Optical Networks: A Practical Perspective - Part 72. 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. | 680 Deployment Considerations Figure The future telecommunications network a Network topology showing a meshed long-haul backbone with metro collector rings b Architecture of a typical backbone node showing an OXC OLT IP router SONET add drop multiplexer and an MSP. c A node on a metro ring served by an MSP. The MSP is used to deliver a variety of services including voice private lines and data services. Designing the Transmission Layer 681 Like SONET rings most MSPs are deployed in ring configurations and include built-in restoration capabilities which are based on SONET mechanisms for the most part. Ring configurations work well for metro networks as the fiber is mostly laid in rings. Laying fiber in ring configurations is economical compared to using other configurations such as a star also called a hub and spoke configuration. A star configuration requires two disjoint fiber routes to be laid between each access node and the central office. In contrast multiple access nodes can be combined on a single fiber ring and additional nodes can be added to the ring as needed without having to lay new fiber routes each time a new node needs to be added. Some MSPs also include built-in WDM interfaces with optical add drop OADM capabilities. Passive optical networks PONs are also emerging as potential candidates to deliver services to small and medium users of bandwidth. In order to prove in they need to be cheaper and more flexible than SONET SDH platforms or MSPs. We studied PONs in Chapter 11. However as of this writing WDM is just beginning to be used in metro networks as its economics are not as compelling as in long-haul networks. More on this in Section . Designing the Transmission Layer We will next look at the choices that service providers have to make in choosing the right tranmission layer. The historical trend has been to increase capacity in the network and at the same time drive down the cost per bit of bandwidth. Service providers .
