Page 208 - Introduction to Information Optics

3.4. Fiber-Optic Networks 193

Metropolitan Area Networks (MANs). MAN is up to approximately 100
kilometers, such as telephone local exchange environments or cable
television distribution systems.
Wide Area Networks (WANs). WAN can be thousands of kilometers.

3.4.2. PHYSICAL TOPOLOGIES AND ROUTING TOPOLOGIES
RELEVANT TO FIBER-OPTIC NETWORKS

We need to know the requirements of physical topologies, which include:
1. Scalability. The term scalability means the ability to expand the network
to accommodate many more nodes than the number in the initial
installation.
2. Modularity. The term modularity means the ability to add just one more
node.
3. Irregularity. The term irregularity means that the topology should not be
forced artificially into some unusual, highly stylized pattern that may not
meet the user's requirement.
Physical topologies take many forms. The most widely used topologies
include start, ring, and bus as shown in Fig. 3.16. Optical or electronic switches
can be added to optical networks to realize the routing operation.

3.4.3. WAVELENGTH DIVISION MULTIPLEXED
OPTICS NETWORKS

14
To fully employ extremely high carrier frequency (i.e., 10 Hz) of light,
multiplexing techniques are used. The most widely used multiplexing technique
is called wavelength division multiplexing (WDM), which is based on the
unique property of light, as described in the following.
Optical beams with different wavelengths propagate without interfering with
one another, so several channels of information (each having a different carrier
wavelength) can be transmitted simultaneously over a single fiber. This scheme,
called wavelength division multiplexing (WDM), increases the information-
carrying capacity by a number of w, where w is the number of wavelengths used
in the optics networks. The critical components needed for WDM optics
networks include the optical multiplexer and the optical demultiplexer. An
optic multiplexer couples wavelengths with different light widths from individ-
ual sources to the transmitting fiber, as illustrated in Fig. 3.17. An optical
demultiplexer separates the different carriers before photodetection of the
individual signals, as shown in Fig. 3.18. The important parameters for optic
multiplexers and demultiplexers are insertion loss and cross talk. The insertion