Propagation of the optical signal across fibers and optical nodes
- the so called transparency of optical networks - provides the
network designers with a number of alternative network architectures
to choose from, e.g., wavelength routing, broadcast-and-select,
and photonic slot routing networks. The common objective of these
architectures is to eliminate, or significantly reduce, the relatively
slow and cumbersome electronic processing of the transmitted
signal at the intermediate nodes.
Among these architectures, wavelength routing networks make use of
Wavelength Division Multiplexing (WDM) to create multiple
coarse-bandwidth channels (i.e., wavelengths) in the fiber.
To efficiently exploit each wavelength bandwidth, traffic grooming
is thus required. It is important to observe that with current technology
traffic grooming is possible only using electronics. Three classes of
traffic grooming solutions are briefly summarized.
A. In conventional First Generation (FG) optical networks, i.e.,
SONET/SDH, traffic grooming is performed at each intermediate
node, thus potentially achieving bandwidth-efficient
solutions at the cost of a large number of Optical Terminals (OT).
B. In Single-Hop (SH) optical networks, less OTs are used as they
are required only at the end nodes of the optical circuit or lightpath.
Once transmitted, the optical signal propagates along the lightpath
without requiring O/E and E/O conversion, until it is received at
the destination node. Grooming is limited among the tributary
signals that share the same source-destination pair.
C. A generalization of both FG and SH network architectures is the
Multi-hop and Multi-rate (M&M) architecture in which the tributary
signal is transmitted from source to destination through multiple
lightpaths, or optical hops, and the transmission rate of each hop
may differ from the others'. Multi-hop transmission yields reduced
number of OTs (and associated electronics) when compared to FG
architecture and reduced number of wavelengths when compared to
SH architecture. In addition, the multi-rate feature provides the
network designer with the flexibility to select the most cost
effective OT on a per-lightpath basis, as opposed to single-rate
solutions in which all lightpaths must be transmitted at the same rate.
It has been shown that in WDM ring, the M&M architecture has the
potential to yield significant cost reductions when compared to
FG and SH rings.
Such cost reductions may be however affected by transmission
impairments induced by available fibers and optical components,
that may significantly restrain the signal transparency and must be
taken into account during the network design. Examples of such
impairments are Group Velocity Dispersion (GVD), Self-Phase Modulation (SPM),
and Polarization Mode Dispersion (PMD). In presence of the above
undesirable effects the quality of the optical signal may degrade
significantly and, practically speaking, the maximum span of
a lightpath may be constrained. In other words, the transparency
degree of the network may be limited if no countermeasures are
taken to compensate for such signal degradation.
The goal of this project is to assess the impact of a number of
transmission impairments on the overall design and cost of FG,
SH, and M&M networks.