19-02-2011, 10:07 AM
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SATELLITE DATA COMMUNICATIONS USING VSAT SYSTEMS – EXTENDING IT NETWORKS TO THE GLOBAL CONTEXT
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 page 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.
SATELLITE DATA COMMUNICATIONS USING VSAT SYSTEMS – EXTENDING IT NETWORKS TO THE GLOBAL CONTEXT
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 page 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.
