The multiaccess and broadcast capabilities of satellites have been historically recognised but it has proved difficult to realise their full potential because of technology limitations. The advent of the VSAT type of system, whether one-way data broadcasting or two-way interactive, represents the congruence of recent advances in several technological areas including higher gain and higher power satellites, relatively inexpensive microwave and RF components, digital modems and protocol processing. This evolution in satellite and earth terminal technology will ensure a role for the VSAT in most telecommunication architectures, whether it is a business data network supporting a major company in the USA or as the backbone of a basic telecommunications service in a Third World country. Most of the early developments in VSAT systems and service concepts evolved in the USA encouraged by a liberal regulatory environment and the availability of space segment at very competitive tariffs. These systems were developed, mainly, to support business requirements for data distribution and two-way interactive data communications where reconfigurability and rapid deployment are important. More recently, the potential for VSATs in support of communications in developing countries has been recognised.

Over the frequency range 1-40 GHz, the solid-state microwave device has come of age and, for small signal and medium power (~ tens of watts), applications must be the automatic choice for systems and circuit engineers. The GaAs FET has been, and remains, the dominant device with the GaAs MMIC, utilising the FET as the active element, causing a revolution in circuit technology. The newer derivatives, e.g. HEMT and PHEMT, will continue this revolution, allowing fully integrated circuits to be realised up to at least 100 GHz with enhanced performance at the lower frequencies.

The purpose of this chapter is to review the theory, design methods and physical implementation of low-noise downconverters for use in VSAT terminals. It would be impossible to consider all of the possible design approaches, so emphasis is placed on an approach consistent with that used in consumer DBS downconverters.

A review of established and new approaches to satellite multiaccess for VSAT applications is presented. A variety of contention and reservation based protocols for use on both slotted and unslotted channels are described and compared in terms of key attributes such as throughput, delay, stability, robustness, operational convenience and implementation complexity. After the survey, detailed performance results for four candidate 'first generation' VSAT protocols are presented (ALOHA, selective reject ALOHA, slotted ALOHA and DAMA with slotted ALOHA access) applied to an example transaction application. It is shown that, among the random class systems considered, SREJ ALOHA generally outperforms both ALOHA, and slotted ALOHA. DAMA is shown to achieve a higher capacity and lower delay variance than the random access alternatives, but this is at the expense of a 0-75 s irreducible delay, poor robustness and higher implementation complexity. In view of the relatively low impact of VSAT to hub space segment cost on overall system economics, it is expected that delay, implementation complexity and robustness will be primary considerations in VSAT access protocol selection, with capacity (within reason able ranges) being an important, but secondary issue.

This chapter presents a system design overview for Ku-band VSAT based interactive data networks. The objective is to provide an understanding of the major issues which arise in Ku-band VSAT network design, in terms of network architecture, VSAT equipment, satellite channel operation and access protocol considerations. Specifically, a reasonable approach to selection of key system components and their parameters over a range of typical traffic environments and performance objectives will be presented. Consideration will be limited to technical design issues, from which detailed cost comparisons with alternative technologies can be obtained, if so desired.

This chapter gives an overview of one of a number of novel systems being developed which bring new services to users and provide new forms of traffic for satellites. PANDATA is a system which provides a flexible data distribution service. Market research and regulatory opinion both suggest that data distribution systems based on small Earth stations are becoming acceptable in a European environment and will be used by PTTs and other licensed public network operators to carry specialised broadcast data traffic.

APOLLO (Article Procurement with On-Line Local Ordering) originates from the desire to complement on-line search and ordering of bibliographical references with an electronic delivery of the required document. Early investigations into the market and application requirements were initiated by the European Commission. It was soon concluded that only a satellite link could provide the envisaged transmission capacity, and since no practical system concept existed at that time, the Commission, Eutelsat, members of the CEPT and the European Space Agency (ESA) joined in a working group. During 1983, this group produced outline specifications for a generalised concept for electronic document delivery. At the end of 1984, the Joint Communication Programme Board of ESA approved APOLLO as a co-operative programme, in which ESA would provide the APOLLO-specific equipments and the participating PTT administrations would provide transmitting earth stations. Detailed specifications for the APOLLO system were then developed by ESA staff in close co-operation with the other participants. The major objective was to realise APOLLO as a data dissemination service with a limited number of source stations and an unlimited number of small, low-cost, receive-only earth stations. Development contracts with industry were awarded during 1985-86. Practically all equipment has already been delivered and used in satellite transmission tests. Overall system demonstrations have been successfully completed during the first half of 1989.

This chapter describes the family of two-way data networking products which provide the AT&T SKYNET® Clearlink Service. The first product, a very small aperture terminal (VSAT) called Clearlink, was introduced by AT&T Tridom in early 1986. The rest of the SKYNET Clearlink family includes various versions and configurations of the Clearlink VSAT, along with the necessary hub station and network management hardware and software to form a complete, end-to-end network that can provide reliable, two-way and one-way data and video broad cast communications.

An innovative adaptive assignment TDMA concept has been invented which allows efficient transfer of burst transaction type data and larger batch type data. This concept has been realised in the NEXTAR VSAT which has been described in this chapter. NEXTAR is emerging as a serious contender in the very competititive 2-way VSAT market.

Satellite-based messaging systems have emerged as a reliable alternative to existing terrestrial facilities especially with the advent of low cost VSATs. Although most packet switched networks implemented through VSATs do provide for messaging needs, it is important to emphasise that their primary role is for providing interactive or transaction oriented services. In fact, today, their most significant application is in providing for point of sale, credit validation and reservation type of transactions. It is essential to distinguish such a role from a much simpler role as required in message transmission. There exist already 'store and forward' systems for telegraph transmission in some countries working on terrestrial links. The primary distinguishing feature of these networks is in the 'access' time, which is not an important criterion as typically a 'few minutes' delay could be tolerated between transmission of a message to its reception at the destination terminal. The terminals are required to work unattended, receive messages and store them in memory for retrieval at a later time. Finally, such a satellite based network can interconnect the existing store and forward switches at appropriate gateways and form an important segment of the national telecommunication network: a typical terminal being implemented for a rural telegraph network in India is described in Appendix 19.7.

Features of satellite communications (satcoms) particularly appropriate to a military role include the ability to communicate worldwide over long or short ranges irrespective of natural ionospheric variations, with no dependence on an existing communications infrastructure, and the ability to establish communications within minutes of a terminal arriving in location. Unlike their civilian counterparts, military ground terminals may also have to contend with an intentionally hostile electromagnetic environment at the onset of jamming, and be required to minimise their risk of detection through interception of the uplink carrier. The military satellite system in which the terminals operate must respond flexibly to rapidly changing requirements of users whose priorities and traffic rates must adapt to meet operational needs. Before reviewing the state of small military ground terminals a relevant perspective can be achieved through an outline of the space segment resources available to the United Kingdom, United States and NATO in the military satellite frequency bands.

Today, there are very few VSATs scattered on the landscapes of the developing countries. One can confidently project a healthy market for VSAT hardware and services in the USA, Europe, Japan, Canada and Australia. The prospects in the developing world are less certain. Perhaps it is best to consider VSATs as presenting new opportunities for those countries understanding the importance of good communication in the struggle towards development. Their use will begin modestly and then grow steadily; there is not likely to be a VSAT 'revolution' in the developing world. But as further arrows in the quiver of developmental communications, they can and will make a significant contribution to the efforts to bring modern communications to all points on the globe.

The use of satellites and VSATs for providing the data link and physical protocol levels of IBM system network architecture (SNA) and X25 networks has gained widespread acceptance since the mid-1980s. Data communications performance characteristics over leased dedicated or multidrop telephone lines are unpredictable, not easily controlled, and are not uniform in many parts of the country. Satellite links greatly simplify the problems of network monitoring and control because the myriad of ground connections is eliminated. VSATs can provide flexible changes, additions or deletions to individual ports of a network that are not possible with telephone lines. This chapter addresses the use of VSATs for connecting remote cluster controllers and terminals to front end processors and hosts in satellite based IBM SNA and X25 networks. GTE Spacenet, an American satellite and network service provider, has worked with 12 VSAT vendors since 1984 and currently services five different types of VSATs. The discussion summarises its experience as a service provider.

Regulation of VSATs is occurring in many areas, not just those relating to national or international telecommunications policy. Control of services and equipment are two very distinct areas and national and international regulatory authorities treat them in different ways. This chapter analyses the background and current state of regulation and deregulatory trends for VSAT networks on a worldwide basis. The concentration on the USA, UK and other European Community countries is a result of the level of deregulatory activity in these areas. Deregulation in the USA has fostered rapid development of this technology, whilst most of Europe is now grappling with unique national and international regulatory issues concerning VSAT networks. A primary consideration in any discussion of VSAT regulation should be that the terminals themselves are rarely the major problem. It is the operation of the network, and the bypass inherent in satellite networks, that is the major issue for most governments and PTTs.