TAILIEUCHUNG - Optical Networks: A Practical Perspective - Part 41

Optical Networks: A Practical Perspective - Part 41. 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. | 370 Client Layers of the Optical Layer mapped into an STS-3c signal. Mappings have been defined in the standards for a variety of signals including IP ATM and FDDI fiber distributed data interface . While SDH employs the same philosophy as SONET there are some differences in terminology and in the multiplexing structure for sub-STM-1 signals. Analogous to SONET virtual tributaries SDH uses virtual containers VCs to accommodate lower-speed non-SDH signals. VCs have been defined in five sizes VC-11 VC-12 VC-2 VC-3 and VC-4. These VCs are designed to carry Mb s DS1 2 Mb s El 6 Mb s E2 45 Mb s E3 and DS3 and 140 Mb s E4 asynchronous plesiochronous streams respectively. However a two-stage hierarchy is defined here where VC-1 Is VC-12s and VC-2s can be multiplexed into VC-3s or VC-4s and VC-3s and VC-4s are then multiplexed into an STM-1 signal. SONET SDH Layers The SONET layer consists of four sublayers the path line section and physical layers. Figure shows the top three layers. Each layer except for the physical layer has a set of associated overhead bytes that are used for several purposes. These overhead bytes are added whenever the layer is introduced and removed whenever the layer is terminated in a network element. The functions of these layers will become clearer when we discuss the frame structure and overheads associated with each layer in the next section. The path layer in SONET and SDH is responsible for end-to-end connections between nodes and is terminated only at the ends of a SONET connection. It is possible that intermediate nodes may do performance monitoring of the path layer signals but the path overhead itself is inserted at the source node of the connection and terminated at the destination node. Each connection traverses a set of links and intermediate nodes in the network. The line layer multiplex section layer in SDH multiplexes a number of path-layer connections onto a single link between two nodes. Thus the line layer is .

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