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Unformatted text preview: d to serve LANs, MANs, and WANs.
We have already seen that satellites are widely used for providing broadcasting,
long distance, and international data communication services to stationary users.
With advances in antenna design, signal reception, and other related technologies,
it has now become possible to provide mobile services using satellites. Many
satellite-based networks using Low-Earth-Oribit (LEO), Medium-Earth-Orbit
(MEO), and Geosynchronous-Earth-Orbit (GEO) have been started to provide
commercial wireless data services at relatively low data rates (below 64 Kbps).
For example, Iridium is a LEO system that uses 66 satellites to provide mobile
communications to every point on earth and within 50 miles above it. In Iridium,
the same satellite may not serve a user throughout the duration of a call; a call may
be handed off to another approaching satellite. The satellites, in concentric orbits,
maintain links with up to four satellites in neighboring orbits. But handoffs are necessary among satellites in counter-rotating orbits to maintain cross-links among
Although there is a large market for satellite-based mobile services especially in
places where no (or poor) cellular/PCS service exists, due to the high initial cost
and per-minute charges, users of satellite-based mobile communications remain a
small minority among a much larger number of wireless users. For example,
Iridium, which started its service in January 1999, has been officially declared
bankrupt and has terminated commercial services because the number of
customers that adopted the service was much smaller than needed to maintain this
We have already seen that microwave systems use very high frequency radio
signals to transmit data through space. Hence for several years, microwave
systems have been and continue to be a popular means of wireless data
communications. Microwave, links operate in higher frequency bands than any
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This document was uploaded on 04/07/2014.
- Spring '14