The idea of using satellites as relay stations for an international microwave radiotelephone system goes back to 1945 when Arthur C. Clarke proposed the scheme in a British technical journal. Clarke, then a young scientist and officer of the Royal Air Force, later became a leading science fiction writer and coauthor of the motion picture 2001: ASpace Odyssey. However, it was not until 1957 that the first satellite was put into orbit.
Although just a beacon whose primary purpose was to announce its presence in the sky, the successful orbital deployment of the 185-pound Russian Sputnik sparked a technological revolution in communications that continues to this day. There are now over 2560 satellites in orbit, along with over 6000 pieces of debris tracked by the National Aeronautics and Space Administration (NASA).
The United States launched the first communication satellites in the early 1960s. Echo 1 and Echo 2 were little more than metallic balloons that simply reflected microwave signals from point A to point B. These passive satellites could not amplify the signals. Reception was often poor and the range of transmission limited. Ground stations had to track them across the sky, and communication between two ground stations was only possible for a few hours a day when both had visibility with the satellite at the same time.
Later, geostationary satellites overcame this problem. Such systems were high enough in orbit to move with the earth’s rotation, in effect giving them fixed positions so that they could provide communications coverage to specific areas. Satellites are now categorized by type of orbit and area of coverage as follows:
- Geostationary-earth-orbit (GEO) satellites orbit the equator in a fixed position about 23,000 miles above the earth. Three GEO satellites can cover most of the planet, with each unit capable of handling 20,000 voice calls simultaneously. Because of their large coverage “footprint,” these satellites are ideal for radio and television broadcasting and long distance domestic and international communications.
- Middle-earth-orbit (MEO) satellites circle the earth at about 6000 miles up. It takes about 12 satellites to provide global coverage. The lower orbit reduces power requirements and transmission delays that can affect signal quality and service interaction.
- Low-earth-orbit (LEO) satellites circle the earth only 600 miles up (Figure S-2). As many as 200 satellites may be required to provide global coverage. Since their low altitude means that they have nonstationary orbits and they pass over a stationary caller rather quickly, calls must be handed off from one satellite to the next to keep the session alive. The omnidirectional antennas of these devices do not have to be pointed at a specific satellite. There is also very little propagation delay. And the low altitude of these satellites means that earthbound transceivers can be packaged as low-powered, inexpensive hand-held devices.
The International Telecommunication Union (ITU) is responsible for all frequency/orbit assignments. Through its International Bureau, the Federal Communications Commission (FCC) regulates all satellite service rates, competition among carriers, and international telecommunications traffic in the United States, ensuring that U.S. satellite operators conform to ITU frequency and orbit assignments. The FCC also issues licenses to domestic satellite service providers.
Each satellite carries transponders, which are devices that receive radio signals at one frequency and convert them to another for transmission. The uplink and downlink frequencies are separated to minimize interference between transmitted and received signals. Satellite channels allow one sending station to broadcast transmissions to one or more receiving stations simultaneously.
In a typical scenario, the communications channel starts at a host computer, which is connected through a traditional telephone company medium to the central office (i.e., master earth station or hub) of the satellite communications vendor. The data from this and other local loops are multiplexed into a fiberoptic or microwave signal and sent to the satellite vendor’s earth station.
This signal becomes part of a composite transmission that is sent by the earth station to the satellite (uplink) and then transmitted by the satellite to the receiving earth stations (downlink). At the receiving earth station, the data are transferred by a fiberoptic or microwave link to the satellite carrier’s central office. The composite signal is then separated into individual communications channels that are distributed over the Public Switched Telephone Network (PSTN) to their destinations.
Satellite communication is very reliable for data transmission. The bit error rate (BER) for a typical satellite channel is in the range of 1 error in 1 billion bits transmitted. However, a potential problem with satellite communication is delay. Round-trip satellite transmission takes approximately 500 milliseconds, which can hamper voice communications and create significant problems for real-time interactive data transmissions.
For voice communications, digital echo cancelers can correct voice echo problems caused by the transmission delay. Anumber of techniques are employed to nullify the effects of delay during data transmissions via satellite. One technique employed by Mentat, Inc., increases the performance of Internet and intranet access over satellites by transparently replacing the Transmission Control Protocol (TCP) over the satellite link with a protocol optimized for satellite conditions.
The company’s SkyX Gateway intercepts the TCP connection from the client and converts the data to a proprietary protocol for transmission over the satellite. The gateway on the opposite side of the satellite link translates the data back to TCP for communication with the server. The result is vastly improved performance while the process remains transparent to the end user and fully compatible with the Internet infrastructure. No changes are required to the client or server, and all applications continue to function without modification.
Very Small Aperture Terminals (VSATs)
VSAT networks have evolved to become mainstream communication networking solutions that are affordable to both large and small companies. Today’s VSAT is a flexible, softwareintensive system built around standard communications protocols. With a satellite as the serving office and using radiofrequency (RF) electronics instead of copper or fiber cables, these systems can be truly considered packet-switching systems in the sky.
VSATs can be configured for broadcast (one-way) or interactive (two-way) communications. The typical star topology provides a flexible and economical means of communications with multiple remote or mobile sites. Applications include broadcasting database information, insurance agent support, reservations systems, retail point-of-sale credit card checking, and interactive inventory data sharing.
Today’s VSATs are used for supporting high-speed message broadcasting, image delivery, integrated data and voice, and mobile communications. VSATs are used increasingly for supporting local area network (LAN)–to-LAN connectivity and LAN-to–wide area network (WAN) bridging, as well as for providing route and media diversity for disaster recovery.
To make VSAT technology more affordable, VSAT providers offer compact hubs and submeter antennas that provide additional functionality at approximately 33 percent less than the cost of full-size systems. Newer submeter antennas are even supporting direct digital TV broadcasts to the home.
The performance of the VSAT network is monitored increasingly at the hub location by the network control system. A failure anywhere on the network automatically alerts the network control operator, who can reconfigure capacity among individual VSAT systems. In the case of signal fade due to adverse weather conditions, for example, the hub detects the weak signal—or the absence of a signal—and alerts the network operations staff so that corrective action can be taken.
Today’s network management systems indicate whether power failures are local or remote. They also increasingly locate the source of communications problems and determine whether the trouble is with the software or hardware. Such capabilities often eliminate the expense of dispatching technicians to remote locations. And when technicians must be dispatched, the diagnostic capabilities of the network management system can ensure that service personnel have the appropriate replacement parts, test gear, software patch, and documentation with them to solve the problem in a single service call.
Overall link performance is determined by the BER, network availability, and response time. Because of the huge amount of information transmitted by the hub station, uplink performance requirements are more stringent. A combination of uplink and downlink availability, coupled with BER and response time, provides the network control operator with overall network performance information on a continuous basis.
The VSAT’s management system provides an interface to the major enterprise management platforms for single-point monitoring and control and offers a full range of accounting, maintenance, and data flow statistics, including those for inbound versus outbound data flow, peak periods, and total traffic volume by node. Also provided are capabilities for identifying fault conditions, performing diagnostics, and initiating service restoration procedures.
Within the VSAT network there are three categories of protocols—those associated with the backbone network, those of the host computer, and those concerned with transponder access. The scope and functionality of protocol handling differ markedly among VSAT network providers. The backbone network protocol is responsible for flow control, retransmissions of bad packets, and running concurrent multiple sessions.
The backbone network protocol could be associated with either the host or the communications link. The host protocol is related to the user application interface, which provides a compatible translation between the backbone protocol and the host communications protocol. Several host protocols are used in VSAT networks, including SNA/SDLC, 3270 Base Station Controller (BSC), Poll Select, and HASP. Multiple protocols can be used at the same VSAT location.
Transponder access protocols are used to assign transponder resources to various VSATs on the network. The three key transponder access protocols that are used on VSAT networks include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA).
Frequency Division Multiple Access With FDMA, the radiofrequency is partitioned so that bandwidth can be allocated to each VSAT on the network. This permits multiple VSATs to simultaneously use their portion of the frequency spectrum.
Time Division Multiple Access With TDMA, each VSAT accesses the hub via the satellite by the bursting of digital information onto its assigned radiofrequency carrier. Each VSAT bursts at its assigned time relative to the other VSATs on the network. Dividing access in this way—by time slots— is inherently wasteful because bandwidth is available to the VSAT in fixed increments whether or not it is needed. To improve the efficiency of TDMA, other techniques are applied to ensure that all the available bandwidth is used, regardless of whether the application contains bursty or streaming data. Areservation technique can even be applied to ensure that bandwidth is available for priority applications.
Code Division Multiple Access With CDMA, all VSATs share the assigned frequency spectrum and also can transmit simultaneously. This is possible through the use of spreadspectrum technology, which employs a wideband channel as opposed to the narrowband channels employed by other multiple access techniques such as FDMAand TDMA. Over the wideband channel, each transmission is assigned a unique code—a long row of numbers resembling a combination to a lock. The outbound data streams are coded so that they can be identified and received only by the station(s) having that code. This technique is also used in mobile communications as a means of cutting down interference and increasing available channel capacity by as much as 20 times.