Tech Tip
Chapter 4 Wireless Technology
175
and satellite. It is caused by the great distance between the satellite and earth
and can be compared to latency. In the past, the Motorola Company attempted to
send a large group of 66 satellites into orbit at approximately 460 miles (740 km)
above the earth to eliminate much of the propagation delay. At this height, the
propagation delay is signifi cantly reduced and is relatively unnoticed by the users.
Unfortunately, the project, called Iridium, proved too costly and was abandoned
before it could be fully implemented.
The term propagation delay is used in satellite transmission in much the same way that the
term latency is used for data delay in network systems.
While the propagation delay that normally occurs is generally short—
only a fraction of a second—the effect of the delay depends on the type of data
transmitted. For example, a delay of 250 milliseconds (ms) for a message consisting
of several pages of text would go completely unnoticed by the end user. The same
delay for data containing a telephone conversation would show slight pauses in
the conversation. A series of frames containing millions of bits of picture or video
data could show serious evidence of the delay. Picture or video data needs to be of
relatively low resolution to limit the total amount of data transmitted. Transmitting
high-resolution picture or video data would only be practical in half-duplex
communication. Full-duplex communication for high-resolution picture or video
data transmission is not yet practical. There are still very noticeable delays in
high-resolution, two-way conference calls using satellite links in a network.
For one-way data transmission, buffering techniques similar to the buffering
techniques used for downloading sound on a slow communications link, such as
Figure 4-26.
A typical satellite in
geosynchronous orbit
is approximately
22,300 miles
(35,860 km) above
the earth’s surface.
At this distance, the
satellite can maintain
a fixed position
above the earth.
68,000 mph
24 hours
22,300 miles (35,860 km)