SONET is an acronym for Synchronous Optical Network . SONET is widely used in telephone network and is one of the first large scale optical transmission systems Digital information is sent through optical fibers using a LED or a laser source. However, most of data processing, switching etc. are done electronically. SONET is primarily used in the North America while Europe and Japan use a modified version, called the Synchronous Digital Hierarchy (SDH). SONET arose out of a need to find a solution of the problem of inter-operability among various vendors and technologies that existed in sixties and seventies. Multiplexing :
In telecommunication, the phrase multiplexing is used to denote the process of combining two or more channels into a single channel. For instance, in encoding a video stream, we need to multiplex audio and video on single channel. One of the ways of doing it is known as time division multiplexing . The basic idea of the process is as follows.
Suppose we have three individual users in Mumbai who wish to send low stream data to three users in Delhi . Assigning physical lines for each will be expensive and such expense would grow exponentially with increasing number of users. If instead, we used a higher bit rate channel, we could use different time slots for the different data set. The figure below illustrates how this is achieved.
In sixties, the International Telecommunication Union (ITU) defined what is now known as a T-1 carrier. The analog voice data was digitized by sampling at a rate which is twice that of the maximum frequency component in the signal. This is known as Pulse Code Modulation (PCM). The T-1 standard was an universal agreement on a sampling rate of 8 kHz and a channel rate of 64 kilo-bit per second. The single voice channel is known as DS-0 signal (DS= Digital Signal). Thus, if we return back to the example of three users, with each transmitting at a rate of 64 kbps, we could transmit them as a sequential stream of data over a single channel capable of transmitting at 192 kbit/sec. .