To satisfy the requirements of ever increasing data rate for diverse applications, ANS developed a standard known as synchronous optical network by utilizing the enormous bandwidth of optical fibber, another very similar standard developed by ITU T is known as synchronous digital hierarchy. It is a synchronous TDM systems controlled by master clock which adds predictability. The comprehensive SONET/ synchronous digital hierarchy standard is expected to provide the transport infrastructure for worldwide telecommunication of or at least the next two or three decades.
The increased configuration flexibility and bandwidth availability of SONEL provides significant advantages over the older telecommunication systems. These advantages include the followings .
a.Reduction in equipment requirement san an increase in network reliability.
b.Provision of overhead and payload bytes the overhead bytes management of the payload bytes on an individual basis and facilitate centralized fault sectionalisation.
c.Definition of a synchronous multiplexing format for carrying lower level digital signals and a synchronous structure that greatly simplifies the interface to digital switches digital cross connect switches and add drop multiplexers.
d.Availability of a flexible architecture capable of accommodating future applications with a variety of transmission rates.
In brief SONET defines optical carrier levels and electrically equivalent synchronous transport signals for the fibber optic based transmission hierarchy.
To a set of synchronous signals the digital transitions in the signals occurs at exactly at the same rate.
If two digital signals are plesiochronous their transitions occur at almost the same rate with any variation being constrained with in tight limits.
In t eh case of asynchronous singles the transitions of the signals don't necessarily occurs a the same nominal rate.