SONET SDH

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.