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3.1.6
Quality Features and Issues
The quality of satellite communications or any communications service must be
defined in user terms. The following sections identify different approaches to mea-
suring quality of transmission as perceived by humans. If the information is analog
in nature, quality is measured objectively by using a signal-to-noise ratio or subjec-
tively by surveying people (subjects) who have been exposed to the service. Quality
in data communications boils down to the quantity of valid data that reaches the
distant end (the bit or packet error rate), along with the time it takes to get there
(the latency).
3.1.6.1
Signal Reproduction
For a single point of transmission, a satellite is nearly ideal for delivering a signal
of the highest quality. Modern GEO FSS satellites radiate sufficient power into the
geographical footprint to be received by ground antennas of diameters in the range
of 45 cm to 10m (18 inches to 32 feet). The GEO and non-GEO MSS systems
3.1
General Features of Satellite Networks
83
introduced in the late 1990s reach antennas the size of your thumb. Because satellites
use line-of-sight transmission in directions perpendicular to the atmosphere, the
frequency and duration of link fades are reduced. Satellite links aimed near the
horizon, at a low elevation angle, do experience fading similar to terrestrial
microwave.
The predominant mode of transmission over satellites is digital, in which infor-
mation is transformed from its analog form into a data stream. Such a process
alters the ideal quality of the signal, introducing a certain level of distortion due
to quantization (assigning the analog signal to discrete voltage levels that can be
represented by numerical values) and compression (altering the actual information
to reduce the required number of bits). As long as the bits are transferred through
the link with little or no error, the quality remains exactly as it was after the initial
conversion process. Whether a satellite or a terrestrial link is used, signal quality
may be maintained at all times.
Many terrestrial networks suffer from man-made noise and various kinds of
short interruptions (‘‘glitches’’) while satellite links experience primarily receiver
noise, which is constant and easily compensated for with power or improved
forward error correction. Satellite-based mobile communication is affected by vari-
ous forms of blockage, which makes these services somewhat more glitchprone
than terrestrial cellular networks. That can cause data dropouts that will affect
the consistency and continuity of the delivered information.
When these factors are properly taken into account, satellite communication
can be engineered for high circuit quality and reliability. Designers can then select
appropriate Earth station equipment that will provide that quality confidently. The
application in which these aspects of quality play the greatest role is video. From
a viewer perspective, the quality of the delivered video signal is the same as the
signal created at the studio or played from the originating video tape machine.
3.1.6.2
Voice Quality and Echo
The issue of quality of voice transmission over satellites has received a lot of
attention because of heavy competition among satellite and terrestrial communica-
tion companies. As discussed in Chapter 2, the use of the GEO for communications
relays was a controversial topic prior to the first use of SYNCOM in 1962 because
of the quarter-second delay introduced by the long transmission path. Voice commu-
nications over a GEO satellite can be made acceptable to over 90% of telephone
subscribers, as has been proved by numerous quality surveys. Terrestrial and LEO
systems that span continents and oceans do not suffer from noticeable delay and
hence are potentially more desirable from that standpoint alone. People who rou-
tinely make phone calls over the Internet often experience delays comparable to
the GEO path.
The delay itself is not as detrimental as the presence of a returned echo to the
speaking party. Both terrestrial and satellite links are capable of producing echo,
as illustrated in Figure 3.9. At the left of the figure, speech from a female talker
is converted into electrical energy in the voice frequency range (300 to 3,400 Hz)
by the handset and passes over the single pair of wires to the telephone equipment
84
Satellite Network Architectures
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