Page 205 - Optical Switching And Networking Handbook

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fiber-based OC-1. As the circuit is taken across the long-haul net-
work, it may pass through amplifiers or regenerators in a
SONET/SDH environment. At location B, a few DS-1s are dropped
off, and new ones are inserted back into the empty time slots to be
carried back to location A or on to location C depending on the need.
This is a better way of viewing the true use of add-drop multiplexing.

SONET/SDH Topologies

Several different topologies can be employed in a SONET/SDH net-
work, using the various multiplexers to satisfy the high-speed needs
of the carrier or the end user. These include the normal topologies
most networks have been accustomed to over the years, including
Point-to-point
Point-to-multipoint

Hub-and-spoke
Ring
Dual counter-rotating rings

These variations provide the flexibility of SONET/SDH in local-
and wide-area networks built by the carriers. These are now becom-
ing the method of choice at many large organizations too. In each of
the topologies, larger organizations are finding the benefits of
installing highly reliable interoperable equipment at the private net-
work interfaces and access to the public networks.

Point-to-Point

The SONET/SDH multiplexer, the entry level for an organization,
acts as a concentrator for multiple lower-speed communications
channels such as DS-1/E-1 and DS-3/E-3. This equipment may be
acquired by the end user, or it may be provided as the customer
premises equipment (CPE) by the carrier (ILEC, CLEC, and IEC). In