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Digital Satellite Systems for Internet Access
Syed Ali, Preetham Peter, & Jin White
Project 1
EE 4984: Telecommunication Networks
April 15, 1997
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Abstract
The objective of this paper is to inform the reader of the ongoing developments in the arena of digital satellite systems for internet access. Iridium, Spaceway, Teledesic, Globalstar, and DirecPC will be examined and compared. The topics will range from the number of satellite in each system consists to the data rate of each system. Out of these systems, only one, DirecPC, is currently available; while the other systems are scheduled for operation in the future, some as soon as next year. DirecPC was established strictly for internet services, while the Iridium and Globalstar systems are planning to extend their range of services to include voice as well as data. In examining this rapidly growing communications sector, the implementation of such a system, with regard to the techniques that will be employed in order to launch and maintain reliable service, especially with regard to the implementation of ATM across such a network will be analyzed. Other areas of equal importance, such as the political aspect, with regard to frequency allocation and international agendas in the field, as well as the more rudimentary problems of launching large satellite networks into the various orbits that each of the varying networks have proposed, will also be examined. Another concern is the demand for such service, which, while being touted by a great many industry insiders, is also being downplayed by a others. Also, the economic viability of such a system as well as the technological viability of these networks will be examined.
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Introduction
The area of satellite communications is a hotbed of economic activity, with analysts estimating that investors have poured in excess of $50 billion into the market for voice and data satellite communications. This space race is deemed to have been sparked by the success of Hughes' DirecTV, a satellite dish TV system launched in 1996; more that 5 million US customers have signed up for DirecTV access. With the coming implementation of IP6, which is employed in great part due to a dearth of IP addresses, the internet sector of the global communications industry is seen as one of great growth by these corporations. A closer examination of the technological, political, and economic factors affecting this market is needed to better understand where it is headed.
The Systems
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We will examine three types of systems: voice and data satellite communications systems, passive satellite receiver systems, and full-duplex satellite communications systems. Iridium and Globalstar are examples of the first type of satellite system; DirecPC is an example of the second type and Teledesic and Spaceway are examples of the third type.
Iridium
Motorola's Iridium system is a project backed by an international consortium of more than a dozen companies that will also provide service in many forms, such as real-time voice, data, fax, paging, messaging, and position location. Iridium will be a global system and will also be non-geostationary. The project will cost $3.7 billion, and will consist of 66 low earth orbit (LEO) satellites plus 6 spares; LEO satellites orbit about 700-2000 km above the earth's surface. Each satellite will contain a powerful switching center, along with intersatellite links to provide intrinsic long distance capability. Iridium will operate in the L and Ka bands. Iridium proposes time division duplex; both downlink and uplink employs a mixture of TDMA (time division multiple access) and FDMA. The data rate offered by this system is only 2.400 kbps, because this system also does not focus on data transmission but on voice. The customer will have to purchase a hand-held mobile terminal from $2500-$3000; and the call rate will be $3 per minute. Iridium expects to be up and running in 1998.
Globalstar
The Globalstar system is being designed by Loral/Qualcomm; and will provide service in many forms, such as real-time voice, data, fax, paging, short message service, and position location. Globalstar will be a global system and it will also be non-geostationary, meaning that the satellites will not rotate with the Earth and will not appear stationary to observers on the Earth. The project will cost $2.2 billion, and will consist of 48 LEO satellites plus 8 spares. These satellites are strictly digital repeaters, with all the switching and call processing facilities on the ground from its 100 to 210 Earth Stations. Globalstar will use Qualcomm's terrestrial CDMA(code division multiple access) technology for the mobile link, and for the feeder link FDM(frequency division multiplexing) uplink and FDMA(frequency division multiple access) downlink. The maximum data rate supported by the system is 9.600 kbps, which is not very fast because this system does not focus on data transmission, but on voice. The customer will have to purchase a hand-held mobile terminal for about $750; and the call rates will range for $0.35 to $0.53(wholesale) per minute. Globalstar is expected to be up and running in 1998.
DirecPC
The third system that will be examined is DirecPC; which is the only system currently on the market. It is owned by Hughes Networks of Germantown, Maryland. This system consists of one satellite called the Galaxy 4, which is located somewhere in the southern sky for most of North America at an altitude of 36,000 km, making it geostationary. The DirecPC Terminal is a two-way Internet access terminal with a 400kbps satellite connection from the Internet. However to access and send requests to the Internet, the customer needs a standard telephone dial-up modem and a local Internet Service Provider (ISP). This is because the 21-inch elliptical satellite dish the customer gets in their start-up kit package is a non-powered receiver, which means it can not transmit data back to the Internet. The other components that are included in the hardware/software package are a 100feet coaxial cable to connect to their PC through a 16-bit ISA bus card adapter, and Microsoft Windows '95-based software. The price of the package is $699, which does not include the monthly charges for this service varying from $24.95 to $129.95, depending on the amount of data the customer downloads. Then there is also the monthly dial-in access fees and telephone charges to the customers local ISP, in other words this system is very costly. However at a data rate of 400kbps, DirecPC Turbo Internet Service gives the customer the highest speed internet access currently available nationwide.
Teledesic
The Teledesic satellite system is the focus of Teledesic corporation. This system will be designed with an emphasis on data transmission, but will also provide service for voice, fax, paging, and video. Teledesic is a global system and will be non-geostationary. The project will cost $9 billion, and will consist of 840 LEO satellites plus up to 84 spares. The reason for the unusually large number of satellites is to provide superior service; each satellite will not have to handle as many customers, the responsibility will be spread out. Each satellite will be like a cell in a cellular phone network, except that the cells can move as well as the callers. The Teledesic system is a broadband system so high data rates, 16-2048 kbps, can be achieved. The network aims to provide 18 simultaneous 1.5Mbps Internet links in each satellite, and 20,000 worldwide. The terminals will use a FDMA uplink and ATDMA(asynchronous TDMA) downlink Also incorporated in this system will be an ATM capability and regeneration of digital signals. The customer will have to purchase a portable terminal for a currently undetermined price; and the call rate will be $0.04 per minute. Teledesic expects to have their system ready and running in 2001.
Spaceway
Spaceway is a system proposed by Hughes Communications, Inc. and will consist of about 20 satellites in geostationary orbit; this is different from the other systems in that the orbital altitude is much greater, around 30,000 km. These satellites will operate in the Ka band and provide from 384 kbps to 6 Mbps uplink rates and up to 108Mbps downlink rates in a bandwidth on demand service. FDM/TDMA serve as access schemes for uplink, TDM for the downlink. The system is fully compatible with a wide range of terrestrial transmission standards such as ATM, ISDN, Frame Relay and X.25 and will utilize hubless full-mesh networking. Spaceway end-users will access the system with a relatively inexpensive Ultra Small Aperture Terminal (USAT), utilizing a 0.7-meter diameter dish antenna. The terminals will cost on the order of <$1000 and are expected to be available in consumer electronics stores.
In comparing these systems, it is obvious that those systems designed specifically for data communications such as Teledesic and Spaceway have a distinct advantage over the systems that support real-time voice as well as data; due mainly to their increased data rates and ability to implement ATM. Other systems that are cast in the same mold as Spaceway are AT&T's Voicespan, Lockheed Martin's AstroLink, Loral's Cyberstar, TRW's Millenium, GE's GEStar and KaStar.
Technical Issues
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There are a range of technical issues that are related to the successful implementation of satellite constellations for internet access; some of these will be examined here.
Aerospace
Launch Vehicles in the US
The actual launching of these large (Teledesic proposes an 840 satellite constellation) systems definitely presents a potential stumbling block to the corporations funding these ambitious ventures. In late January of this year, the Iridium project hit a potential snag when a McDonnell Douglas Delta II rocket blew up shortly after its launch from Cape Canaveral. Though the rocket was not the one slated to carry the first three of Iridium's planned 66 satellites, the explosion--just seconds into liftoff--puts the $5 billion project on hold. This is due to the potentially long turnaround times for the Air Force, which is conducting a classified investigation, to produce a detailed report regarding the cause of the accident. Nonetheless, Motorola maintains that its system will be able to adhere to its timetable of offering commercial satellite phone service by late 1998. Such developments have raised concerns about the viability of launching large satellite constellations, especially with regard to their seeming proneness to failure and the required frequency of these launches. For example, the two compatible launch vehicles available currently, the Delta II and the Lockheed Martin's Atlas and Titan, have launch rates far smaller than those required for implementation of Teledesic by 2001; and this is without consideration being given for the other proposed relatively large systems. This new demand has increased the amount of private investment in commercial space; Orbital Sciences Corp. of Fairfax, Virginia recently became the first company to privately develop and launch a spacecraft.
Non-US Options
Europe, China and Russia also have proven commercial launch capabilities. Europe's family of Ariane vehicles has been the US's chief competitor in the international launch market, dominating the market by launching 55% of all commercial payloads between 1990 and 1995. China's Long March vehicle captured 9% of commercial payloads, in the first half of the decade, compared to the US's 36% for the same period.1 Russia has joined the commercial launch market, offering the Proton through a consortium with Lockheed Martin called International Launch Services (ILS). Russia is expected to launch its first commercial payload in 1999. Sea Launch Services, an international collaboration of the Boeing Company, Russia's RSC Energia, Ukraine's NPO Yuzhnoye, and Kvaerner of Europe, will offer space launches from a mobile ocean platform beginning in 1998. India, Israel, Japan, and Australia round out the list of countries with proven space launch capabilities. These countries have yet to offer international commercial services, however.
The great demand for launchers is plainly obvious when one considers the pending deal between Teledesic Corporation and the International Corporation for Space Transport Systems, a newly formed Russian space agency, which may see former Soviet SS-18 ICBMs refitted and pressed into service as inexpensive satellite boosters. The deal will help Teledesic deliver on its ambitious goals in satellite-based Internet access. The International Corporation for Space Transport Systems includes the Russian Space Agency, the Ukraine's National Space Agency and several engineering, scientific research, and ICBM-building firms. The coalition was formed on 31 January to "convert and deploy a multi-purpose" launch vehicle based on the SS-18 missile, officials said in a statement. Analysts seem to agree that this is a credible development given the ruggedness of the Russian ICBMs. The SS-18 missile conversions will also allow the Russians to fulfill their obligations under the 1993 Strategic Arms Reduction Treaty, which allows the use of converted ICBMs for commercial launches. For its part, Teledesic is only too happy to take part in this defense-to-commercial conversion. "The Russians have a demonstrated track record and reliability. [The rocket] has a 97 percent success rate in 140 test launches," according to an analyst.
Constant Motion Problems
A problem unique to Teledesic and other geosynchronous systems is due to their being in motion with respect to each other. This increases the complexity of tracking and communications systems by a great deal. To the user terminal, it creates a requirement for tracking the satellite and handing off to a second satellite. To the satellite, it requires constellation tracking and dynamic traffic routing. However, this drawback is being compensated for with new technology and software.
ATM Implementation
Fiber ATM vs. Satellite ATM
An important protocol that must be maintained in order to have efficient communications over great distances is Asynchronous Transfer Mode (ATM) technology. Fiber optics played a very important role in the definition of the Asynchronous Transfer Mode (ATM) technology. The extremely low bit error characteristics of fiber were assumed when the ATM parameters were defined, hence the very bare error correcting mechanisms have been included in the ATM protocol. The error characteristics of digital transmission in a fiber medium are that the low bit rates and more importantly random errors. The error characteristics in a satellite environment where modems deploy convolutional coding, the errors are in bursts and as such may impact negatively the performance of ATM transmission over satellite. No error correcting mechanism is allowed in the payload by the core network (this is left to the end-to-end termination equipment). In the header, only one-bit error correction is allowed as the assumption is that more than one bit error -- and thus discard of the cell -- will rarely occur. This unique cell structure allows ATM to multiplex and transport voice, data, image, video and multimedia information. Voice and video may not require re-transmission when errors are introduced by the core network whereas data would, and this is done by the protocol stacks above the ATM level in the termination equipment.
Error Detection & Correction
In practice, convolutional coding is used to achieve high throughput while lowering the antenna size and/or power requirements thus containing costs. In the case of ATM, multiple errors sometimes spread over both the payload and the header. This burst of errors is the major concern for adapting ATM technology over satellite. Research indicates that for optic fiber media, about one cell is lost per century in a 155 Mbps link whereas in an ATM satellite link of the same rate, this probability jumps to about one per minute. Therefore different error-detection methods must be implemented when using ATM over satellite networks. If the errors are likely to be all bursty, one could envision using the cell header to protect the data and the incorrect cells would be discarded. However, if the errors tend to be both bursty and random, then other schemes have to used to avoid these errors and protect the cells as well as the data within them. Most of these schemes are still prone to having a correct header and corrupted cells, but it is assumed that higher level protocols such as TCP/IP will take care of this type of error.
Dealing with Delays
Another problem with using ATM over satellite links is due to the delay of the links. These are usually relatively large compared to wireline or terrestrial links. The long delays can cause a large number of cells or even higher layer data units to be outstanding on the links. To account for this, the numbering schemes in ATM may not be sufficiently large to allow the satellite links to operate continuously. Sequence number starvation, the nomenclature applied to this problem, can be resolved by allocating more bits for cells or by numbering larger data units than just cells. As part of the European RACE II program, the CATALYST project successfully demonstrated the use of 10 to 150 Mbps ATM connections over a geosynchronous satellite for various broadband applications, including multimedia communications. Round trip delays without the satellite in the path were on the order of 200 ms; with the satellite, this increased to almost 900 ms, indicating that the on-board satellite processing latency was around 200 ms. Since all these processes require larger buffers and processing units, they must be designed such that they minimize weight due to the high cost per kilogram of material put into orbit (from $10,000/kg to $40,000/kg). Such factors have caused some independent analysts such as Mitre Corp. of Fairfax, Virginia to estimate that the true cost of a system such as Teledesic could reach $60 billion.
The Real World
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The Market
For some years, various companies have been considering how to use satellites to implement worldwide mobile radio communications. Some vendors are focusing their networks on North America, while others are planning worldwide services. Of course, these plans affect existing markets and standardization work, as well the supremacy in the radio sector of the participating countries. The capital investment required for the different satellite networks vary considerably. They are, however, broadly comparable with the costs of setting up terrestrial digital mobile radio networks. The variation in costs depends on the type of system and the geographical coverage.
The main advantages of LEO system are the wide radio coverage, the transmission quality, and greater subscriber capacity worldwide. Potential users are already apparent today in the familiar sectors and aviation radio. To these can be added:"High-income" business travelers, international professionals, especially in regions with inadequate telecommunication infrastructure, organizations and professional groups employed in disaster relief work, private pilots and yachtsman, international travel organizations and users in new housing estates which cannot economically be provided with terrestrial networks . Questions about user demand with regard to global communication can now be responded to with reference to surveys and analyses, such as the conducted by Infraset for Germany in 1991. With subscribers becoming increasingly mobile internationally, the demand for satellite-aided networks with global radio coverage will grow. Further analysis reveals potential use in the areas of disaster relief and to provide service to areas with poor telecommunications infrastructure: for certain regions, the combination of a terrestrial network with a satellite network can be a very economical solution. Thus, various sources confirm that the potential number of users for satellite PCN during the next decade is one the order of several millions.
The number of subscribers to mobile satellite services (MSS) will grow from 130,000 in 1998 to 8 million in 2002, according to the latest report from Ovum magazine, LEOs MEOs and GEOs: the Market Opportunity for Mobile Satellite Services. The report announced on January 15, predicts that revenues for the global MSS market by the year 2002 will be $8.5 billion. Of this figure, MSS operators will account for $3.7 billion, handset revenues for $3.3 billion, and services providers for $1.5 billion.
In 1995, the revenues generated by U.S. companies from activities directly related to space, like satellite communications services, ground equipment sales, satellite manufacturing, launch services, and satellite remote sensing, reached over $7 billion. The largest portion of that revenue was generated by satellite communications services, followed by ground equipment sales and satellite manufacturing. According to Hughes Electronics estimate, worldwide demand for Satellite Communications has increased by 50 percent from 1995 to 1996, fuelled by technology advances that have created new applications and improved the cost-efficiency of existing ones. In th past two to three years the internet boom has played a very important part in pushing satellite technology toward the public. Two companies--Iridium LLC and Globalstar LP--appear to be neck-and-neck in the race for space. One another American big LEO is licensed, TRW's CDMA-based Odyssey system. However, it doesn't plan to offer service until 2000. Peter Hadinger, director of telecommunications policy for TRW, said his company does not anticipate a strong market for the services until 2000 or 2001. The Odyssey system is aimed at a larger, lower-cost market than the Iridium or Globalstar services, and the company's market will grow once the developing world opens access, he said.
Political Issues and Related News
Assuming successful satellite deployments, the highest hurdles for Iridium, Globalstar and other satellite network underwriters, over the next two years will be licensing by each of the nations where they intend to operate. The industry hopes 1997 meetings of the World Trade Organizations, the International Telcommunications Union and other international forums will open markets to their services and increase foreign licensing. The ITU's Radio Regulation 2613 gives GEOs absolute priority over LEOs. For Spaceway, Hughes is now demanding an exclusive license for the full five gigahertz available in Ka-band worldwide, leaving no room for Teledesic or any Ka-band LEO. But recently, Teledesic was granted a license by the Federal Communications Commission to provide telecommunications services in the Ka band. The FCC license allows the company to build and launch the Teledesic Network, as well as make use of 500 MHz of domestic radio frequency in the 28 GHz band - the uplink portion of the Ka-band - and a corresponding 500 MHz of downlink spectrum. In July 1996, the FCC adopted a domestic band plan that designated that spectrum for primary use by non-geostationary fixed satellite services (NGSO FSS) such as Teledesic's.
The merger frenzy that has been sweeping the corporate world seems to be affecting the satellite industry as well. Intel Corp. announced that it is teaming up with European satellite giant SES (Societe European des Satellites) to promote satellite delivery of Internet content. In doing this Intel is embracing high-speed satellite delivery as a reasonably priced alternative what's available using today's technology. In other news, Loral Space & Communications announced a deal to buy AT&T's Skynet Satellite Services for $712.5 million. Skynet provides Loral with an entry into the satellite service business, bringing it four orbiting satellites that beam TV programs, phone calls and computer data around the country. Loral also has signed an agreement with Alcatel of France to combine its geostationary system with Alcatel's proposed SATIVOD LEO internet satellite system.
Analysis & Conclusion
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As can be seen from the above references, the question of whether digital satellite systems can and will be used for internet access can be answered with a firm "yes." The amount of capital, both human, and economic, invested in these ventures will most certainly ensure their success. On the technological front, although challenges exist in implementing these systems, our research has made it obvious that it is simply a matter of time before these challenges can be overcome and successfully mastered. It must be noted, however, that whoever meets these challenges first will be able to reap the rewards of the information age at the cost of their competitors. |
