The
Synchronous optical network, commonly known as SONET, is a standard for communicating
digital information using lasers or light emitting diodes (LEDs) over optical
fiber as defined by GR-253-CORE from Telcordia. It was developed to replace the
PDH system for transporting large amounts of telephone and data traffic and to
allow for interoperability between equipment from different vendors.The more recent
Synchronous Digital Hierarchy (SDH) standard developed by ITU (G.707 and its extension
G.708) is built on experience in the development of SONET. Both SDH and SONET
are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world.
SDH is growing in popularity and is currently the main concern with SONET now
being considered as the variation.SONET differs from PDH in that the exact rates
that are used to transport the data are tightly synchronized to network based
clocks.
Thus an entire network can operate synchronously, though the presence
of different timing sources allow for different circuits within a SONET signal
to be timed off of different clocks (through the use of pointers and buffers.)
SDH was made possible by the existence of atomic clocks.Both
SONET and SDH can be used to encapsulate earlier digital transmission standards,
such as the PDH standard, or used directly to support either ATM or so-called
Packet over SONET networking. As such, it is inaccurate to think of SONET as a
communications protocol in and of itself, but rather as a generic and all-purpose
transport container for moving both voice and data.
