Page 12 - Optical Communications Essentials

Basic Concepts of Communication Systems

2 Chapter One

However, since the physical characteristics of both free-space and electric
wire-based communication systems impose an upper bound on the transmission
capacities, alternative transmission media were investigated. A natural exten-
sion was the use of optical links. After extensive research and development on
the needed electrooptical components and the glass equivalent of a copper wire,
optical fiber communication systems started to appear in the 1970s. It is this
technology that this book addresses.
To exchange information between any two devices in a communication system,
some type of electric or optical signal which carries this information has to be
transmitted from one device to the other via a communication channel. This
channel could consist of a wire, radio, microwave, satellite, infrared, or optical
fiber link. Each of the media used for such communication channels has unique
performance characteristics associated with it. Regardless of its type, the medium
degrades the fidelity of the transmitted signal because of an imperfect response
to the signal and because of the presence of electrical and/or optical noise and
interference. This can lead to misinterpretations of the signal by the electronics
at the receiving end. To understand the various factors that affect the physical
transfer of information-bearing signals, this chapter gives a basic overview of
fundamental data communication concepts. With that as a basis, the following
chapters will describe how information is transferred using lightwave technology.

1.1. Definitions
We start by giving some concepts and definitions used in data communications
and the possible formats of a signal. The signal format is an important factor in
efficiently and reliably sending information across a network.
A basic item that appears throughout any communications book is the prefix
used in metric units for designating parameters such as length, speed, power
level, and information transfer rate. Although many of these are well known, a
few may be new to some readers. As a handy reference, Table 1.1 lists standard
prefixes, their symbols, and their magnitudes, which range in size from 10 24 to
10 24 . As an example, a distance of 2 10 9 m (meters) 2nm (nanometers).
The three highest and lowest designations are not especially common in com-
munication systems (yet!), but are included in Table 1.1 for completeness.
Next let us define some terms and concepts that are used in communications.
■ Information has to do with the content or interpretation of something such as
spoken words, a still or moving image, the measurement of a physical charac-
teristic, or values of bank accounts or stocks.
■ A message may be considered as the physical manifestation of the information
produced by the source. That is, it can range from a single number or symbol
to a long string of sentences.
■ The word data refers to facts, concepts, or instructions presented as some type
of encoded entities that are used to convey the information. These can include

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