1.4 WDM OPTICAL NETWORKING EVOLUTION
1.4.1 WDM Point-to-Point Link
WDM point-to-point links, an example of which is shown in Fig. 1.7, are being deployed by several telecommunication companies due to the increasing demands on communication bandwidth. The capacity of the fiber link A B is now increased by a factor of 2, the number of wavelength channels used. These links are more cost-effective, when the demand exceeds the capacity in existing fibers, compared to installing new fiber. WDM mux/demux (multiplexer/demultiplexer) in point-to-point links with 64 channels is currently available .
Figure 1.7 WDM point-to-point link.
1.4.2 Wavelength Add/Drop Multiplexer
While WDM point-to-point links provide very large capacity between two widely spaced (300 to 600 km) (end) points (or terminals), in many networks it is necessary to drop some traffic at intermediate points along the route between the end points. Using a wavelength add/drop multiplexer (WADM), which can be "inserted" on a fiber link as shown in Fig. 1.8, one can add/drop necessary traffic (wavelengths) at the WADM location. A WADM can be realized using a demultiplexer, 2 × 2 switches (one switch per wavelength), and a multiplexer. If a 2 × 2 switch (S1 in the figure) is in "bar" state, then the signal on the corresponding wavelength passes through the WADM. If the switch (S0 in the figure) is in "cross" state, then the signal on the corresponding wavelength is "dropped" locally, and another signal can be "added" on to the same wavelength at this WADM location.
Figure 1.8 Wavelength add/drop multiplexer.
1.4.3 Wavelength Crossconnect
In order to build a flexible multipoint WDM optical network, apart from WADMs, we need another optical network element called a wavelength crossconnect. Functionally, WADM and wavelength cross-connects are quite similar, differing mainly in the number of input fibers that need to be handled . The function of each element is to provide, under network control, the ability to connect (switch) any input wavelength channel from an input fiber (port) to any one of the output fibers (ports) in optical form or to drop a channel. The wavelength crossconnect is also referred to as a wavelength selective crossconnect (WXC) or wavelength routing switch. Figure 1.9 shows a 2 × 2 wavelength crossconnect which can be realized by demultiplexers, optical switches, and multiplexers. Note that a WXC may also allow addition and dropping of wavelengths.
Figure 1.9 Wavelength crossconnect.