Optical networking Here we explains about SONET -Synchronous Digital Network, that has been greeted with unparalleled enthusiasm throughout the world. It also explains how it came into existence and in which way it differs from others. What does synchronous mean?" Bits from one telephone call are always in the same location inside a digital transmission frame". This material is assumed to be comfortable to the reader as the basic concepts of a public telecommunications network, with its separate functions of transmission and switching, and is assumed to be aware of the context for the growth of broadband traffic. In the early 1970's digital transmission systems began to appear, utilizing a method known as Pulse Code Modulation (PCM), first proposed by STC in 1937. As demand for voice telephony increased, and levels of traffic in the network grew ever higher, it became clear that standard 2 Mbit/s signal was not sufficient. To cope with the traffic loads occurring in the trunk network. As the need arose, further levels multiplexing were added to the standard at much higher speed and thus SONET came into existence. For the first time in telecommunications history there will be a worldwide, uniform and seamless transmission standard for service delivery. SONET provides the capability to send data at multi-gigabit rate over today's single-mode fiber-optic links As end-users become ever more dependent on effective communications, there has been an explosion in the demand for sophisticated telecom services. Services such as videoconferencing remote database access, and multimedia file transfer require a flexible network with the availability of virtually unlimited bandwidth. The complexity of the network, means that network operators are unable to meet this demand. At present SONET is being implemented for long-haul traffic, but there is no reason it cannot be used for short distances