Optical Networks: A Practical Perspective - Part 45. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 410 WDM Network Elements OLT includes multiplexers demultiplexers and transponders. These transponders constitute a significant portion of the system cost. Consider what is needed at node B. Node B has two OLTs. Each OLT terminates four wavelengths and therefore requires four transponders. However only one out of those four wavelengths is destined for node B. The remaining transponders are used to support the passthrough traffic between A and C. These transponders are hooked back to back to provide this function. Therefore six out of the eight transponders at node B are used to handle passthrough traffic a very expensive proposition. Consider the OADM solution shown in Figure b . Instead of deploying point-to-point WDM systems we now deploy a wavelength-routing network. The network uses an OLT at nodes A and C and an OADM at node B. The OADM drops one of the four wavelengths which is then terminated in transponders. The remaining three wavelengths are passed through in the optical domain using relatively simple filtering techniques without being terminated in transponders. The net effect is that only two transponders are needed at node B instead of the eight transponders required for the solution shown in Figure a . This represents a significant cost reduction. We will explore this subject of cost savings in detail in Section . In typical carrier networks the fraction of traffic that is to be passed through a node without requiring termination can be quite large at many of the network nodes. Thus OADMs perform a crucial function of passing through this traffic in a cost-effective manner. Going back to our example the reader may ask why transponders are needed in the solution of Figure a to handle the passthrough traffic. In other words why can t we simply eliminate the transponders and connect the WDM multiplexers and demultiplexers between the two OLTs at node B directly as shown in Figure b rather than designing a separate OADM Indeed this is .
