deploying and managing ip over wdm networks - Index of

Management of the WDM Network Layer 67
In the following section, performance-management methods are identified
that are relevant for all-optical WDM networks.
4.4.3.1 Spectral Analysis
Spectral analysis is the main method used for performance management,
because it only monitors the parameters of the optical carrier alone, without
concern for the modulated signal itself. Hence, it is immediately compatible
with all of the requirements for performance management in all-optical networks.
The method consists of decoupling a sample of the optical carrier from
the transmission fiber and feeding it to an optical spectral analyzer. Here, the
results of the analysis are compared against a predefined set of results defined
for correct operation. The supervision is carried out at the various network elements.
For example, at a switching node every input and output fiber is monitored
by the network analyzer. The analyzer uses a selector mechanism to get a
sample of the optical signal from every port one at a time. This method only
allows the measurement of power-related figures such as power per carrier, carrier
wavelength, and optical signal-to-noise ratio. While the first two characteristics
can be measured quite accurately using this method, the optical signalto-noise
ratio may only be an estimate of the real value due to several different
factors, such as the presence of four wave mixing or cross phase modulation or
even the presence of demultiplexers or other optical filters [14].
The performance of the network depends directly on a number of the
optical carrier parameters. If one (or more) of these parameters is found to be
below (or different) from given thresholds, the performance of the network may
be severely affected (depending on the extent of the degradation), hampering its
service quality as seen by the users. The main optical carrier parameters that
must be monitored in a WDM network are:
• The number of carriers, indicating an unbroken path between the carrier
generation and measurement point—this could indicate that more
than one carrier is used at the same frequency;
• Total optical power, defined as the sum of all carrier power plus noise;
• Carrier identification, which helps identify the carrier by manipulating
some of the carrier parameters at certain network elements;
• Optical power per carrier, measurements indicate whether the optical
power is within the predefined range;
• Carrier-to-noise ratio, per carrier, indicating that the noise accumulated
along the transmission path does not exceed the value beyond which
the transmission is no longer error free;
• Center wavelength drift, defined as the difference between the nominal
carrier wavelength and its actual value;