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Basic Concepts of Communication Systems
12 Chapter One
such as glass, prompted researchers to consider optical fiber transmission
technology.
1.6.2. Baseband signals
Baseband refers to the technology in which a signal is transmitted directly onto
a channel without modulating a carrier. For example, this method is used on
standard twisted-pair wire links running from an analog telephone to the near-
est switching interface equipment.
The same baseband method is used in optical communications; that is, the opti-
cal output from a light source is turned on and off in response to the variations
in voltage levels of an information-bearing electric signal. As is described in
Chap. 6, for data rates less than about 2.5 Gbps, the light source itself can be
turned on and off directly by the electric signal. For data rates higher than
2.5 Gbps, the optical output from a source such as a laser cannot respond fast
enough. In this case an external device is used to modulate a steady optical out-
put from a laser source.
1.7. Signal Multiplexing
Starting in the 1990s, a burgeoning demand on communication network assets
emerged for services such as database queries and updates, home shopping,
video-on-demand, remote education, audio and video streaming, and video con-
ferencing. This demand was fueled by the rapid proliferation of personal com-
puters coupled with a phenomenal increase in their storage capacity and
processing capabilities, the widespread availability of the Internet, and an
extensive choice of remotely accessible programs and information databases. To
handle the ever-increasing demand for high-bandwidth services from users
ranging from home-based computers to large businesses and research organiza-
tions, telecommunication companies worldwide are implementing increasingly
sophisticated digital multiplexing techniques that allow a larger number of
independent information streams to share the same physical transmission
channel. This section describes some common techniques. Chapter 2 describes
more advanced methodologies used and proposed for optical fiber transport sys-
tems.
Table 1.2 gives examples of information rates for some typical voice, video, and
data services. To send these services from one user to another, network providers
combine the signals from many different users and send the aggregate signal over
a single transmission line. This scheme is known as time-division multiplexing
(TDM). Here N independent information streams, each running at a data rate of
R bps (bits per second), are interleaved electrically into a single information
stream operating at a higher rate of N R bps. To get a detailed perspective of
this, let us look at the multiplexing schemes used in telecommunications.
Early applications of fiber optic transmission links were mainly for large-
capacity telephone lines. These digital links consisted of time-division multiplexed
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