- Email: [email protected]
Multimedia systems based on satellite technology
Computer Networks and ISDN Systems 30 (1998) 1543–1549
Multimedia systems based on satellite technology ´ ngel Ferna´ndez del Campo, Carlos Miguel Nieto, Francisco Javier Ruiz Pin˜ar Ł , A Leo´n Vidaller Siso´, Antonio Martı´nez Mas, Juan Antonio Carral Pelayo Departamento de Ingenierı´a de Sistemas Telema´ticos, ETSI Telecomunicacio´n, Universidad Polite´cnica de Madrid, Ciudad Univeritaria, 28040 Madrid, Spain
Abstract In this paper we review the evolution of satellite based multimedia systems. We focus especially on digital TV technology over satellite. This architecture takes advantage of the broadcasting capability of satellite systems to provide a low cost broadcast link towards a huge population of receivers. This ability makes the system especially suitable for the provision of multimedia services targeted to the mass public, although a return channel must be provided in order to support interactive services. Terrestrial networks like the Integrated Services Digital Network (ISDN) may provide this return channel. 1998 Elsevier Science B.V. All rights reserved. Keywords: Satellite; Broadcast; Multimedia; Digital TV technology; Terrestrial return path
1. Introduction Satellite communications have a great advantage over any other communication network whenever the information is targeted to a large population of receivers or when they are spread over big areas. That is the reason why they are specially suitable for the provision of multimedia applications like Tele-education and Tele-conferencing. In this work we present the architecture of multimedia systems based on satellite. We focus especially on architectures based on satellite broadcast and terrestrial return paths. The case of digital TV technology is studied in more detail. The digital TV technology was designed to target a huge market. Therefore, its cost will beneŁ Corresponding
author. Tel.: C34-91-5495700-474; Fax: C3491-3367333; E-mail: [email protected].
fit from economics of scale, allowing the use of low cost equipment (compared to the cost of traditional VSAT (Very Small Aperture Terminal) [1] equipment). Furthermore, the integration with other services available in the digital platform (sharing the satellite link) is an added value to this solution.
2. VSAT networks The main characteristic of communication systems using satellite technology, compared with traditional terrestrial networks, is the ability of broadcasting, the capability to transmit a message to a large number of destinations that may be spread over a broad area. The transmission costs are independent from both the number of receivers and its geographical distribution [1].
0169-7552/98/$ – see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 7 5 5 2 ( 9 8 ) 0 0 1 8 9 - 5
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F.J. Ruiz Pin˜ar et al. / Computer Networks and ISDN Systems 30 (1998) 1543–1549
Most of the satellite networks deployed through the past years are based on VSAT technology. VSAT networks are generally arranged in a star topology. In this architecture the satellite resources are organised in an outbound carrier (received by every VSATs in the network) and, possibly, several inbound channels (shared between the VSATs to access the hub of the star). The hub transmits on the outbound carrier, typically using ATDM (Asynchronous Time Division Multiplex). The information transmitted in this link can be targeted to a unicast destination address (only one VSAT station), a multicast destination address (a subset of the VSAT stations) or a broadcast destination address (every VSAT station in the network) [1]. A satellite-only VSAT network has a set of inbound channels to provide the return path from the VSATs to the hub. These channels can be organised as a set of carriers (FDMA, Frequency Division Multiple Access) and as a sequence of time slots (TDMA, Time Division Multiple Access). Each carrier or time slot can be assigned to a VSAT station on demand (DAMA, Demand Assignment Multiple Access) or shared between a group of VSATs using a contention protocol. The second approach result in better performance (in terms of bandwidth) when the traffic load is low and bursty, otherwise DAMA methods are more suited. In this star topology direct communication (one satellite hop) between VSATs is not possible. It requires a double hop communication (VSAT–Hub– VSAT). At the beginning, the reasons for this limitation were mainly technological (maximum transmitted power by the VSATs), and precluded the use of star VSAT networks to support applications very sensitive to a high propagation delay (two satellite hops D 0.54 s). However, today technology offers the possibility of one-hop communication between VSATs, reducing the delay to 0.27 s. In addition to the broadcasting capability provided by the hub, Meshed VSAT networks also support point to point communication [1].
3. VSAT-based multimedia systems There are several examples of VSAT networks supporting multimedia applications that benefit from the broadcasting capabilities of satellite technology.
As an example we will examine the CODE VSAT network [2]. The CODE network was a joint development of the Department of Telematic Systems Engineering of the Technical University of Madrid and Telefo´nica Sistemas de Sate´lites, under contract of the European Space Agency. It has been used to support many Tele-education projects, like TEN (Telematics ET1022, Trans European TeleEducation Network), or ETSIT [3], a Spanish project in which the Telecommunication Engineering Schools of Spain were interconnected using CODE to build a Teleeducation network. CODE is a star/mesh VSAT network, with an outbound carrier allowing transmission up to 2 Mbps, and a set of FDMA carriers, up to 64 kbps each, for the provision of inbound and mesh channels. Inbound channels can be shared by a set of VSATs using a contention protocol, or can be allocated to a VSAT upon request. At present, a configuration with 1.5 Mbps outbound channel, and 56 kbps inbound channels is used in the TEN network. In order to integrate CODE with other networks, CODE nodes behave as IP routers, with one interface to the satellite network, and support for additional interfaces to other networks, like LAN Ethernet. In this way, CODE can be used to provide interconnection of TCP/IP networks. Furthermore, support of IP multicast [6] is included in order to take advantage of the broadcast capability of the satellite. The Tele-education network supported by CODE is made up of a central node (the one acting as teacher) that broadcasts the class to all the participants (students). Broadcast information includes not only audio and video from the teacher, but also it may include some other information like slides, keyed text, scanned text and figures, and multimedia application control messages. The participants can ask questions during the class, and both the teacher and every other participant in the class receive their interventions. To support this service the CODE network is configured so that both audio and video from the teacher are broadcast to the remote participants using the outbound channel. Since every VSAT receives this channel, they will all be able to receive the class. Whenever a student wants to ask a question or make a remark, the network must provide a return link back to the teacher. In CODE an inbound channel is allocated to
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Fig. 1. TEN system based on CODE network.
the remote VSAT so that the student’s audio and video can reach the hub of the network. From there, they are broadcast to the other VSATs. In this way every student in the class can watch and listen to the question. The whole effect is that of a virtual classroom. CODE is configured to work in the star topology since no direct communication between VSATs is needed. Fig. 1 shows the system architecture. We can see in Fig. 1 that there is a strong asymmetry between the outbound (1.5 Mbps) and the inbound channels (56 kbps), and so are the information flows from the teacher to the students and from a student to the class. However, it does not affect an application such Tele-education where the information flows are also asymmetric. To support the concept of a virtual classroom, we need to provide a fairly good quality of audio and video from the teacher to the students. But this is not true the other way around. The students take an active roll just to make a question or a comment. It is important that every other class attendee can listen to the question but the video image is not so important. To create the
‘feeling of presence’ of the remote participant a low quality video image or even a still picture is enough. So, the total information flow from the participant will require much less bandwidth than that of the teacher, and an inbound channel can handle it. This participant flow must also be broadcast through the outbound channel back to the other participants, but this does not affect the performance of the system as long as the aggregate bandwidth does not exceed the outbound capacity. A VSAT station with full satellite transmission capability is needed to support a satellite based return channel and so the total cost of each VSAT is quite high (a modulator and a power amplifier are needed). This transmission equipment is only used for point to point communication (from the VSAT to the Hub), while the actual broadcast is done in the Hub. Point to point communications are better handled (in terms of cost) by terrestrial networks. So a hybrid architecture with satellite broadcasting and terrestrial return channels is envisaged as a cost-effective solution.
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Fig. 2. TEN system based on CODE network with mixed ISDN and VSAT return.
4. VSAT-based multimedia systems with terrestrial return channel As we mentioned above, the use of terrestrial links (like the Integrated Services Digital Network, ISDN) instead of satellite inbound channels allows the design of receive-only VSAT stations. The return channel is provided through an ISDN basic access (two B-channels, 64 C 64 kbps) so a better quality can be provided in the communication from student to teacher. The only concern with this solution is the availability of an ISDN access due to geographic issues or underdeveloped regions. In this way, an ISDN access card can replace the transmission chain of the VSAT station. This will decrease the equipment cost because the ISDN hardware for the PC platform has reached the mass-
market status. Additional ISDN hardware will be needed in the hub so that it can provide the remote access service. Fig. 2 shows the architecture of a system with hybrid return. There is a mix of terrestrial return stations and satellite return stations. ISDN return links are used whenever they are available otherwise a full transmit–receive VSAT station is used. The equipment in the reception chain of the outbound channel is common to both cases since this channel provides the necessary broadcast capability to support the Teleeducation application. This approach was chosen for the TEN network which is now in operation and with fairly good valuation from the users. Although the TEN network was designed to support a Tele-education application there are many other examples that use this architecture. Perhaps the
F.J. Ruiz Pin˜ar et al. / Computer Networks and ISDN Systems 30 (1998) 1543–1549
best known of them is the Hughes DirecPC [7]. This system is used to provide a low cost Internet access. It uses the satellite broadcast link to support forward channels (service provider to clients) of up to 400 kbps, even more under a ‘business access’ contract. Return access (from the client to the service provider) is provided by a terrestrial network like the telephone network, using a conventional analogue modem. User equipment costs are low because the antenna is very small and the communication equipment is based on low cost PC cards. This system is strongly asymmetric but so is the Internet access for residential users. An ‘average’ user receives much more data than it sends. Web browsing is a clear example of this behaviour. The Hughes system takes advantage of the coverage provided by satellite networks to provide highspeed access to areas where the only available infrastructure is the phone network (with its speed limitations). And it does it at an affordable cost since the user equipment is mass market.
5. Multimedia systems based on satellite digital TV technology The Hughes system proves that the key aspect in the development of a cost-effective satellite based multimedia service is the use of a low-cost massmarket production technology. Another technology that fits very well with that rule is the newly designed digital TV platforms, based on satellite broadcasting. Digital TV via satellite allows the transmission of different programmes over a multiplexed transport stream. The stream is composed of several audio and video flows. Furthermore, other streams are provided to control the mapping of the different audio and video streams to each programme. This stream known as the MPEG-2 transport stream is defined in the ISO 13818-1 standard (ITU-T Rec. H.262) [4]. As defined in the standard, the MPEG-2 multiplex can be used not only for TV audio and video transmission, but also for the transmission of data flows carrying any kind of information. This is due to the digital nature of the MPEG-2 multiplex, which acts as a container of any information. Taking advantage of this capability a system based on satellite digital TV for the broadcasting link and terrestrial return is
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envisaged. Again, low cost mass-market production equipment is the key to reduce the receiver stations cost to a minimum. The remote station can now be a multimedia PC equipped with a digital TV receiver and demultiplexer card attached to the corresponding dish antenna, and with a ISDN card or telephone modem to provide the return channel. This scheme can be seen in Fig. 3. Several equipment manufacturers and software companies have already foreseen the benefits of such an arrangement. For instance, Comstream [9] and Philips [10] have developed PC cards for MPEG-2 satellite reception, to be used in data broadcasting architectures. Microsoft has also developed a specification of protocol stack for PCs equipped with these receivers and the corresponding drivers [8]. Two approaches are possible for the transmission of the multimedia flows: (1) The different Tele-education application data flows are handled as a whole. They are encapsulated into an MPEG-2 transport stream as described in ISO 13818-6 [5]. The receiver stations are PCs equipped with MPEG-2 demultiplexer cards. They extract this flow from the multiplexed transport stream received from the satellite. The demultiplexed flow is then delivered to the multimedia application. The application must handle the demultiplexing of the different data flows: video, audio, slides, control, etc. (2) Audio and video flows are handled as belonging to a TV programme, MPEG-2 encoded and inserted into the multiplexed stream in their corresponding flows. Other information flows (slides, scanners, application control) are encapsulated in a different MPEG-2 flow. With this scheme the receiver can be based in a standard set-top box and the audio and video are presented in a TV set as any other TV programme. The other information flows can be extracted and passed to a PC via an RS-232 interface or via a PCMCIA card. Many commercial set-top boxes support this configuration. Both scheme benefits from a mass-market technology thus decreasing the total cost of the network, compared with that of the network based on proprietary satellite technology and protocols. Work is currently undertaken to migrate the TEN Tele-education application to a satellite environment
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Fig. 3. Tele-education system based on digital TV satellite technology.
based on digital TV technology. A PC running Windows NT 4.0 operating system has replaced the hub. This PC hosts ISDN cards to provide the return access service to the remote terminal users. In addition, it hosts a card based on transputer technology, which is responsible for the encapsulation of IP packets into an MPEG-2 transport stream. The card provides a HOTLink interface [11] to which a DVB satellite modulator is connected. The output of this modulator is then passed to the transmission chain and broadcast to the remote terminals. Remote terminals are multimedia PCs (equipped with audio and video cards, camera and microphone). These PCs also have an ISDN card, for provision of the return channel, and a Comstream DVB demultiplexer card. This card is connected to the receiving chain (dish antenna and LNB), allowing the reception of the MPEG-2 stream. The drivers for this card are responsible for the reconstruction of the original IP packets transmitted at the central
node and to deliver them to the multimedia application have also been developed.
6. Conclusions In this paper we have studied different satellite architectures to support multimedia services. We focused on those services with strong requirements of broadcast or multicast, like Tele-education. A case study based on the TEN Tele-education system has been presented. Several architectures have been examined, each with lower cost and greater possibilities than the previous one. The final one, expected to play a major role in the near future, makes use of a low cost technology such as digital TV over satellite. This makes it possible to build low cost multimedia systems based on satellite technology, very suitable for multimedia applications like Tele-education and Tele-conference.
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References [1] J. Everett (Ed.), VSATs, Very Small Aperture Terminals, Peter Peregrinus Ltd., 1992. ´ lvarez, E. Go´mez-Leal, L. Vidaller, A. Ferna´ndez, [2] M.L. A The use of transputers in VSAT systems, in: Proc. Olympus Utilisation Conf., Seville, Spain, April 1993. [3] L. Vidaller, A. Ferna´ndez, A. Martı´nez, C. Miguel, E. Go´mez-Leal, A. Ruiz, Tele-education using the CODE network, in: Proc. Olympus Utilisation Conf., Seville, Spain, April 1993. [4] ISO-13818-1, Information technology, Generic coding of moving pictures and associated audio information, Part 1: Systems, International Standard. [5] ISO-13818-6, Information technology, Generic coding of moving pictures and associated audio information, Part 6: Extension for digital storage media command and control (DSM-CC), International Standard. [6] S. Deering, Host extensions for IP multicast, RFC 1112, 1988. [7] Hughes DirecPC Personal Edition, Web document, Hughes, http://www.dssdirectv.com/dirpc.htm. [8] Introduction to broadcast architecture, Web document, Microsoft, January 1998, http://www.eu.microsoft.com/hwdev /desinit/bcast1.htm. [9] Comstream MediaCast Satellite PC/Server Receiver Card, Product data sheet, Comstream Co., 1997. [10] Philips, CleverCastPC high speed multimedia broadcasting to the PC, Product information, Philips Electronics N.V., 1997, http://www.dvs.be.philips.com/dvs/products/dct.htm. [11] Cypress Semiconductor, HOTLink design considerations, Application note, 3/5/1995. Francisco Javier Ruiz Pin˜ar received a degree in Telecommunication Engineering in 1990 and the Ph.D. degree in Telecommunication Engineering in 1994. He is Associate Professor of the Department of Telematic Systems Engineering (DIT) of the Madrid Technical University (UPM). Since 1990 he has been involved in R&D activities related to the following topics: software design, protocol design and satellite communications. Angel Ferna´ndez del Campo received a degree in Telecommunication Engineering in 1979 and the Ph.D. degree in Telecommunication Engineering in 1987. He is Associate Professor of the Department of Telematic Systems Engineering (DIT) of the Madrid Technical University (UPM). Since 1979 he has been involved in R&D activities related to the following topics: software design, software engineering, digital transmission,
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data communication, local and metropolitan networks, formal description techniques and parallel architectures for communication equipments. He has given courses, conferences, authored about twenty publications on national and international magazines and congress, and one book. Carlos Miguel Nieto received a degree in Telecommunication Engineering in 1986 and the Ph.D. degree in Telecommunication Engineering in 1991. He is an Associate Professor of the Department of Telematic Systems Engineering (DIT) of the of the Madrid Technical University (UPM). Since 1986 he has been involved in R&D activities related to the following topics: software design, software engineering, digital transmission, data communication, local and metropolitan networks, formal description techniques and parallel architectures for communication equipments. Leo´n Vidaller Siso´ received a degree in Telecommunication Engineering in 1974 and the Ph.D. degree in Telecommunication Engineering in 1977. He is Professor of the Department of Telematic Systems Engineering (DIT) of the Madrid Technical University (UPM). Since 1974 he has been involved in R&D activities related to the following topics: electronic design, analog and digital transmission, error correcting codes, data communication, local and metropolitan networks, criptography and security. He has been project leader of more than 16 industry and public supported contracts. He is an industry consultant on data communications systems, and book reviewer for IEEE Communication Magazine. He has given courses, conferences, authored about fifty publications on national and international magazines and congress, and four books. Antonio Martı´nez Mas received a degree in Telecommunication Engineering in 1984 and the Ph.D degree in Telecommunication Engineering in 1989. He is an Associate Professor of the Department of Telematic Systems Engineering (DIT) of the Madrid Technical University (UPM). Since 1984 he has been involved in R&D activities related to the following topics: electronic design, analog and digital transmission, integrated communication, local and metropolitan area networks, performance of multiple access protocols. His current interest includes high speed data networks, multiple access and protocol simulation. Juan Antonio Carral Pelayo received a degree in Telecommunication Engineering in 1993. He is a research assistant at the Department of Telematic Systems Engineering (DIT) of the Technical University of Madrid (UPM). Since 1992 he has been involved in R&D activities related to the following topics: high speed networking, multimedia systems and digital TV.
Multimedia systems based on satellite technology ´ ngel Ferna´ndez del Campo, Carlos Miguel Nieto, Francisco Javier Ruiz Pin˜ar Ł , A Leo´n Vidaller Siso´, Antonio Martı´nez Mas, Juan Antonio Carral Pelayo Departamento de Ingenierı´a de Sistemas Telema´ticos, ETSI Telecomunicacio´n, Universidad Polite´cnica de Madrid, Ciudad Univeritaria, 28040 Madrid, Spain
Abstract In this paper we review the evolution of satellite based multimedia systems. We focus especially on digital TV technology over satellite. This architecture takes advantage of the broadcasting capability of satellite systems to provide a low cost broadcast link towards a huge population of receivers. This ability makes the system especially suitable for the provision of multimedia services targeted to the mass public, although a return channel must be provided in order to support interactive services. Terrestrial networks like the Integrated Services Digital Network (ISDN) may provide this return channel. 1998 Elsevier Science B.V. All rights reserved. Keywords: Satellite; Broadcast; Multimedia; Digital TV technology; Terrestrial return path
1. Introduction Satellite communications have a great advantage over any other communication network whenever the information is targeted to a large population of receivers or when they are spread over big areas. That is the reason why they are specially suitable for the provision of multimedia applications like Tele-education and Tele-conferencing. In this work we present the architecture of multimedia systems based on satellite. We focus especially on architectures based on satellite broadcast and terrestrial return paths. The case of digital TV technology is studied in more detail. The digital TV technology was designed to target a huge market. Therefore, its cost will beneŁ Corresponding
author. Tel.: C34-91-5495700-474; Fax: C3491-3367333; E-mail: [email protected].
fit from economics of scale, allowing the use of low cost equipment (compared to the cost of traditional VSAT (Very Small Aperture Terminal) [1] equipment). Furthermore, the integration with other services available in the digital platform (sharing the satellite link) is an added value to this solution.
2. VSAT networks The main characteristic of communication systems using satellite technology, compared with traditional terrestrial networks, is the ability of broadcasting, the capability to transmit a message to a large number of destinations that may be spread over a broad area. The transmission costs are independent from both the number of receivers and its geographical distribution [1].
0169-7552/98/$ – see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 7 5 5 2 ( 9 8 ) 0 0 1 8 9 - 5
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F.J. Ruiz Pin˜ar et al. / Computer Networks and ISDN Systems 30 (1998) 1543–1549
Most of the satellite networks deployed through the past years are based on VSAT technology. VSAT networks are generally arranged in a star topology. In this architecture the satellite resources are organised in an outbound carrier (received by every VSATs in the network) and, possibly, several inbound channels (shared between the VSATs to access the hub of the star). The hub transmits on the outbound carrier, typically using ATDM (Asynchronous Time Division Multiplex). The information transmitted in this link can be targeted to a unicast destination address (only one VSAT station), a multicast destination address (a subset of the VSAT stations) or a broadcast destination address (every VSAT station in the network) [1]. A satellite-only VSAT network has a set of inbound channels to provide the return path from the VSATs to the hub. These channels can be organised as a set of carriers (FDMA, Frequency Division Multiple Access) and as a sequence of time slots (TDMA, Time Division Multiple Access). Each carrier or time slot can be assigned to a VSAT station on demand (DAMA, Demand Assignment Multiple Access) or shared between a group of VSATs using a contention protocol. The second approach result in better performance (in terms of bandwidth) when the traffic load is low and bursty, otherwise DAMA methods are more suited. In this star topology direct communication (one satellite hop) between VSATs is not possible. It requires a double hop communication (VSAT–Hub– VSAT). At the beginning, the reasons for this limitation were mainly technological (maximum transmitted power by the VSATs), and precluded the use of star VSAT networks to support applications very sensitive to a high propagation delay (two satellite hops D 0.54 s). However, today technology offers the possibility of one-hop communication between VSATs, reducing the delay to 0.27 s. In addition to the broadcasting capability provided by the hub, Meshed VSAT networks also support point to point communication [1].
3. VSAT-based multimedia systems There are several examples of VSAT networks supporting multimedia applications that benefit from the broadcasting capabilities of satellite technology.
As an example we will examine the CODE VSAT network [2]. The CODE network was a joint development of the Department of Telematic Systems Engineering of the Technical University of Madrid and Telefo´nica Sistemas de Sate´lites, under contract of the European Space Agency. It has been used to support many Tele-education projects, like TEN (Telematics ET1022, Trans European TeleEducation Network), or ETSIT [3], a Spanish project in which the Telecommunication Engineering Schools of Spain were interconnected using CODE to build a Teleeducation network. CODE is a star/mesh VSAT network, with an outbound carrier allowing transmission up to 2 Mbps, and a set of FDMA carriers, up to 64 kbps each, for the provision of inbound and mesh channels. Inbound channels can be shared by a set of VSATs using a contention protocol, or can be allocated to a VSAT upon request. At present, a configuration with 1.5 Mbps outbound channel, and 56 kbps inbound channels is used in the TEN network. In order to integrate CODE with other networks, CODE nodes behave as IP routers, with one interface to the satellite network, and support for additional interfaces to other networks, like LAN Ethernet. In this way, CODE can be used to provide interconnection of TCP/IP networks. Furthermore, support of IP multicast [6] is included in order to take advantage of the broadcast capability of the satellite. The Tele-education network supported by CODE is made up of a central node (the one acting as teacher) that broadcasts the class to all the participants (students). Broadcast information includes not only audio and video from the teacher, but also it may include some other information like slides, keyed text, scanned text and figures, and multimedia application control messages. The participants can ask questions during the class, and both the teacher and every other participant in the class receive their interventions. To support this service the CODE network is configured so that both audio and video from the teacher are broadcast to the remote participants using the outbound channel. Since every VSAT receives this channel, they will all be able to receive the class. Whenever a student wants to ask a question or make a remark, the network must provide a return link back to the teacher. In CODE an inbound channel is allocated to
F.J. Ruiz Pin˜ar et al. / Computer Networks and ISDN Systems 30 (1998) 1543–1549
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Fig. 1. TEN system based on CODE network.
the remote VSAT so that the student’s audio and video can reach the hub of the network. From there, they are broadcast to the other VSATs. In this way every student in the class can watch and listen to the question. The whole effect is that of a virtual classroom. CODE is configured to work in the star topology since no direct communication between VSATs is needed. Fig. 1 shows the system architecture. We can see in Fig. 1 that there is a strong asymmetry between the outbound (1.5 Mbps) and the inbound channels (56 kbps), and so are the information flows from the teacher to the students and from a student to the class. However, it does not affect an application such Tele-education where the information flows are also asymmetric. To support the concept of a virtual classroom, we need to provide a fairly good quality of audio and video from the teacher to the students. But this is not true the other way around. The students take an active roll just to make a question or a comment. It is important that every other class attendee can listen to the question but the video image is not so important. To create the
‘feeling of presence’ of the remote participant a low quality video image or even a still picture is enough. So, the total information flow from the participant will require much less bandwidth than that of the teacher, and an inbound channel can handle it. This participant flow must also be broadcast through the outbound channel back to the other participants, but this does not affect the performance of the system as long as the aggregate bandwidth does not exceed the outbound capacity. A VSAT station with full satellite transmission capability is needed to support a satellite based return channel and so the total cost of each VSAT is quite high (a modulator and a power amplifier are needed). This transmission equipment is only used for point to point communication (from the VSAT to the Hub), while the actual broadcast is done in the Hub. Point to point communications are better handled (in terms of cost) by terrestrial networks. So a hybrid architecture with satellite broadcasting and terrestrial return channels is envisaged as a cost-effective solution.
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Fig. 2. TEN system based on CODE network with mixed ISDN and VSAT return.
4. VSAT-based multimedia systems with terrestrial return channel As we mentioned above, the use of terrestrial links (like the Integrated Services Digital Network, ISDN) instead of satellite inbound channels allows the design of receive-only VSAT stations. The return channel is provided through an ISDN basic access (two B-channels, 64 C 64 kbps) so a better quality can be provided in the communication from student to teacher. The only concern with this solution is the availability of an ISDN access due to geographic issues or underdeveloped regions. In this way, an ISDN access card can replace the transmission chain of the VSAT station. This will decrease the equipment cost because the ISDN hardware for the PC platform has reached the mass-
market status. Additional ISDN hardware will be needed in the hub so that it can provide the remote access service. Fig. 2 shows the architecture of a system with hybrid return. There is a mix of terrestrial return stations and satellite return stations. ISDN return links are used whenever they are available otherwise a full transmit–receive VSAT station is used. The equipment in the reception chain of the outbound channel is common to both cases since this channel provides the necessary broadcast capability to support the Teleeducation application. This approach was chosen for the TEN network which is now in operation and with fairly good valuation from the users. Although the TEN network was designed to support a Tele-education application there are many other examples that use this architecture. Perhaps the
F.J. Ruiz Pin˜ar et al. / Computer Networks and ISDN Systems 30 (1998) 1543–1549
best known of them is the Hughes DirecPC [7]. This system is used to provide a low cost Internet access. It uses the satellite broadcast link to support forward channels (service provider to clients) of up to 400 kbps, even more under a ‘business access’ contract. Return access (from the client to the service provider) is provided by a terrestrial network like the telephone network, using a conventional analogue modem. User equipment costs are low because the antenna is very small and the communication equipment is based on low cost PC cards. This system is strongly asymmetric but so is the Internet access for residential users. An ‘average’ user receives much more data than it sends. Web browsing is a clear example of this behaviour. The Hughes system takes advantage of the coverage provided by satellite networks to provide highspeed access to areas where the only available infrastructure is the phone network (with its speed limitations). And it does it at an affordable cost since the user equipment is mass market.
5. Multimedia systems based on satellite digital TV technology The Hughes system proves that the key aspect in the development of a cost-effective satellite based multimedia service is the use of a low-cost massmarket production technology. Another technology that fits very well with that rule is the newly designed digital TV platforms, based on satellite broadcasting. Digital TV via satellite allows the transmission of different programmes over a multiplexed transport stream. The stream is composed of several audio and video flows. Furthermore, other streams are provided to control the mapping of the different audio and video streams to each programme. This stream known as the MPEG-2 transport stream is defined in the ISO 13818-1 standard (ITU-T Rec. H.262) [4]. As defined in the standard, the MPEG-2 multiplex can be used not only for TV audio and video transmission, but also for the transmission of data flows carrying any kind of information. This is due to the digital nature of the MPEG-2 multiplex, which acts as a container of any information. Taking advantage of this capability a system based on satellite digital TV for the broadcasting link and terrestrial return is
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envisaged. Again, low cost mass-market production equipment is the key to reduce the receiver stations cost to a minimum. The remote station can now be a multimedia PC equipped with a digital TV receiver and demultiplexer card attached to the corresponding dish antenna, and with a ISDN card or telephone modem to provide the return channel. This scheme can be seen in Fig. 3. Several equipment manufacturers and software companies have already foreseen the benefits of such an arrangement. For instance, Comstream [9] and Philips [10] have developed PC cards for MPEG-2 satellite reception, to be used in data broadcasting architectures. Microsoft has also developed a specification of protocol stack for PCs equipped with these receivers and the corresponding drivers [8]. Two approaches are possible for the transmission of the multimedia flows: (1) The different Tele-education application data flows are handled as a whole. They are encapsulated into an MPEG-2 transport stream as described in ISO 13818-6 [5]. The receiver stations are PCs equipped with MPEG-2 demultiplexer cards. They extract this flow from the multiplexed transport stream received from the satellite. The demultiplexed flow is then delivered to the multimedia application. The application must handle the demultiplexing of the different data flows: video, audio, slides, control, etc. (2) Audio and video flows are handled as belonging to a TV programme, MPEG-2 encoded and inserted into the multiplexed stream in their corresponding flows. Other information flows (slides, scanners, application control) are encapsulated in a different MPEG-2 flow. With this scheme the receiver can be based in a standard set-top box and the audio and video are presented in a TV set as any other TV programme. The other information flows can be extracted and passed to a PC via an RS-232 interface or via a PCMCIA card. Many commercial set-top boxes support this configuration. Both scheme benefits from a mass-market technology thus decreasing the total cost of the network, compared with that of the network based on proprietary satellite technology and protocols. Work is currently undertaken to migrate the TEN Tele-education application to a satellite environment
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Fig. 3. Tele-education system based on digital TV satellite technology.
based on digital TV technology. A PC running Windows NT 4.0 operating system has replaced the hub. This PC hosts ISDN cards to provide the return access service to the remote terminal users. In addition, it hosts a card based on transputer technology, which is responsible for the encapsulation of IP packets into an MPEG-2 transport stream. The card provides a HOTLink interface [11] to which a DVB satellite modulator is connected. The output of this modulator is then passed to the transmission chain and broadcast to the remote terminals. Remote terminals are multimedia PCs (equipped with audio and video cards, camera and microphone). These PCs also have an ISDN card, for provision of the return channel, and a Comstream DVB demultiplexer card. This card is connected to the receiving chain (dish antenna and LNB), allowing the reception of the MPEG-2 stream. The drivers for this card are responsible for the reconstruction of the original IP packets transmitted at the central
node and to deliver them to the multimedia application have also been developed.
6. Conclusions In this paper we have studied different satellite architectures to support multimedia services. We focused on those services with strong requirements of broadcast or multicast, like Tele-education. A case study based on the TEN Tele-education system has been presented. Several architectures have been examined, each with lower cost and greater possibilities than the previous one. The final one, expected to play a major role in the near future, makes use of a low cost technology such as digital TV over satellite. This makes it possible to build low cost multimedia systems based on satellite technology, very suitable for multimedia applications like Tele-education and Tele-conference.
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References [1] J. Everett (Ed.), VSATs, Very Small Aperture Terminals, Peter Peregrinus Ltd., 1992. ´ lvarez, E. Go´mez-Leal, L. Vidaller, A. Ferna´ndez, [2] M.L. A The use of transputers in VSAT systems, in: Proc. Olympus Utilisation Conf., Seville, Spain, April 1993. [3] L. Vidaller, A. Ferna´ndez, A. Martı´nez, C. Miguel, E. Go´mez-Leal, A. Ruiz, Tele-education using the CODE network, in: Proc. Olympus Utilisation Conf., Seville, Spain, April 1993. [4] ISO-13818-1, Information technology, Generic coding of moving pictures and associated audio information, Part 1: Systems, International Standard. [5] ISO-13818-6, Information technology, Generic coding of moving pictures and associated audio information, Part 6: Extension for digital storage media command and control (DSM-CC), International Standard. [6] S. Deering, Host extensions for IP multicast, RFC 1112, 1988. [7] Hughes DirecPC Personal Edition, Web document, Hughes, http://www.dssdirectv.com/dirpc.htm. [8] Introduction to broadcast architecture, Web document, Microsoft, January 1998, http://www.eu.microsoft.com/hwdev /desinit/bcast1.htm. [9] Comstream MediaCast Satellite PC/Server Receiver Card, Product data sheet, Comstream Co., 1997. [10] Philips, CleverCastPC high speed multimedia broadcasting to the PC, Product information, Philips Electronics N.V., 1997, http://www.dvs.be.philips.com/dvs/products/dct.htm. [11] Cypress Semiconductor, HOTLink design considerations, Application note, 3/5/1995. Francisco Javier Ruiz Pin˜ar received a degree in Telecommunication Engineering in 1990 and the Ph.D. degree in Telecommunication Engineering in 1994. He is Associate Professor of the Department of Telematic Systems Engineering (DIT) of the Madrid Technical University (UPM). Since 1990 he has been involved in R&D activities related to the following topics: software design, protocol design and satellite communications. Angel Ferna´ndez del Campo received a degree in Telecommunication Engineering in 1979 and the Ph.D. degree in Telecommunication Engineering in 1987. He is Associate Professor of the Department of Telematic Systems Engineering (DIT) of the Madrid Technical University (UPM). Since 1979 he has been involved in R&D activities related to the following topics: software design, software engineering, digital transmission,
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data communication, local and metropolitan networks, formal description techniques and parallel architectures for communication equipments. He has given courses, conferences, authored about twenty publications on national and international magazines and congress, and one book. Carlos Miguel Nieto received a degree in Telecommunication Engineering in 1986 and the Ph.D. degree in Telecommunication Engineering in 1991. He is an Associate Professor of the Department of Telematic Systems Engineering (DIT) of the of the Madrid Technical University (UPM). Since 1986 he has been involved in R&D activities related to the following topics: software design, software engineering, digital transmission, data communication, local and metropolitan networks, formal description techniques and parallel architectures for communication equipments. Leo´n Vidaller Siso´ received a degree in Telecommunication Engineering in 1974 and the Ph.D. degree in Telecommunication Engineering in 1977. He is Professor of the Department of Telematic Systems Engineering (DIT) of the Madrid Technical University (UPM). Since 1974 he has been involved in R&D activities related to the following topics: electronic design, analog and digital transmission, error correcting codes, data communication, local and metropolitan networks, criptography and security. He has been project leader of more than 16 industry and public supported contracts. He is an industry consultant on data communications systems, and book reviewer for IEEE Communication Magazine. He has given courses, conferences, authored about fifty publications on national and international magazines and congress, and four books. Antonio Martı´nez Mas received a degree in Telecommunication Engineering in 1984 and the Ph.D degree in Telecommunication Engineering in 1989. He is an Associate Professor of the Department of Telematic Systems Engineering (DIT) of the Madrid Technical University (UPM). Since 1984 he has been involved in R&D activities related to the following topics: electronic design, analog and digital transmission, integrated communication, local and metropolitan area networks, performance of multiple access protocols. His current interest includes high speed data networks, multiple access and protocol simulation. Juan Antonio Carral Pelayo received a degree in Telecommunication Engineering in 1993. He is a research assistant at the Department of Telematic Systems Engineering (DIT) of the Technical University of Madrid (UPM). Since 1992 he has been involved in R&D activities related to the following topics: high speed networking, multimedia systems and digital TV.