Page 297 - Optical Communications Essentials

Optical Networks

Optical Networks 287

Optical layer

Wavelength
channels

Physical layer

Optical
fiber links

SONET/SDH nodes
Figure 17.5. The optical layer is a wavelength-based
concept and lies just above the physical layer.

can be routed from end to end across a packet-switched network. Here the IP
network views the SONET network merely as a set of physical point-to-point
links between IP routers. However, as described in Sec. 17.2, the internal
switching and routing operations of the SONET itself encompass physical, data
link, and network layer functions.

17.1.4. Optical layer
When dealing with optical network concepts, one hears the words optical layer
used to describe various network functions or services. The optical layer is a
wavelength-based concept, and it lies just above the physical layer, as shown in
Fig. 17.5. This means that whereas the physical layer provides a physical con-
nection between two nodes, the optical layer provides lightpath services over that
link. A lightpath is an end-to-end optical connection that may go through one or
more intermediate nodes. For example, in an eight-channel WDM link there are
eight lightpaths, which may go over a single physical line. Note that for a spe-
cific lightpath the wavelengths between various node pairs may be different.
As shown in Fig. 17.6, the optical layer may carry out processes such as wave-
length multiplexing, adding and dropping of wavelengths, and support of optical
cross-connects or wavelength switching. Networks which have these optical
layer functions are referred to as wavelength-routed networks.

17.2. SONET/SDH
With the advent of fiber optic transmission lines, the next step in the evolution
of the digital time division multiplexing (TDM) scheme was a standard signal

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