The first generation of equipment for optical fiber transmission was proprietary and no standards were available for the interconnection of equipment from different vendors.

The Concept of SONET Multiplexing

To meet the urgent need for standards to interconnect optical transmission system, the Synchronous Optical Network (SONET) standards were developed in North America. The CCIT later developed a corresponding set of standards called Synchronous Digital Hierarchy(SDH) which was used in Europe and many other countries.

SONET/SDH is a Synchronous network. A single clock is used to handle the timing of transmissions and equipment across the entire network. SONET, a multiplexed transport mechanism as can be used as the carrier for broadband services, particularly ATM and B-ISDN. 

Synchronous Transport Signal

SONET defines a hierarchy of signaling levels called synchronous transport signal (STS). Each STS level (STS-1 to STS-192) supports a certain data rate, specified in megabits per second.

The physical links defined to carry each level of STS are called optical carriers (OCs).

Sonet Mulitplexing

Lower rate STSs can be multiplexed to make them compatible with higher rate systems.

For example, three STS-1s can be combined into one STS-3s can be multiplexed into one STS-12, and so on. The general format STS-n is made up of lower rate STSs. However, that in actual practice lower rate STSs is interleaved.