| Synchronous
Optical Networking |
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.
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