Page 243 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
14

Performance Measures

A major challenge in the operation of WDM networks is how to verify that the
system is functioning properly. Thus there is a crucial need to monitor each
wavelength intelligently in order to meet network reliability requirements and
to guarantee a specific quality of service (QoS) to the end customer, as spelled out
in a service-level agreement (SLA). The key performance parameters to monitor
are wavelength, optical power, and optical signal-to-noise ratio (OSNR).
This chapter examines the parameters needed to evaluate the performance of
a network. Section 14.1 discusses the concept of bit error rate (BER) for meas-
uring the performance of digital systems, which encompass the predominant
application of fiber optic links. Since the BER depends on the OSNR, Sec. 14.2
addresses that topic. Next, Sec. 14.3 discusses the performance of analog links,
which is given in terms of a carrier-to-noise ratio. How to measure these param-
eters is the theme of Sec. 14.4. Chapter 18 gives more details on performance
monitoring and control of optical communication links.

14.1. Digital Link Performance

In the operation of a digital optical fiber link, first a light source launches a cer-
tain amount of optical power into a fiber. As the optical signal travels along the
link, it becomes attenuated due to loss mechanisms in the fiber, at connectors,
and in other components. There also may be signal degradation mechanisms
due to factors such as chromatic dispersion, polarization mode dispersion, non-
linear effects in the fiber, and various electrical and optical noises. As described
in Chap. 7, there is a lower limit as to how weak an optical signal a receiver can
detect in the presence of noise and interference. The optical power level at the
end of a link defines the signal-to-noise ratio at the receiver, which commonly is
used to measure the performance of both analog and digital communication sys-
tems. Note that the signal-to-noise ratio is designated as either S/N or SNR.
Here we will use the abbreviation SNR.

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