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Sec. 8–5 Satellite Communication Systems 583
8–4 CAPACITIES OF PUBLIC SWITCHED TELEPHONE NETWORKS
The wideband channels used to connect the toll offices consist of one predominant type:
fiber-optic cable. Table 8–2 lists some of the wideband systems that are used, or have been
used in the past, and indicates the capacity of these systems in terms of the number of VF
channels they can handle and their bit rate.
Historically, open-wire pairs, which consist of individual bare wires supported by glass insulators
on the cross arms of telephone poles, provided wideband service via FDMSSB signaling.
Fiber-optic cable with OOK signaling has overtaken twisted-pair cable, coaxial cable, and
microwave relay because of its tremendous capacity and relatively low cost. As shown in Table
8–2, data rates on fiber-optic links continue to increase as technology advances. Typically,
a 2.5-Gbs data rate is employed on an OOK optical carrier, as is the case with the FT-2000
system. This is equivalent to 32,000 VF telephone circuits. Data rates of 10 Gbs on each carrier
are also possible. For even higher capacity, multiple carriers at different wavelengths are used on
a single fiber. Dense wavelength division multiplexing (DWDM) systems include as many as
176 optical carriers on one fiber. For example, the FlashWave7700 DWDM system achieves a
capacity of 1.76 Tbs, or 27.5 million VF circuits, by using 176 carriers. However, fiber-optic
cable provides service only from one fixed point to another. Conversely, communication satellites
provide wideband connections to any point on the globe. Service to isolated locations can
be provided almost instantaneously by the use of portable ground stations. This is described in
more detail in the next section.
8–5 SATELLITE COMMUNICATION SYSTEMS
The number of satellite communication systems has increased over the last few years.
Satellites have made transoceanic relaying of television signals possible. Satellite communications
provide the relaying of data, telephone, and television signals and now provide
national direct-into-the-home television transmission via satellite.
Satellite technology has changed tremendously over the past 15 years. The major change
is the transition from analog signaling to digital signaling. Also, the type of information transmitted
has changed. In the United States, satellites are now used mainly for TV distribution and
to provide communication links to isolated areas. Satellite systems are ideally suited for global
positioning system (GPS) applications where the satellite signal is used by the recipient to
determine the user’s exact geometric coordinates (latitude, longitude and altitude) on Earth
[Pratt et al, 2003; Kaplan, 2005].
As indicated in Table 8–2, modern fiber optic-systems have tremendously more capacity
than modern satellites. The WaveStar fiber optic system has about 40 times the number of equivalent
VF channels than does the Intelsat X system. Bundles of fiber can be laid with very little
increase in cost, and thus, the initial fiber-cable capacity can be expanded to almost any capacity
imaginable. The per channel costmaintenance of a fiber system is orders of magnitude less
than that for a satellite system. Consequently, in the United States almost all telephone and
internet data are carried by fiber-optic systems. Satellite systems are used to distribute TV programs
to cable TV companies and directly to the home (e.g., DirecTV), for satellite radio (Sirius
XM), and to provide private VSAT (very small aperture terminals) networks for business.