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LAN interconnection by satellite and the COST 226 project
Computer Networks and ISDN Systems 21 (1991) 285-292 North-Holland
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LAN interconnection by satellite and the COST 226 project O. Koudelka Institut fiir Angewandte Systemtechnik (IAS), Joanneum Research, Inffeldgasse 12, A-8010 Graz, Austria
R.A. Harris Directorate of Telecommunications, European Space Agency, Estec, Postbus 299, 2200 A G Noordwijk, The Netherlands
Abstract Koudelka, O. and R.A. Harris, LAN interconnection by satellite and the COST 226 project, Computer Networks and ISDN Systems 21 (1991) 285-292. There is an urgent need for international wideband networks to interconnect the rapidly growing number of wideband local communication networks. Suitable terrestrial networks will not be available for a considerable time, but satellites could meet the demand. This paper describes activities within the framework of the COST 226 project to study and demonstrate the design and performance of suitable satellite networks.
Keywords. LANs, COST, satellites, B-ISDN, standards, TDMA, multi-media.
1. Introduction
Wideband local communication networks are becoming very common and are helping to change the face of research, development and production. There is an urgent need for appropriate means to interconnect these local networks, but suitable wideband international networks are not yet available. This paper is concerned primarily with the interconnection of LANs by satellite, in the framework of the recently established COST 226 project. Other aspects of the project are described briefly. In the long term, most of the needs for wideband international connections are foreseen to be met by the Broadband ISDN. This will not, however, achieve wide coverage for quite some time and there will be a substantial period during which users' needs are not met. Satellite networks can however be used to provide wideband coverage of large areas rapidly. In many cases there will be a requirement for private networks, with satellite earth-stations on users' premises. One of the objectives of the COST 226 project is to explore ways of doing this efficiently and to demonstrate a
representative system. As B-ISDN is deployed, it can be expected that there will be a need to bridge B-ISDN islands using satellite networks. Gradually, communications equipment, including computer systems, will become B-ISDN based. It is therefore desirable that satellite networks achieve a degree of B-ISDN compatibility. This is also being studied in the project. Several of the participants in COST 226 have been involved in the SATINE a programme and this experience will be made use of. The scope of COST 226 is, however, significantly broader.
2. Applications, local networks and user needs
User requirements for data communications are growing rapidly, as a result of the increasing internationalisation of work, the explosive growth of cheap computing power and the developments of standards which allow the interconnection of equipment from different manufacturers. 1 Satellite Internetworking Experiment, which demonstrated the feasibility of LAN interconnection by satellite.
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Most commercial and academic organisations have installed communication networks at their various sites. Many of these are (or include) wideband Local Area Networks (LANs). Increasingly, they are being used to connect powerful single- or multi-user computers which are capable of communicating at high speed. Graphics and other applications which need these high speeds, are becoming increasingly important. In particular, interactive operation may require large amounts of data to be transferred in a short time. Of course, such networks will normally serve a variety of users, including many with small individual demands such as electronic mail, and others with large files to transfer but no strong time constraint. A striking example of this trend is the spread of desktop workstations and high-end PC's running UNIX and linked by Ethernet. Computers from different manufacturers happily share the same LAN, using standard protocols. With the increasing use of X-Windows, it is becoming easier for applications software from different suppliers to communicate. Other LAN technologies have been standardised or are being developed, some operating at much higher speeds than Ethernet. It can be expected that these will become increasingly important in the future, particularly to support sophisticated modelling and design processes and for distributed processing. Another important development is the use of voice and video as ad-
juncts to interactive data processing. The availability of fast, high-resolution graphics displays opens the door to true multi-media terminals and considerable R & D work is going on in this area. Wideband communications are relatively easy and cheap to provide within a local site. Increasingly though, many medium- and large-sized companies conduct their R&D and design work in several countries and there is a growing collaboration between companies and other groups across Europe. There is an urgent need to interconnect the L A N islands by wideband telecommunications networks. The user should ideally not perceive any difference in performance between local and long-distance communications. It can be expected that the provision of international wideband links will open up new ways of working, potentially offering important cost and time savings. It seems probable that voice and video support will be of particular value for some types of international connections.
3. The (developing) telecommunications infrastructure
Present public telecommunications systems offer little in the way of wideband international links, let alone networks. Compared with the United States, the prices charged by European pubhc network operators, for those wideband links that are available, are still very high. Although the
Otto Koudelka graduated from the Technical University Graz in 1980 and joined the Department of Communications and Wave Propagation in the same year. Since 1982 he has been Deputy Head of the Department. He received his Ph.D. in communications in 1986. He has been carrying out research work in satellite communications and is also lecturing in communications and networking. He is project manager for the data communications activities of the associated Institute of Applied Systems Technology which included the STELLA and SATINE experiments. He is chairman of project COST 226, a member of the "Telecommunications" Technical Committee of the EC and of two working groups of ESA. Bob Harris joined ESA (then ESRO) in 1970. Since 1978 he has been head of the Transmission Techniques
Section in ESA's Directorate of Telecommunications. He specialises in digital transmission methods and has participated in the design and specification of most of ESA's communications satellites. In recent years his activities have extended to include networking aspects of satellite communications. He supported part of the SATINE internetworking experiment and has been active in helping to set up COST 226.
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trend is favourable, the pricing policy for both terrestrial and satellite links must change significantly if wideband long-distance communications are to become attractive. While some aspects of the telecommunications infrastructure are being developed quite rapidly, it is not expected that the requirements for wideband international data services will be satisfied in the near future. In PTT planning, these will have to wait for the deployment of Broadband ISDN links. This is expected to take at least 10 years, during which time the demand for such services is certain to grow enormously, fuelled by developments in computing power, networking and graphics applications. It is possible that Metropolitan Area Networks (MANs) will be implemented in some countries in a shorter timescale and this would certainly improve the situation for some users. Their limited geographical coverage will however not be adequate for others. The situation is likely to be particularly bad for those countries on the periphery of the EC which have poor links to the rest of the community, but whose requirements are growing rapidly. In addition to developments in Western Europe, recent changes in Central and Eastern Europe are expected to lead to a substantial demand for new data transmission capacity in an area with a poor telecommunications infrastructure. Western companies setting up new enterprises or helping to restructure existing factories can be expected to import their own data-processing systems and want to connect them to their home base. Cooperation with Eastern Europe on large science projects is already expanding. As well as support for data communications, voice and video links are likely to be needed in their own right in such situations. The CEC Green Paper on Satellite Communications Policy has recently been put forward in the framework of the "Single Market" of 1992. If implemented, it would open up additional possibilities for the use of satellites. It promises to liberalise the earth-segment, provide unrestricted access to space segment capacity and to give full commercial freedom to space-segment providers. It opens the door to small earth-stations on user premises, with operators being given freedom to market their services directly to customers anywhere in Europe. By increasing competition, it offers the prospect of lower tariffs.
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The key to such a policy is the development of standards to ensure safe operation of such systems with the minimum of day-to-day control. The Green Paper emphasises the role of ETSI in this process. It also proposes close collaboration with ESA in the development of satellite technologies for both private and public applications. The Green Paper anticipates that satellite systems will increasingly be used for short-term deployment and for "distinct specialist niche markets". The provision of private networks, linking wideband local data network "islands" fits well into this concept. It is an application which is particularly well suited to satellites. Satellites can provide wide coverage quickly, with investment on the ground only where it is needed and with delivery straight to the end user. The satellite acts as a natural "concentrator" of the bursty data traffic, in contrast to the inefficient use of multiple wideband links in a "mesh" terrestrial network.
4. The COST 226 project The COST 226 project (Integrated Space/ Terrestrial Networks) was defined by the working group on satellite communications of the "Telecommunications" Technical Committee of the Commission of European Communities. The project was kicked off in May 1990. The following countries and organisations participate: Austria, Belgium, Denmark, Italy, Spain, Sweden, Yugoslavia, and ESA. Hungary applied recently and Germany and Switzerland have expressed interest in participating. The objectives of the project are: (a) to carry out research in the area of integration of terrestrial public or closed networks and satellite networks with emphasis on intelligent satellites (with on-board switching and processing) and the relevant earth segment; (b) to study problems of internetworking and gateways between heterogeneous networks, network management and control, traffic management techniques and strategies for network evolution; (c) to define transmission signal interfaces, signal and routing interfaces, ISDN and LAN access methods; (d) to investigate efficient satellite access
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schemes taking into account the properties of existing and future satellite systems as well as different ground station environments (small user stations and medium/large terminals); (e) to investigate quality of service parameters for the requirements of multi-media traffic (voice, video, data); (f) to elaborate concepts for integrated terrestrial and satellite networks serving the communications needs of the 1990s and to prepare for demonstration systems, and (g) to set up pilot systems for demonstration which should lead to operational services. The following working group areas have been identified: - Integration of satellite and public networks (e.g., integration of ISDNs by VSATs), - Integration of satellite and closed (private) networks (e.g., integration of LANs by satellite), and - I n t e g r a t i o n of ground systems by on-board processing satellites. The standard method of working in COST projects is for participants to sign a Memorandum of Understanding which commits them to a minimum level of participation and to make all results obtained available to the other participants. Individual participants are responsible for their own funding, but the Commission provides secretarial support. Participants appoint a Management Committee (which meets four times a year) to coordinate the project. The management committee of COST 226 includes representatives of network operators, research organisations, universities and industry. Collaboration between the working group members is informal, with regular meetings to exchange results. An annual workshop is planned, at which results will be presented for the whole project and directions will be set for the following year. Close coordination and information exchange will be established with the other relevant COST and EC research projects; furthermore, regular contact will be held with CEPT, CCITT and ETSI.
5. L A N
interconnection
by satellite
The remainder of this paper is concerned with the second of the working group areas described
above, i.e., the use of satellites to connect LANs in a private network. As indicated in the previous section, COST 226 aims to take a rather broad view, in the study phase at least. This will cover users' needs, alternative terrestrial and satellite systems including cost aspects, current and future LANs, local and intersite traffic characteristics, protocol architecture issues, integration of voice and video, compatibility with emerging terrestrial wideband systems, satellite access anu capacity allocation procedures, transmission methods, earth-station possibilities and satellite requirements and availability. Although the main thrust in the early stages will be to concentrate on technical and economic issues, it is clear that operational aspects must be considered from the beginning. Of course, considerable work has been done in most of these areas. The purpose of the study phase is take a fresh look at what has been achieved already, to make use of it where appropriate and to identify where new work is needed. In particular, experience gained from the SATINE project will form an important input. The FODA (FIFO Ordered Demand Assignment) satellite access protocol developed for SATINE contains many of the features needed and may provide a suitable base for further development. Attention will be given to existing standards, which will be used whenever appropriate. When it is felt that existing standards are inadequate, this will be discussed with the appropriate standards bodies. One area in which assistance is particularly sought is that of identifying the real requirements of users. It is easy to develop a scenario of the type presented in Section 2, but if the work of COST 226 is to be of real value, it must be based on actual requirements. After the study phase, it is intended to proceed with a demonstration programme. The aim of this is to show the use and advantages of satellites for this application. It will clearly not be possible to cover such a wide field as is planned for the study phase and selections will have to be made. Nevertheless, it is considered essential that the demonstration be complete, in the sense that it should show a representative set of applications using standard computer and LAN equipment, together with voice and video support. Any specially-developed units such as satellite access and transmission equipment must be of high quality and repre-
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sentative of what would be needed in an operational system. It is hoped to start the demonstration programme in one to two years, making extensive use of earlier developments.
5.1. Networking for data, voice and video Data In any completely new computer network, it is desirable to conform to the I S O / O S I reference model and to the associated standard protocols. In the present application, however, there is a requirement to link existing local networks, many of which use non-OSI protocols. For instance, within the computer industry, many of the products already installed and being installed in large numbers, are based on TCP and IP transport and internet protocols, which must be considered as a de facto standard. TCP has mechanisms built into it which make it well suited to satellite links, even at relatively high speeds. It is, therefore, considered desirable to adopt a design approach which allows both a pure OSI implementation and support for T C P / I P . As far as most users are concerned, the issue of compatibility applies only to the upper layers of the OSI model, for such applications as electronic mail, file transfer and remote login. TCP and IP require only minor external modifications (convergence sublayers) to make them conform to the OSI standard interfaces. The ISODE software package which is available in the public domain for development work, as well as commercially, adopts this approach. Its use is being investigated for this application. The convergence sublayer presents the OSI upper layers with a TP0 interface, providing a high-quality connection-oriented transport subsystem. The protocol stack can therefore conveniently be divided into three distinct subsystems, with standard interfaces between them. Layers 5 to 7 form a Utility Subsystem, layers 3 and 4 are the Transport Subsystem and layers 1 and 2 form the Transmission Subsystem. With this approach, OSI and non-OSI utility subsystems can, in principle, coexist above a T C P / I P (or OSI) transport subsystem and the transmission subsystem can be changed without affecting the subsystems above it. The local network at any site may not be homogeneous. It may consist of several different subnetworks, connected together by gate-
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ways to handle the necessary address and protocol conversion. The satellite network can then be regarded as simply another subnetwork, with its own gateways connecting it to the various terrestrial subnetworks. Data transmission normally requires a very high link integrity, which may be achieved in a variety of ways. The most effective method is normally to use ARQ. The transmission subsystem quality required is then mainly determined by the need for a high throughput efficiency, rather than by the overall link integrity target. End-to-end flow control is required on each connection, to prevent information loss at the receiver. It must, therefore, be controlled by the receiver.
Voice and video Most existing LANs, which are designed primarily to link computers, feature highly variable transmission delay. They are not well suited for the transmission of voice and video traffic, which require essentially constant delay for satisfactory performance. Some existing LANs do offer constant delay and it can be expected that future LAN standards will allow the integrated transmission of data, voice and video. At present though, separate local links or subnetworks will usually be needed, with integration only taking place at the satellite gateways. Voice and video of the types appropriate for this application do not in general require such good overall transmission quality as data. ARQ is in any case inappropriate as it introduces variable delay. If necessary, Forward Error Correction (FEC) can be used to improve the quality of any (part of a) link. Voice and video are essentially stream traffic. While the source may be able to transmit data at various rates, the receivers are designed to accommodate the maximum rate. Signalling from the source may be required to assist the receiver, but receiver-initiated flow control of the type needed for data transmission is not needed on an end-toend basis. Although current voice and video coders typically use algorithms which generate data at variable rate, according to talker activity or picture content, they do not normally exploit this for transmission and there is significant redundancy in their outputs. Future coders, however, will deliver data in bursts, as they are produced. This, for
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instance, is the approach adopted for codecs being developed for use in ATM Networks.
5.2. Satellite access and transmission
While the general requirement is rather clear, a considerable amount of work has to be done to identify satellite capabilities and the best ways to use them. In the longer term, it can be expected that onboard switching (OBP) satellites will be used for such applications, but they are not likely to become available much before wideband terrestrial networks become widespread. In the shorter term, only transparent satellites will be available. The activity proposed here is strictly aimed at such transparent satellites, but it overlaps in several ways with current work on OBP systems. A variety of system scenarios is being considered, ranging from high bit-rate operations involving the dedicated use of a satellite transponder, to shared transponder usage for moderate data-rates, supporting for instance links at a few Mbps between a few Ethernets. Access and transmission concepts are being studied, with the objective of minimising earth- and space-segment costs for a given system capacity. As a general principle, it is assumed that earth-stations should be installed on the users' premises. Consideration is being given to a range of possible satellites with different coverages and powers. The access methods used in LANs invariably depend on the transmission time being small. They are not therefore suited for wideband satellite systems in which the round-trip-delay is about 250 ms. Pure contention access schemes are also unsuitable if the satellite capacity is to be used efficiently. Given that satellite capacity is a limited resource, but that the offered traffic load will normally be highly variable, it is clear that some form of controlled access is essential. This means that flow control will be needed on data paths into the satellite gateways. For (the relatively high data rate) video signals, source-rates may need to be varied, under the control of the access controller, to limit traffic peaks. An access method which offers the necessary flexibility and control, is reservation-based TDMA. Capacity reservation and allocation necessarily involve a substantial delay however and so
a hybrid of preassigned- and reservation-TDMA is being studied, to provide each earth-station with a (relatively small) fixed pool of rapid-access capacity. The capacity reservation scheme should ideally be capable of providing fixed transmission times (constant transmission delay) for each voice or video call. In order to ensure that existing calls and sessions are properly handled, information about them must be made available to the access control system. In many cases it seems likely that individual earth-stations will not need to access the full capacity of the network, although they will usually need connectivity to all other earth-stations. In this case it may be possible to minimise the transmitter power and reduce overall costs by operating a multifrequency (MF)-TDMA system. This is essentially a set of separate T D M A systems operating on different frequencies, but with coordinated access to the different channels. Each earth-station is able to access one channel at a time, hopping channels for each burst, as directed by the access controller. Preliminary link budgets indicate that transmission of several mega.bits/s should be feasible with full European coverage, using quite modest earthstations. The limit is primarily set by a desire to use solid-state transmitters. The use of M F - T D M A would however allow the system capacity to be several times larger than the transmit capability of each earth-station. In principle, the number of satellite carriers could be increased as necessary to match the required system capacity. Such a system could, for instance, allow interconnection of a number of Ethernets. In conventional T D M A transmission, each burst is only coarsely synchronised to the T D M A frame. It is, therefore, preceded by a synchronisation preamble, allowing the receiver to identify the carrier phase and symbol timing of the burst and the start of the data. Guard-times must also be provided between bursts, to prevent collisions. The guard-times and preambles represent wasted capacity, corresponding to increased transmit symbol rate and system dimensions. The usual way to minimise this is to extend the burst length, keeping the overhead to an acceptable fraction of the useful traffic. In order to optimise the transmission design, powerful FEC coding will be needed for many applications, with demodulators operating at low signal to noise ratios. This will
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require significantly longer preambles and correspondingly increased burst lengths. In order to achieve adequate burst lengths, it will typically be necessary to group traffic from one earth-station into a small number of bursts. Given the variability of the traffic flow, the burst lengths will vary from frame to frame and at the boundaries of voice and video calls. Bursts occurring late in the T D M A frame will therefore be moved back and forth according to the total duration of the bursts preceding them. This makes it impossible to allocate fixed transmission times to the voice and video traffic, although strategies for minimising the delay variation have been devised. The FODA protocol, for instance, separates delay-sensitive and delay-insensitive traffic into separate bursts. All delay-sensitive traffic bursts are transmitted at the start of each frame, with the delay-insensitive traffic at the end. Various methods are being studied to improve the situation. With digital demodulator implementation, the burst preambles can be eliminated, avoiding the need to group traffic into long bursts. Traffic units can then be transmitted individually, with each earth-station transmitting many bursts per frame. If the traffic is broken into fixed-length cells, the possibility exists of truly constant delay transmission. A further possibility would be to operate the satellite network on a symbol-synchronous basis. This would require each earth-station to monitor the timing of its own transmissions, using a long loop through the satellite and to maintain them to within a small fraction of a symbol interval. Preliminary investigations indicate that this can be done satisfactorily. Symbol-synchronous operation appears to offer a number of advantages for the design and performance of the receivers. At high microwave frequencies, atmospheric attenuation is a problem. This is severe at 30/20 G H z and needs to be taken into account in the design of the system. Simply providing a power margin is inefficient and some form of adaptive system is normally preferable. At 14/12 GHz, attenuations are smaller and fade adaptation will normally not be needed. Fade adaptation within fixed power limits inevitably leads to a reduction in overall system capacity. It therefore interacts with the capacity allocation process and any changes in the transmission mode involve signalling. Studies will be made on how to detect atmo-
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spheric fading reliably and on the best ways to adapt to it. These are likely to be different for the different classes of traffic. 5.3. Compatibility with terrestrial transmission systems
As indicated earlier, one of the objectives of the COST 226 project is to investigate and promote the integration of future satellite and terrestrial systems. One of the tasks is, therefore, to follow work on the Broadband ISDN network, with the aim of harmonising the network interfaces and if appropriate adopting some of the same access and transmission concepts. As far as network interfaces are concerned, it was suggested earlier that voice and video equipment based on ATM techniques is likely to be available fairly soon and that this would offer some advantages in terms of capacity utilisation. Transmission of fixed-length cells over a symbol-synchronous satellite network obviously has a number of similarities with the ATM transmission mode. The ATM cell size appears to be well suited to satellite transmission. At first sight it also appears to be a suitable basic unit for segmenting data packets. In this case, ATM techniques may be useful for the multiplexing of multiple data, voice and video sources in the satellite gateways. The use of ATM-related techniques for multiplexing and transmission would obviously require a special interface for existing LANs. The underlying processes in the gateway would however be close to those in the eventual B-ISDN and this should make long-term compatibility easier. There are, however, several important differences between a (capacity-limited) satellite network and the broadband transmission medium foreseen for B-ISDN and it will be necessary to investigate these carefully.
6. Conclusion
There is a rapidly growing need for wideband international private networks to interconnect LAN islands. There is little prospect of this requirement being satisfied by terrestrial facilities in the near future, but satellite systems could be deployed rapidly for this purpose. One of the objectives of the newly-established COST 226 pro-
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ject is to identify appropriate system designs and to demonstrate a representative system, including suitable applications. Eventually, the Broadband ISDN is foreseen to satisfy most of the requirements, but for a substantial period it is expected that an integrated satellite/B-ISDN approach would offer significant advantages. COST 226 will, therefore, study compatibility issues. Early indications are that there may be some advantages in adopting some B-ISDN concepts for the satellite network.
Acknowledgements A project of this nature obviously involves the active participation of many people and it is not possible to name them all here. Nevertheless, we want to acknowledge the particular contributions to this work of Mervyn Hine (CERN), Erina Ferro and Nedo Celandroni (CNUCE), Chris Adams (Univ. Buckingham/RAL) and Horst Clausen (Univ. Salzburg).
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LAN interconnection by satellite and the COST 226 project O. Koudelka Institut fiir Angewandte Systemtechnik (IAS), Joanneum Research, Inffeldgasse 12, A-8010 Graz, Austria
R.A. Harris Directorate of Telecommunications, European Space Agency, Estec, Postbus 299, 2200 A G Noordwijk, The Netherlands
Abstract Koudelka, O. and R.A. Harris, LAN interconnection by satellite and the COST 226 project, Computer Networks and ISDN Systems 21 (1991) 285-292. There is an urgent need for international wideband networks to interconnect the rapidly growing number of wideband local communication networks. Suitable terrestrial networks will not be available for a considerable time, but satellites could meet the demand. This paper describes activities within the framework of the COST 226 project to study and demonstrate the design and performance of suitable satellite networks.
Keywords. LANs, COST, satellites, B-ISDN, standards, TDMA, multi-media.
1. Introduction
Wideband local communication networks are becoming very common and are helping to change the face of research, development and production. There is an urgent need for appropriate means to interconnect these local networks, but suitable wideband international networks are not yet available. This paper is concerned primarily with the interconnection of LANs by satellite, in the framework of the recently established COST 226 project. Other aspects of the project are described briefly. In the long term, most of the needs for wideband international connections are foreseen to be met by the Broadband ISDN. This will not, however, achieve wide coverage for quite some time and there will be a substantial period during which users' needs are not met. Satellite networks can however be used to provide wideband coverage of large areas rapidly. In many cases there will be a requirement for private networks, with satellite earth-stations on users' premises. One of the objectives of the COST 226 project is to explore ways of doing this efficiently and to demonstrate a
representative system. As B-ISDN is deployed, it can be expected that there will be a need to bridge B-ISDN islands using satellite networks. Gradually, communications equipment, including computer systems, will become B-ISDN based. It is therefore desirable that satellite networks achieve a degree of B-ISDN compatibility. This is also being studied in the project. Several of the participants in COST 226 have been involved in the SATINE a programme and this experience will be made use of. The scope of COST 226 is, however, significantly broader.
2. Applications, local networks and user needs
User requirements for data communications are growing rapidly, as a result of the increasing internationalisation of work, the explosive growth of cheap computing power and the developments of standards which allow the interconnection of equipment from different manufacturers. 1 Satellite Internetworking Experiment, which demonstrated the feasibility of LAN interconnection by satellite.
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Most commercial and academic organisations have installed communication networks at their various sites. Many of these are (or include) wideband Local Area Networks (LANs). Increasingly, they are being used to connect powerful single- or multi-user computers which are capable of communicating at high speed. Graphics and other applications which need these high speeds, are becoming increasingly important. In particular, interactive operation may require large amounts of data to be transferred in a short time. Of course, such networks will normally serve a variety of users, including many with small individual demands such as electronic mail, and others with large files to transfer but no strong time constraint. A striking example of this trend is the spread of desktop workstations and high-end PC's running UNIX and linked by Ethernet. Computers from different manufacturers happily share the same LAN, using standard protocols. With the increasing use of X-Windows, it is becoming easier for applications software from different suppliers to communicate. Other LAN technologies have been standardised or are being developed, some operating at much higher speeds than Ethernet. It can be expected that these will become increasingly important in the future, particularly to support sophisticated modelling and design processes and for distributed processing. Another important development is the use of voice and video as ad-
juncts to interactive data processing. The availability of fast, high-resolution graphics displays opens the door to true multi-media terminals and considerable R & D work is going on in this area. Wideband communications are relatively easy and cheap to provide within a local site. Increasingly though, many medium- and large-sized companies conduct their R&D and design work in several countries and there is a growing collaboration between companies and other groups across Europe. There is an urgent need to interconnect the L A N islands by wideband telecommunications networks. The user should ideally not perceive any difference in performance between local and long-distance communications. It can be expected that the provision of international wideband links will open up new ways of working, potentially offering important cost and time savings. It seems probable that voice and video support will be of particular value for some types of international connections.
3. The (developing) telecommunications infrastructure
Present public telecommunications systems offer little in the way of wideband international links, let alone networks. Compared with the United States, the prices charged by European pubhc network operators, for those wideband links that are available, are still very high. Although the
Otto Koudelka graduated from the Technical University Graz in 1980 and joined the Department of Communications and Wave Propagation in the same year. Since 1982 he has been Deputy Head of the Department. He received his Ph.D. in communications in 1986. He has been carrying out research work in satellite communications and is also lecturing in communications and networking. He is project manager for the data communications activities of the associated Institute of Applied Systems Technology which included the STELLA and SATINE experiments. He is chairman of project COST 226, a member of the "Telecommunications" Technical Committee of the EC and of two working groups of ESA. Bob Harris joined ESA (then ESRO) in 1970. Since 1978 he has been head of the Transmission Techniques
Section in ESA's Directorate of Telecommunications. He specialises in digital transmission methods and has participated in the design and specification of most of ESA's communications satellites. In recent years his activities have extended to include networking aspects of satellite communications. He supported part of the SATINE internetworking experiment and has been active in helping to set up COST 226.
O. Koudelka, R.A. Harris / LAN interconnection by satellite and the COST 226 project
trend is favourable, the pricing policy for both terrestrial and satellite links must change significantly if wideband long-distance communications are to become attractive. While some aspects of the telecommunications infrastructure are being developed quite rapidly, it is not expected that the requirements for wideband international data services will be satisfied in the near future. In PTT planning, these will have to wait for the deployment of Broadband ISDN links. This is expected to take at least 10 years, during which time the demand for such services is certain to grow enormously, fuelled by developments in computing power, networking and graphics applications. It is possible that Metropolitan Area Networks (MANs) will be implemented in some countries in a shorter timescale and this would certainly improve the situation for some users. Their limited geographical coverage will however not be adequate for others. The situation is likely to be particularly bad for those countries on the periphery of the EC which have poor links to the rest of the community, but whose requirements are growing rapidly. In addition to developments in Western Europe, recent changes in Central and Eastern Europe are expected to lead to a substantial demand for new data transmission capacity in an area with a poor telecommunications infrastructure. Western companies setting up new enterprises or helping to restructure existing factories can be expected to import their own data-processing systems and want to connect them to their home base. Cooperation with Eastern Europe on large science projects is already expanding. As well as support for data communications, voice and video links are likely to be needed in their own right in such situations. The CEC Green Paper on Satellite Communications Policy has recently been put forward in the framework of the "Single Market" of 1992. If implemented, it would open up additional possibilities for the use of satellites. It promises to liberalise the earth-segment, provide unrestricted access to space segment capacity and to give full commercial freedom to space-segment providers. It opens the door to small earth-stations on user premises, with operators being given freedom to market their services directly to customers anywhere in Europe. By increasing competition, it offers the prospect of lower tariffs.
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The key to such a policy is the development of standards to ensure safe operation of such systems with the minimum of day-to-day control. The Green Paper emphasises the role of ETSI in this process. It also proposes close collaboration with ESA in the development of satellite technologies for both private and public applications. The Green Paper anticipates that satellite systems will increasingly be used for short-term deployment and for "distinct specialist niche markets". The provision of private networks, linking wideband local data network "islands" fits well into this concept. It is an application which is particularly well suited to satellites. Satellites can provide wide coverage quickly, with investment on the ground only where it is needed and with delivery straight to the end user. The satellite acts as a natural "concentrator" of the bursty data traffic, in contrast to the inefficient use of multiple wideband links in a "mesh" terrestrial network.
4. The COST 226 project The COST 226 project (Integrated Space/ Terrestrial Networks) was defined by the working group on satellite communications of the "Telecommunications" Technical Committee of the Commission of European Communities. The project was kicked off in May 1990. The following countries and organisations participate: Austria, Belgium, Denmark, Italy, Spain, Sweden, Yugoslavia, and ESA. Hungary applied recently and Germany and Switzerland have expressed interest in participating. The objectives of the project are: (a) to carry out research in the area of integration of terrestrial public or closed networks and satellite networks with emphasis on intelligent satellites (with on-board switching and processing) and the relevant earth segment; (b) to study problems of internetworking and gateways between heterogeneous networks, network management and control, traffic management techniques and strategies for network evolution; (c) to define transmission signal interfaces, signal and routing interfaces, ISDN and LAN access methods; (d) to investigate efficient satellite access
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schemes taking into account the properties of existing and future satellite systems as well as different ground station environments (small user stations and medium/large terminals); (e) to investigate quality of service parameters for the requirements of multi-media traffic (voice, video, data); (f) to elaborate concepts for integrated terrestrial and satellite networks serving the communications needs of the 1990s and to prepare for demonstration systems, and (g) to set up pilot systems for demonstration which should lead to operational services. The following working group areas have been identified: - Integration of satellite and public networks (e.g., integration of ISDNs by VSATs), - Integration of satellite and closed (private) networks (e.g., integration of LANs by satellite), and - I n t e g r a t i o n of ground systems by on-board processing satellites. The standard method of working in COST projects is for participants to sign a Memorandum of Understanding which commits them to a minimum level of participation and to make all results obtained available to the other participants. Individual participants are responsible for their own funding, but the Commission provides secretarial support. Participants appoint a Management Committee (which meets four times a year) to coordinate the project. The management committee of COST 226 includes representatives of network operators, research organisations, universities and industry. Collaboration between the working group members is informal, with regular meetings to exchange results. An annual workshop is planned, at which results will be presented for the whole project and directions will be set for the following year. Close coordination and information exchange will be established with the other relevant COST and EC research projects; furthermore, regular contact will be held with CEPT, CCITT and ETSI.
5. L A N
interconnection
by satellite
The remainder of this paper is concerned with the second of the working group areas described
above, i.e., the use of satellites to connect LANs in a private network. As indicated in the previous section, COST 226 aims to take a rather broad view, in the study phase at least. This will cover users' needs, alternative terrestrial and satellite systems including cost aspects, current and future LANs, local and intersite traffic characteristics, protocol architecture issues, integration of voice and video, compatibility with emerging terrestrial wideband systems, satellite access anu capacity allocation procedures, transmission methods, earth-station possibilities and satellite requirements and availability. Although the main thrust in the early stages will be to concentrate on technical and economic issues, it is clear that operational aspects must be considered from the beginning. Of course, considerable work has been done in most of these areas. The purpose of the study phase is take a fresh look at what has been achieved already, to make use of it where appropriate and to identify where new work is needed. In particular, experience gained from the SATINE project will form an important input. The FODA (FIFO Ordered Demand Assignment) satellite access protocol developed for SATINE contains many of the features needed and may provide a suitable base for further development. Attention will be given to existing standards, which will be used whenever appropriate. When it is felt that existing standards are inadequate, this will be discussed with the appropriate standards bodies. One area in which assistance is particularly sought is that of identifying the real requirements of users. It is easy to develop a scenario of the type presented in Section 2, but if the work of COST 226 is to be of real value, it must be based on actual requirements. After the study phase, it is intended to proceed with a demonstration programme. The aim of this is to show the use and advantages of satellites for this application. It will clearly not be possible to cover such a wide field as is planned for the study phase and selections will have to be made. Nevertheless, it is considered essential that the demonstration be complete, in the sense that it should show a representative set of applications using standard computer and LAN equipment, together with voice and video support. Any specially-developed units such as satellite access and transmission equipment must be of high quality and repre-
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sentative of what would be needed in an operational system. It is hoped to start the demonstration programme in one to two years, making extensive use of earlier developments.
5.1. Networking for data, voice and video Data In any completely new computer network, it is desirable to conform to the I S O / O S I reference model and to the associated standard protocols. In the present application, however, there is a requirement to link existing local networks, many of which use non-OSI protocols. For instance, within the computer industry, many of the products already installed and being installed in large numbers, are based on TCP and IP transport and internet protocols, which must be considered as a de facto standard. TCP has mechanisms built into it which make it well suited to satellite links, even at relatively high speeds. It is, therefore, considered desirable to adopt a design approach which allows both a pure OSI implementation and support for T C P / I P . As far as most users are concerned, the issue of compatibility applies only to the upper layers of the OSI model, for such applications as electronic mail, file transfer and remote login. TCP and IP require only minor external modifications (convergence sublayers) to make them conform to the OSI standard interfaces. The ISODE software package which is available in the public domain for development work, as well as commercially, adopts this approach. Its use is being investigated for this application. The convergence sublayer presents the OSI upper layers with a TP0 interface, providing a high-quality connection-oriented transport subsystem. The protocol stack can therefore conveniently be divided into three distinct subsystems, with standard interfaces between them. Layers 5 to 7 form a Utility Subsystem, layers 3 and 4 are the Transport Subsystem and layers 1 and 2 form the Transmission Subsystem. With this approach, OSI and non-OSI utility subsystems can, in principle, coexist above a T C P / I P (or OSI) transport subsystem and the transmission subsystem can be changed without affecting the subsystems above it. The local network at any site may not be homogeneous. It may consist of several different subnetworks, connected together by gate-
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ways to handle the necessary address and protocol conversion. The satellite network can then be regarded as simply another subnetwork, with its own gateways connecting it to the various terrestrial subnetworks. Data transmission normally requires a very high link integrity, which may be achieved in a variety of ways. The most effective method is normally to use ARQ. The transmission subsystem quality required is then mainly determined by the need for a high throughput efficiency, rather than by the overall link integrity target. End-to-end flow control is required on each connection, to prevent information loss at the receiver. It must, therefore, be controlled by the receiver.
Voice and video Most existing LANs, which are designed primarily to link computers, feature highly variable transmission delay. They are not well suited for the transmission of voice and video traffic, which require essentially constant delay for satisfactory performance. Some existing LANs do offer constant delay and it can be expected that future LAN standards will allow the integrated transmission of data, voice and video. At present though, separate local links or subnetworks will usually be needed, with integration only taking place at the satellite gateways. Voice and video of the types appropriate for this application do not in general require such good overall transmission quality as data. ARQ is in any case inappropriate as it introduces variable delay. If necessary, Forward Error Correction (FEC) can be used to improve the quality of any (part of a) link. Voice and video are essentially stream traffic. While the source may be able to transmit data at various rates, the receivers are designed to accommodate the maximum rate. Signalling from the source may be required to assist the receiver, but receiver-initiated flow control of the type needed for data transmission is not needed on an end-toend basis. Although current voice and video coders typically use algorithms which generate data at variable rate, according to talker activity or picture content, they do not normally exploit this for transmission and there is significant redundancy in their outputs. Future coders, however, will deliver data in bursts, as they are produced. This, for
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instance, is the approach adopted for codecs being developed for use in ATM Networks.
5.2. Satellite access and transmission
While the general requirement is rather clear, a considerable amount of work has to be done to identify satellite capabilities and the best ways to use them. In the longer term, it can be expected that onboard switching (OBP) satellites will be used for such applications, but they are not likely to become available much before wideband terrestrial networks become widespread. In the shorter term, only transparent satellites will be available. The activity proposed here is strictly aimed at such transparent satellites, but it overlaps in several ways with current work on OBP systems. A variety of system scenarios is being considered, ranging from high bit-rate operations involving the dedicated use of a satellite transponder, to shared transponder usage for moderate data-rates, supporting for instance links at a few Mbps between a few Ethernets. Access and transmission concepts are being studied, with the objective of minimising earth- and space-segment costs for a given system capacity. As a general principle, it is assumed that earth-stations should be installed on the users' premises. Consideration is being given to a range of possible satellites with different coverages and powers. The access methods used in LANs invariably depend on the transmission time being small. They are not therefore suited for wideband satellite systems in which the round-trip-delay is about 250 ms. Pure contention access schemes are also unsuitable if the satellite capacity is to be used efficiently. Given that satellite capacity is a limited resource, but that the offered traffic load will normally be highly variable, it is clear that some form of controlled access is essential. This means that flow control will be needed on data paths into the satellite gateways. For (the relatively high data rate) video signals, source-rates may need to be varied, under the control of the access controller, to limit traffic peaks. An access method which offers the necessary flexibility and control, is reservation-based TDMA. Capacity reservation and allocation necessarily involve a substantial delay however and so
a hybrid of preassigned- and reservation-TDMA is being studied, to provide each earth-station with a (relatively small) fixed pool of rapid-access capacity. The capacity reservation scheme should ideally be capable of providing fixed transmission times (constant transmission delay) for each voice or video call. In order to ensure that existing calls and sessions are properly handled, information about them must be made available to the access control system. In many cases it seems likely that individual earth-stations will not need to access the full capacity of the network, although they will usually need connectivity to all other earth-stations. In this case it may be possible to minimise the transmitter power and reduce overall costs by operating a multifrequency (MF)-TDMA system. This is essentially a set of separate T D M A systems operating on different frequencies, but with coordinated access to the different channels. Each earth-station is able to access one channel at a time, hopping channels for each burst, as directed by the access controller. Preliminary link budgets indicate that transmission of several mega.bits/s should be feasible with full European coverage, using quite modest earthstations. The limit is primarily set by a desire to use solid-state transmitters. The use of M F - T D M A would however allow the system capacity to be several times larger than the transmit capability of each earth-station. In principle, the number of satellite carriers could be increased as necessary to match the required system capacity. Such a system could, for instance, allow interconnection of a number of Ethernets. In conventional T D M A transmission, each burst is only coarsely synchronised to the T D M A frame. It is, therefore, preceded by a synchronisation preamble, allowing the receiver to identify the carrier phase and symbol timing of the burst and the start of the data. Guard-times must also be provided between bursts, to prevent collisions. The guard-times and preambles represent wasted capacity, corresponding to increased transmit symbol rate and system dimensions. The usual way to minimise this is to extend the burst length, keeping the overhead to an acceptable fraction of the useful traffic. In order to optimise the transmission design, powerful FEC coding will be needed for many applications, with demodulators operating at low signal to noise ratios. This will
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require significantly longer preambles and correspondingly increased burst lengths. In order to achieve adequate burst lengths, it will typically be necessary to group traffic from one earth-station into a small number of bursts. Given the variability of the traffic flow, the burst lengths will vary from frame to frame and at the boundaries of voice and video calls. Bursts occurring late in the T D M A frame will therefore be moved back and forth according to the total duration of the bursts preceding them. This makes it impossible to allocate fixed transmission times to the voice and video traffic, although strategies for minimising the delay variation have been devised. The FODA protocol, for instance, separates delay-sensitive and delay-insensitive traffic into separate bursts. All delay-sensitive traffic bursts are transmitted at the start of each frame, with the delay-insensitive traffic at the end. Various methods are being studied to improve the situation. With digital demodulator implementation, the burst preambles can be eliminated, avoiding the need to group traffic into long bursts. Traffic units can then be transmitted individually, with each earth-station transmitting many bursts per frame. If the traffic is broken into fixed-length cells, the possibility exists of truly constant delay transmission. A further possibility would be to operate the satellite network on a symbol-synchronous basis. This would require each earth-station to monitor the timing of its own transmissions, using a long loop through the satellite and to maintain them to within a small fraction of a symbol interval. Preliminary investigations indicate that this can be done satisfactorily. Symbol-synchronous operation appears to offer a number of advantages for the design and performance of the receivers. At high microwave frequencies, atmospheric attenuation is a problem. This is severe at 30/20 G H z and needs to be taken into account in the design of the system. Simply providing a power margin is inefficient and some form of adaptive system is normally preferable. At 14/12 GHz, attenuations are smaller and fade adaptation will normally not be needed. Fade adaptation within fixed power limits inevitably leads to a reduction in overall system capacity. It therefore interacts with the capacity allocation process and any changes in the transmission mode involve signalling. Studies will be made on how to detect atmo-
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spheric fading reliably and on the best ways to adapt to it. These are likely to be different for the different classes of traffic. 5.3. Compatibility with terrestrial transmission systems
As indicated earlier, one of the objectives of the COST 226 project is to investigate and promote the integration of future satellite and terrestrial systems. One of the tasks is, therefore, to follow work on the Broadband ISDN network, with the aim of harmonising the network interfaces and if appropriate adopting some of the same access and transmission concepts. As far as network interfaces are concerned, it was suggested earlier that voice and video equipment based on ATM techniques is likely to be available fairly soon and that this would offer some advantages in terms of capacity utilisation. Transmission of fixed-length cells over a symbol-synchronous satellite network obviously has a number of similarities with the ATM transmission mode. The ATM cell size appears to be well suited to satellite transmission. At first sight it also appears to be a suitable basic unit for segmenting data packets. In this case, ATM techniques may be useful for the multiplexing of multiple data, voice and video sources in the satellite gateways. The use of ATM-related techniques for multiplexing and transmission would obviously require a special interface for existing LANs. The underlying processes in the gateway would however be close to those in the eventual B-ISDN and this should make long-term compatibility easier. There are, however, several important differences between a (capacity-limited) satellite network and the broadband transmission medium foreseen for B-ISDN and it will be necessary to investigate these carefully.
6. Conclusion
There is a rapidly growing need for wideband international private networks to interconnect LAN islands. There is little prospect of this requirement being satisfied by terrestrial facilities in the near future, but satellite systems could be deployed rapidly for this purpose. One of the objectives of the newly-established COST 226 pro-
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ject is to identify appropriate system designs and to demonstrate a representative system, including suitable applications. Eventually, the Broadband ISDN is foreseen to satisfy most of the requirements, but for a substantial period it is expected that an integrated satellite/B-ISDN approach would offer significant advantages. COST 226 will, therefore, study compatibility issues. Early indications are that there may be some advantages in adopting some B-ISDN concepts for the satellite network.
Acknowledgements A project of this nature obviously involves the active participation of many people and it is not possible to name them all here. Nevertheless, we want to acknowledge the particular contributions to this work of Mervyn Hine (CERN), Erina Ferro and Nedo Celandroni (CNUCE), Chris Adams (Univ. Buckingham/RAL) and Horst Clausen (Univ. Salzburg).