Page 326 - Optical Communications Essentials

Network Management

316 Chapter Eighteen

a variety of hardware technologies from different vendors. In addition, the BER
information can be integrated with other operations support system (OSS) func-
tions such as order processing, billing, and service provisioning.
Note that since the BER measurement is done after the received signal is
transformed back to the electrical domain, this metric only gives an indication
of the overall performance of a link. It does not tell whether a change in link
quality was caused by optical power reduction, degradation in the optical signal-
to-noise ratio, or component aging or failure. To measure these effects, one
needs to invoke the element management procedures described in Sec. 18.5.3.

18.5.2. Wavelength management
To deploy new services and applications rapidly to their customers, service
providers must possess a complete end-to-end management capability of the optic-
al access link. For example, the SLA can specify that certain wavelengths will be
available during specific time periods or only on weekends when normal traffic
flow is low. Thus, network managers need the ability to reassign wavelengths or
portions of the capacity of a wavelength quickly, thereby temporarily or perman-
ently increasing bandwidth allocation according to customer requirements.
Wavelength management is especially important in metro networks where
there typically is a high degree of dynamic change in customer bandwidth
requirements. By using dynamic lightpath reconfiguration rather than depending
on a fixed physical infrastructure, network management personnel can provision
for these needs by means of a point-and-click feature on their management con-
sole. Such capabilities are part of large network management programs that are
available commercially. Among the features in these programs are capabilities
such as path tracing for individual wavelengths, power tracing and remote power
adjustment for individual wavelengths, and optical-layer topology discovery that
permits fault isolation to specific network sections.

18.5.3. Element monitoring
Since the signal quality of an optical network depends critically on the proper
operation of all its constituent elements, monitoring techniques that can be per-
formed directly in the optical domain are a key requirement. The three main
parameters for any element are wavelength, optical power, and OSNR (optical
signal-to-noise ratio). The measurement instruments are based on spectrum
analysis techniques (see Chap. 19) and are known by a variety of names. For
example, one may see the names optical channel monitor (OCM), optical per-
formance monitor (OPM), or optical channel analyzer (OCA). The original
intent was to designate slightly different monitoring functions, but the differ-
ences among these functions are becoming blurred. Therefore for simplicity we
will refer to them as optical performance monitors.
An OPM taps off a small portion of the light signals in a fiber and separates
the wavelengths or scans them onto a detector or detector array. This enables
the measurement of individual channel powers, wavelength, and OSNR. These

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