Chapter 19

Satellite Communications

The telecommunications industry experienced some spectacular failures around
the end of the twentieth century and the start of the twenty-first, but few could
compare to satellite companies Iridium and Teledesic. Iridium opened for business
in late 1998 with a series of low Earth orbit (LEO) satellites, which delivered
phone connectivity anywhere on the face of the Earth without the delay of a geosynchronous
satellite. Led by Motorola, Iridium spent some $5 billion, but did not
attract enough subscribers to make it pay. In 2000, Iridium filed for bankruptcy
and was on the verge of destroying its satellites until a group of private investors
acquired them for $25 million. With lower rates and a tolerable financial structure,
the service remains alive through its network of 66 satellites.
Teledesic did not get quite that far. Its original plans called for a network of
840 satellites. This dropped to 288 satellites, then in February 2002 Teledesic
announced that it was planning 20 medium Earth orbit (MEO) satellites. In July
2003 the company gave up its frequency slots and put the venture on hold.
A merger with another MEO company, ICO, was considered and dropped. ICO
had raised $3.1 billion before filing for Chapter 11 protection in 1999. A group of
investors acquired control of ICO’s assets for $1.2 billion. The network is not yet
in service at this writing.
Satellite communications had its beginning in 1962 with AT&T’s launch of
its Telstar 1. Orbiting the Earth in about 2 h, Telstar was visible from the Earth
station for less than half an hour, as the antennas followed its track across
the sky. Although Telstar demonstrated that the technology was feasible, loworbiting
satellites were abandoned in favor of geosynchronous satellites (GEO).
Orbiting the equator at an altitude of 22,239 miles (35,790 km) GEO satellites
appear stationary to the Earth station.
The LEO was still a good idea, however, because sending a radio signal up
to a repeater station that far from the Earth results in a round-trip delay of about
327
270 ms. Most data protocols can work around this kind of latency, but for voice
many people find the delay disconcerting. The growth of fiber optics has captured
the voice market except for those countries where nothing but satellite is available.
Although the delay makes satellites less than ideal for voice communications,
they have plenty of applications that cannot readily be filled by any other
alternative. Communications satellites are used for global positioning, communications
with ships at sea, telemetering data from trucks in transit, and for many
other applications where the user is either moving or in a remote area or both.
Another application, direct broadcast television, was a long time in coming, but
now high-quality TV can be received, even in remote locations. Very small aperture
terminal (VSAT) enables users to mount small antennas on rooftops to run
a multitude of applications such as point-of-sale, which need low bandwidth
facilities distributed over a wide range.
SATELLITE TECHNOLOGY
Figure 19-1 shows the relative position of the three orbit classes—LEO, MEO, and
GEO. At geosynchronous orbit, the satellite travels at the same speed as the Earth’s
rate of spin, so the orbiting vehicle remains at a fixed position with relation to
the Earth station. From geosynchronous orbit, three satellites can theoretically
cover the Earth’s surface, except for the polar regions, with each satellite subtending
a radio beam 17° wide. The portion of the Earth’s surface that a satellite
328 PART 3 Transmission Technologies
F I G U R E 19-1
Communication Satellite Orbits
Satellite dish
Satellite dish
GEO: Altitude
35,790 km
MEO: Altitude
10,400 km
LEO: Altitude
700-1400 km
Earth's Surface
illuminates is called its footprint. MEO and LEO satellites have the advantage of
low delay, but at the expense of needing more satellites and tracking antennas to
provide the same coverage.
Satellites fall into three general categories—domestic, regional, and international.
Domestic satellites carry traffic within one country. Regional satellites
span a geographical area, such as Europe, and international satellites are intended
for intercontinental traffic. Although undersea fiber-optic systems carry most of
the international voice traffic, international television is still a large and growing
market for satellites. International satellite communications are provided by
Intelsat Ltd, which became a private company in 2001 following 37 years as an
intergovernmental organization. Intelsat operates more than 20 satellites and
serves about 150 countries at this writing.
As Table 19-1 shows, the frequencies available for communication satellites are
limited. The lower frequency is always used from the satellite to the ground because
Earth stations’ transmitting power can overcome the greater path loss of the higher
frequency, but solar battery capacity limits satellite output power. The 4- and 6-GHz
C-band frequencies are the most desirable from a transmission standpoint because
they are the least susceptible to rain absorption. Satellites share the C-band frequencies
with common carrier terrestrial microwave, requiring close coordination of spacing
and antenna positioning to prevent interference. Interference between satellites
and between terrestrial microwave and satellites is prevented by using highly directional
antennas. Currently, satellites are spaced about the equator at 2° intervals.
The Ku-band of frequencies has come into more general use as the C-band
becomes congested. K-band frequencies are exclusive to satellites, allowing users
to construct Earth stations almost anywhere, even in metropolitan areas where
congestion often precludes placing C-band Earth stations. The primary disadvantage
of the Ku-band is rain attenuation, which results in lower reliability. With
identical 2° spacing for both C and Ku-bands, most satellites carry transponders
for both bands.
Ka-band satellites are becoming more attractive as the lower frequencies
become congested. Although the higher frequency of Ka-band means a higher
probability of fading, it is possible to use smaller antennas and inexpensive Earth
stations, which offset fading to some degree. Although considerable bandwidth
CHAPTER 19 Satellite Communications 329
TA B L E 19-1
Principal Communication Satellite Frequency Bands
Band Uplink Downlink
C 5925 to 6425 MHz 3700 to 4200 MHz
Ku 14.0 to 14.5 GHz 11.7 to 12.2 GHz
Ka 27.5 to 31.0 GHz 17.7 to 21.2 GHz
is available in Ka and higher frequencies, further development is needed before
these come into general use.
Satellites have several advantages over terrestrial communications. These
include:
_ The receiving station can be mobile, which offers coverage from a single
satellite.
_ Within the coverage range of a single satellite, circuit cost is independent
of distance.
_ Impairments that accumulate on a per-hop basis on terrestrial
microwave circuits are avoided with satellites because the Earth-stationto-
Earth-station path is a single hop through a satellite repeater.
_ Sparsely populated or inaccessible areas can be covered by a satellite
signal, providing high-quality communications service to areas that
are otherwise difficult or impossible to reach.
_ Coverage is also independent of terrain and other obstacles that may
block terrestrial communications.
_ Earth stations can verify their own data transmission accuracy by listening
to the return signal from the satellite.
_ Because satellites broadcast a signal, they can cover wide areas.
_ Large amounts of bandwidth are available over satellite circuits, making
voice, video, and high-speed data circuits available.
_ The satellite signal can be brought directly to the end user, bypassing
the local telephone facilities that are expensive and limit bandwidth.
_ Multipath reflections that impair terrestrial microwave communications
have little effect on satellite radio paths.
Satellites are not without limitations, however. The greatest drawback is the
lack of frequencies. If higher frequencies can be developed with reliable paths,
plenty of spectrum is available, but atmospheric limitations may prevent their use
for commercial-grade telecommunications service. Other limitations include:
_ Multihop satellite connections impose delay that is detrimental to voice
communications and is generally avoided.
_ Path loss is high (about 200 dB) from Earth to satellite.
_ Rain absorption affects path loss, particularly at higher microwave
frequencies.
_ Frequency crowding in the C-band is high with potential for interference
between satellites and terrestrial microwave operating on the same
frequency.
The rapid growth of fiber-optic systems has had an adverse effect on satellites’
share of the telecommunications market, but the technology shows no signs of
330 PART 3 Transmission Technologies
dying. Though the satellites’ market share may be dropping, the traffic carried by
communications satellites continues to increase and will do so in the future.
Satellite Systems
A satellite circuit has five elements as shown in Figure 19-2: two terrestrial links,
an uplink, a downlink, and a satellite repeater. The satellite has six subsystems
described below:
_ physical structure
_ transponder
_ attitude control apparatus
_ power supply
_ telemetry equipment
_ station-keeping equipment
Physical Structure
The size of communications satellites has been steadily increasing since the launch of
the first commercial satellite in 1965. Size is limited by the capacity of launch vehicles
and the need to carry enough solar batteries and fuel to keep the system alive for its
design life of 5 to 10 years. Advances in space science such as launch vehicles that
can carry greater payloads are making larger satellites technically feasible. A large
physical size is desirable. Not only must the satellite contain the radio and support
equipment, but it must also provide a platform for large antennas to obtain the high
gain needed to overcome the path loss between the Earth station and the satellite.
Transponders
A transponder is a radio relay station on board the satellite. Transponders are technically
complex, but their functions are identical with those of terrestrial microwave
CHAPTER 19 Satellite Communications 331
F I G U R E 19-2
Communication Satellite System
Earth Station
Earth Station
Terrestrial
Link Terrestrial
Link
Uplink Downlink
radio relay stations. A receiving antenna picks up the signal from the Earth station
and amplifies it with a low noise amplifier (LNA), which boosts the received signal.
The LNA output is amplified and converted to the downlink frequency. The downlink
signal is applied to a high-power amplifier, using a traveling wave tube or
solid-state amplifier as the output device. The output signal couples to the downlink
or transmitting antenna. Solid-state amplifiers are popular because of their high
reliability. Most satellites carry multiple transponders, each with a bandwidth of
36 to 72 MHz. For example, Echostar 9 (also known as Telstar 13 and Intelsat
Americas 13), which was launched in August 2003, has 2 Ka-, 32 Ku-, and 24 C-band
transponders. It covers the United States from 121° west longitude and operates
with 120 W of power.
Attitude Control Apparatus
Satellites must be stabilized to prevent them from tumbling through space and to
keep antennas precisely aligned toward Earth. Satellite stabilization is achieved
by two methods. A spin-stabilized satellite rotates on its axis at about 100 rpm.
The antenna is despun at the same speed to provide constant positioning and
polarization toward Earth. The second method is three-axis stabilization, which
consists of a gyroscopic stabilizer inside the vehicle. Accelerometers sense any
change in position in all axes and fire positioning rockets to keep the satellite at a
constant attitude.
Power Supply
Satellites are powered by solar batteries. Power is conserved by turning off
unused equipment with signals from the Earth. On spin-stabilized satellites, the
cells mount outside the unit so that one-third of the cells always face the sun.
Three-axis stabilized satellites have cells mounted on solar panels that extend like
wings from the satellite body. Solar cell life is a major factor that limits the working
life of a satellite. Solar bombardment gradually weakens the cell output until
the power supply can no longer power the on-board equipment.
A nickel–cadmium battery supply is also kept on board most GEO satellites
to power the equipment during solar eclipses, which occur during two 45-day
periods for about an hour per day. The eclipses also cause wide temperature
changes that the on-board equipment must withstand.
Telemetry Equipment
A satellite contains telemetry equipment to monitor its position and attitude and
to initiate correction of any deviation from its assigned station. Through telemetry
equipment, the Earth control station initiates changes to keep the satellite at its
assigned longitude and inclination toward Earth. Telemetry also monitors the
received signal strength and adjusts the receiver gain to keep the uplink and
downlink paths balanced.
332 PART 3 Transmission Technologies
Station-Keeping Equipment
Small rockets are installed on GEO vehicles to keep them on station. When the satellite
drifts, rockets fire to return it to position. The tasks that keep the satellite on position
are called station-keeping activities. The fuel required for station keeping is the
factor, with solar cell life that limits the design life of the satellite. Moveable antennas
track MEO and LEO satellites, so station-keeping equipment is not required.
EARTH STATION TECHNOLOGY
Earth stations vary from simple, inexpensive, receive-only stations that individual
consumers can purchase to elaborate two-way communications stations such as
the one in Figure 19-3 that offer commercial satellite service. An Earth station
includes microwave radio relay equipment, terminating multiplex equipment,
and a satellite communications controller. The Earth stations for MEO and LEO
satellites link to the PSTN for dial-up telephone service.
Radio Relay Equipment
The radio relay equipment used in an Earth station is similar to terrestrial
microwave equipment except that the transmitter output power is considerably
CHAPTER 19 Satellite Communications 333
F I G U R E 19-3
Satellite Earth Station on Majuro (Photo by Author)
higher. In addition, antennas up to 30 m in diameter in GEO Earth stations provide
the narrow beam width required to concentrate power on the targeted satellite.
Because the Earth station’s characteristics are more easily controllable than
the satellite’s and because power is not limited on Earth as it is in space, the Earth
station plays the major role in overcoming the path loss between the satellite and
Earth. Path loss for GEO satellites ranges from about 197 dB at 4 GHz to about
210 dB at 12 GHz. In addition, the higher the frequency, the greater the loss from
rainfall absorption. Therefore the uplink always operates at the higher frequency
where higher transmitter output power can overcome absorption, while the lower
frequency is reserved for the downlink.
GEO antennas are adjustable to compensate for slight deviations in satellite
positioning. Antennas at commercial stations are normally adjusted automatically
by motor drives, while inexpensive antennas are adjusted manually as needed.
Thirty-meter antennas provide an extremely narrow beam width, with half-power
points 0.1° wide. LEO and MEO antennas are moveable to track the satellite in
its orbit.
Satellite Communications Control
A satellite communications controller (SCC) apportions the satellite’s bandwidth,
processes signals for satellite transmission, and interconnects the Earth station
microwave equipment to terrestrial circuits. The SCC formats the received signals
into a single integrated bit stream in a digital satellite system or combines FDM
signals into an analog FM signal. The multiplex interface of an Earth station is conventional.
Satellite circuits use either analog or digital modulation, with interfaces
to frequency division and time division terrestrial circuits.
Access Control
Satellites employ several techniques to increase the traffic-carrying capacity and
provide access to that capacity. Some FDMA satellites divide the transponder
capacity into multiple frequency segments between endpoints. One disadvantage
of FDMA is that users are assigned a fixed amount of bandwidth that cannot
be adjusted rapidly or easily assigned to other users when it is idle. In addition,
the guard bands between channels use part of the capacity.
TDMA time-shares the total transponder capacity. Earth stations transmit
only when permitted by the access protocol. When the Earth station receives
permission to transmit, it is allotted the total bandwidth of the transponder for the
duration of the station’s assigned time slot. A master station controls the access
or the Earth station listens to which station transmitted last and sends its burst in a
preassigned sequence. Each Earth station receives all transmissions but decodes
only those addressed to it. TDMA provides priority to stations with more traffic to
334 PART 3 Transmission Technologies
transmit by assigning those stations more time slots than it assigns to low-priority
stations. Therefore, a station with a growing amount of traffic can be allotted a
greater share of total transmission time.
Demand-assigned multiple access (DAMA) is an alternative to preassigned
multiple access. DAMA equipment keeps a record of idle radio channels or time
slots. Channels are assigned on demand by one of the three methods—polling,
random access with central control, and random access with distributed control.
Control messages are sent over a separate terrestrial channel or contained in a
control field in the transmitted frame.
Signal Processing
The SCC conditions the signals for transmission between the terrestrial and satellite
links. The type of signal conditioning depends on the service provider and may
include voice compression, echo cancellation, forward error correction, and digital
speech interpolation to avoid transmitting the silent periods of a voice signal.
GEO SATELLITE TRANSMISSION
Much of the previous discussion is of only academic interest to those who use
satellite services. However, satellite circuits and terrestrial circuits have different
transmission characteristics.
Satellite Delay
The quarter-second delay between two Earth stations is noticeable in voice
communications circuits, but most people become accustomed to it and accept it as
normal if the circuit is confined to one satellite hop but delay affects many data protocols.
TCP/IP, for example, sees a delay in packet acknowledgement as a sign of
congestion or packet loss and may force retransmission of packets that have actually
been correctly received. Furthermore, TCP has a slow-start characteristic that sets
its packet acknowledgement window at a single packet and gradually opens it.
Therefore, performance depends on the transfer rate and the round-trip delay.
Performance problems begin to occur when TCP/IP operates over what the
industry calls a “long fat pipe.” For example, a T1 satellite channel has a bandwidth
delay product of 100 outstanding TCP segments of 1200 bytes each, which
requires a large buffer to contain unacknowledged packets. RFC 1323 discusses
recommended TCP modifications to improve performance on satellite circuits.
One solution is “spoofing,” in which the Earth stations acknowledge the packets
to the endpoints and communicate between themselves in the satellite link with a
protocol that is not delay susceptible. Satellite circuits are unsatisfactory for VoIP
because the combined delays are well outside the recommended range.
CHAPTER 19 Satellite Communications 335
Rain Absorption
Rain absorption has a dual detrimental effect on satellite communications. Heavy
rains increase the path loss and may change the signal polarization enough to
impair the cross-polarization discrimination ability of the receiving antennas.
Rain absorption can be countered by these methods:
_ choosing Earth station locations where heavy rain is least likely
_ designing sufficient margin into the path to enable the circuits to tolerate
the effects of rain
_ locating a diversity Earth station at a sufficient distance from the main
station
Technical considerations may limit the first two options. Transmit power
and antenna gain from the satellite can be increased only within the limits dictated
by the size of the satellite and the transmit power available. Locations with low
precipitation cannot always deliver service where required. These considerations
mandate the use of Earth station diversity at higher frequencies.
Sun Transit Outage
During the spring and fall equinoxes for periods of about 10 min/day for 10 days,
the sun is positioned directly behind the satellite and focuses a considerable
amount of high-energy radiation directly on the Earth station antenna. This solar
radiation causes a high noise level that renders the circuits unusable during this
time. If the outage cannot be tolerated, traffic must be rerouted through a backup
satellite.
Carrier-to-Noise Ratio
Satellite transmission quality is based on carrier-to-noise ratio, which is analogous
to signal-to-noise ratio on terrestrial circuits. The ratio is relatively easy to
improve on the uplink portion of the satellite circuit because transmitter output
power and antenna gain can be increased to offset noise. On the downlink portion
of a circuit, the effective isotropic radiated power (EIRP), which is a measurement
of the transmitter output power that is concentrated into the downlink footprint,
can be increased only within the size and power limits of the satellite or by using
spot beams to concentrate signal strength.
REPRESENTATIVE SATELLITE SERVICES
In this section, three different types of services are discussed to illustrate the
versatility of communications satellites. LEO is a new service that avoids the delay
inherent with other satellite services. Maritime radio service is an excellent example
336 PART 3 Transmission Technologies
of a service that cannot be provided in any other feasible way: communication with
aircraft and ships at sea. VSAT replaces conventional terrestrial communications
and offers the advantage of bringing signals directly to the user without requiring
the last link in a communications path—the local telephone loop—that is often
expensive and bandwidth limiting.
Low Earth Orbiting Satellite
Iridium launched commercial voice, fax, paging, and narrow-band data service in
2001, using a network of 66 operational satellites plus six in-orbit backups. The
system operates at an altitude of 780 km (485 miles). Its Earth station links and
intersatellite links operate in the Ka-band. Voice is digitized and compressed to
2.4 Kbps. Dial-up data can be transmitted at a maximum speed of 2.4 Kbps, with
Internet access at 10 Kbps. Short messages of up to 160 characters can be received
on the telephone. In addition, Iridium offers short-burst data message service,
which supports messages up to 1960 bytes.
Satellites are connected via intersatellite links to the four nearest neighbors
and to an Earth station. Calls can be connected to landline telephones or to other
satellite phones. The satellites use the L-band (1616 to 1626.5 MHz) for communication
with the subscriber terminals. Subscribers can receive and place telephone
calls and pages while roaming anywhere in the world that the service is
authorized.
International Maritime Satellite Service
Inmarsat began life in 1979 as an intergovernmental organization. In 1999, it was
converted to private ownership with headquarters in Britain. The company provides
phone, fax, and data communications to more than 287,000 ship, vehicle, aircraft,
and other mobile users through a network of nine satellites. Inmarsat provides
service for all types of ocean-going vessels ranging from merchant ships to yachts.
The system has a network of coastal Earth stations that can communicate with
ships at sea. The ship’s Earth station mounts above decks and automatically stays
in position with satellite tracking equipment.
Inmarsat offers a variety of fleet services, providing both ISDN and mobile
packet data service (MPDS). MPDS charges by packet volume and not for the
time spent online, which makes it convenient for real-time access. The service can
be used for video conferencing, store-and-forward video, remote monitoring,
chart and weather updates, telemedicine, and distress and safety signals.
In the past, the principal methods of communication from ships were telex
and high-frequency radio, which were unreliable and expensive. Now data
circuits are replacing those modes of communication. Voice circuits replace the
high-frequency ship-to-shore radio that often suffered from poor reliability. Ship
locations can be monitored precisely through polling equipment. Distress calls
CHAPTER 19 Satellite Communications 337
can be received and rebroadcast to ships that are in the vicinity but out of radio
range. Broadcasts such as storm warnings can be made to all ships in an area.
Through their I-4 satellites, Inmarsat provides Broadband Global Area
Network (BGAN) service, which supports up to 432 Kbps for Internet access,
mobile multimedia, and similar applications. BGAN is also compatible with thirdgeneration
cellular service.
Inmarsat is the wholesaler of satellite airtime, which is sold by hundreds of
partners. In addition, Inmarsat provides satellite services to commercial airliners
and corporate jets. Inmarsat’s Swift64 offers Mobile ISDN- and IP-based MPDS
connectivity at 64 Kbps. United Airlines has announced the use of Swift64 to
provide video surveillance of the secured flight deck via satellite. Scandinavian
Airlines System is offering high-speed Internet access on several of its long-range
aircraft. The availability of satellite services for Internet access will become
increasingly common in the future.
Very Small Aperture Terminal
VSATs are named for the size of the transmitting antennas, which are much
smaller than those used in conventional Earth stations. VSAT antennas are
normally 1.8 m (6 ft) or less in diameter, which makes them easy to conceal on
rooftops and in areas with zoning restrictions. A VSAT network is star-connected
with a hub at the center and dedicated lines running to the host computer as
shown conceptually in Figure 19-4. The hub has a larger antenna, often 4 to 11 m
in diameter, aimed at the satellite. Hubs are complex and expensive, so only the
largest organizations can justify a privately owned hub. Usually, the VSAT vendor
338 PART 3 Transmission Technologies
F I G U R E 19-4
A VSAT System
Earth Station
VSAT Terminal
VSAT Terminal
VSAT Terminal
VSAT Master
Station
Server
owns the hub, or one organization owns it and shares it with others. Not only is a
shared hub more cost-effective for most companies, but it also relieves the company
of the necessity of managing the hub, which may require one or two people
per 100 nodes. Generally, a privately owned hub is feasible only when 200 or more
remote stations share the service.
The hubs control demand assignment to the satellite and monitor and diagnose
network performance. Demand is allocated in one of the four ways—pure aloha,
slotted aloha, TDMA, or spread spectrum. The first three methods generally are used
on Ku-band and the last on C-band. Pure aloha is an inefficient method of regulating
access. Stations transmit at will, and when their transmissions collide they must
retransmit. Slotted aloha is somewhat more efficient in that stations can transmit only
during allotted time slots. TDMA and spread spectrum are the most effective ways
of allocating access. VSAT provides bandwidth as high as T1/E1 and as low as the
customer needs to go. It is used for voice, video, and data transmission.
The remote station has an antenna and a receiving unit, which is about the
size of a personal computer base unit. The receiving unit contains a modulator/
demodulator, a packet assembler/disassembler, and a communication controller.
The remote transmitter operates with an output power of about 1 W. The receiver
uses a low-noise amplifier.
The primary application for VSAT is data, although it can also carry voice
and video. Typically, C-band VSATs carry 9.6 Kbps data, and Ku-band VSATs
carry 56 or 64 Kbps data; some systems carry a full or fractional T1/E1. Most
applications are two-way interactive. The primary advantage of VSAT is its ability
to support multiple locations. For a few locations, the terrestrial link from the host
computer to the hub plus the investment in remote stations may make VSAT
prohibitively expensive. As the number of remote sites increases, however, VSAT
becomes more attractive.
SATELLITE APPLICATION ISSUES
In one sense, satellite applications diminish as terrestrial and undersea fiber-optic
circuits become more plentiful and economical, but satellite services are still
uniquely suited for many applications. The heavy investments the major providers
are making show that they expect demand to remain healthy.
Satellite Service Evaluation Considerations
Satellite space vehicle evaluation criteria are complex, technical, and of interest
only to designers, owners, and manufacturers of satellites and on-board equipment.
Therefore, this discussion omits these criteria. Likewise, common carrier
Earth station equipment evaluations are omitted from this discussion. Evaluation
criteria discussed in Chapter 18 on microwave equipment generally apply to satellite
services except that multipath fading is not a significant problem in satellite
CHAPTER 19 Satellite Communications 339
services. In addition, alarm and control systems in terrestrial microwave are
different from those used in satellite systems.
The following factors should be considered in evaluating satellite services
and privately owned Earth station equipment.
Availability
Circuit availability is a function of path and equipment reliability. To the user of
capacity over a carrier-owned Earth station, equipment reliability is a secondary
consideration. The important issue is circuit reliability measured as percent errorfree
seconds in digital services and percent availability within specified noise
limits for analog services.
These same availability criteria apply with privately owned Earth stations,
but the carrier can quote availability based only on path reliability. Equipment
availability depends on MTBF and MTTR and must be included in the reliability
calculation. The frequency and duration of any expected outages because of
solar radiation or solar eclipse should be evaluated. Availability figures of 99.5 to
99.9% are typical.
Data Bandwidths
Satellite carriers typically provide transponder bandwidth in 128 Kbps segments.
The number of stations that can be supported in this bandwidth depends on the
amount of activity. VSAT is generally cost-competitive with frame relay in large
networks and offers a similar type of service, but with somewhat lower throughput
for a given bandwidth of the access facility. The applications of VSAT are
similar to those for frame relay with certain exceptions. First, the cost of a frame
relay access channel varies with the distance from the carrier’s point-of-presence.
VSAT is not distance-sensitive except for the cost of a backhaul circuit to carry
data from the Earth station to the customer’s site. The backhaul circuit can be a
point-to-point circuit, IP, or frame relay.
Access Method
Satellite carriers employ several techniques to increase the information-carrying
capacity of the space vehicle. Techniques such as DAMA can result in congestion
during peak load periods and the possibility that Earth station buffer capacity
can be exceeded or access to the system blocked. Users should determine what
methods the carrier uses to apportion access, whether blockage is possible, and
whether transmission performance will meet objectives.
Transmission Performance
The carrier’s BER, loss, noise, echo, envelope delay, and absolute delay objectives
should be evaluated. To support TCP/IP, the carrier should provide spoofing.
Except for absolute delay, which cannot be reduced except by using terrestrial
340 PART 3 Transmission Technologies
facilities to limit the number of satellite hops, satellite transmission evaluation
should be similar to terrestrial circuits.
Earth Station Equipment
Earth station equipment is evaluated against the following criteria:
_ equipment reliability
_ support for specific protocols such as TCP/IP
_ technical criteria, such as antenna gain, transmitter power, and receiver
sensitivity, that provide a sufficiently reliable path to meet availability
objectives
_ antenna positioning and tracking equipment that is automatically
or manually adjustable to compensate for positional variation in the
satellite
_ physical structure that can withstand the wind velocity and ice-loading
effects for the locale
_ the availability of radome or deicing equipment to ensure operation
during snow and icing conditions
Network Management Capability
Network management is important in VSAT networks where many Earth stations
are under the control of a single hub. The service provider should be able to
reconfigure the network rapidly from a central location. Monitoring and control
equipment should be able to diagnose problems and detect degradations before
hard faults occur. The network management package should collect statistics
on network use and provide information for predicting when growth additions
will be required. Determine whether the network provider can service all network
components including routers.