174
Networking Fundamentals
geosynchronous
orbit
an orbit in which a
satellite’s rotational
speed is synchro-
nized with the earth’s
rotational speed,
making the satel-
lite appear to be in a
stationary position.
propagation delay
the time it takes for
data to be trans-
mitted from the
earth and satellite.
Microwave Transmission and Networking
The term microwave is used to describe radio waves in the electromagnetic
spectrum that have a wavelength from 1 millimeter (mm) to 30 centimeters (cm)
and radio wave frequencies between 1 GHz and 300 GHz. The amount of data
carried by a radio wave is directly proportional to its frequency. In short, the
higher the frequency, the more data that can be transmitted in a given period of
time. The portion of the electromagnetic spectrum identifi ed as microwave is the
preferred technology used for wireless networking. Its application is described in
the IEEE 802.11 standards. Microwave radio wave transmission can be broadcast
directly between wireless devices or routed through satellites orbiting the earth.
Satellites are often used as part of a WAN distribution. Satellites can offer
the advantage of providing a wireless network connection to remote or mobile
locations that cannot be achieved using conventional methods.
Look at Figure 4-26. The satellite in this illustration is positioned 22,300
miles (35,880 km) above the earth’s surface and moves at a speed of approximately
68,000 mph. At this distance, it takes the satellite exactly 24 hours to make one
revolution around the earth. This time is equal to the time of the earth’s rotation.
This equality causes the satellite to appear in a stationary position above the
earth and is said to be in geosynchronous orbit. In other words, the satellite’s
speed is synchronized with the earth’s rotational speed. If the satellite were in
a lower orbit, it would need to move at a much higher orbital speed. This would
result in the need to track the satellite as it moves across the sky. Tracking a
satellite in stationary position is easy when compared with tracking a constantly
moving satellite.
One signifi cant disadvantage to satellite communications is propagation delay.
Propagation delay is the time it takes for data to be transmitted from the earth
Figure 4-25.
An example of using
cellular technology
to send a message
from a computer to
a mobile network
device such as a
PDA.
Meet me at Harry’s
at 8:00 for dinner. Rich.
Meet me at
Harry’s at 8:00
for dinner. Rich.
Meet me at
Harry’s at 8:00 for
dinner. Rich.
Meet me at Harry’s at 8:00 for
dinner. Rich.
Meet me at
Harry’s at 8:00
for dinner. Rich.
Meet me at
Harry’s at 8:00
for dinner. Rich.
Internet
ISP
Telephone
company
Pager PDA
Message is typed
in at keyboard Message is received at pager or PDA
Paging service
company