Satellite Data Communications using VSAT Systems – Extending IT Networks to the Global Context

Bruce Elbert

Application Technology Strategy, Inc.

Abstract

This paper provides a review and assessment of satellite technologies for providing broadband data communications using very small aperture terminal (VSAT) network systems. In this context, “broadband” means that the application requires a data transfer rate greater than 100 kbps and should allow broadcast, multi- and uni-cast, and interactive bi-directional services to fixed locations worldwide. The applications considered are: Internet access over satellite, digital content distribution, wide area network (WAN) connectivity, video teleconferencing, distance learning, and telephony. The systems examined include digital broadcasting (e.g., DVB) with IP encapsulation, and bi-directional VSAT star networks. Detailed comparisons of various transmission parameters are provided to help evaluate currently available satellite and ground equipment capabilities. It is observed that improved forward correction is desirable, namely the turbo product codes now being introduced widely in satellite ground equipment.

1                    Broadband service – a definition

A broadband data communications service is one that requires a transfer rate greater than that afforded by a dial-up telephone line using a V.92 modem. This places the minimum data transfer rate at about 100 kbps, which is typical of current high-speed access (HSA) services from Digital Subscriber Line (DSL) in its many forms, cable modems, and comparable fixed wireless and satellite HSA services. There is also the question of whether the two directions of transmission are of equal speed (symmetrical) or asymmetrical such that the inbound speed from server to user is greater than the outbound speed from user to server. From an IT perspective, broadband service supports standard office applications including email and file transfer, and major software systems like Enterprise Resource Planning and distance education. Organizations are structuring many of their IT applications for use within a standard Web browser, allowing employees and partners to access services within the Intranet and from the external Internet as well. This makes applications seem relatively similar to the network, but the detailed structure cannot be ascertained in general. HSA can provide video distribution, telephony and video conferencing, although these may not be deliverable through a browser since they require specialized user terminal devices or other appliances.

2                    Role of Operating GEO satellites

The single most critical element and technology in broadband satellite communications is the satellite itself, since every link within a common footprint must pass through it. Spacecraft designed and constructed in recent years are larger in physical size and mass, and provide substantially more power than their predecessors. This results from improvements in launchers, on-board power systems, high performance components, and radio-frequency high power amplifiers. Other components used within the microwave repeater have improved as well, with benefits showing up in reduced component mass, lower signal loss, and enhancement of transmission quality. A typical satellite weighs almost 5000 kg on top of the launch vehicle, has a lifetime of 15 years, and provides between 50 and 90 channels of wideband transmission (commonly referred to as transponders) with individual power levels of up to 200 RF watts, each. A modern GEO satellite may serve relatively small antennas throughout a large area such as the entire Euro-Asia continent or the full breath of the Pacific region (illustrated in Figure 1 for the PanAmSat 2 satellite).

Figure 1. A typical satellite transmit (downlink) footprint of the PanAmSat 2 satellite, located at 169º East Longitude; satellite radiated EIRP values are approximate (courtesy PanAmSat Corp).

Digital communications and GEO satellites have long partnered and in fact innovations such as TDMA, CDMA, digital speech interpolation and video compression were applied to space-ground links well ahead of terrestrial networks. The Digital Video Broadcast (DVB) standard fits tightly to the satellite’s natural ability to transmit the same high-quality signal across a wide region, rendering the cost per location to an infinitesimally small number (not including the cost of the dish and set-top box installation). Moving forward, this platform provides broadband data delivery and facilitates return channel service if remote sites are suitably configured. Competition from new entrants like PanAmSat, SES and Loral has impacted satellite operation such that quasi-governmental operators like Intelsat and Eutelsat have become commercial enterprises. Suitable GEO satellite capacity is now available throughout Asia-Pacific for networks that serve most any location.

3                    Application Interface Standards

A summary of applications and interface standards is provided in Table 1. For digitized content, quality is set at the source encoder and transmission only introduces time delay. For a satellite hop, this delay is small compared to that of compression/decompression. The terrestrial interface concerned with DVB and digital TV in general is called ASI, a high speed serial connection used primarily on the uplink side. Telephone service is usually delivered on an analog basis (2-wire or 4-wire). A properly engineered satellite voice circuit meets the currently accepted standard of 400 ms total delay, including the added delay for speech processing (e.g., compression and decompression, if applied), routing and switching. Importantly, such a satellite circuit will sound better to subscribers than casual Voice over IP connections through the Internet.

Table 1. User applications and their interface standards applied in satellite communication networks.

4                    Internet Protocol

The Internet itself is the last and probably most important interface in the context of data communications. Organizations in the private and public sector have either converted their data communications over to the Internet Protocol, or are in the process of doing so. The interface that is growing to dominate the data world is the simple RJ-45 modular jack associated with the Ethernet standards, 10baseT and 100baseT. Higher rates than 100 Mbps demand Gigabit Ethernet or the optical speeds of the Synchronous Digital Hierarchy (e.g., OC-3, OC-48 and the like). Such speeds are presently beyond a practical HSA service from currently operating C and Ku band GEO satellites. This could be the domain of the coming generation of broadband satellites employing Ka band spot beams and on-board processing.

4.1             Broadcast, Multicast and Unicast

Terrestrial networks, including the Internet, are effective for point-to-point transfer of digital media and content. Multicast service over the Internet must employ several point-to-point links to emulate a broadcast system, and therefore has difficulty assuring timely delivery of content to all receivers. A broadcasting station from a local radio tower or GEO satellite affords timely delivery of content with a consistent bandwidth. Guaranteeing delivery is usually less of a problem because receivers are designed to directly play the content (a local recording device can allow later playback, if desired).

Included in the DVB standard is a data transfer capability called Internet Protocol Encapsulation (IPE). This allows a single broadcast carrier to transfer both television programming and Internet content on the same transport stream. At the subscriber end, the carrier is detected by an integrated receiver decoder (IRD) that extracts the data and delivers it a local PC or LAN. This vehicle allows satellite broadcasters to introduce broadband data into their multiplexed transmissions. The data that rides the MPEG stream may be encrypted along with the digital video and audio, or can be processed with its own unique encryption system. To this may be added a terrestrial return channel for bi-directional service to the desktop or other computational device. Many applications can be supported in this asymmetrical manner since the greater demand is for megabit per second transfer over the satellite in the outbound direction. One must not neglect the potential of this mode for reaching locations that cannot transmit directly over the satellite. Provision of a satellite return channel, in the inbound direction, is discussed next.

4.2             Interactive bi-directional data

Interactive data communications are the foundation of must corporate and government uses of telecommunications. These needs can be addressed by properly engineered bi-directional satellite links that involve multiple transmitting earth stations. The Very Small Aperture Terminals (VSATs) used by fueling stations and discount department store chains in the Americas, Europe and parts of Asia demonstrate that such networks are practical (e.g., easy to install and centrally manage), reliable (e.g., 99.9% availability) and cost/effective (e.g., saving users as much as 20% over what an equivalent terrestrial network would cost). The architecture of a typical VSAT network employing a star topology (e.g., all communications through a central hub) is illustrated in Figure 2.
 

Figure 2. Architecture for a typical VSAT network employing a common hub and star topology.

In 2002, VSATs are becoming attractive to smaller enterprises and for big organizations that wish to push the use of satellite communication down further in their operation. The cost of equipment per site has dropped from over US$10,000 in 1998 to around US$2000 in 2002. Consumer versions that provide HSA to the Internet are offered in the US for under $500.

5                    Review of equipment and network suppliers in the marketplace

The market for satellite communications ground equipment as introduced above is served by specialist manufacturers and systems integrators. A partial listing is provided in Table 2. Several have been in business for more than a decade; however, some of the more interesting technology is offered by relatively new companies that didn’t exist prior to 1998. This can make it more challenging to convert architecture into a real network, but the methodology reviewed here should make the task less formidable.

Table 2. Suppliers of Technology for Broadband Satellite Communications.