Page 11 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
1

Basic Concepts of

Communication Systems

Ever since ancient times, people continuously have devised new techniques and
technologies for communicating their ideas, needs, and desires to others. Thus,
many forms of increasingly complex communication systems have appeared
over the years. The basic motivations behind each new one were to improve the
transmission fidelity so that fewer errors occur in the received message, to
increase the transmission capacity of a communication link so that more infor-
mation could be sent, or to increase the transmission distance between relay sta-
tions so that messages can be sent farther without the need to restore the signal
fidelity periodically along its path.
Prior to the nineteenth century, all communication systems operated at a very
low information rate and involved only optical or acoustical means, such as signal
lamps or horns. One of the earliest known optical transmission links, for exam-
ple, was the use of a fire signal by the Greeks in the eighth century B.C. for send-
ing alarms, calls for help, or announcements of certain events. Improvements of
these systems were not pursued very actively because of technology limitations at
the time. For example, the speed of sending information over the communication
link was limited since the transmission rate depended on how fast the senders
could move their hands, the receiver was the human eye, line-of-sight transmis-
sion paths were required, and atmospheric effects such as fog and rain made the
transmission path unreliable. Thus it turned out to be faster, more efficient, and
more dependable to send messages by a courier over the road network.
The invention of the telegraph by Samuel F. B. Morse in 1838 ushered in a
new epoch in communications—the era of electrical communications. In the ensu-
ing years increasingly sophisticated and more reliable electrical communication
systems with progressively larger information capacities were developed and
deployed. This activity led to the birth of free-space radio, television, microwave,
and satellite links, and high-capacity terrestrial and undersea wire lines for
sending voice and data (and advertisements!) to virtually anywhere in the world.
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