Synchronous optical network (SONET) is a standard for optical telecommunications transport that was created in 1984. SONET enable internetworking among the existing proprietary systems possible. SONET is an American National Standards Institute (ANSI) standard and its counterpart Synchronous Digital Hierarchy (SDH) an International Telecommunications Union-Telecommunications Standardization Sector (ITU-T) standard. SONET/SDH standard is expected to provide the transport infrastructure for worldwide telecommunications for at least the next few decades.
The configuration flexibility and bandwidth availability offers significant advantages over older telecommunications system. These advantages include the following:.
• Reduction in equipment requirements and increase network reliability.
• The overhead bytes permit management of the payload bytes on an individual basis and facilitate centralized fault sectionalization.
• Physical specifications and frame design include mechanisms that allow it to carry signals from incompatible tributary systems (asynchronous services) for a universal connectivity.
• Networkwide synchronization adds predictability that coupled with a powerful frame design, enables individual channels to be multiplexed, thereby improving speed and reducing cost.
• A multiplexed transport mechanism and as such can be carrier for broadband services (ATM, B-ISDN).
Basic SONET Signal.
SONET defines a hierarchy of signaling levels called synchronous transport signals (STSs). Each STS level (STS-1 to STS-192) supports a certain data rate in megabits per second. Each STS signal has an optical counterpart, designated optical carrier level (OC-1 to OC-192).
SDH defines a similar system called synchronous transport module (STM). STM is compatible with existing European hierarchies. The lowest STM level, STM-1, is at 155.