tend to invent more and more new categories. ● Sensor: A very simple wireless device is represented by a sensor transmitting state information. One example could be a switch sensing the office door. If the door is closed, the switch transmits this to the mobile phone inside the office which will not accept incoming calls. Without user interaction, the semantics of a closed door is applied to phone calls. ● Embedded controllers: Many appliances already contain a simple or some- times more complex controller. Keyboards, mice, headsets, washing machines, coffee machines, hair dryers and TV sets are just some examples. Why not have the hair dryer as a simple mobile and wireless device (from a communication point of view) that is able to communicate with the mobile phone? Then the dryer would switch off as soon as the phone starts ringing – that would be a nice application ● Pager: As a very simple receiver, a pager can only display short text mes- sages, has a tiny display, and cannot send any messages. Pagers can even be integrated into watches. The tremendous success of mobile phones, has made the pager virtually redundant in many countries. Short messages have replaced paging. The situation is somewhat different for emergency services 3 Putting a mainframe on a truck does not really make it a mobile device.02Chap01 8804 (1-24) 30/5/03 11:03 am Page 8 8 Mobile communications where it may be necessary to page a larger number of users reliably within short time. ● Mobile phones: The traditional mobile phone only had a simple black and white text display and could send/receive voice or short messages. Today, mobile phones migrate more and more toward PDAs. Mobile phones with full color graphic display, touch screen, and Internet browser are easily available. ● Personal digital assistant: PDAs typically accompany a user and offer simple versions of office software (calendar, note-pad, mail). The typical input device is a pen, with built-in character recognition translating hand- writing into characters. Web browsers and many other software packages are available for these devices. ● Pocket computer: The next steps toward full computers are pocket comput- ers offering tiny keyboards, color displays, and simple versions of programs found on desktop computers (text processing, spreadsheets etc.). ● Notebook/laptop: Finally, laptops offer more or less the same performance as standard desktop computers; they use the same software – the only tech- nical difference being size, weight, and the ability to run on a battery. If operated mainly via a sensitive display (touch sensitive or electromagnetic), the devices are also known as notepads or tablet PCs. The mobile and wireless devices of the future will be more powerful, less heavy, and comprise new interfaces to the user and to new networks. However, one big problem, which has not yet been solved, is the energy supply. The more features that are built into a device, the more power it needs. The higher the perfor- mance of the device, the faster it drains the batteries (assuming the same technology). Furthermore, wireless data transmission consumes a lot of energy. Although the area of mobile computing and mobile communication is developing rapidly, the devices typically used today still exhibit some major drawbacks compared to desktop systems in addition to the energy problem. Interfaces have to be small enough to make the device portable, so smaller key- boards are used. This makes typing difficult due to their limited key size. Small displays are often useless for graphical display. Higher resolution does not help, as the limiting factor is the resolution capacity of the human eye. These devices have to use new ways of interacting with a user, such as, e.g., touch sensitive displays and voice recognition. Mobile communication is greatly influenced by the merging of telecommu- nication and computer networks. We cannot say for certain what the telephone of the future will look like, but it will most probably be a computer. Even today, telephones and mobile phones are far from the simple ‘voice transmission 4 devices’ they were in the past. Developments like ‘voice over IP’ and the gen- eral trend toward packet-oriented networks enforce the metamorphosis of telephones (although voice services still guarantee good revenue). While no one 4 Chapter 4 will present more features of modern mobile phone systems, including the growing demand for bandwidth to use typical Internet applications via the mobile ‘phone’.02Chap01 8804 (1-24) 30/5/03 11:03 am Page 9 Introduction 9 can predict the future of communication devices precisely, it is quite clear that there will still be many fixed systems, complemented by a myriad of small wire- less computing devices all over the world. More people already use mobile phones than fixed phones 1.2 A short history of wireless communication For a better understanding of today’s wireless systems and developments, a short history of wireless communication is presented in the following section. This cannot cover all inventions but highlights those that have contributed fun- damentally to today’s systems. The use of light for wireless communications reaches back to ancient times. In former times, the light was either ‘modulated’ using mirrors to create a cer- tain light on/light off pattern (’amplitude modulation’) or, for example, flags were used to signal code words (’amplitude and frequency modulation’, see chapter 2). The use of smoke signals for communication is mentioned by Polybius, Greece, as early as 150 BC. It is also reported from the early (or west- ern) Han dynasty in ancient China (206 BC–24 AD) that light was used for signaling messages along a line of signal towers towards the capitol Chang’an (Xi’an). Using light and flags for wireless communication remained important for the navy until radio transmission was introduced, and even today a sailor has to know some codes represented by flags if all other means of wireless com- munication fail. It was not until the end of the 18th century, when Claude Chappe invented the optical telegraph (1794), that long-distance wireless com- munication was possible with technical means. Optical telegraph lines were built almost until the end of the following century. Wired communication started with the first commercial telegraph line between Washington and Baltimore in 1843, and Alexander Graham Bell’s invention and marketing of the telephone in 1876 (others tried marketing before but did not succeed, e.g., Philip Reis, 1834–1874, discovered the tele- phone principle in 1861). In Berlin, a public telephone service was available in 1881, the first regular public voice and video service (multimedia) was already available in 1936 between Berlin and Leipzig. All optical transmission systems suffer from the high frequency of the car- rier light. As every little obstacle shadows the signal, rain and fog make communication almost impossible. At that time it was not possible to focus light as efficiently as can be done today by means of a laser, wireless communi- cation did not really take off until the discovery of electromagnetic waves and the development of the equipment to modulate them. It all started with Michael Faraday (and about the same time Joseph Henry) demonstrating elec- tromagnetic induction in 1831 and James C. Maxwell (1831–79) laying the theoretical foundations for electromagnetic fields with his famous equations (1864). Finally, Heinrich Hertz (1857–94) was the first to demonstrate the wave02Chap01 8804 (1-24) 30/5/03 11:03 am Page 10 10 Mobile communications character of electrical transmission through space (1886), thus proving Maxwell’s equations. Today the unit Hz reminds us of this discovery. Nikola Tesla (1856–1943) soon increased the distance of electromagnetic transmission. The name, which is most closely connected with the success of wireless communication, is certainly that of Guglielmo Marconi (1874–1937). He gave the first demonstration of wireless telegraphy in 1895 using long wave transmis- sion with very high transmission power ( 200 kW). The first transatlantic transmission followed in 1901. Only six years later, in 1907, the first commer- cial transatlantic connections were set up. Huge base stations using up to thirty 100 m high antennas were needed on both sides of the Atlantic Ocean. Around that time, the first World Administration Radio Conference (WARC) took place, coordinating the worldwide use of radio frequencies. The first radio broadcast took place in 1906 when Reginald A. Fessenden (1866–1932) trans- mitted voice and music for Christmas. In 1915, the first wireless voice transmission was set up between New York and San Francisco. The first com- mercial radio station started in 1920 (KDKA from Pittsburgh). Sender and receiver still needed huge antennas and high transmission power. This changed fundamentally with the discovery of short waves, again by Marconi, in 1920 (In connection with wireless communication, short waves have the advantage of being reflected at the ionosphere.) It was now possible to send short radio waves around the world bouncing at the ionosphere – this technique is still used today. The invention of the electronic vacuum tube in 1906 by Lee DeForest (1873–1961) and Robert von Lieben (1878–1913) helped to reduce the size of sender and receiver. Vacuum tubes are still used, e.g., for the amplification of the output signal of a sender in today’s radio stations. One of the first ‘mobile’ transmitters was on board a Zeppelin in 1911. As early as 1926, the first tele- phone in a train was available on the Berlin-Hamburg line. Wires parallel to the railroad track worked as antenna. The first car radio was commercially available in 1927 (‘Philco Transitone’); but George Frost an 18-year-old from Chicago had integrated a radio into a Ford Model T as early as 1922. Nineteen twenty-eight was the year of many field trials for television broadcasting. John L. Baird (1888–1946) transmitted TV across the Atlantic and demonstrated color TV, the station WGY (Schenectady, NY) started regular TV broadcasts and the first TV news. The first teleteaching started in 1932 from the CBS station W2XAB. Up until then, all wireless communication used amplitude modulation (see section 2.6), which offered relatively poor quality due to interference. One big step forward in this respect was the invention of frequency modulation in 1933 by Edwin H. Armstrong (1890–1954). Both fundamental modulation schemes are still used for today’s radio broadcasting with frequency modulation resulting in a much better quality. By the early 1930s, many radio stations were already broadcasting all over the world. After the Second World War, many national and international projects in the area of wireless communications were triggered off. The first network in Germany was the analog A-Netz from 1958, using a carrier frequency of 160 MHz. Connection setup was only possible from the mobile station, no02Chap01 8804 (1-24) 30/5/03 11:03 am Page 11 Introduction 11 handover, i.e., changing of the base station, was possible. Back in 1971, this system had coverage of 80 per cent and 11,000 customers. It was not until 1972 that the B-Netz followed in Germany, using the same 160 MHz. This network could initiate the connection setup from a station in the fixed telephone net- work, but, the current location of the mobile receiver had to be known. This system was also available in Austria, The Netherlands, and Luxembourg. In 1979, the B-Netz had 13,000 customers in West Germany and needed a heavy sender and receiver, typically built into cars. At the same time, the northern European countries of Denmark, Finland, Norway, and Sweden (the cradle of modern mobile communications) agreed upon the nordic mobile telephone (NMT) system. The analogue NMT uses a 450 MHz carrier and is still the only available system for mobile communication in some very remote places (NMT at 900 MHz followed in 1986). Several other national standards evolved and by the early 1980s Europe had more than a handful of different, completely incompatible analog mobile phone standards. In accordance with the general idea of a European Union, the European coun- tries decided to develop a pan-European mobile phone standard in 1982. The new system aimed to: ● use a new spectrum at 900 MHz; 5 ● allow roaming throughout Europe; ● be fully digital; and ● offer voice and data service. The ‘Groupe Spéciale Mobile’ (GSM) was founded for this new development. In 1983 the US system advanced mobile phone system (AMPS) started (EIA, 1989). AMPS is an analog mobile phone system working at 850 MHz. Telephones at home went wireless with the standard CT1 (cordless telephone) in 1984, (fol- lowing its predecessor the CT0 from 1980). As digital systems were not yet available, more analog standards followed, such as the German C-Netz at 450 MHz with analog voice transmission. Hand-over between ‘cells’ was now possible, the signalling system was digital in accordance with the trends in fixed networks (SS7), and automatic localization of a mobile user within the whole network was sup- ported. This analog network was switched off in 2000. Apart from voice transmission the services offered fax, data transmission via modem, X.25, and electronic mail. CT2, the successor of CT1, was embodied into British Standards published in 1987 (DTI, 1987) and later adopted by ETSI for Europe (ETS, 1994). CT2 uses the spectrum at 864 MHz and offers a data channel at a rate of 32 kbit/s. The early 1990s marked the beginning of fully digital systems. In 1991, ETSI adopted the standard digital European cordless telephone (DECT) for digital cordless telephony (ETSI, 1998). DECT works at a spectrum of 1880–1900 MHz with a range of 100–500 m. One hundred and twenty duplex channels can carry 5 Roaming here means a seamless handover of a telephone call from one network provider to another while crossing national boundaries.02Chap01 8804 (1-24) 30/5/03 11:03 am Page 12 12 Mobile communications up to 1.2 Mbit/s for data transmission. Several new features, such as voice encryp- tion and authentication, are built-in. The system supports several 10,000 2 users/km and is used in more than 110 countries around the world (over 150 million shipped units). Today, DECT has been renamed digital enhanced cord- less telecommunications for marketing reasons and to reflect the capabilities of DECT to transport multimedia data streams. Finally, after many years of discus- sions and field trials, GSM was standardized in a document of more than 5,000 pages in 1991. This first version of GSM, now called global system for mobile communication, works at 900 MHz and uses 124 full-duplex channels. GSM offers full international roaming, automatic location services, authentication, encryption on the wireless link, efficient interoperation with ISDN systems, and a relatively high audio quality. Furthermore, a short message service with up to 160 alphanumeric characters, fax group 3, and data services at 9.6 kbit/s have been integrated. Depending on national regulations, one or several providers can use the channels, different accounting and charging schemes can be applied etc. However, all GSM systems remain compatible. Up to now, over 400 providers in more than 190 countries have adopted the GSM standard (over 70 per cent of the world’s wireless market). It was soon discovered that the analog AMPS in the US and the digital GSM at 900 MHz in Europe are not sufficient for the high user densities in cities. While in the US, no new spectrum was allocated for a new system, in Europe a new frequency band at 1800 MHz was chosen. The effect was as follows. In the US, different companies developed different new, more bandwidth-efficient tech- nologies to operate side-by-side with AMPS in the same frequency band. This resulted in three incompatible systems, the analog narrowband AMPS (IS-88, (TIA, 1993a)), and the two digital systems TDMA (IS-136, (TIA, 1996)) and CDMA (IS-95, (TIA, 1993b)). The Europeans agreed to use GSM in the 1800 MHz spectrum. These GSM–1800 networks (also known as DCS 1800, digital cellular system) started with a better voice quality due to newer speech codecs. These net- works consist of more and smaller cells (see chapters 2 and 4). GSM is also available in the US as GSM–1900 (also called PCS 1900) using spectrum at 1900 MHz like the newer versions of the TDMA and CDMA systems. Europe believes in standards, while the US believes in market forces – GSM is one of the few examples where the approach via standardization worked. So, while Europe has one common standard, and roaming is possible even to Australia or Singapore, the US still struggles with many incompatible systems. However, the picture is different when it comes to more data communication- oriented systems like local area networks. Many proprietary wireless local area network systems already existed when ETSI standardized the high performance radio local area network (HIPERLAN) in 1996. This was a family of standards and recommendations. HIPERLAN type 1 should operate at 5.2 GHz and should offer data rates of up to 23.5 Mbit/s. Further types had been specified with type 4 going up to 155 Mbit/s at 17 GHz. However, although coming later than HIPERLAN in 1997, the IEEE standard 802.11 was soon the winner for local area02Chap01 8804 (1-24) 30/5/03 11:03 am Page 13 Introduction 13 networks. It works at the license-free Industrial, Science, Medical (ISM) band at 2.4 GHz and infra red offering 2 Mbit/s in the beginning (up to 10 Mbit/s with proprietary solutions already at that time). Although HIPERLAN has better per- formance figures, no products were available while many companies soon offered 802.11 compliant equipment. Nineteen ninety-eight marked the beginning of mobile communication using satellites with the Iridium system (Iridium, 2002). Up to this time, satel- lites basically worked as a broadcast distribution medium or could only be used with big and heavy equipment – Iridium marked the beginning of small and truly portable mobile satellite telephones including data service. Iridium consists of 66 satellites in low earth orbit and uses the 1.6 GHz band for communication with the mobile phone. In 1998 the Europeans agreed on the universal mobile telecommunications system (UMTS) as the European proposal for the International Telecommunication Union (ITU) IMT-2000 (international mobile telecommunications). In the first phase, UMTS combines GSM network technol- ogy with more bandwidth-efficient CDMA solutions. The IMT-2000 recommendations define a common, worldwide framework for future mobile communication at 2 GHz (ITU, 2002). This includes, e.g., a framework for services, the network architecture including satellite communica- tion, strategies for developing countries, requirements of the radio interface, spectrum considerations, security and management frameworks, and different transmission technologies. Nineteen ninety nine saw several more powerful WLAN standards. IEEE published 802.11b offering 11 Mbit/s at 2.4 GHz. The same spectrum is used by Bluetooth, a short-range technology to set-up wireless personal area networks with gross data rates less than 1 Mbit/s. The ITU dropped the plan of a single, worldwide standard for third generation mobile phone systems and decided on the IMT-2000 family concept that includes several technologies (UMTS, cdma2000, DECT etc. see chapter 4). The wireless application protocol (WAP) started at the same time as i-mode in Japan. While WAP did not succeed in the beginning, i-mode soon became a tremendous success (see chapter 10). The year 2000, came with higher data rates and packet-oriented transmis- sion for GSM (HSCSD, GPRS – see chapter 4). It should not be forgotten that the late nineties was the time when a lot of hype about the communications busi- ness started. Thus it was relatively easy for marketing people to portray third generation technology as high-performance Internet on mobile phones. In Europe, UMTS was announced as capable of handling live, interactive video streaming for all users at 2 Mbit/s. All technically-oriented people knew that this promise could not be fulfilled by the system, but the auctions and beauty con- tests for licensing 3G spectrum started. In Europe alone more than €100 billion had been paid before the disillusionment set in. Companies that had never run a network before paid billions for licenses. Many of these companies are now bankrupt and the remaining companies suffer from the debts. Most of the hype is over, but the third generation of mobile communication02Chap01 8804 (1-24) 30/5/03 11:03 am Page 14 14 Mobile communications Figure 1.3 cellular phones satellites cordless wireless Overview of some phones LAN wireless communication systems 1980: CT0 1981: NMT 450 1982: Inmarsat-A 1983: AMPS 1984: CT1 1986: NMT 900 1987: CT1+ 1988: Inmarsat-C 1989: CT 2 1991: 1991: 1991: CDMA D-AMPS DECT 199x: 1992: 1992: proprietary GSM Inmarsat-B Inmarsat-M 1993: PDC 1997: 1994: IEEE 802.11 DCS 1800 1998: Iridium 1999 802.11b, Bluetooth 2000: 2000: GPRS IEEE 802.11a 2001: IMT d-2000 analog digital 200?: Fourth Generation (Internet based) started in 2001 in Japan with the FOMA service, in Europe with several field trials, and in, e.g., Korea with cdma2000 (see Figure 4.2 for the evolution of 3G systems). IEEE released a new WLAN standard, 802.11a, operating at 5 GHz and offering02Chap01 8804 (1-24) 30/5/03 11:03 am Page 15 Introduction 15 gross data rates of 54 Mbit/s. This standard uses the same physical layer as HiperLAN2 does (standardized in 2000), the only remaining member of the HIPERLAN family. In 2002 new WLAN developments followed. Examples are 802.11g offering up to 54 Mbit/s at 2.4 GHz and many new Bluetooth applications (headsets, remote controls, wireless keyboards, hot syncing etc.). The network providers continued to deploy the infrastructure for 3G networks as many licens- ing conditions foresee a minimum coverage at a certain date. While digital TV via satellite has existed for several years, digital terrestrial TV (DVB-T, see chapter 6) started as regular service in Berlin in November 2002. This system allows for high- quality TV on the move and requires only an antenna of a few centimeters. Figure 1.3 gives an overview of some of the networks described above, and shows the development of cellular phone systems and cordless phones together with satellites and LANs. While many of the classical mobile phone systems con- verged to IMT-2000 systems (with cdma2000 and W-CDMA/UMTS being the predominant systems), the wireless LAN area developed more or less indepen- dently. No one knows exactly what the next generation of mobile and wireless system will look like, but, there are strong indicators that it will be widely Internet based – the system will use Internet protocols and Internet applications. While the current third generation systems still heavily rely on classical tele- phone technology in the network infrastructure, future systems will offer users the choice of many different networks based on the internet (see chapter 11). However, no one knows exactly when and how this common platform will be available. Companies have to make their money with 3G systems first. The dates shown in the figure typically indicate the start of service (i.e., the systems have been designed, invented, and tested earlier). The systems behind the acronyms will be explained in the following chapters (cellular and cordless 6 phones in chapter 4, satellites in chapter 5, WLANs in chapter 7). 1.3 A market for mobile communications Although the growth in wireless and mobile communication systems has slowed down, these technologies have still a huge market potential. More and more people use mobile phones, wireless technology is built into many cars, wireless data services are available in many regions, and wireless local area net- works are used in many places. Figure 1.4 shows the increasing number of subscribers to mobile phone ser- vices worldwide (GSM World, 2002). This figure shows the tremendous growth rates up to 2000. That growth continues today, mainly due to China that has the largest number of users. Figure 1.5 shows the cellular subscribers per region (GSM World, 2002). 6 Note that analog systems are not described.02Chap01 8804 (1-24) 30/5/03 11:03 am Page 16 16 Mobile communications Figure 1.4 1200 Mobile phone service subscribers worldwide 1000 (in millions) 800 600 400 200 0 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 Figure 1.5 Middle East; 1.6 Cellular subscribers Africa; 3.1 per region (June 2002) Americas (incl. USA/Canada; Asia Pacific; 22 36.9 Europe; 36.4 While the shares of Europe and China are almost equal, the market in Europe is saturated with second-generation GSM systems (mobile penetration is about 70 per cent). Countries such as Germany and France exhibited growth rates of 40 per cent or more in 1998. Europe’s share will decrease compared to China, the Americas, and Africa. 1.4 Some open research topics Although this book explains many systems supporting mobility and explores many solutions for wireless access, a lot remains to be done in the field. We are only at the beginning of wireless and mobile networking. The differences between wired, fixed networks and wireless networks open up various topics. The reader may find even more in, e.g., the book of the wireless world research forum (WWRF, 2002):02Chap01 8804 (1-24) 30/5/03 11:03 am Page 17 Introduction 17 ● Interference: Radio transmission cannot be protected against interference using shielding as this is done in coaxial cable or shielded twisted pair. For example, electrical engines and lightning cause severe interference and result in higher loss rates for transmitted data or higher bit error rates respectively. ● Regulations and spectrum: Frequencies have to be coordinated, and unfor- tunately, only a very limited amount of frequencies are available (due to technical and political reasons). One research topic involves determining how to use available frequencies more efficiently, e.g., by new modulation schemes (see chapter 2) or demand-driven multiplexing (see chapter 3). Further improvements are new air interfaces, power aware ad-hoc networks, smart antennas, and software defined radios (SDR). The latter allow for soft- ware definable air interfaces but require high computing power. ● Low bandwidth: Although they are continuously increasing, transmission rates are still very low for wireless devices compared to desktop systems. Local wireless systems reach some Mbit/s while wide area systems only offer some 10 kbit/s. One task would involve adapting applications used with high-bandwidth connections to this new environment so that the user can continue using the same application when moving from the desktop out- side the building. Researchers look for more efficient communication protocols with low overhead. ● High delays, large delay variation: A serious problem for communication protocols used in today’s Internet (TCP/IP) is the big variation in link char- acteristics. In wireless systems, delays of several seconds can occur, and links can be very asymmetrical (i.e., the links offer different service quality depending on the direction to and from the wireless device). Applications must be tolerant and use robust protocols. ● Lower security, simpler to attack: Not only can portable devices be stolen more easily, but the radio interface is also prone to the dangers of eaves- dropping. Wireless access must always include encryption, authentication, and other security mechanisms that must be efficient and simple to use. ● Shared medium: Radio access is always realized via a shared medium. As it is impossible to have a separate wire between a sender and each receiver, differ- ent competitors have to ‘fight’ for the medium. Although different medium access schemes have been developed, many questions are still unanswered, for example how to provide quality of service efficiently with different com- binations of access, coding, and multiplexing schemes (Fitzek, 2002). ● Ad-hoc networking: Wireless and mobile computing allows for spontaneous networking with prior set-up of an infrastructure. However, this raises many new questions for research: routing on the networking and application layer, service discovery, network scalability, reliability, and stability etc. A general research topic for wireless communication (and a source for endless discussion) is its effect on the human body or organisms in general. It is unclear if, and to what extent, electromagnetic waves transmitted from wireless devices can influence organs. Microwave ovens and WLANs both operate at the same frequency of 2.4 GHz. However, the radiation of a WLAN is very low (e.g.,02Chap01 8804 (1-24) 30/5/03 11:03 am Page 18 18 Mobile communications 100 mW) compared to a microwave oven (e.g., 800 W inside the oven). Additionally, as chapter 2 shows in more detail, propagations characteristics, absorption, directed antennas etc. play an important role. Users, engineers, researchers and politicians need more studies to understand the effect of long- term low-power radiation (Lin, 1997), BEMS (2002), COST (2000), NIEHS (2002). The World Health Organization (WHO) has started a worldwide project on elec- tromagnetic fields (WHO, 2002). 1.5 A simplified reference model This book follows the basic reference model used to structure communication systems (Tanenbaum, 2003). Any readers who are unfamiliar with the basics of communication networks should look up the relevant sections in the recom- mended literature (Halsall, 1996), (Keshav, 1997), (Tanenbaum, 2003), (Kurose, 2003). Figure 1.6 shows a personal digital assistant (PDA) which provides an example for a wireless and portable device. This PDA communicates with a base station in the middle of the picture. The base station consists of a radio trans- ceiver (sender and receiver) and an interworking unit connecting the wireless link with the fixed link. The communication partner of the PDA, a conventional computer, is shown on the right-hand side. Underneath each network element (such as PDA, interworking unit, com- puter), the figure shows the protocol stack implemented in the system according to the reference model. End-systems, such as the PDA and computer in the example, need a full protocol stack comprising the application layer, transport layer, network layer, data link layer, and physical layer. Applications Figure 1.6 Simple network and reference model used in this book Application Application Transport Transport Network Network Network Network Data Link Data Link Data Link Data Link Physical Physical Physical Physical Radio Medium02Chap01 8804 (1-24) 30/5/03 11:03 am Page 19 Introduction 19 on the end-systems communicate with each other using the lower layer services. Intermediate systems, such as the interworking unit, do not necessarily need all of the layers. Figure 1.6 only shows the network, data link, and physical layers. As (according to the basic reference model) only entities at the same level communicate with each other (i.e., transport with transport, network with net- work) the end-system applications do not notice the intermediate system directly in this scenario. The following paragraphs explain the functions of each layer in more detail in a wireless and mobile environment. ● Physical layer: This is the lowest layer in a communication system and is responsible for the conversion of a stream of bits into signals that can be transmitted on the sender side. The physical layer of the receiver then transforms the signals back into a bit stream. For wireless communication, the physical layer is responsible for frequency selection, generation of the carrier frequency, signal detection (although heavy interference may disturb the signal), modulation of data onto a carrier frequency and (depending on the transmission scheme) encryption. These features of the physical layer are mainly discussed in chapter 2, but will also be mentioned for each system separately in the appropriate chapters. ● Data link layer: The main tasks of this layer include accessing the medium, multiplexing of different data streams, correction of transmission errors, and synchronization (i.e., detection of a data frame). Chapter 3 discusses different medium access schemes. A small section about the specific data link layer used in the presented systems is combined in each respective chapter. Altogether, the data link layer is responsible for a reliable point-to- point connection between two devices or a point-to-multipoint connection between one sender and several receivers. ● Network layer: This third layer is responsible for routing packets through a network or establishing a connection between two entities over many other intermediate systems. Important topics are addressing, routing, device loca- tion, and handover between different networks. Chapter 8 presents several solutions for the network layer protocol of the internet (the Internet Protocol IP). The other chapters also contain sections about the network layer, as routing is necessary in most cases. ● Transport layer: This layer is used in the reference model to establish an end-to-end connection. Topics like quality of service, flow and congestion control are relevant, especially if the transport protocols known from the Internet, TCP and UDP, are to be used over a wireless link. ● Application layer: Finally, the applications (complemented by additional layers that can support applications) are situated on top of all transmission- oriented layers. Topics of interest in this context are service location, support for multimedia applications, adaptive applications that can handle the large variations in transmission characteristics, and wireless access to the world wide web using a portable device. Very demanding applications are video (high data rate) and interactive gaming (low jitter, low latency).

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