Introduction
In 1962, the American telecommunications giant AT&T launched the world's first true communications satellite, called Telstar. Since then, countless communications satellites have been placed into earth orbit, and the technology being applied to them is forever growing in sophistication.
Basic Elements
Satellite communications are comprised of 2 main components:
- The Satellite
- The Ground Station.
This is the earth segment. The ground station's job is two-fold. In the case of an uplink, or transmitting station, terrestrial data in the form of baseband signals, is passed through a baseband processor, an up converter, a high powered amplifier, and through a parabolic dish antenna up to an orbiting satellite. In the case of a downlink, or receiving station, works in the reverse fashion as the uplink, ultimately converting signals received through the parabolic antenna to base band signal.
The satellite itself is also known as the space segment, and is composed of three separate units, namely the fuel system, the satellite and telemetry controls, and the transponder. The transponder includes the receiving antenna to pick-up signals from the ground station, a broad band receiver, an input multiplexer, and a frequency converter which is used to reroute the received signals through a high powered amplifier for downlink. The primary role of a satellite is to reflect electronic signals. In the case of a telecom satellite, the primary task is to receive signals from a ground station and send them down to another ground station located a considerable distance away from the first. This relay action can be two-way, as in the case of a long distance phone call. Another use of the satellite is when, as is the case with television broadcasts, the ground station's uplink is then downlinked over a wide region, so that it may be received by many different customers possessing compatible equipment. Still another use for satellites is observation, wherein the satellite is equipped with cameras or various sensors, and it merely downlinks any information it picks up from its vantagepoint.
Various Uses of Satellite Communications
Traditional Telecommunications
Cellular
Television Signals
C-band
Ku-Band
- Fixed Satellite Service (FSS)
- Broadcasting Satellite Service (BSS)
DBS
Marine Communications
Spacebourne Land Mobile
Satellite Messaging for Commercial Jets
Global Positioning Services
Since the beginnings of the long distance telephone network, there has been a need to connect the telecommunications networks of one country to another. This has been accomplished in several ways. Submarine cables have been used most frequently. However, there are many occasions where a large long distance carrier will choose to establish a satellite based link to connect to transoceanic points, geographically remote areas or poor countries that have little communications infrastructure. Groups like the international satellite consortium Intelsat have fulfilled much of the world's need for this type of service.
Various schemes have been devised to allow satellites to increase the bandwidth available to ground based cellular networks. Every cell in a cellular network divides up a fixed range of channels which consist of either frequencies, as in the case of FDMA systems, or time slots, as in the case of TDMA. Since a particular cell can only operate within those channels allocated to it, overloading can occur. By using satellites which operate at a frequency outside those of the cell, we can provide extra satellite channels on demand to an overloaded cell. These extra channels can just as easily be, once free, used by any other overloaded cell in the network, and are not bound by bandwidth restrictions like those used by the cell. In other words, a satellite that provides service for a network of cells can allow its own bandwidth to be used by any cell that needs it without being bound by terrestrial bandwidth and location restrictions.
C-Band (3.7 - 4.2 GHz) - Satellites operating in this band can be spaced as close as two degrees apart in space, and normally carry 24 transponders operating at 10 to 17 watts each. Typical receive antennas are 6 to 7.5 feet in diameter. More than 250 channels of video and 75 audio services are available today from more than 20 C-Band satellites over North America. Virtually every cable programming service is delivered via C-Band.
Ku Band (11.7 - 12.2 GHz) - Satellites operating in this band can be spaced as closely as two degrees apart in space, and carry from 12 to 24 transponders that operate at a wide range of powers from 20 to 120 watts each. Typical receive antennas are three to six feet in diameter. More than 20 FSS Ku-Band satellites are in operation over North America today, including several "hybrid" satellites which carry both C-Band and Ku-Band transponders. PrimeStar currently operates off Satcom K-2, an FSS or so-called "medium-power" Ku-Band satellite. AlphaStar also uses an FSS-Ku Band satellite, Telestar 402-R.
Ku-Band (12.2 - 12.7 GHz) - Satellites operating in this band are spaced nine degrees apart in space, and normally carry 16 transponders that operate at powers in excess of 100 watts. Typical receive antennas are 18 inches in diameter. The United States has been allocated eight BSS orbital positions, of which three (101, 110 and 119 degrees) are the so-called prime "CONUS" slots from which a DBS provider can service the entire 48 contiguous states with one satellite. A total of 32 DBS "channels" are available at each orbital position, which allows for delivery of some 250 video signals when digital compression technology is employed.
DBS (Direct Broadcast Satellite) -The transmission of audio and video signals via satellite direct to the end user. More than four million households in the United States enjoy C-Band DBS. Medium-power Ku-Band DBS surfaced in the late 1990s with high power Ku-Band DBS launched in 1994.
In the maritime community, satellite communication systems such as Inmarsat provide good communication links to ships at sea. These links use a VSAT type device to connect to geosynchronous satellites, which in turn link the ship to a land based point of presence to the respective nations telecommunications system.
Along the same lines as the marine based service, there are VSAT devices which can be used to establish communication links even from the world's most remote regions. These devices can be hand-held, or fit into a briefcase. Digital data at 64K ISDN is available with some (Inmarsat).
Another service provided by geosyncronous satellites are the ability for a passenger on an airbourne aircraft to connect directly to a landbased telecom network.
Another VSAT oriented service, in which a small apparatus containing the ability to determine navigational coordinates by calculating a triangulating of the signals from multiple geosynchronous satellites.
Technological Overview
Satellites for Data
Characteristics
- Latency (propagation delay): Due to the high altitudes of satellite orbits, the time required for a transmission to navigate a satellite link (more than 2/10ths of a second from earth station to earth station) could cause a variety of problems on a high speed terrestrial network that is waiting for the packets.
- Poor Bandwidth: Due to radio spectrum limitations, there is a fixed amount of bandwidth allocable to satellite transmission.
- Noise:A radio signals strength is in proportion to the square of the distance traveled. Due to the distance between ground station and satellite, the signal ultimately gets very weak. This problem can be solved by using appropriate error correction techniques, however.
Error Correction
- Forward-error-correction (FEC)
- Automatic-repeat-request (ARR)
- Stop and Wait (SW)
- Go-back-N (GBN)
- Selective-repeat (SR)
Hybrid Networks
Incorporating satellites into terrestrial networks is often hindered by three characteristics possessed by satellite communication.
Due to the high noise present on a satellite link, numerous error correction techniques have been tested in on such links. They fall into the two categories of forward-error-correction (FEC) and automatic-repeat-request (ARQ):
In this method a certain number of information symbols are mapped to new information symbols, but in such a way as to get more symbols than were original had. When these new symbols are checked on the receiving end, the redundant symbols are used to decipher the original symbols, as well as to check for data integrity. The more redundant symbols that are included in the mapping, the better the reliability of the error correction. However it should be noted that the more redundant symbols that are used to achieve better integrity, the more bandwidth that is wasted. Since this method uses relatively a large amount redundant data, it may not be the most efficient choice on a clear channel. However when noise levels are high, FEC can more reliably ensure the integrity of the data.
In this method, data is broken into packets. Within each packet is included an error checking key. This key is often of the cyclic redundancy check (CRC) sort. If the error code reflects a loss of integrity in a packet, the receiver can request the sender to resend that packet. ARR is not very good in a channel with high noise, since many retransmissions will be required, and the noise levels that corrupted the initial packet will be likely to cause corruption in subsequent packets. ARR is more suitable to relatively noise free channels.
With this form of ARR, the sender must wait for an acknowledgement of each packet before it can send a new one. This can take upwards of 4/10ths of a second per packet since it takes 2/10ths seconds for the receiver to get the packet an another 2/10th seconds for the sender to receive the acknowledgement.
This method of ARR is an improvement over stop and wait in that it allows the sender to keep sending packets until it gets a request for a resend. When the sender gets such a request, it sends packets starting at the requested packet over again. It can again send packets until it receives another retransmit request, and so on.
This ARR protocol is an improvement over GBN in that it allows the receiver to request a retransmit of only that packet that it needs, instead of that packet and all that follows it. The receiver, after receiving a bad packet and requesting a retransmit, can continue to accept any good packets that are coming. This method is the most efficient method for satellite transmissions of the three ARR methods discussed.
ARR methods can be demonstrated to provide a usable error correction scheme, but it is also the most expensive, in terms of hardware. This is in part due to the buffering memory that is required, but more importantly to the cost of the receiver, which needs to be able to transmit re-requests. Systems such as the Digital Broadcast Satellites used for television signal distribution would become inordinately expensive if they had to make use of ARR, since the home based receiver would now need to be a transmitter, and the 18 inch dish would be inadequate for the requirements of transmitting back to a satellite.
In today's global networking landscape, there are many ways to transmit data from one place to another. It is desirable to be able to incorporate any type of data transmission media into a network, especially in networks that encompass large areas. A hybrid network is one that allows data to flow across a network, using many types of media, either satellite, wireless or terrestrial, transparently. Since each type of media will have different characteristics, it is necessary to implement a standard transmission protocol. One that is normally used in hybrid networks is TCP/IP. In addition, much work is being done to use TCP/IP over ATM for the satellite segments of hybrid networks, about which more will be discussed later.
One way to get around the need in ARR for the receiver to have to request retransmit via an expensive and slow satellite link is to use a form of hybrid network. In one form of hybrid network, the reciever transmits its requests back to the sender via a terrestrial link. Terrestrial link allows for quicker, more economical and less error prone transmission from the reciever, and the costs associated with the receivers hardware are greatly reduced when compared to the costs involved if it had to transmit back over the satellite link. There are products on the market today that allow a home user to get intenet access at around 400MB via digital satellite, while its retransmit signals are sent via an inexpensive modem or ISDN line.
In fact, a product currently being marketed by Direct PC called Turbo Internet uses a form of hybrid network. The system uses two network interfaces; one connects via a special ISA bus PC adapter to a receive-only Very Small Aperture Terminal (VSAT), while the other is a modem attached to a serial port. Inbound traffic comes down to the VSAT, while outbound traffic goes through the modem link. The two interfaces are combined to appear as a single virtual interface to upper layer TCP/IP protocol stacks by a special NDIS compliant driver. The Serial Line Internet Protocol (SLIP) is used to connect the modem-based link with an internet service provider. Packets, which are encapsulated by the terminal such that the desired ip address of the destination host is embedded underneath the IP address of the Direct PC Gateway, to which all packets leaving the terminal must go. Once at the gateway, the outer packet is stripped, and the gateway contacts the destination address within. Upon the gateway's receiving the request from the host, it then prepares the packet for satellite transmission, which is then used to send the packet back to the terminal.
ATM Over Satellite
- SATATM Type 1 - Fixed ATM Direct Access
- SATATM Type 2 - Fixed ATM Network Interconnect
- SATATM Type 3 - Mobile ATM Direct Access
- ATM network access by mobile terminals. The radio interface provides for 64 kbit/s - E1 for moving, 64 kbit/s - NxE1 for portable terminals and the rervice interfaceallows for 75 bit/s - E1 for moving, 75 bits/s - NxE1 for portable terminals.
- SATATM Type 4 - Mobile ATM Network Interconnect
- High speed interconnections between mobile and fixed networks or between two mobile networks providing fast moving land-mobile data rates < NxE1 and slow-moving airborne data rates of < 622 Mbit/s.
SATIN - Satellite Integrated Terrestrial Network:
VSAT Networks
Orbits
GEO
LEO
Constellations
- Global Voice Communications
- There are currently several consortiums that are working on global voice via satellite. One of the most prominant is the IRIDIUM constellation, which will consist of 66 interconnected satellites orbiting 420 nautical miles above the earth. The satellites will use a LEO orbit so that very small handheld terminals can be used by ground-based cutomers. The system will use intersatellite crosslink transmissions that will take place in the Ka frequency band between 23.18 and 23.38 GHz. The IRIDIUM system will use a combination of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) signal multiplexing to make the most efficient. The L-Band (1616-1626.5 MHz), is used to link the satellite and IRIDIUM the subscribers equipment. The Ka-Band (19.4-19.6 GHz for downlinks and 29.1-29.3 GHz for uplinks) links the satellite and the gateways and earth terminals.
- Global Broadband Networks
There are basically two types of networks being proposed here, namely LEO based and GEO based ones.
- LEO
LEO networks use low orbits, which allows for much less latency that do GEO based networks. One problem that these satellites have is that since the are not geostationary (they are contsantly orbiting around the earth) they cannot talk continuously to that same ground station. The way this is overcome is by using intesatellite communications, so that the sattellites function together as a blanket of coverage. A major player in this are is Teledesic.
The Teledesic Network uses a constellation of 840 operational interlinked low-Earth orbit satellites. The system is planned to provide "on-demand" channel rates from 16 Kbps up to 2.048 Mbps ("E1"), and for special applications up to 1.24416 Gbps ("OC-24"). The network uses fast packet switching technology based on the Asynchronous Transfer Mode (ATM) using fixed-length (512) bit packets.. Each satellite in the constellation is a node in the fast packet switch network, and has intersatellite communication links with eight adjacent satellites. Each satellite is normally linked with four satellites within the same plane (two in front and two behind) and with one in each of the two adjacent planes on both sides. Each satellite keeps the same position relative to other satellites in its orbital plane. The Teledesic Network uses a combination of multiple access methods to ensure efficient use of the spectrum. Each cell within a supercell is assigned to one of nine equal time slots. All communication takes place between the satellite and the terminals in that cell during its assigned time slot . Within each cell’s time slot, the full frequency allocation is available to support communication channels. The cells are scanned in a regular cycle by the satellite’s transmit and receive beams, resulting in time division multiple access (TDMA) among the cells in a supercell. Since propagation delay varies with path length, satellite transmissions are timed to ensure that cell N (N=1, 2, 3,...9) of all supercells receive transmissions at the same time. Terminal transmissions to a satellite are also timed to ensure that transmissions from the same numbered cell in all supercells in its coverage area reach that satellite at the same time. Physical separation (space division multiple access (SDMA) and a checkerboard pattern of left and right circular polarization eliminate interference between cells scanned at the same time in adjacent supercells. Guard time intervals eliminate overlap between signals received from time-consecutive cells.
Within each cell’s time slot, terminals use Frequency Division Multiple Access (FDMA) on the uplink and Asynchronous Time Division Multiple Access (ATDMA) on the downlink. On the uplink, each active terminal is assigned one or more frequency slots for the call’s duration and can send one packet per slot each scan period (23.111 msec). The number of slots assigned to a terminal determines its maximum available transmission rate. One slot corresponds to a standard terminal’s 16 Kbps basic channel with its associated 2 Kbps signaling and control channel. A total of 1800 slots per cell scan interval are available for standard terminals. The terminal downlink uses the packet’s header rather than a fixed assignment of time slots to address terminals.TELEDESIC
- GEO
GEO's high points are that it's satellites are geostationary, which means that the difficulties of intersatellite communications are avoided. The problem arises due to the latency delays caused by the high orbit. Applications which rely on steady bandwidth, like multimedia, will definately be affected.
There are problems, however. ATM's relatively large propagation delays can significantly increase the latency of feedback mechanisms essential for congestion control. acquisition time, cell in-synch time and cell discard probability.Solutions to these issues are still being explored.
The group that is currently working to develop interoperability specifications that facilitate ATM access and ATM network interconnect in both fixed and mobile satellite networks is known as The TIA/SCD/CIS - WATM group. As of March, 1997 they have proposed the following standards:
- Fixed network access via satellite that is characterized by a large number of small inexpensive user terminals and a small number of gateway earth stations. Provides for a radio interface of 64 kbit/s - NxE1 and a service interface of 2.4 kbit/s - NxE1 while providing no mobility support
The group also has established requirements for dealing with the physical layer, the media access control layer and the data link control layer.
- The goal of SATIN is to create a fully integrated hybrid network in which the method of communication, which can incorporate networks of local, metropolitan and wide area scope, Broadband ISDN, Integrated Network Management, AIN (Advanced Intelligent Networks) and PCS (Personal Communications Services), in addition to ATM (Asynchronous Transfer Mode) over satellite, is totally transparent to the user. The difficulties inherent in this are obvious. Differences in latency, noise, bandwidth and reliability must be equalized in all the media that will encompass the network.
VSAT stands for Very Small Aperture Terminal. Although this acronym has been used amongst telecom groups for some time now to describe small earth stations, the concepts of VSAT are being applied to modern hand held satellite communications units, such as GPS (Global Positioning System), portable Inmarsat phones and other types of portable satellite communication devices.
GEO stands for Geostationary Earth Orbit. This refers to satellites that are placed in orbit such that they remain stationary relative to a fixed spot on earth. If a satellite is placed at 35,900 km above the earth, its angular velocity is equal to that of the earth, thereby causing it to appear to be over the same point on earth. This allows for them to provide constant coverage of the area and eliminate blackout periods of ordinary orbiting satellites, which is good for providing television broadcasting. However their high altitude causes a long delay, so two way communications, which would need to be uploaded and then downloaded over a distance of 72,000 km, are not often used with this type of orbit.
LEO stands for Low Earth Orbit, and it refers to satellites in orbit at less that 22300 miles above the earth. This type of an orbit reduces transmission times as compared to GEO. A LEO orbit can also be used to cover a polar region, which the GEO cannot accomplish. Since it does not appear stationary to earth stations, however, earth stations need an antenna assembly that will track the motion of the satellite.
