Optical Transport Networks from TDM to Packet

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This Handbook discusses the technologies covered by ITU-T Recommendations related to Synchronous Digital Hierarchy (SDH), Optical Transport Network (OTN) and Ethernet over Transport (EoT). Each chapter of this Handbook is dedicated to a specific aspect (architecture, multiplexing hierarchy, protection, synchronization, etc.) of SDH, OTN and EoT optical transport networks. Plesiochronous Digital Hierarchy networks, which are at present considered obsolete, and Multiprotocol Label Switching - Transport Profile, which is at a preliminary stage of standardization, are only mentioned in the Preface. Finally, the last chapter of the Handbook discusses automatically switched optical networks (ASON), based on the control planes introduced in addition to transport planes and management planes.



Frame structures, mapping and multiplexing

Today's optical transport technologies are focused on efficient and reliable transport of information that is digitized for transport. Whether it is voice, video, or raw data of some other sort, the digital content of those signals must be carried in a way that can be transported, monitored, and protected. To avoid requiring different transport equipment for every different type of signal that might be carried, all types of digital information are adapted to fit into standardized containers and carried over the same equipment. In this way, only the adaptation function is unique to each payload or client signal type. These transport containers, called frames, are made up of the digital payload and additional bytes of data called overhead. The overhead is used to identify the payload, to carry control information used for encapsulating and extracting the data, for performance monitoring of the frame, and for carrying control/management messages. The frames are structured in such a way that the transmitting and receiving equipment can identify the start and end of each frame and extract the overhead and payload reliably. The frames are defined in terms of rows and columns of bytes, where each frame is transmitted and received one byte at a time, one row at a time, until all rows in the frame are sent and received, with the next frame following immediately after. This chapter describes the most commonly used optical transport protocols defined in ITU-T Recommendations, namely synchronous digital hierarchy (SDH), optical transport network (OTN) and Ethernet over transport (EoT).


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