Optical Networks: A Practical Perspective - Part 42. 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. | 380 Client Layers of the Optical Layer quite common to use multiple overlaid rings particularly in backbone networks each operating over a different wavelength provided by an underlying optical layer. Two types of ring architectures are used unidirectional path-switched rings UPSRs and bidirectional line-switched rings BLSRs . The BLSRs can use either two fibers BLSR 2 or four fibers BLSR 4 . We will discuss these architectures and the protection mechanisms that they incorporate in detail in Chapter 10. In general UPSRs are used in the access part of the network to connect multiple nodes to a hub node residing in a central office and BLSRs are used in the interoffice part of the network to interconnect multiple central offices. Another major component in the SONET infrastructure is a digital crossconnect DCS . A DCS is used to manage all the transmission facilities in the central office. Before DCSs arrived the individual DSls and DS3s in a central office were manually patched together using a patch panel. Although this worked fine for a small number of traffic streams it is quite impossible to manage today s central offices which handle thousands of such streams using this approach. A DCS automates this process and replaces a patch panel by crossconnecting these individual streams under software control. It also does performance monitoring and has grown to incorporate multiplexing as well. DCSs started out handling only PDH streams but have evolved to handle SONET streams as well. Although the overall network topology including the DCSs is a mesh note that only rings have been standardized so far. A variety of DCSs are available today as shown in Figure . Typically these DCSs have hundreds to thousands of ports. The term grooming refers to the grouping together of traffic with similar destinations quality of service or traffic type. It includes multiplexing of lower-speed streams into high-speed streams as well as extracting lower-speed streams from different .
