Apple 1004
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`Apple 1004
`
`

`
`Jochen H. Schiller
`
`
`
`VAV Addison-Wesley
`
`An imprint of PEARSON EDUCATION
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`
`hat will computers look like in ten years, in the next century?
`
`No wholly accurate prediction can be made, but as a general feature,
`
`most computers will certainly be portable. How will users access net-
`
`works with the help of computers or other communication devices? An
`
`ever-increasing number without 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 characteristics 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.
`ln 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 passen-
`gers. Furthermore, 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.
`Before presenting more applications, definitions of the terms ‘mobile’ and
`
`‘wireless’ as used throughout this book should be given. There are two different
`
`kinds of mobility: user mobility and device portability. User mobility refers to a
`user who has access to the same or similar telecommunication services at differ-
`
`ent 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).
`With device portability} the communication device moves (with or with-
`out a user). Many mechanisms in the network and inside the device have to
`
`make sure that communication is still possible while it 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 station) 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.
`
`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
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`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:
`
`0
`
`Fixed and wired: This configuration describes the typical desktop computer
`in an office.
`
`0
`
`0 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 modern.
`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.
`0 Mobile and wireless: This is the most interesting case. No cable restricts
`the user, who can roam between different wireless networks. Most technolo-
`gies discussed in this book deal with this type of devices and the networks
`supporting them.
`
`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 millions
`of users of wireless devices today, the market potential is still enormous.
`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 the
`details of each technology and systems presented.
`
`1.1 Applications
`
`Although wireless networks and mobile communications can be used for many
`applications, particular application environments seem to be predestined for
`their use. Some of them will be enumerated in the following sections ~ it is left
`
`to you to imagine more.
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`1.1.1 Vehicles
`
`‘Tomorrow's cars will comprise many wireless communication systems and
`mobility aware applications. Music, news, road conditions, weather reports, and
`other broadcast information is received via digital audio broadcasting (DAB)
`with 1.5 Mbit/s. For personal communication, a global system for mobile com-
`munications (GSM) phone might be available offering voice and data
`connectivity with 384l<bit/s. For remote areas satellite communication can be
`
`used, while the current position of the car is determined via global positioning
`system (GPS). Additionally, cars driving in the same area build a local ad hoc
`network for fast information exchange in emergency situations or to help each
`other keeping a safe distance. In case of an accident, not only will the airbag be
`triggered, but also an emergency call to a service provider informing ambulance
`and police. Cars with this technology are already available. 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 the accident. Buses, trucks,
`and trains are already transmitting maintenance and logistic information to
`their home base, which helps to improve organization (fleet management), and
`thus save 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). Additionally, satellite communication links can be used. The net-
`
`works between cars and also inside a car will more likely work in an ad hoc
`fashion. Wireless pico networks inside a car can comprise PDAS, laptops, or
`mobile phones, e. g., connected with each other using the Bluetooth technology.
`
`DAB, GSM,
`
` UMTS, WLAN,
` 1.
`Bluetooth,
`
`Personal Travel Assistant,
`DAB, PDA, laptop,
`GSM, UMTS, WLAN,
`
`Figure 1.1
`
`A typical application of
`mobile communications:
`road traffic
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`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 appen-
`dix A, which contains the complete list.
`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, current trains up to 350 km/h, many technologies cannot operate 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.
`
`1.1.2 Emergencies
`
`Just imagine the possibilities of an ambulance with a high-quality wireless con-
`nection to a hospital. After an accident, vital information about injured persons
`can be sent to the hospital immediately. There, all necessary steps for this partic-
`ular type of accident can be prepared or further specialists can be consulted for
`an early diagnosis. Furthermore, wireless networks are the only means of com-
`munication in the case of natural disasters such as hurricanes or earthquakes. In
`the worst cases only decentralized, wireless ad hoc networks survive. The break-
`down 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
`
`Today’s typical travelling salesman needs instant access to the company's data-
`base: to ensure that files on his or her laptop reflect the actual state, to enable
`the company to keep track of all activities of their travelling employees, to keep
`databases consistent etc. With wireless access, the laptop can be turned into a
`true mobile office.
`
`1.1.4 Replacement of wired networks
`
`In some cases, wireless networks can also be used to replace wired networks, as
`for remote sensors, for tradeshows, or in historic buildings. Due to economic
`reasons, it is often impossible to wire remote sensors in cases such as weather
`
`forecast, earthquakes detection, or 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, therefore, use WLANs as a replacement
`for cabling. Other cases for wireless networks are computers, sensors, or infor-
`mation displays in historical buildings, where it is crucial not to add more
`cabling than necessary to avoid the destruction of 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
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`_ 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 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. Another growing field of wireless
`network applications lies in entertainment and games in order to enable, e.g., ad
`hoc gaming networks as soon as people meet to play together.
`
`1.1.6 Location dependent services
`
`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 9.1), 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 it might be that the user needs location information
`for further activities. Several services that might depend on the actual location
`
`can be distinguished:
`
`o
`
`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-
`rects incoming calls.-2 Using mobile computers, a follow—on service could,
`for instance, offer the same desktop environment wherever you are around
`the world. All e-mail would automatically be forwarded, all changes to your
`
`desktop and documents stored, at a central location at your company. If
`someone wanted to reach you using a multimedia conferencing system, this
`
`call would then also be forwarded to your current location.
`
`0
`
`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! But without additional informa-
`
`tion about your environment, this might be the only thing you can do.
`Therefore, services are needed that provide information about the capabili-
`ties of your environment. For instance, there could be a service in the hotel
`
`Figure 1.2
`A location-aware travel
`
`guide with wireless
`network access
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`announcing that a standard laser printer is available in the lobby, a colour
`printer in a hotel meeting room etc. In return, your computer might then
`transmit your personal profile to your environment, so that the hotel can
`charge 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. So there might
`be locations and/or times when you want to exclude certain services from
`reaching you, thereby telling the caller that you do not want to be dis-
`turbed. Furthermore, although you want to utilize location dependent
`services, you might not want the environment to know exactly who you
`are. Imagine a hotel monitoring all guests and selling these profiles to com-
`panies for advertisements.
`Information services: While walking around in a city you could always use
`your wireless travel guide to ‘pull’ information from a service, e.g., ‘Where
`is the next Mexican restaurant to my current position?’ But a service could
`also actively ‘push’ information on your travel guide, e.g., that the Mexican
`restaurant just around the corner has a special taco offer.
`Support services: Finally, many small additional mechanisms can be inte-
`grated to support a mobile device. Intermediate results of calculations, state
`information, or cache contents could ‘follow’ the mobile node through the
`fixed network. As soon as the mobile node reconnects, all information is
`available again. This helps to reduce access delay and traffic within the fixed
`network. The alternative would be a central location for user information
`and a user accessing this information through the (possibly large and con-
`gested) network all the time as it is typically done today.
`
`1.1.7 Mobile and wireless devices
`Even though many mobile and wireless devices are already available, we will see
`many more in the future. There is no precise classification of such devices, by
`size, shape, weight, or computing power. Currently, laptops are considered to be
`the upper end of the mobile device range.3 The following list gives some exam-
`ples of mobile and wireless devices graded by increasing performance (CPU,
`memory, display, input devices etc.).
`
`0
`
`0
`
`Sensor: A very simple wireless device is represented by a sensor transmitting
`state information. An example for such a sensor could be a switch sensing
`the office door. If the door is closed, the switch transmits this state to the
`mobile phone inside the office and the mobile phone will not accept
`incoming calls. Thus, 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.
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`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 phone could switch off the dryer 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.
`0 Mobile phones: The traditional mobile phone only had a simple black and
`white text display and could send/receive voice or short messages. Today,
`however, mobile phones migrate more and more toward PDAs. Mobile
`phones with full colour graphic display, touch screen, and Internet browser
`are available.
`
`0
`
`a
`
`Personal digital assistant: PDAS typically accompany a user and offer Very
`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 already available for these devices.
`Palmtop/pocket computer: The next step toward full computers are
`pocket computers offering tiny keyboards, colour displays, and simple ver-
`sions of programs found on desktop computers (text processing, spread
`sheets etc.).
`
`o Notebook/laptop: Finally, laptops offer more or less the same performance
`as standard desktop computers, use the same software, the only technical
`difference being size, weight, and the ability to run on a battery.
`
`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 been solved yet, is the energy supply. The more features
`are built into a device, the more power it needs. The higher the performance 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 devel-
`oping 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. Thus, smaller keyboards are used,
`which are frequently clumsy for typing due to their limited key size. Furthermore,
`small displays are often useless for graphical display, and a higher resolution of
`the display does not help as the limiting factor is the resolution capacity of the
`human eye. Therefore, 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
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`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. While no-one can predict the future of communication devices pre-
`cisely, it is quite clear, that there will still be many fixed systems, complemented
`by a myriad small wireless computing devices all over the world.
`
`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. Using light and flags for wireless communi-
`cation remained important
`for the navy until
`radio transmission was
`introduced. But even today a sailor has to know some codes represented by flags
`if all other means of wireless communication fail.
`
`it was not until the end of the 18th century, when Claude Chappe
`invented the optical telegraph (1794), that long-distance wireless‘communica-
`tion was possible. Almost until the end of the following century optical
`telegraph lines were built.
`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 the 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. Furthermore, at that time it was not possible
`to focus light as efficiently as can be done today by means of a laser. Therefore,
`Wireless communication did not really take off until the discovery of electro-
`magnetic waves and the development of equipment to modulate them. It all
`started with Michael Faraday (and about the same time Joseph Henry) demon-
`strating electromagnetic 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 demon-
`
`“
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`Finally, in 1965, the first commercial geostationary communication satellite
`INTELSAT 1, also known as ‘Early Bird’, went into operation. It was in service
`for 1.5 years, Weighing 68 kg and offering 240 duplex telephone channels or
`alternatively a single TV channel. INTELSAT 2 followed in 1967, INTELSAT 3 in
`
`1969 already offering 1,200 telephone channels. While communication on land
`always provides the alternative of using wires, this is not the case for ships at
`sea. Therefore, three MARISAT satellites went into operation in 1976 which
`offered worldwide maritime communication. Still, sender and receiver had to be
`
`installed on the ships with large antennas (1.2 m antenna, 40 W transmit
`power). The first mobile satellite telephone system, INMARSAT-A, was intro-
`duced in 1982. Six years later, INMARSAT-C followed as the first satellite system
`offering mobile phone and data services. (Data rates are about 600 bit/s, inter-
`faces to the X25 packet data network exist.) In 1993, satellite telephone systems
`finally became fully digital. The actual mobility, however, was relative from a
`user's point of view, as the devices needed for communication via geostationary
`satellites are heavy (several kilograms) and need a lot of transmit power to
`achieve decent data rates. Nineteen ninety-eight marked the beginning of a new
`age of satellite data communication with the introduction of global satellite sys-
`tems for small mobile phones, such as, e.g., Iridium and Globalstar (see section
`5.7). The current number of almost 200 geostationary satellites for commercial
`use shows the impressive growth of satellite communication over the last 30
`years (Miller, 1998), (Maral, 1998), (Pascall, 1997).
`
`5.2 Applications
`
`Traditionally, satellites have been used in the following areas:
`
`0
`
`a Weather forecasting: Several satellites deliver pictures of the earth using,
`e.g., infrared or visible light. Without the help of satellites forecasting of
`hurricanes would be impossible.
`Radio and TV broadcast satellites: Hundreds of radio and TV programmes are
`available via satellite. This technology competes with cable in many places, for
`it is cheaper to install and in most cases no extra fees have to be paid for this
`service. Today's satellite dishes have diameters of 30-40 cm in central Europe,
`whereas the diameters in northern countries are slightly larger.
`0 Military satellites: One of the earliest applications of satellites was their use
`for carrying out espionage. In addition to that, many communication links
`are managed via satellites for they are much safer from attack by enemies.
`Satellites for navigation: Even though it was only used for military pur-
`poses in the beginning, the global positioning system (GPS) is nowadays
`well~known and available for everyone. The system allows for precise local-
`ization worldwide, and with some additional techniques, the precision is in ‘
`the range of some metres. Almost all ships and aircraft rely on GPS as an
`
`o
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`A lthough this book mostly deals with different communication techno1o~
`
`gies allowing individual two-way communication, it is important to
`understand the role of unidirectional broadcast systems within future
`mobile communication scenarios. Typical broadcast systems, such as radio and
`television, distribute information regardless of the needs of individual users. As
`an addition to two-way communication technologies, broadcasting information
`can be very cost effective.
`Future television and radio transmissions will be fully digital. Already sev-
`eral radio stations produce and transmit their programmes digitally via the
`Internet or digital radio (see later sections in this chapter). Digital television is
`on its way. Besides transmitting video and audio, digital transmission allows for
`the distribution of arbitrary digital data, i.e., multimedia information can
`accompany radio and TV programmes at Very low cost compared to individual
`wireless connections.
`
`The following sections give a general introduction into asymmetric commu-
`nication up to the extreme case of unidirectional broadcasting. One important
`issue is the cyclic repetition of data (as discussed in the sections about broadcast
`disks). A broadcasting system which will be explained in detail is digital audio
`broadcasting (DAB), which is already standardized and in use. One interesting
`feature with respect to data communication is the ability of DAB to carry multi-
`media information. Finally, one possible successor to today's analog TV system
`is presented: digital Video broadcasting (DVB). In combination with satellite
`transmission and the use of the Internet, this system is able to deliver high
`bandwidth to individual customers at low cost (ETSI, 1999).
`
`6.1 Overview
`
`Unidirectional distribution systems or broadcast systems are an extreme version
`of asymmetric communication systems. Quite often, bandwidth limitations, dif-
`ferences in transmission power, or cost factors prevent a communication system
`from being symmetrical. Symmetrical communication systems offer the same
`transmission capabilities in all communication directions, i.e., the channel char-
`acteristics from A to B are the same as from B to A (e.g., bandwidth, delay, costs).
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`6.4 Digital video broadcasting
`
`The logical consequence of applying digital technology to radio broadcasting is
`doing the same for the traditional television system. The analog system used
`today has basically remained unchanged for decades. The only invention worth
`mentioning was the introduction of colour TV for the mass market back in the
`
`1960s. Therefore, television still uses the low resolution of 768x576 for the
`
`European PAL system or 720x460 for the US NTSC respectively. The display is
`interlaced with 25 or 30 frames per second respectively. So, compared with
`today's computer displays with resolutions of 1,Z80><1,024 and more than 75 Hz
`frame rate, non—interlaced, TV performance is not very impressive.
`There have been many attempts to change this and to introduce digital TV
`with higher resolution, better sound and additional features, but no approach
`has yet been truly successful. One reason for this failure is the huge base of
`installed old systems that will not be replaced as fast as is done with computers
`(we can watch the latest movie on an old TV, but it is impossible to run new
`software on older cornputersl). Furthermore, varying political and economic
`interests are counterproductive to a common standard for digital TV. One
`approach toward such a standard is presented in the following sections.
`After some national failures in introducing digital TV,
`the so—called
`European Launching Group was founded in 1991 with the aim of developing a
`common digital television system for Europe. In 1993 these common efforts
`were named digital video broadcasting (DVB) (Reimers, 1998), (DVB, 1999).
`Although the name shows a certain affinity to DAB, there are some fundamental
`differences regarding the transmission technology, frequencies, modulation etc.
`The goal of DVB is to introduce digital television broadcasting using satellite
`transmission (DVB—S, (ETSI, 1997c)), cable technology (DVB—C, (ETSI, 1998b)),
`and at a later stage also terrestrial transmission (DVB—T, (ETSI, 1997b)).
`Figure 6.7 shows components that should be integrated into the DVB archi-
`tecture. The centre point is an integrated receiver-decoder (set—top box)
`connected to a high-resolution monitor. This set—top box can receive DVB sig-
`nals via satellites, terrestrial local/regional senders (multipoint distribution
`systems, terrestrial receiver), cable, B-ISDN, ADSL, or other possible future tech-
`nologies. Cable, ADSL, and B-ISDN connections also offer a return channel, i.e.,
`a user can send data such as channel selection, authentication information, or a
`shopping list. Additionally, audio/Video streams can be recorded, processed, and
`replayed using digital Versatile disk (DVD), digital video tape recorder
`(DVTR) or multimedia PCS. Different levels of quality are envisaged: standard
`definition TV (SDTV), enhanced definition TV (EDTV), and high definition
`TV (HDTV) with a resolution of up to 1,920><1,08O pixels.
`Similar to DAB, DVB also transmits data using flexible containers. These
`containers are basically MPEG-2 frames that do not restrict the type of informa-
`tion. DVB sends service information contained in its data stream, which
`specifies the content of a container. The following contents have been defined:
`
`
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`Figure 6.7
`
`Digital video
`broadcasting scenario
`
`Multipoint
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