Chapter 7 - Other New Optical Networks

7.1 THE OPTICAL TRANSPORT NETWORK

The Optical Transport Network (OTN) is relatively a new ITU-T recommendation
(G.709, G.872 and G.959) that was developed for long-haul transport at data rates
from 2.5 to 40 Gbit/s.

OTN, like the NG-S, defines a synchronous payload within a fixed frame length,
a comprehensive overhead to support a variety of client payloads, and FEC to provide
margin for fixed fiber span or increased fiber span at BER objectives that can
be met.

The key application of OTN is long-haul transport of massive data at high bit
rates and with as few as possible regenerators. Therefore, it includes forward error
correction, a practice that has been used successfully in transoceanic fiber-optic
communications.

In OTN multiplexed frames such as SONET/SDH, when M STM-16/STS-48
signals are to be interleaved to achieve M × 2.5 Gbit/s transmission data rates, the
FEC encoding function is performed in the terminal transmission equipment (TTE)
at each STM-16 signal and before the interleaver. At the receiving TTE, de-interleaving
is performed to separate the STM-16 signals. Thus, in typical systems, FEC
is performed in the electronic regime. Clearly, when FEC is performed in regenerators,
the latter are optical–electrical–optical (OEO), a function that adds to the cost
of the system.

If the number of errors exceeds the capability of the FEC, then uncorrected errors
tend to form bursts of errors. Thus, as these errors propagate to the next receiver,
the next FEC may add to the number of burst errors. Since the FEC is at the
physical layer, it may not be possible to report uncorrected errors to the next data
link layer.

The number of errored bits in an ensemble of transmitted bits over a medium
(any medium) has a probabilistic expected value. Probabilistically, the amount of
errors increases as the traveled distance increases. However, in many optical com-
munications links, the signal has to travel many kilometers without regeneration
and arrive at the receiver with a predetermined (very low) error content, measured
in bit error rate (BER), and, depending on application, BER may be orders of mag-
nitude lower than 10^-12. Thus, when the probability for errored bits is higher than
expected, error detection and correction (EDC) codes are used to reduce the number
of errored bits and meet the expected BER. A consequence of the application of
EDCs is that the fiber distance without signal regeneration may be increased, which
clearly lowers the capital expense and the maintenance cost.

< Previous Excerpt Next Excerpt >

Purchase This Book

TABLE OF CONTENTS

Chapter 7 - Other New Optical Networks

Contact Preferences

This is embarrasing...

Customize Your GlobalSpec Experience

Select Your Free Newsletters

Industry Newsletters

Select Your Free Product Alerts

This is embarrasing...