1. INTRODUCTION
Today, there is a general consensus that, in the near future, wide area networks
(WAN)(such as, a nation wide backbone network) will be based on Wavelength
Division Multiplexed (WDM) optical networks. One of the main advantages of a WDM
WAN over other optical technologies, such as, Time Division Multiplexed (TDM)
optical networks, is that it allows us to exploit the enormous bandwidth of an optical
fiber (up to 50 terabits bits per second) with requiring electronic devices, which operate
at extremely high speeds.
The concept of light tree is introduced in a wavelength routed optical network,
which employs wavelength -division multiplexing (WDM).
Depending on the underlying physical topology networks can be classified into three
generations:
 First Generation: these networks do not employ fiber optic technology; instead
they employ copper-based or microwave technology. E.g. Ethernet.
 Second Generation: these networks use optical fibers for data transmission but
switching is performed in electronic domain. E.g. FDDI.
 Third Generation: in these networks both data transmission and switching is
performed in optical domain. E.g. WDM.
WDM wide area networks employ tunable lasers and filters at access nodes and
optical/electronic switches at routing nodes. An access node may transmit signals on
different wavelengths, which are coupled into the fiber using wavelength multiplexers.
An optical signal passing through an optical wavelength-routing switch (WRS) may be
routed from an output fiber without undergoing opto-electronic conversion.


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