Page 229 - Optical Communications Essentials

Constructing the WDM Network Puzzle

Constructing the WDM Network Puzzle 219

■ Each wavelength must emerge from an optical amplifier with the same power
level, to prevent an increasing skew in power levels from one wavelength to
another as the signals pass through successive amplifiers.
■ Strict device temperature and light frequency controls are required of laser
transmitters to prevent crosstalk between channels.
■ High-rate transmission over long distances requires fast modulators and
receivers, forward-error-correction (FEC) schemes, and optical signal-
conditioning techniques such as chromatic dispersion and polarization-mode
dispersion compensations.
Although these performance requirements result in expensive components,
the cost is distributed over many information channels.

13.1.2. Narrowband metro WDM network
Metro topologies can be viewed as consisting of core networks and access net-
works, as illustrated in Fig. 13.2. Nominally a metro core network consists of
point-to-point connections between central offices of carriers that are spaced 10
to 20km apart. These connections typically are configured as SONET/SDH
rings. The core ring usually contains from six to eight nodes and nominally is
from 80 to 150km in circumference. The metro access network consists of links
between end users and a central office. The ring configurations in this case
range from 10 to 40km in circumference and typically contain three or four
nodes. Optical add/drop multiplexers provide the capability to add or drop mul-
tiple wavelengths to other locations or networks. A router or other switching
equipment allows interconnections to a long-haul network.
In contrast to the stringent performance specifications imposed on wideband
long-haul WDM systems, the shorter transmission spans in metro and LAN
applications relax some of the requirements. In particular, if coarse WDM is

Figure 13.2. Metro topologies can be viewed as consisting of
core networks and access networks.

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