wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Media 2-7
2.2.6 Standards
Numerous standards in different versions were developed over the years which are used in the area of
industrial communication. Table 2.1 gives an overview of some often used systems.
2.2.7 Data Transmission Utilizing Existing Cable Infrastructure
In special cases, e.g., within buildings, existing cable infrastructure primarily not intended for data
transmission can be employed for. Especially used for home applications, power line communication
or power line networking allows data transfer via AC power supply lines. Applications range from narrowband
home control up to broadband home networking and internet access with data rates ≥1.Mbps.
Power line data transmission is based on medium to high carrier frequencies (narrowband 9–500.kHz,
broadband 1.6–80.MHz) modulated by different modulation formats, e.g., OFDM (see Section 2.4.3).
Induced by device on and off switching and by nonlinearities causing harmonics, power lines are inherently
a noisy environment. Power lines are unshielded and, therefore, radiate signals they carry which
may interfere with nearby equipment. Power line technology can also be used for in-vehicle networks.
Other cables which can be employed for data transmissions are telephone wires using Digital Subscriber
Line (DSL) technology.
2.3 Optical Links
2.3.1 Physical Properties
The core and cladding are the two key elements of an optical fiber. The core is the inner part of the fiber,
through which light is guided. The cladding surrounds it completely. The refractive index of the core is
higher than that of the cladding, so light in the core strikes the boundary with the cladding at a glancing
angle, confined in the core by the principle of total internal reflection.
Optical communication is the fastest communication. The tendency of the light to travel in packets
through the narrow core is not because of photon size but of speed; photons are many orders of magnitude
smaller than the core. Transmission of light by optical fibers is not 100% efficient. Some light is lost,
causing attenuation of the signal. Attenuation measures the reduction in signal strength by comparing
output power with input power. Measurements are made in decibels (dB). Optical fibers are unique
in allowing high-speed signal transmission at low attenuation. The degree of attenuation depends on
the wavelength of light transmitted. This makes operating wavelength an important feature of a fiber
system. Light intensity attenuation has no direct effect on the bandwidth of the electrical signals being
transported. There is a direct correlation, between the S/N of the fiber receiver electronic circuits and
the usable recovered optical signal. For commercially available fibers, attenuation ranges from approximately
0.15–0.5.dB/km for single-mode fibers. The gradual reduction in attenuation losses in all the four
generations of optical communication systems (OCSs) have been described in Table 2.2.
Dispersion is the spreading or broadening of light pulses as they propagate through fiber. Dispersion
limits fiber transmission capacity. Dispersion increases with distance, so the maximum transmission rate
decreases with the distance. There are different forms of dispersion, including material and waveguide. The
materials used to create the fiber cable use different refractive indices; therefore, each wavelength moves
at a different speed inside the fiber cable. This means that some wavelengths arrive prior to others and a
signal pulse disperses over a broader range. This is also called smearing. The center core creates the
waveguide. The shape and the refractive index inside the core can create the dispersion or spreading
of the pulse. Information-carrying capacity (band width/bit rate) is very important in all types of communications.
The more bits that can pass through a system in a given time, the more information it can
carry. The band width enhancement has been the major area of research since 1974. The developments in
bandwidth are shown in chronological order in Table 2.2.
© 2011 by Taylor and Francis Group, LLC