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222 Chapter 9

Transparent Optical Networks

For more than 10 years, optical systems have been tested and built
based on certain theories and test beds. Transparency is a desired
feature in developing optical networks because the signal is carried
through the entire network transparently (optically) without having
to perform conversions.

Opaque Optical Networks

In the past, conversion was needed at the amplifiers.The change was
from optical to electrical and then back to optical again (OEO).
Because this conversion was necessary, the following was present:

A risk of degradation of the signal
Added equipment required to perform the conversions
Latency built into the overall performance of the network

This conversion adds to the overall cost of the network, increases
the maintenance of equipment, and increases the risk and complex-
ity. The alternative to this was to use opaque networks.
Several advantages and disadvantages exist for both transparent
and opaque networking strategies. One also can imagine that in
between the two end points, several hybrid or in-between models
exist. The carriers and manufacturers are involved in providing the
best cost advantages as soon as possible so that network build-out
can be accommodated.Yet the risk of having to bring in a forklift and
remove older equipment is one that is minimized wherever possible.
It is through this combined approach of the ends that the networks
have been deployed.The advantage of using transparent networks is
shown in Figure 9-5 in that it is less expensive to operate. Transpar-
ent networks also operate independently of the data rate passing
through them. Introducing a new data rate requires little changes.
This provides for the overall “future proofing” of the network so that
mass equipment changes are not required.
Of course, the disadvantages also must be viewed as the other side
of the coin, as shown in Figure 9-6. The following are several of the
disadvantages of transparent networks: