Table of Contents

  • Telecommunication networks are continuously evolving towards providing seamless connectivity, in response to the apparently insatiable demands of people around the world to communicate in many different ways. Evolutionary technologies – especially optical technologies – are providing an excellent foundation upon which present-day and future networks can become robust, scalable and increasingly "intelligent". These advances have been enabled by the continuous hard work within technical standardization organizations, and ITU's Telecommunication Standardization Sector (ITU-T) is at the forefront of such efforts.

  • The ITU-T, beginning from the mid-1970s, has standardized several transport networks following the evolution of telecommunication services and of transport technologies. The content of this manual is based mainly on the content of the Recommendations issued by the ITU-T on optical transport networks.

  • The primary purpose of a transport network is to transfer user information from a sender at one location to a receiver at another location. The transport network must also transfer various kinds of network control information such as signalling, and operations and maintenance information that is required to operate the transport network.

  • 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).

  • The acronym OAM is frequently used in the telecommunication industry. It is used to identify an important and integral part of telecommunication technologies. The quality of service (QoS) depends for a large part on the OAM of the used technology.

  • Protection for survivability is a term that refers to the ability of the network to maintain an acceptable level of service during a network or equipment failure or traffic signal degradation. There are several survivability mechanisms covering a wide range of network architectures, technologies and allocation of network resources.

  • In today's telecommunication networks, the clocks in transmission and switching equipment are often required to operate at equal or almost equal frequencies in order to transport signals between them that carry digital information, and to do so without introducing single-bit errors or bursts of errors. Synchronous operation of equipment that is spread out over a large geographic area requires a distribution network for synchronization information.

  • The ITU-T has specified a methodology to describe equipment for transport networks which is based on a set of generic basic functional blocks referred to as atomic functions, a bigger combination of these basic functions referred to as compound functions, and a set of rules for combining these functions. Using this methodology, equipment are described by an equipment functional specification (EFS) which lists the atomic functions and their interconnection.

  • Up through the late 1990s, transport networks and equipment were wholly configured and provisioned via centralized management systems, with commissioning and installation of new equipment being a tedious error-prone process. Each new generation of technology and equipment had offered additional capabilities to facilitate evolution from full manual operation towards increased automation. However, while a transport network could be re-arranged for network maintenance or improved utilization, it was considered as a relatively static entity (relative to a timescale of less than months) that offered leased line services.