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14 Basic Measurement and Monitoring Techniques
status of all parts of their network. Basically the network health is assessed by means
of a continuous in-line BER measurement. The information obtained from this test
is used to assure that the quality-of-service (QoS) requirements are met. Another
standard network management function is fault monitoring, which checks to see
where and why a network failure has occurred or is about to take place.
Optical performance monitoring (OPM) adds to these standard network management concepts by checking the status of elements in the physical layer to examine
the temporal behavior of the basic performance factors that affect signal quality.
Depending on the desired network control complexity and the system cost constraints,
optical performance monitoring can range anywhere from simply checking the
optical power level of each WDM channel to a highly sophisticated system that
identifies the origins of a wide range of signal impairments and assesses their impact
on network performance.
Problems
14.1 Consider the NRZ and RZ waveforms shown in Fig. 14.3. If the peak power
in each waveform is 0.5 mW, show that the average powers are 0.25 and
0.125 mW for the NRZ and RZ patterns, respectively.
14.2 An engineer wants to find the attenuation at 1310 nm of an 1895-m long
fiber. The only available instrument is a photodetector, which gives an output
reading in volts. Using this device in a cutback-attenuation setup, the engineer
measures an output of 3.31 V from the photodiode at the far end of the fiber.
After cutting the fiber 2 m from the source, the output voltage from the
photodetector now reads 3.78 V. Show that the attenuation of the fiber is
0.31 dB/km.
14.3 A field engineer has a 2400-m long optical cable that has connectors on
both ends. Using an insertion loss technique and an optical power meter, the
emerging optical power at the output end of the fiber is measured as 0.150
mW. If the power launched into the fiber is 0.65 mW, show that the cable
attenuation (including connectors) is 6.37 dB.
14.4 Suppose an optical network element with attached flyleads is inserted into the
link in Fig. 14.6a. Assume the flyleads are terminated with optical connectors.
Consider the case when the power at the photodetector prior to inserting the
component is P 1 = 0.42 mW and the power level with the optical element in
the link is P 2 = 0.35 mW. Show that the insertion loss of the network element
is 0.79 dB.
14.5 The optical power in a fiber at a distance x from the input end is given by
Eq. (14.6). By assuming that the loss coefficient is uniform along the fiber,
use this equation to derive Eq. (14.2).
14.6 Assuming that Rayleigh scattering is approximately isotropic (uniform in all
directions), show that the fraction S of scattered light trapped in a multimode
fiber in the backward direction is given by
14 Basic Measurement and Monitoring Techniques
status of all parts of their network. Basically the network health is assessed by means
of a continuous in-line BER measurement. The information obtained from this test
is used to assure that the quality-of-service (QoS) requirements are met. Another
standard network management function is fault monitoring, which checks to see
where and why a network failure has occurred or is about to take place.
Optical performance monitoring (OPM) adds to these standard network management concepts by checking the status of elements in the physical layer to examine
the temporal behavior of the basic performance factors that affect signal quality.
Depending on the desired network control complexity and the system cost constraints,
optical performance monitoring can range anywhere from simply checking the
optical power level of each WDM channel to a highly sophisticated system that
identifies the origins of a wide range of signal impairments and assesses their impact
on network performance.
Problems
14.1 Consider the NRZ and RZ waveforms shown in Fig. 14.3. If the peak power
in each waveform is 0.5 mW, show that the average powers are 0.25 and
0.125 mW for the NRZ and RZ patterns, respectively.
14.2 An engineer wants to find the attenuation at 1310 nm of an 1895-m long
fiber. The only available instrument is a photodetector, which gives an output
reading in volts. Using this device in a cutback-attenuation setup, the engineer
measures an output of 3.31 V from the photodiode at the far end of the fiber.
After cutting the fiber 2 m from the source, the output voltage from the
photodetector now reads 3.78 V. Show that the attenuation of the fiber is
0.31 dB/km.
14.3 A field engineer has a 2400-m long optical cable that has connectors on
both ends. Using an insertion loss technique and an optical power meter, the
emerging optical power at the output end of the fiber is measured as 0.150
mW. If the power launched into the fiber is 0.65 mW, show that the cable
attenuation (including connectors) is 6.37 dB.
14.4 Suppose an optical network element with attached flyleads is inserted into the
link in Fig. 14.6a. Assume the flyleads are terminated with optical connectors.
Consider the case when the power at the photodetector prior to inserting the
component is P 1 = 0.42 mW and the power level with the optical element in
the link is P 2 = 0.35 mW. Show that the insertion loss of the network element
is 0.79 dB.
14.5 The optical power in a fiber at a distance x from the input end is given by
Eq. (14.6). By assuming that the loss coefficient is uniform along the fiber,
use this equation to derive Eq. (14.2).
14.6 Assuming that Rayleigh scattering is approximately isotropic (uniform in all
directions), show that the fraction S of scattered light trapped in a multimode
fiber in the backward direction is given by