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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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