Satellite communications for speech and data, as well as for television, have been with us for many years. The typical satellite system design uses a large central hub station and smaller size remote stations. If the remote stations are designed to receive satellite television, they can be quite small and simple, with a dish size of less than 1 metre across. However, if the remote stations are designed to transmit and receive high-speed data, then originally they had to be very large and complex, with dish sizes of 10 metres or more. The march of technology owever, has meant that such remote stations can now be much smaller and simpler, with dish sizes in the range of 2 to 3 metres.

Such a small dish satellite transmit/receive station is called a Very Small Aperture Terminal - in short, VSAT.

A VSAT network exploits the requirement for centralised corporate communication by providing an asymmetrical satellite link in which one earth station is larger and the other one is smaller than the size required in a symmetrical link.

The typical application for VSATs is in a so-called "closed" network. An organisation places a hub station at its head office and a VSAT at each of its branch offices. Then data can flow between the branches and the head office, for example to the mainframe computer at head office. If the organisation has a number of branches measured in the hundreds, then VSAT can be a very competitive technology - in North America, at least.

VSATs can also be one-way (reception of data only), and in that case they are even smaller and simpler. If the requirement is to "broadcast" data out to hundreds of locations, then one-way VSAT is a far cheaper approach than any other; and even in Europe there are several applications of this sort.

A typical one-way VSAT terminal employs a 60 cm to 90 cm dish, whilst two-way terminals utilise dishes of 1.2 m to 1.8 m diameter. In both cases, all the necessary radio equipment is combined with the antenna to form an outdoor unit installed on the roof of a user's premises. The outdoor unit is connected to an indoor unit which provides for communication and control functions and interfacing with data terminals and user equipment. Note that there is no need to use any PTT facilities to connect the VSAT to the organisation's own LAN. So does VSAT makes one independent of the PTTs? Not entirely, not yet, because so far in Europe many of the VSAT networks are run by PTTs - although things are changing. However, there is no need for local (or national) connections via PTT networks to the earth stations, since they are at user sites. In the past the only way to use satellite communications was to have large multi-user business earth stations or large trunk earth stations with access via PTT networks - this largely eliminated the savings from using satellite networks. Not so nowadays.

For a centralised network (so-called star topology) where one end of all links terminates in one location, it is possible to use one large central (hub) earth station and a multitude of small earth stations, one in each network node. Since the number of remote VSAT terminals located at network nodes is large, economy of scale has been utilised to make these terminals affordable.

The result of these basic technologically determined economics has been a deployment of VSATs in recent years in their tens of thousands - even in Europe there are over 50,000 if one takes a broad classification.

VSAT terminals can be considered, at present, as satellite networking tools mainly for communications from a large number of peripheral nodes to the common hub station in a closed network. The hub station implements satellite access architectures, provides user interfacing and protocol conversion and performs centralised control of the network as a whole. Because of these functions and its large size the hub station is relatively expensive - hence the network has to employ a sufficiently large number of VSAT terminals to bring the network cost per node below its terrestrial counterparts.

In terms of traffic configuration, current VSAT networks are divided into one-way and two-way networks. Originally the concept of VSAT network was developed for one-way transmission of data and for a few years one-way networks were the only type in operation.

However, two-way VSAT networks have been used in large numbers for the last 5 or 6 years, mainly in corporate data communication applications. In a two-way VSAT network, a remote data terminal or a personal computer can interact with a host computer or server which is located at the hub station or at the corporate headquarters connected to the hub stations via a leased land line.

One widely used configuration is designed to connect a number of different VSAT networks via the same hub station to their corresponding headquarters. In this case the hub is termed a "shared hub", an approach which has proved more popular and cost-effective for smaller VSAT networks.

The following are the key issues to be addressed in selecting a satellite network for any application:

Another important issue is the network topology. The two main choices are:

1 star, where all communication between VSATs is via the hub station

Almost all existing VSAT networks are of star topology. However, there are requirements, such as voice, which would be ideally satisfied by small mesh networks. A mesh network eliminates the need for an expensive hub station and operates with a distributed management and control architecture. Note that a mesh network can be configured as a centralised star network or a hybrid network which is a combination of star and mesh topologies.

Since a mesh network is based on direct communication between VSAT terminals, it offers a distinct advantage in real-time communications, particularly voice. In a star network the signal from one VSAT to another travels to satellite and back twice before reaching its destination VSAT. This "double hop" link utilises twice the satellite capacity compared to a single-hop link and produces twice as much delay - this is objectionable for voice and some real-time data services.

Since VSATs are much more prevalent in the USA, we consider that region first. After that, we look at Europe.

A large number of star VSAT networks have been deployed in the USA. This is because of their need for continent-wide communications, a deregulated environment and technological lead. The Table shows a few examples of these networks.

The main sectors of activities using VSATs are:

The largest network (SIS) is used for video distribution to betting shops numbering over 10,000 in the UK and over 3000 in the rest of Europe. Most users have a 1.3 metre dish, via which they receive video signals plus voice and data. The video signals originate from two horse-racing courses and one greyhound course. The voice and data traffic are carried by the video sub-carriers. All racing information is distributed via a 9.6 kbit/s data channel. SIS are now developing racing services to Russia.

It is important to realise that the emergence of a single European market and the liberalisation in Eastern Europe is transforming the nature and size of the European telecommunication market. Networking by satellite is has played a vital role in providing quick and flexible solutions in Eastern Europe where the terrestrial infrastructure was not sufficiently developed.

The European VSAT Monitor reports the following numbers of VSATs operating in Europe:

Thus mesh networks are still in a small minority. Note also that terminals for "business television" (essentially satellite TV broadcasting to closed user groups) numbered over 25,000.

The main reasons for using VSAT in Europe are:

The main rivals to VSAT networks are terrestrial networks of various types:

JANUS was a VSAT-based network that operated across Europe in the period 1993-96. It linked a number of university and commercial sites and provided education and training services to students. A major difference between JANUS and many other satellite networks developed in and around universities was that JANUS used off-the-shelf satellite equipment and selected networks and equipment suppliers after thorough costings studies and procurement processes.

The aim of the JANUS project was to build, using a mix of satellite and terrestrial networking technology, a pilot network similar to the terrestrial academic Internet networks but pan-European and particularly oriented to serving the needs of remote regions. The network was trialled with distance education users, based both at home and at local study centres (small colleges etc).

The main application used on JANUS was FirstClass, a distributed electronic mail/bulletin board system. The "customers" of JANUS were students and trainees at various universities and companies.

The general structure of the JANUS network was a combination of:

The JANUS VSAT network entered operation with its first node in October 1993. At the end of the project it had six nodes, all linked in a private TCP/IP network. These JANUS VSAT nodes were in five European countries as shown below.

The VSAT technology used in JANUS was the Hughes Olivetti Telecom "LanAdvantage" earth stations. These were leased from Unisource Satellite Services (part of the Unisource joint venture of European PTTs). Each VSAT connected to the others via a hub station, situated in Burum in the north Netherlands. At each site, the VSAT was connected to an Ethernet LAN, running a mix of protocols.

Any satellite network has propagation delays caused by the signal taking time to travel to the satellite and back again. This round trip delay is about 0.25 second. JANUS VSAT network operates in double-hop mode (node to hub to node - see below), leading to a total transit time of 0.5 second. Many messages have to go all the way across the network and back again, leading to a further doubling of response time to around 1 second. This figure ignores any further delays in the LANs at each end, at the hub, and in gaining access to the VSAT network in conditions of satellite link congestion. In practice the round trip delay, as measured by a user, is more like 2 or 3 seconds.

Future networks considering the use of satellite technology will expect VSAT technology to deliver: