Moreover, SONET also provides mechanisms that ensure a
fault is reported within 10 ms, making the ???60 ms??? number (from time of fault to
protection switch completion) perhaps more important than the well-known ???50
ms??? number associated with SONET. SONET includes linear (i.e., 1+1) protection
and two varieties of ring protection: Unidirectional Path Switched Ring (UPSR)
and Bidirectional Line Switched Ring (BLSR).
In the late 1980s, several vendors recognized these benefits and began developing
SONET systems. In particular, SONET??™s synchronous multiplexing and ring protection
capabilities enabled vendors to build very low cost multiplexers??”systems that could
sit on an optical fiber ring (the preferred deployment topology because of its low fiber
cost and inherent route diversity) and add and drop traffic at each location on the ring.
These multiplexers, called SONET Add Drop Multiplexers (ADMs), ushered in a new
paradigm in optical access and transport. The ADM has served as the primary building
block for optical transport networks for nearly 20 years. Figure 11.2 illustrates the
SONET ADM.
The combination of SONET??™s benefits and the advent of the ADM has resulted in
the widespread deployment of SONET technology in service provider networks over
the past 20 years, with North American service providers deploying hundreds of thousands
of SONET network elements (most of them ADMs) over that time.
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