Page 288 - Optical Communications Essentials

Optical Link Design

278 Chapter Sixteen

The most popular error correction codes are cyclic codes. These are desig-
nated by the notation (n, m), where n equals the number of original bits m plus
the number of redundant bits. Although many types of cyclic codes have been
considered over the years in electrical systems, the Reed-Solomon FEC codes
are among the best suited for optical signals. They have a low overhead and
high coding gain, and they can correct bursts of errors. The RS (255, 239) code
is used widely. This code changes 239 data bits into 255 bits, thereby adding
about 7 percent overhead. This additional redundancy enables an otherwise
unacceptable BER to be sent over a channel by providing a 6- to 10-dB coding
gain. A 6-dB coding gain can double the WDM channel count or could increase
the transmission distance.

16.6. Modeling and Simulation Tools
With the increased complexity of optical links and networks, computer-based
simulation and modeling tools that integrate component, link, and network
functions can make the design process more efficient, less expensive, and
faster. As described in Chap. 1, such simulation programs are available com-
mercially. The tools typically are based on graphical programming languages
that include a library of icons containing the operational characteristics of
devices such as optical fibers, couplers, light sources, optical amplifiers, and
optical filters, plus the measurement characteristics of instruments such as
optical spectrum analyzers, power meters, and bit-error-rate testers. To check
the capacity of the network or the behavior of passive and active optical
devices, network designers invoke different optical power levels, transmission
distances, data rates, and possible performance impairments in the simulation
programs.
Associated with this book is an abbreviated version of the software-based
tool VPItransmissionMaker from VPIsystems, Inc. The full module is a
design and simulation tool for optical devices, components, subsystems, and
transmission systems. It enables the user to explore, design, simulate, verify,
and evaluate active and passive optical components, fiber amplifiers, dense
WDM transmission systems, and broadband access networks. Familiar meas-
urement instruments offer a wide range of settable options when displaying
data from multiple runs, optimizations, and multidimensional parameter
sweeps.
The abbreviated version of the VPItransmissionMaker simulation tool is
called VPIplayer and contains predefined component and link configurations
that allow interactive concept demonstrations. Results are shown in a format
similar to the displays presented by laboratory instruments. VPIplayer can be
downloaded free from the VPIphotonics web site at www.VPIphotonics.com. In
addition, at www.PhotonicsComm.com there are numerous interactive exam-
ples of optical communication components and links related to topics in this
book that the reader can download and simulate.

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