Sonet (Synchronous Optical Network) is a high-speed, physical layer network designed to carry large volumes of traffic over relatively long distances on fiber optic cabling using lasers or light-emitting diodes (LEDs). It provides a standard interface for communications carriers to connect networks based on fiber optic cable to handle multiple data types (voice, video, and so on). Sonet defines a technology for carrying many signals of different capacities through a synchronous, flexible, optical hierarchy. This is accomplished by means of a byte-interleaved multiplexing scheme. Byte-interleaving simplifies multiplexing and offers end-toend network management. Sonet was proposed by Bellcore in the mid-’80s and now an ANSI standard. It is a successor to other wellknown communication technology implemented on fiber optics network called PDH (Plesiochronous Digital Hierarchy). It possesses several characteristics that make it good for the internet today: It defines clear interoperability standards between different vendors’ products. It can carry nearly any higher-level protocol (including IP), and includes built-in support for ease of management and maintenance. Sonet has emerged as a powerful protocol which is extensively used for large and high performance networks.