Direct broadcast satellite
Direct broadcast satellite (DBS) is a term used to refer to satellite television broadcasts intended for home reception.
A designation broader than DBS would be direct-to-home signals, or DTH. This was initially meant to distinguish the transmissions directly intended for home viewers from cable television distribution services that sometimes carried on the same satellite. The term DTH predates DBS and is often used in reference to services carried by lower power satellites which required larger dishes (1.7m diameter or greater) for reception.
In Europe, prior to the launch of Astra 1A in 1988, the term DBS was commonly used to describe the nationally-commissioned satellites planned and launched to provide TV broadcasts to the home within several European countries (eg BSB in the UK, TV-Sat in Germany). These services were to use the D-Mac and D2-Mac format and BSS frequencies with circular polarization from orbital positions allocated to each country. Before these DBS satellites, home satellite television in Europe was limited to a few channels, really intended for cable distribution, and requiring dishes typically of 1.2m SES Astra launched the Astra 1A satellite to provide services to homes across Europe receivable on dishes of just 60 cm-80 cm and, although these mostly used PAL video format and FSS frequencies with linear polarization, the DBS name slowly came to applied to all Astra satellites and services too.
Terminology confusion
As a technical matter, DBS (also known by the International Telecommunications Union as Broadcasting Satellite Service, or BSS) refers only to services transmitted by satellite in specific frequency bands: 11.7-12.2 GHz in ITU Region 3 (Asia, Australia), 10.7 - 12.75 GHz in ITU Region 1 (Europe, Russia, Africa), and 12.2-12.7 GHz ITU Region 2 (North and South America). In 1977, the ITU adopted an international BSS Plan under which each country was allocated specific frequencies at specific orbital locations for domestic service. Over the years, this plan has been modified to, for example, accommodate new countries, increase coverage areas, and reflect digital (rather than analog) technology. At present, numerous countries have brought into use their BSS Plan allocations.
By contrast, DTH can apply to similar services transmitted over a wider range of frequencies (including standard Ku band and Ka band) transmitted from satellites that are not part of any internationally planned band. Nonetheless, the term DBS is often used interchangeably with DTH to cover both analog and digital video and audio services (including video-on-demand and interactive features) received by relatively small dishes (less than 1 meter). A "DBS service" usually refers to either a commercial service, or a group of free channels available from one orbital position targeting one country. In certain regions of the world, especially in North America, DBS is used to refer to providers of subscription satellite packages, and has become applied to the entire equipment chain involved.
Commercial DBS services
The first commercial DBS service, Sky Television plc (now BSkyB after its merger with British Satellite Broadcasting's five-channel network), was launched in 1989. Sky TV started as a four-channel free-to-air analogue service on the Astra 1A satellite, serving the United Kingdom and Republic of Ireland. By 1991, Sky had changed to a conditional access pay model, and launched a digital service, Sky Digital, in 1998, with analogue transmission ceasing in 2001. Since the DBS nomenclature is rarely used in the UK or Ireland, the popularity of Sky's service has caused the terms "minidish" and "digibox" to be applied to products other than Sky's hardware. BSkyB is controlled by News Corporation.
PrimeStar began transmitting an analog service to North America in 1991, and was joined by DirecTV (then owned by a division of General Motors, GM Hughes Electronics), in 1994. At the time, DirecTV's introduction was the most successful consumer electronics debut in American history. Although PrimeStar transitioned to a digital system in 1994, it was ultimately unable to compete with DirecTV, which required a smaller satellite dish and could deliver more programming. DirecTV purchased PrimeStar in 1999 and moved all PrimeStar subscribers to DirecTV equipment. In a series of transactions consummated in 2003, Hughes Electronics was spun out of GM and the News Corporation purchased a controlling interest in the new company, which was renamed The DIRECTV Group. In 2008, Liberty Media Corporation purchased News Corporation's controlling interest in DIRECTV.
In 1996, EchoStar's Dish Network went online in the United States and, as DirecTV's primary competitor, achieved similar success. AlphaStar also started but soon went under. Astro was also started, using a direct broadcast satellite system.
Dominion Video Satellite Inc.'s Sky Angel also went online in the United States in 1996 with its DBS service geared toward the faith and family market. It has since grown from six to 36 TV and radio channels of family entertainment, Christian-inspirational programming, and 24-hour news. Dominion, under its former corporate name Video Satellite Systems Inc., was actually the second from among the first nine companies to apply to the FCC for a high-power DBS license in 1981, and it is the sole surviving DBS pioneer from that first round of forward-thinking applicants. Sky Angel, although a separate and independent DBS service, uses the same satellites, transmission facilities, & receiving equipment used for Dish Network through an agreement with Echostar. Because of this, Sky Angel subscribers also have the option of subscribing to Dish Network's channels as well.
In 2003, EchoStar attempted to purchase DirecTV, but the FCC and U.S. Department of Justice denied the purchase based on anti-competitive concerns.
As of 2010, India has the most competitive Direct-broadcast satellite market with 7 operators vying for more than 400 million TV homes.
Free DBS services
Germany is likely the leader in free-to-air (FTA) DBS, with approximately 40 analogue and 100 digital channels broadcast from the SES Astra 1 position at 19.2E. These are not marketed as a DBS service, but are received in approximately 12 million homes, as well as in any home using the German commercial DBS system, Sky Deutschland.
The United Kingdom has approximately 160 digital channels (including the regional variations of BBC and ITV channels) broadcasting without encryption from the Astra 28.2°E satellite position, and receivable on any DVB-compliant receiver. Most of these channels are included within the Sky Digital EPG, and an increasing number within the Freesat EPG. They include a handful of FTA HDTV channels.
India's national broadcaster, Doordarshan, promotes a free-to-air DBS package as "DD Direct Plus", which is provided as in-fill for the country's terrestrial transmission network.
While originally launched as backhaul for their digital terrestrial television service, a large number of French channels are free-to-air on 5W, and have recently been announced as being official in-fill for the DTT network.
In North America (USA, Canada and Mexico) there are over 80 FTA digital channels available on Galaxy 19. (The majority of them are ethnic or religious.) Other popular FTA satellites include AMC-4, AMC-6, Galaxy 18, and Satmex 5. A company called GloryStar promotes FTA religious broadcasters on G-19 and AMC-4.
The Special Challenges of Satellite Communications Systems
Satellite systems hold forth the promise of true "anywhere, anytime" access to communications, even in the most rural and remote areas of the globe. A number of world-wide satellite telephone systems have been proposed, and several are already in operation. These systems provide communications coverage over very wide areas, including over the ocean. In general, they fall into three broad classes: geosynchronous (GEO), "big" low earth orbit (LEO), and "little" LEOs.
Geosynchronous systems, in which the satellites maintain a high orbit that keeps them over a fixed spot on the Earth's equator, have several advantages in terms of long satellite life and wide area coverage by a small number of satellites. They have the disadvantages of round trip latencies that exceed a half a second, poor coverage and inadequate elevation angles (to avoid building radio shadows in urban areas) at the high latitudes. These weaknesses are addressed by the low earth orbit systems, which follow elliptical orbits, allowing them to provide reduced delays and better coverage and elevation angles when close to their orbital perigee. However, LEOs require substantially greater numbers of satellites to provide adequate coverage, and these will need more frequent replacement.
Geosynchronous systems include Inmarsat and OmniTRACS. The former is geared mainly for analog voice transmission (it was used by reporters to transmit from Bagdad during the Gulf War). The first generation Inmarsat-A system was designed for large (1m parabolic dish antenna) and rather expensive terminals. Newer generations of Inmarsats are incorporating digital techniques for use with smaller, less expensive terminals (i.e., the size of briefcase). The Inmarsat system uses allocations in the 6 Ghz band for the ground station to the satellite,1.5 Ghz for the satellite to terminal downlink, 1.6 Ghz for the terminal to satellite uplink, and 4 Ghz for the satellite to ground station.
Qualcomm's OmniTRACS provides two-way communications as well as location positioning. The system operates in the 12/14 Ghz bands. The downlink data rate is between 5 Kbps and 15 Kbps while the uplink is between 55 bps and 165 bps. The system is used extensively for alphanumeric messaging and on-board sensor reading for trucking fleets.
Little LEOs are intended to be relatively small and inexpensive satellites that provide low cost, low data rate, two-way digital communications (but not voice) and location positioning to small, handheld terminals. The frequency allocations are in the VHF band below 400 MHz. The advantage of little LEOs are their small size and relatively low costs. Systems include Leosat, Orbcomm, Starnet, and Vitasat. For example, the Orbcomm system requires 34 satellites for reliable, full world coverage, and provides 2400 bps on the uplink and 4800 bps on the downlink.
Big LEOs are larger, more expensive satellites that provide voice communications as well as moderate to high speed data communications (56 Kbps). Proposals include Aries, Ellipso, Globalstar, Iridium, and Odyssey. Frequency allocations are above 1 GHz. For example, Motorola's Iridium system will offer worldwide cellular phone service from 66 satellites placed in 6 polar orbits.
Unlike the proposed LEO systems, the Hughes DBS system provides several unique communications capabilities. The system is no longer proposed; it has been launched, it is in operation, and it provides coverage for most of North American. Beyond its current ability to distribute digital video, it is ripe for data communications experimentation and pilot applications development. Of the commercially available GEO systems, only DBS has the potential for multi-megabit per second transmissions, although latencies are high (i.e., greater than 500 ms for the downlink alone).
However, the system presents considerable challenges for applications development. Unlike the other mobile satellite systems, DBS is not intended to be used as a two-way system over the satellite segment; the data communications uplink is provided though wireline networks such as the public switched telephone system (PSTN) and Internet gateways. It is not suitable for mobile wireless communications; its 18-inch satellite dish, though rapid to deploy, will not support communications to users continuously on the move. And unlike the other systems, which have moderately asymmetric satellite segment uplinks and downlinks, the DBS is a highly asymmetric communications system that spans hybrid links (i.e., satellite downlink, wired uplink).
This makes DBS a particularly attractive technology for wide-area data distribution or asymmetric data access, in which significantly more information can profitably be broadcast on the downlink than on the uplink. Note that this characteristic is not unique to satellite systems. The Berkeley InfoPad project is building a wireless in-building system in which the downlink radios can transmit at 100 mbps while the pad's uplink radios transmit at only 2 mbps. Numerous important applications can take advantage of broadcast-based and asymmetric access. Examples of the former include distribution to a dealer network of updates to maintenance manuals or product catalogs, and the latter include World Wide Web access.
The Soda Hall/Hughes DBS Experimental Testbed System
In order to investigate the issues of applications support for the DBS system, we are integrating DBS capabilities into our in-building wireless access testbed. This figure shows the planned organization of the testbed. Within the Computer Science Division's new facility in Soda Hall (see figure), we have already deployed a wireless local area network using commercially available radio modems (AT&T WaveLan devices) and PC-based basestations and laptop computers running the BSDI Unix-based operating system for x86 PCs. To this we are adding a DBS satellite dish mounted on the roof of the building, and a DirectPC basestation integrated with our wireline network. This testbed allows us to experiment with the integration of high data rate downlink transmissions from the satellite with distribution through the local area wireline network and forwarding to mobile devices roaming in the wireless in-building network. Uplink traffic will be routed back through the wireless network, to the wireline infrastructure, and then through the Internet to the DBS gateway.
At first it may appear that there is no apparent reason to combine in-building wireless access with satellite-based information distribution. However, our applications testbed depends critically on the computational and storage resources of the in-building wireline infrastructure. Applications running on the mobile devices have full access to the capabilities of building's network of workstations (NOW) processor clusters, distributed workstations, and file and tertiary storage servers. In aggregate, these represent processing resources that far exceed the capabilities of any single device. For example, this testbed configuration will allow us to experiment with using the satellite broadcast channel to aggressively prestage data for cached storage in the local environment. America On-Line, for example, maintains a cache of all of the World Wide Web pages accessed by its 1 million+ users. The size of this cache is well less than 10 GBytes today. It would not take long at 10 mbps DBS transmission speeds to distribute this vast volume of data to user sites with the capacity to store it for local high speed access. We hope to be able to understand these kinds of system-level implications of the satellite technology by integrating DBS with a rich local computing environment.
Applications Enabled by Direct Broadcast Satellite Technology
Before we describe our strategies for supporting applications within the DBS system, it is important to gain a better understanding of the kinds of applications that are suited to broadcast-based information dissemination. Consider the following applications scenarios, which illustrate some of the possibilities inherent in the DBS system:
• Rapid Information Dissemination: A major forest fire is burning out of control in a National Park. Firefighters are being brought in from several adjacent states, and they are not familiar with the disaster area. Detailed forest service maps are needed to help them plan their disaster response, but there are not enough to go around. A DBS dish is rapidly installed in the disaster management command center, a complete set of digital maps are quickly downloaded over the DBS broadcast channel. Depending on where the individual teams are to be sent, selected maps are further distributed to mobile computers (perhaps over a WLAN in the command center) or computers embedded in support vehicles (perhaps via a campus-area packet relay network that spans the depot around the command center) that are then taken into the field by the firefighter teams.
• Integrated Broadcast Video and Interactive Data Services: Rural health centers around the country are equipped with DBS dishes. A "medical practitioners channel" is established to transmit DirectTV broadcasts of the latest medical procedures. During these broadcasts, a viewing physician can use the system's data communications capabilities to interactively select from a collection of medical journal articles that describe the procedure or treatment currently on view be downloaded into his or her personal computer.
• Information on Demand: Up-to-the-minute weather reports are captured for the entire country and fed into a DBS "digital weather channel." For major metropolitan areas (or areas in which the weather is changing rapidly or is particularly severe), these reports are frequently scheduled. They may include high resolution "moving" weather maps (e.g., in MPEG) as well as detailed textual descriptions of the current weather conditions, perhaps specialized to specific regions of the metropolitan area. DirectPC users can enable filtering programs to capture the reports for their areas (or where they are planning to travel to). Users in less populated regions will receive their reports on a much less frequent basis. Nevertheless, they can use the system's uplink capabilities to request a report "on demand," and the network will schedule its delivery for a future time slot. If multiple users request the same information, its priority can be increased, and a sooner slot can be allocated to transmit the requested information.
DBS systems can be installed rapidly, even in areas without a well developed communications infrastructure, although some uplink path will be needed, perhaps through the public switched telephone system. This makes DBS ideal for establishing communications in support of emergency response activities, and the system is particularly effective at distributing critical information to the field.
DBS was developed primarily to deliver video to users. However, the ability to integrate interactive data access with simultaneous video broadcasts opens new opportunities for information dissemination combined with television. Distance learning applications that combine broadcast telelectures on DirectTV with simultaneous access to instructional materials on DirectPC is but one example.
The final scenario shows how a broadcast channel could be efficiently structured to combine frequent and less frequent data retrieval requests.
