Optical Networks: A Practical Perspective - Part 71. 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. | 670 Deployment Considerations M Figure A typical carrier backbone network based on SONET SDH showing SONET SDH add drop multiplexers ADMs and digital crossconnects DCSs along with optical line terminals OLTs and routers a The network topology which consists of interconnected rings in the backbone with feeder metro rings b Architecture of a typical node including OLTs stacked up SONET ADMs and DCSs. The Evolving Telecommunications Network 671 Figure Bandwidth wasted when two rings built using ADMs share the same fiber route. Half the bandwidth on each ring along the shared route is reserved for protection. This network was designed primarily to carry voice and private line traffic. The network provides guaranteed latency and bandwidth and well-established protection schemes ensure high network availability. SONET SDH also provides extensive performance monitoring and fault management capabilities. The network is mostly static with switching provided by the DCSs in order to provision connnections. The switching is done at the time a connection is set up. Once set up connections remain for months or years but may have to be switched in the interim to deal with network failures or for maintenance purposes. However as we see the increasing dominance of data traffic and the emergence of new optical layer equipment several deficiencies of the SONET SDH-based network architecture become evident It consists primarily of static rings where capacity is provisioned in a static manner. It does not allow the rapid provisioning of services end to end across the network on time scales of tens of milliseconds for fast protection switching to seconds for rapid provisioning . The traffic demands themselves are more meshed and the ring architecture is not the most efficient at supporting an inherently meshed traffic demand for several reasons. Multiple rings need to be interconnected and the interconnection is fairly complex and done through digital crossconnects. Half
