Page 254 - Optical Communications Essentials

Performance Measures

244 Chapter Fourteen

purposes. Since the tapped-off signal will not be added back to the main data
stream, there are no effects on the properties of the transmitted data and thus
OCPM provides a noninvasive measurement process. Chapter 18 presents fur-
ther details of how OCPM devices are used in an optical network.

14.5. Summary
WDM network operators need to monitor each wavelength intelligently to ver-
ify that the system is functioning properly. This is crucial in order to meet net-
work 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.
The most common figure of merit for digital links is the bit error rate or BER.
This is the number of bit errors N E occurring over a specific time interval,
divided by the total number of bits N T sent during that interval. BER is
expressed by a number, such as 10 9 , which states that on the average one error
occurs for every billion pulses sent. Typical error rates specified for optical fiber
telecommunication systems range from 10 9 to 10 15 . The eye diagram tech-
nique is a standard measurement method for assessing the data-handling abil-
ity of a digital transmission system. The eye pattern measurements allow the
effects of waveform distortion to be shown immediately on the display screen of
standard BER test equipment.
An important point about the BER is that it is determined principally by the
optical signal-to-noise ratio. Therefore it is the OSNR that is measured when a
WDM link is installed and when it is in operation. The OSNR does not depend
on factors such as the data format, pulse shape, or optical filter bandwidth, but
only on the average optical signal power and the average optical noise power.
In certain applications it is desirable to transmit analog signals directly over
the fiber without first converting them to a digital form. These applications
include cable television (CATV) distribution and microwave links such as con-
nections between remotely located antennas and base stations. Traditionally, in
an analog system a carrier-to-noise ratio analysis is used, since the information
signal normally is superimposed on a radio-frequency (RF) carrier through an
optical intensity modulation scheme. The main noise sources are due to thermal
noise, shot noise in the photodetector, and relative intensity noise from the
laser.

Further Reading

1. The Telecommunications Industry Association: TIA/EIA-526-19 Standard, OFSTP-19, Optical
Signal-to-Noise Ratio Measurement Procedures for Dense Wavelength-Division Multiplexed
Systems, October 2000.
2. G. Keiser, Optical Fiber Communications, 3d ed., McGraw-Hill, Burr Ridge, Ill., 2000.
3. B. A. Forouzan, Introduction to Data Communications and Networking, 2d ed., McGraw-Hill,
Burr Ridge, Ill., 2001.

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